Designing Radio Control Model Aircraft
It is probably safe to say that everyone involved in this hobby has dreamt of some really cool
model airplane designs. I've had ideas for glow engine powered FrisbeesTM, a canard sort of
thing with a sleek migrating bird shaped fuselage and an inverted gull wing having twin ducted
fans in pods slung underneath, a Bücker Triplane and a lot of things I can't remember.
The Frisbee thing may not work out but I once tried bolting a Cox .049 to the center of one. It
was a resounding failure that revolved slowly all the way to the ground. The rest of the designs
could be made to fly but I may never build them.
Most of my designs that I've built have flown. My early attempts were not stellar examples of
anything other than what can be produced by a fledgling modeler with flawed design ideas.
Nevertheless, I've always found designing model aircraft to be enjoyable. I have no interest in
advanced aerodynamics so I do what many others do — live by rules of thumb, learn from
experience and apply the laws of aerodynamics that I do know and understand.
If any of my designs were ever subjected to wind-tunnel testing I would probably not be pleased
by the results. I'm sure my designs have aerodynamic flaws that I don't know about. But that's
why I don't care. I've learned a lot over the years and I'm very pleased with how my models fly
for the most part.
Regardless of my lack of expert aerodynamic knowledge, I design most of my own R/C aircraft
simply for the reason that it's fun and I can do anything I want. I began designing my own
models because I had wood lying around that needed something done with it and I figured I'd try
my hand at it.
I continued to design models because at the time kits weren't all that great. Most of them were
poorly die-cut and often the wood was warped, too heavy or had other defects. Now I design
because I enjoy it and to have models that are unique and are purpose-designed to do what they
do better than anything commercially available.
It is not my intention to encourage everyone to design model airplanes. I simply want to pass
along what I know for those who want to try it. These articles may help you learn how various
design parameters affect a model even if you have no interest in designing your own.
This series will cover is a variety of model aircraft design topics while leaving you to put it all
together and do the critical thinking necessary to package it into your own successful design.
Introduction to Radio Control Model Aircraft Design
At the risk of stating the obvious, the first step in creating a model aircraft design is to decide
what kind of aircraft you are designing. Stating that you want to design a trainer would be
meaningless if it weren't for the fact that everyone knows what a trainer is.
Mention the word trainer and most of us picture a model that looks pretty much like any trainer
we've ever seen and we think it's obvious that's what a trainer is and should be.
But what if you were to design the first R/C trainer ever? If what any given aircraft type
"should" be is immediately obvious then our less successful ancestors would not have chosen to
cover their arms with feathers and jump off a cliff. Trainers haven't changed much over the
years, but other types have.
For example, look at precision aerobatic models. If everyone knew what it should be then it
would not have taken so long to develop them to the point where they are now (and there is
much more development still to come — look back to today 20 years from now).
If you want to design an exceptional model then you can't just choose a type of model and expect
everything to fall into place.
Generic goals such as, "I want an aircraft that flies well and is aerobatic" sound good but are
meaningless.
A Sig Kadet flies well and is aerobatic. Coincidentally, this also describes a Sukhoi, a Piper
Cub, an F-15 and almost anything else that can fly in a controllable manner. It's easy to create a
successful design if you let the model define the design after the fact even though it might not
even be close to what you originally wanted.
Additionally, the commonly asked question, "How big should such and such model be for
<insert engine size here> be?" is fundamentally flawed. A much better approach is to choose a
powerplant, then set a target weight based on various factors. From there a target wing loading
is chosen, again based on whatever flight characteristics you want your model to have. The size
of the wing is calculated from this information.
Sorry, but that don't count
If you want a standard sport model, then copy a good standard sport model and change the
outlines to suit your tastes. If you don't think anyone does this, just look at all the "new and
original" designs in some of the R/C magazines that are nearly exact copies of 100 other "new
and original" designs published by the same magazines over the past couple of years.
I don't consider changing outlines to be "designing your own." At best it should be considered
re-styling. But it will get you started so it is not a horrible thing to do either. However, please be
courteous enough not to submit it to a magazine. There are more than enough Kadet, Ugly Stik
and Kaos clones out there already that people are taking credit for even though they don't
deserve it.
I read a web page a while back entitled, "How to Design a Radio Control Airplane." As I read
through the page I realized that the title is misleading and wrong. The author of the article
provides a set of cookie-cutter parameters for one design — you guessed it — an Ugly Stik or
Kaos clone depending on the wing location and whether you decide to taper it (Kaos). A better
title for the page would be, "Copy my design please."
All moments, areas and ratios are pre-determined leaving the "designer" only to determine the
size of the aircraft and choose an airfoil. All aircraft built from the given parameters will
basically be the same aircraft.
As far as I'm concerned that's not really designing an airplane. It's simply copying someone
else's design. When you think about it there are very few new ideas in our realm. We are
standing on the shoulders of giants. That doesn't mean we can't mix it up a little to design
something somewhat out of the ordinary or just to further improve an already existing design.
I believe that if someone is going to say this is how to design a model airplane then he should
cover how things actually work and how each parameter affects the airplane. A set of prepackaged numbers doesn't do that.
I encourage you to build your original design. There is absolutely no reason why you can't.
Build a canard, a flying wing, something out of Star Wars or whatever. You will realize it is not
a black art and it doesn't require an aerospace degree when you see your creation take to the
skies. Even if your design doesn't fly (which is unlikely) you'll still learn a lot. An honest
attempt is never a failure regardless of the outcome.
The one thing that you need to know is that if you design something that looks right it will fly. I
can almost guarantee your success if you build the airframe straight, strong, light and get the
center of gravity close to correct. If the model is a standard planform then put the CG between
27% and 33% MAC to start.
THE PROCESS
Establishing a Design Specification for a Radio Control Model Aircraft
This is the first and most important step to designing anything. It is also the most neglected.
A specification allows you to take a vague concept and turn it into specifically what you want. It
should detail everything that is important to include and exclude from your design.
The following specification template is not all inclusive but should give you some ideas as to
what a specification can include.
What you are doing is defining what the model should be and especially what it should do as
closely as possible.
If you are building a model primarily for flying characteristics, then that is what you
design around.
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Purpose of the model
Style — Modern, Old Timer, Scale, Sleek, etc.
Powerplant class
Flight time
Stability — Should the model be self-stabilizing, neutrally stable or somewhere in
between?
Airspeed envelope
Vertical performance
Control response
Stall characteristics
Construction methods — Traditional wood, composite, etc.
Control system
Landing gear system
Break-down for transportation
Etc.
With the exception of trainers, surviving a crash has no place on a specification. Design and
build your models to fly very well and expect them to be a write-off in a crash. If you really
want one badly enough again, you'll build it.
There may be problems that need to be solved regarding building techniques or installations that
you already know you don't have the answers to.
For example, I might want to build a model having an airfoiled-tube-shape fuselage. I honestly
don't know how I would do that. The first thought that comes to mind is fiberglass, but it's not
something that is worth the expense of investing in all the supplies to get started making molds.
I could turn a male mold from foam, but it would be prone to warping at the thin areas... etc.
The above is a problem that I am going to solve before I even consider any other part of the
design. If I can't solve it then the project is over.
I can also take the attitude that I have supreme confidence in my building-problem solving and
my building skills and then go ahead and start building knowing that somehow I'll complete the
mission.
Model Aircraft Design Step-By-Step
This article describes the process I currently use to design a new model aircraft design. The
information will be edited over time as I learn more.
As I mentioned on the Model Aircraft Design home page, my early designs were nothing to brag
about. I did not define goals for each model nor did I have a sound design philosophy. I had
vague ideas and started building. This often resulted in having conflicting goals that weren't well
thought out.
There are three things you should get from this discussion if nothing else:
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Compromise is a recurring theme in model aircraft design.
A model aircraft designed to excel in every flight category won't. It simply isn't possible.
Before you do anything else you should work out specific goals and a specification for
your design.
Most non-specific sport models such as Stiks and Super Sportsters are good middle of the road
designs. They aren't exceptional at anything but they also don't have any particularly bad habits.
They're easy to build and fly but that's as much as can be said for them.
When you purpose-design a model it will do what it is intended to do very well, but other flight
characteristics may be precluded altogether. Additionally there may be the risk of one or more
devastating flight characteristics such as vicious tip stalls under some conditions. If you
understand these risks and don't fly your model in those realms you will be pleased with your
design.
With Each Step — Establish Engineering and Construction Methods
This is something you should be mulling over throughout. Always stay focused on your design
goals, target loadings and finished weight. The structure should be strong enough for only the
intended flight envelope plus a reasonable safety margin.
Challenge your building skills but stay within the realm of what you can accomplish
You may have an idea for a beautiful blended-body design, but can you actually build it? Maybe
you can't now but as your skills increase it might be very possible in the future.
Step 1 — Create a Specification
I can not stress strongly enough that this is the most important step to creating your new design.
If you haven't read Creating a Design Specification for a Radio Control Model Aircraft then I
suggest you do so before you begin your new design.
Step 2 — Choose a Powerplant
I am firmly against designing a model for a wide range of engines. This practice results in a
model that has too many engineering compromises. The model must be structurally designed
for the largest engine in the range but it won't necessarily be the best aerodynamically. I hate to
be the bearer of bad news, but you can't have it all.
For example, if the model is designed for a .25 - .40 engine then the model has to be strong
enough for the largest engine in the range, a .40 in this case, which will result in an airframe that
is too heavily built for the .25 engine.
Step 3 — Establish Vertical Performance and Airspeed Envelope
Rate of climb is determined by the powerplant, propeller and the aircraft's ready-to-fly weight.
What kind of climb do you want your model to have? Should it accelerate going straight up even
from zero airspeed? Do you want it to have enough power to climb a few hundred feet before it
runs out of momentum? Should the model loop from level flight or is diving to gain speed
acceptable? Decide how heavy the model should be then design and build toward that goal.
Airspeed (minimum and maximum) is determined by the engine, propeller and wing.
Rate of climb and airspeed influence each other to the extent that they always compromise each
other. If you want the model to fly fast and also be able to climb straight up indefinitely you will
need to build the model lighter. A fast (high pitch) propeller doesn't have the same lugging
power as a slow (low pitch) prop.
Ultimately you may need to compromise rate of climb to achieve the desired airspeed or viceversa.
Fill in the blanks
Weight Ready to Fly ounces
Minimum Airspeed
mph
Maximum Airspeed
mph
Rate
of
Climb
(description)
Step 4 — Design the Wing
One thing I want to make very clear is that the wing is the airplane and by far the most
important component. Many parameters must be considered at the same time and weighed
against each other. Again we will need to make many compromises to determine what are we
willing to give up to gain something else.
You can begin by choosing a family of airfoils that can conceivably meet the specification.
Pinning down a specific foil will be dependent on which specific design goals are most important
to achieve and which ones can be compromised.
For example, if aerobatics are a primary goal then you would certainly choose a symmetrical
airfoil. The specific airfoil within that family will depend on other factors such as airspeed and
desired stall characteristics.
Rule out airfoils that can not stall as desired. An airfoil that has a gentle, difficult to enter stall
may be a poor choice, but an airfoil with an unpredictable or vicious stall could mean a short life
for the model.
The table below is intended to demonstrate how easy it is to become bogged down in a quagmire
of indecision. Note that each parameter in the chart affects every flight characteristic somewhat.
Characteristics that are marginally affected are ignored here.
Design Parameters
Flight
Characteristic
Airspeed
Roll Rate
Stall
Stability
Airfoil Wing Loading
Aspect Ratio
Dihedral Washout
Aileron
(Area/Style)
Lift
Capability
Lift/Drag
Ratio
Aerobatics
Affects characteristic
Let's wade ourselves out of the bog by concentrating on what's most important, taking it to the
extreme of efficiency and then scaling it back as necessary so that the model is practical to build
and to avoid creating a related characteristic that is devastating.
Wing Loading
How to Calculate
Wing loading is a goal you should design and build to — not a surprising discovery at the end of
construction.
The wing loading is a compromise of several flight characteristics — low speed flight,
predictable landing approach, rate of climb (lift from the wing, not pull from the engine), control
response, and how easily the plane is upset in flight.
The lower the wing loading, the slower the model can fly. The higher the wing loading, the
more predictable the airplane is on landing approach. Light airplanes are strongly affected by
pockets of rising and sinking air which makes it very difficult to spot land the airplane. By the
same token a heavier model is less affected by wind but is also slower to respond to control
inputs and must fly faster to stay in the air.
Designing to a light wing loading may restrict the plane to lower top end speeds to prevent wing
failure during high-G maneuvers. For example, the wing may fold if you build a large, light
wing and then yank the plane out of a dive. This is not because the aircraft is light but because it
may be more frail due to the lightweight structure. But the lower inertia of a light airframe also
imposes less load on the wing. It is very possible to build a wing that is both light and strong.
Wing Area
How to Calculate
Wing Area is not included in the chart because it is virtually meaningless. All the wing area
does is allow us to calculate the wing loading. It is better to determine the wing area based on
the target wing loading which is based on target weight.
For this example we're building a model to weigh 7 lbs with a wing loading of 20 oz./ft2.
Plug those numbers into the wing loading equation to find the wing area:
Given:
Wing
Loading
Target Weight = 7 lbs
=
oz./ft2
20
Find the Wing Area:
Wing
Loading
=
(Note that Weight is in pounds)
(Weight
x
2304)
÷
Wing
Area
Rearrange the equation to find the wing area:
Wing Area = (Weight x 2304) ÷ Wing Loading
Plug in given parameters:
Wing Area = (7 x 2304) ÷ 20
Wing Area = 806.4 in2
Now we know what to do — build a 7 lb airplane having about 800 square inches of wing.
When building a kit you have to hope and pray that the design and included materials make it
possible.
Often a kit comes in at a weight higher than the manufacturer specified. While I don't know this
for a fact, I suspect the published recommended weight is based on a prototype built by the
designer with hand-selected wood which isn't what comes in the kit.
You should easily be able to stay at or under the target weight of your design because it's your
engineering and you select the materials.
Aspect Ratio
How to Calculate
In the chart above you can see that aspect ratio
strongly affects nearly every flight characteristic. It
is one of the most important decisions you will make
so consider it well.
A banked turn describes a
cone. The lower wing tip (the
pointy end of the cone) is
moving through the air at a
The aspect ratio of the wing affects several areas of
slower rate than the opposite tip
flight such as roll rate, lift-to-drag ratio and pitch
(the open end of the cone).
sensitivity.
When the lower tip stalls, the other end of
Not the least important flight characteristic strongly the wing is still lifting.
affected by the aspect ratio is the wing's propensity to
tip stall — particularly in banked turns. Wermacht The stalled tip falls from lack of lift but
lift from the wing that's still flying flips
explains in the sidebar.
the plane over. The aircraft may enter a
Most of us aren't terribly concerned with fuel spin from which it may not be able to
efficiency, but for some specialized tasks this is a recover.
high priority concern. The most aerodynamically
efficient wing in terms of lift to drag ratio will be one This slick maneuver is an especially
entertaining way for someone else's plane
of extremely high aspect ratio.
to crash.
For example, short, stubby wings are not even considered an option for these types:
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Sailplanes
Go a long way using very little fuel.
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Airlines
Money spent on fuel is less money in the bank.
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Voyager by Burt Rutan.
It's sole purpose was to fly non-stop around the world without refueling and then retire.
A high aspect ratio wing has a better lift to drag ratio and is generally more efficient than a low
aspect ratio wing. If the aspect ratio is too high the plane will have a sluggish rate of roll and is
easier to break.
As the aspect ratio of a wing becomes lower the aircraft becomes more maneuverable in roll and
less efficient in lift. That's why you never see fighter aircraft having high aspect ratio wings and
you don't see bombers with low aspect ratio wings with some special exceptions.
If the aspect ratio is too low the plane may be twitchy about the roll axis and slow down
excessively in turns. Low aspect ratio wings have tremendous drag as angle of attack increases.
Low aspect ratio wings are inefficient and not good for load lifting.
Sounds like we need to compromise again.
Wing Taper
Elliptical wings are very efficient but difficult to build — particularly elliptical wings having
elliptical thickness. Wood doesn't like compound curves. Some designs get around this by
adjusting the airfoil (rib height) to create a straight taper in thickness from root to tip which
never looks right.
The way to build a wing easily while approaching the efficiency of an elliptical wing is to build a
tapered wing.
The Taper Ratio of a wing is simply the Tip chord divided by the Root chord. High aspect
ratio wings with low taper ratios (tip chord much less than root chord) are extremely prone to tip
stalls so it is best to avoid using both on the same wing.
If you want a highly tapered wing then keep the aspect ratio down. If you want a high aspect
ratio wing then keep the taper ratio closer to 1 (same root and tip chord).
Knowing the taper ratio, aspect ratio and wing area allows you to calculate the root and tip
chords assuming the wing does not have multiple tapers.
Wing Sweep
I am told that sweeping a wing rearward is equivalent to adding dihedral (each 2-1/2° of sweep is
equivalent to approximately 1° of dihedral). Presumably it works even when the aircraft is
inverted. Sweep also makes an aircraft more stable because it causes the aircraft to pitch down
in a stall.
Sweep somewhat broadens the CG range of an aircraft as well as moving the CG rearward.
Lastly, sweep makes the aircraft appear sleeker.
I have never built a forward swept wing and don't expect I will ever design a model that uses
one. You'll have to find another source of information if you want to build a forward swept
wing.
Dihedral
Dihedral has two distinct aerodynamic purposes that immediately come to mind:
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Increased stability
Allows an aircraft to be steered with the rudder alone (no ailerons)
As a corollary to the second bullet, dihedral can also add undesired control coupling. Control
coupling occurs when one control causes the aircraft rotate about a different axis than intended
such as pitching or rolling when rudder is applied. Aerobatic ships in particular should have as
little coupling as possible. Often the dihedral needs to be adjusted to remove roll coupling
caused by the rudder.
Unfortunately there is no way to know in advance how much dihedral will be necessary so we
make our best guess based on previous experience. As Don Lowe will tell you, if you want your
ship to be tuned for competition you need to be willing to cut the wing apart to adjust the
dihedral.
I have no interest in competition or precision aerobatics and I don't cut my wings apart, but you
might need to.
I've found that approximately 5° of dihedral works very well for a rudder-only model. Too little
dihedral will make turns sluggish. Too much dihedral will make the wing inefficient.
Washout
Washout is a deliberate warp built into the wing so that the wing tips fly at a lower angle of
attack than the wing root. The purpose is to delay or prevent tip stalls.
I have never found washout to be necessary for sport models — especially constant chord wings
which by their nature are very resistant to tip stalls.
Washout on an aerobatic ship is a bad move because these types of models should fly as
neutrally as possible. An aircraft having washout will have wash-in when the model is flying
inverted.
Cases where washout will be beneficial are aircraft such as those with high aspect ratio wings
(sailplanes, airliners), heavy scale models, utility aircraft (camera platforms) and other models
not intended to perform precision aerobatics.
Aileron Style and Area
Strip ailerons are easier to build and in my experience have a better roll rate than barn door
ailerons. Tapered strip ailerons tend to work best with the least possibility of flutter.
Aileron area is usually 10% - 20% for strip ailerons and up to 25% for barn doors. Again, it
depends on what you want the plane to do. Don't do anything too radical for your first designs.
Once you have some time on the prototype you can adjust things on the second prototype.
Fill in the blanks
Wing Loading
oz./ft2
Wing Area
in2
Aspect Ratio
:1
Taper Ratio
Root Chord
Tip Chord
Wing Sweep
Dihedral
°
Stall performance
(description)
Washout
Airfoil (root)
Airfoil (tip)
Step 5 — Determine Pitch and Yaw Rates
Untested Theory
It is my belief that (theoretically) the
horizontal stabilizer can have any amount
of area and any length moment.
As the pitch moment becomes longer,
the stabilizer can become smaller. As the
moment becomes shorter the area must
increase.
However, I believe that this rule ensures
that there is wide latitude for placement of
the center of gravity.
If we mix and match we can have a short
pitch moment and a small horizontal
stabilizer. However, this will make the
The fuselage is nothing more than a lever to mount CG much more critical — perhaps to the
flying surfaces, the powerplant and various systems. point that a change in fuel level will move
the CG enough to make the aircraft too
As far as I'm concerned the only consideration for the unstable to fly.
length of the fore or nose moment is so that the
aircraft will balance as closely as possible without We also have to consider that if we take
adding ballast.
this idea to the extreme then we also have
to depend on the air being perfect (which
The length of the pitch and roll moments determines it isn't) while having perfectly uniform
the rate at which the aircraft rotates about these axis. density (which it doesn't).
The moment itself does not make a model more or
less aerobatic, but determines the rate and size of the I haven't tested this theory so if you want
maneuver.
to, then you do so at your own risk.
Longer moments make for larger, smoother aerobatics than a shorter moment.
Most people will tell you that the combined area of the fixed stabilizer and elevator should be
about 20% of the wing area. You can't go wrong with that number but it is a very conservative
and safe area that doesn't always make the most efficient setup.
The vertical stabilizer, including rudder, are usually about 8% - 15% of the wing area. As with
the horizontal stabilizer, the size can decrease as the moment increases and vice-versa.
A good starting point for ratio of moveable surface to fixed surface is that 10-40% of the overall
surface should move. You can adjust that however you like. 3D aircraft often have moving
surfaces that are larger than the fixed surface as well as extreme control throws.
Even though I don't like doing it, I will cut off tail surfaces and replace them with new ones to
improve a design. Many people consider a design successful just because it flies, but frankly
that's not much of a feat. If you want your designs to be excellent then consider the prototype to
be a test bed and be willing to cut it up.
Step 6 — Lay it Out on Paper
At this point we don't care about the outlines of the design. We can make it pretty later.
Start by drawing a reference line on your paper. Locate the wing as desired. For a more stable
design, move the wing higher. For a neutrally stable design place the wing on the reference line
(as well as the stabilizer and engine).
Determine the horizontal stabilizer location and draw it.
At this point I start sketching outlines. If you want your design to achieve high speeds then the
fuselage should be aerodynamically clean and have as little frontal area as possible. Fillets help
prevent turbulent, high drag airflow.
If you want the fuselage to support knife edge flight then it should have plenty of side area with a
large percentage of that area toward the front of the aircraft.
If you want to prevent speed build up then the best way is to design a thicker wing. You can also
increase the drag of the fuselage by designing more frontal area or adding a blunt wind screen
and cowl rather than a streamlined cowl. These items create turbulent drag which is why they
are not as good of a choice as building thicker flying surfaces.
I honestly have no way to determine in advance how long the nose moment should be. I
normally flesh out the design from the wing to the tail and then visualize it in 3D in the bare
construction.
From there I locate the engine where it looks like it will balance the model. I'm usually pretty
close although my models tend to come in nose heavy. Another consideration is that I try to
make the fuel tank fit entirely in its compartment rather than allowing it to extend into the radio
compartment.
Step 7 — Build It
Then fly it a lot. Learn what you can, make practical adjustments and then build another. When
the aircraft flies as you originally specified then it's ready for prime time.
A Model Aircraft Design Case Study
To give you an idea of how I put my design philosophy into practice I will go through the steps I
recommend to create a radio control model aircraft design.
The resulting model invariably differs from the original specification for a variety of reasons.
Small aircraft in particular have limited space for mounting everything. The only way to know
for certain that everything will fit as intended is to either draw a plan or build things larger to
cope with unforeseen problems.
Additionally, something that seemed to work out ok in theory may not look right in practice. A
built model always looks larger than in a plan view. I have a bad habit of drawing fins that are
too small because at the "right" size they tend to look huge on paper. As I'm building the model
the fin starts to looks too small. That may be a problem unique to me. I have tossed out more
than one fin and built one that is larger.
The point being you should be disciplined about creating a design specification and building
towards it, but you should also be flexible to change things that aren't right. If something looks
wrong it very well may be. Adding a few inches of area to a flying surface won't add appreciable
weight.
Case Study 1
Background
My buddy Mike has an idea for a model that I think is very interesting. He has been spending
his time locked away in a secret location hashing out the details so I won't give anything away
here. This design is a cousin to his.
The airplane is intended to be very pure — sleek, semi-fast, very smooth and aerobatic but not
touchy on the controls.
Specifications
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Aerodynamically pure and clean.
Neutrally stable.
Not compromised aerodynamically to improve aesthetics.
Cruises efficiently at 50-70 MPH with a top speed of 90-110 MPH.*
Strong to unlimited vertical performance.
Crisp, smooth and predictable control response in all axis.
Predictable stall characteristics.
Minimal control coupling. E.g. rudder does not cause pitch or roll.
Controls and linkages designed and built to prevent flutter within airspeed envelope.
Ailerons controlled by individual servos to reduce slop and allow flaperons.
Flying elevators will be considered for feasibility. Each elevator half may be controlled
by its own servo to allow the incorporation of ailervators.
Fast mini servos of adequate torque will be used throughout.
Installed systems to be simple, accessible and reliable.
Meets specifications powered by a strong sport .40 2-Stroke engine. A racing engine or
tuned exhaust system is not required, but the plane will have the best aerodynamics with
a rear-exhaust piped engine.
Capable of 10 minute flight times.
Structure stressed to handle high speed maneuvering.
Built utilizing conventional construction techniques and readily available materials.
Wing panels remove for transportation.
Landing gear is optional. If used it should be retractable or extremely sleek.
A color/trim scheme will be used that clearly differentiates the top from the bottom of the
aircraft to prevent disorientation.
* Given speeds are what I believe them to be as I really don't have any idea how fast 100 MPH
is. I just know how fast I think it is and that's what the design should achieve.
Non Concerns
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Low speed flight
Knife edge flight
3D aerobatics
Crash resistance
Notes
This is simply a specification that I will strive to accomplish. The end result will most likely
differ somewhat for practicality. For example, I probably don't want to hand-launch a .40
powered airplane. A .40 can pull pretty hard and there will be a safety concern trying to hold the
model while launching it into a desirable flight attitude.
Even though a landing gear may compromise the aerodynamic integrity of the design I may not
have much choice about it. I'll do the best I can.
Another possibility is that the aircraft will not fly as intended. However that possibility is
significantly reduced simply because I have clearly stated what I want. I will consider
everything I know about each parameter to help ensure I reach my goal successfully.
Note that the specification did not mention the size of anything except the engine. Ultimately the
specification must be met as closely as possible using the engine as a base point.
This is a conventional aircraft with no particularly radical features. The model will be similar to
pod and boom sailplanes but having a lower aspect ratio wing that can handle higher flight loads.
The aircraft will be aesthetically pleasing due to its aerodynamically clean appearance and
simplicity. All I care about is what the air sees. The design will not be compromised just for
looks.
The model will be primarily of wood construction, fully skinned, fiberglassed and painted. The
airframe can be built to its target weight with proper engineering, material selection and
construction practices.
Target Weight
The vertical performance requirement dictates a target weight (dry, ready to fly) of no more than
4 lbs.
Wing Loading
I can build to any wing loading I want. What I don't want is the model bouncing around in the
air with a low wing loading or the sluggish roll rate of a high wing loading (assuming at least
some of the weight is in the wing).
Therefore the wing loading will be in the range of 16 to 18 oz/ft2. I could choose a higher or
lower wing loading and build to it. This wing loading is a good compromise to achieve the most
desired flight characteristics.
I never go over my target weight and often beat it by up to 20% because I stay focused on the
purpose of the model and don't let myself get off track doing the "what if's". For example, I don't
start thinking about maybe putting a .46 engine in the model or building it to survive something
it isn't intended to do.
If the model isn't intended to be yanked out of terminal velocity power dives, then don't build the
wing to survive it. You're giving away weight. Of course it goes without saying that if you don't
design the model to do something then you probably shouldn't turn around and attempt whatever
that is with the model.
Remember that if you build the model too light you can always add ballast.
Design Specifics
The Airfoil
There is no question that a symmetrical airfoil is the best choice for this model. The only
question is how thick it should be. I've had good success with the NACA 00XX airfoils and will
probably select one in the 11% to 14% range. Depending on how much I choose to taper the
wing I may use a thicker tip airfoil than at the root to help prevent tip stalls.
I really don't think tip stalls will be a problem though so I will probably use one airfoil for the
entire panel. I'll look at the charts before making a final selection but my first thought is to use a
NACA 0013.
The Wing
The wing will taper and have an aspect ratio in the range of 7 to 8:1. It will be all wood
construction with plenty of ribs, sitka spruce main spars, shear webs and a full skin. A second
pair of spars may be necessary for the aft portion of the wing. The wing panels will slide onto
tubes permanently glued into the pod fuselage.
Knowing the weight and wing area allows me to determine the wing area which in turn allows
me to calculate the wing span based on the aspect ratio (A/R). Figures in the table below are
rounded. The chords given below are the average chords. Again, the target weight is 4 lbs (64
ounces).
Wing Loading Wing Area
Span (Chord) w/7:1 A/R Span (Chord) w/8:1 A/R
18 oz/ft2
512 in2
60" (8.5")
64" (8")
16 oz/ft2
576 in2
63" (9")
68" (8.5")
The wing will be thinner than a typical aerobatic sport design in order to achieve higher
airspeeds. Narrow strip ailerons will be shaped as part of the airfoil rather than flat plates. The
ailerons will be sealed to prevent efficiency losses, drag and reduce the possibility of flutter. The
ailerons will be driven by individual servos that are buried in the wing to prevent drag. Another
flutter reduction measure will be ending the ailerons short of the wing tips.
The wing will probably be too thin to house retracts that are reliable so I might build this plane to
be a hand launch/belly lander. Other options are wire gear mounted in the wings or a dural gear
mounted to the pod.
Fuselage Specifics
The fuselage will be pod and boom construction. Formers can be turned using a drill or drill
press. The pod will be planked with balsa and can be built using the carbon fiber tube as a jig.
The engine will be mounted upright and fully cowled similar to that of control line stunt ships.
The pod must hold an 8 oz. fuel tank, three servos (four if dual elevator servos), receiver and
battery pack.
The model will be as symmetrical as possible about all axis. The thrust line, wing centerline and
horizontal stabilizer centerline will be located along the centerline of the pod and boom. A small
degree of right thrust will be incorporated for trim purposes.
The boom will be a lightweight carbon fiber tube. In this case I will have to find a tube that isn't
too heavy but also isn't too flexible. Off the top of my head I would say a thin wall 1/2" diameter
tube should be close to the right size. Another consideration is that the tube isn't weakened too
much by the exits for the control system.
The model will have a generous tail moment with small, thin airfoiled flying surfaces. The fin
can not extend too far below the boom or it will be damaged on landing. I can make the fin
longer in chord and shorter in span to get the area I want while putting some of it below the
boom. This will allow me to balance the areas better than if it had a longer span and shorter
chord. Sort of like an arrow.
Using the "that looks about right" method, I'll start by designing a horizontal stabilizer having
approximately 16% of the wing area. From there I'll adjust it in proportion and size until it looks
right. The model may have a fully flying horizontal stabilizer depending on whether it can be
practically implemented.
Evolution and Radio Control Aircraft Design Development
Myself and others often design a model and take what we get with no intention of further
developing the design. It's not the way to create a design that is as good as it can be, but if the
model is for our own personal use and we're happy with it then that's all that matters. With
enough experience and a relatively standard design it isn't too difficult to achieve fair success
with an initial prototype.
Unfortunately, many who publish their plans as well as manufacturers appear to take this
approach with their designs. It's obvious the model hasn't been properly developed when the
flight trimming phase reveals that the wing incidence, decalage, dihedral, thrust line and other
items need to be significantly changed for the model to trim properly.
95% of all sport kits on the market can be built by the average builder on their bench with no
plans. They are simple, standard designs. That's why you don't see many of those kits in my
gallery. Why pay for a kit that I can build from better materials without financially driven
compromises imposed by manufacturers?
As soon as you begin building you should be looking at what you could do better the next time.
My designs are usually better developed when I draw even minimal plans rather than making
things up as I go along.
Either way I often find that in practice I could have improved some things by doing them
differently. This aspect mainly involves engineering, contruction and control systems.
From an aerodynamics standpoint, design development begins with the first flight of each
prototype. It is important that you take the time to properly trim your new model. Most guys get
the plane flying straight and level, make an adjustment or two to the CG and then consider their
aircraft trimmed. It is all done in a weekend or two. Talk to any aerobatic pilot and he'll tell you
that it takes gallons of fuel to get an aircraft in trim.
Get a trim chart and use it. If you fail to trim your model then anything else you do to correct it
may be an improvement to your model, but not to the design because you are basically treating
the symptoms and not the disease.
One thing that always bugs me is when I hear the comment, "<Insert aircraft here> is the best
model I have ever flown" - especially when what I am looking at is basically a rehash of a Stik
model. My first thought is, 'It is probably the only one you ever built that was close to straight
and close to being in trim.' The fact is that most models will fly very well if built and trimmed
properly.
After the model is trimmed fly the model as much as possible to learn about it. Specifically,
perform sets of maneuvers you want it to be able to do to determine which characteristics need
improvement. Dedicate entire flights or even flying days to learning about one particular
characteristic of your model.
Always use the previous prototype as a starting point
If you want to build a 3D aircraft that compliments your flying style, then the wrong way to go
about it is to design a model, decide it does not do what you want and then design a whole new
model. Work on improving the prototype and you will be more successful than you will be if
you keep starting from scratch.
Make one change at a time to improve the prototype. If that means cutting off the tail feathers
and building new ones of different proportions then do it. If the ailerons need to be a different
shape, then make new ones. Try different propellers, changes to the CG, thrust line, wing
incidence, control throws/differential, etc.
Trim charts do not apply only to aerobatic models. Use one for reference to help you determine
why a model flies the way it does and what to do about it.
Build the next prototype only when changes to the previous prototype are no longer a significant
improvement or changes would be impossible or impractical to make.
Normally it is a poor practice to make more than one change at one time. However, if there are
several flight characteristics you want to improve then you should make all the changes you
think will improve the design unless you want to build one hundred prototypes (which would
probably be better in theory, but a nightmare in practice).
Rule out — do not guess — as many potential reasons for a performance problem as possible.
For example, the prototype may have a sluggish roll-rate. This could indicate one or more of the
following:

Control surfaces
o Too large or too small
o Excessive hinge line gap
o Sloppy hinges
o Flexible control surfaces
o Surface deflection too small
o Blow-back (weak servo or flexible linkage)
o Slow servo

Wing Design or Construction
o Aspect ratio is too high
o Excessive mass in wings or heavy outboard sections/tips
o Flexible wing (twists under flight loads)

Etc.
Don't give up on the prototype even if the problem is severe. Several of the above possibilities
can be ruled out without having to build a new model:

Seal the hinge line
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Adjust the linkages for more or less control surface throw
Strengthen the linkages
Replace the servo with one that is stronger or faster
Replace the hinges with hinges having less play
Build new control surfaces
If the problem still exists, then the only way to resolve it may be building another prototype.
Before you begin building the next prototype you should continue ruling out process for each
flight quality you want to improve. Then when you build the next mark you can significantly
improve the design.
Now that you have ruled out as many potential reasons for undesirable flight qualities as possible
you need to consider remaining possibilities and choose which one(s) may be the cause. This is
where experience is helpful because you might have to guess, but if you eliminated items then
the choices should be very limited which increases your chances of picking the correct fix. No
matter what you choose you don't want to make the problem worse but that is always a
possibility.
In the above example of an aircraft having poor roll response, you may have eliminated all
linkage/servo possibilities leaving only aerodynamic causes such as airfoil or control surface size
and proportions. If you are stumped at this point then look at other models similar to yours that
have the response you want yours to have. Adjust the control surface design to more closely
match that of the other designs.
Model aircraft design is not as complicated as it may seem, but it does require some experience
with different aircraft to learn how each parameter affects an airplane's performance.
Design and Build Model Aircraft for Light Weight and Strength
I have finally heard one conversation too many discussing the weight of glues in the context of
having any significant effect on overall weight of a flying model airplane.
I could build an entire model using nothing but epoxy, which is probably the heaviest glue there
is, and still have a lightweight airframe. The weight of glue in an R/C model is inconsequential
when properly used even if the wrong glue is used.
So why not use epoxy throughout? Because it simply isn't the best glue to use in most cases.
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Epoxy is often harder than the materials it is bonding which makes sanding a smooth
seam difficult.
Epoxy is expensive.
Epoxy is less convenient than single part adhesives.
Epoxy takes longer to grab than water or solvent based glues.
Etc.
But this is not an article about what kind of glue to use in your models. It's about how I've
learned to consistently design and build models that are significantly lighter than comparable
models from other sources and how you can do the same. Generally speaking, if my model isn't
lighter, then it has more features for the same finished weight.
By the way, these are the building techniques I use and advocate. They are not my original ideas
and concepts. Almost every technique I discuss in this article has been around for a very long
time.
These building methods have stood the test of time from when they were developed back in the
days when radios weighed as much as a car battery and engines had half the power they have
now. Builders had no choice but to use building methods that kept the weight low and the
strength high.
New designs, especially those put out by major manufacturers, take advantage of the fact that
radios are lighter, engines are more powerful and the customer base is primarily the instantgratification, it's-not-worth-it-if-there's-actually-work-involved crowd. Many of the most
popular kits today are poorly engineered to the extreme. More about this to come.
The problem that I am seeing is that most designers base their engineering methods on kits that
they've built. In this case it means they are emulating poor designs not knowing that anything is
wrong with them.
Consequently, model aircraft structural design is doing a Darwin in reverse.
Please note that my philosophy doesn't necessarily build a model that will withstand your
piloting or your flying field.
If you know your planes will take a lot of abuse you will have to make them beefier to withstand
it unless you like doing lots of repairs.
Engineering Radio Control Aircraft Structures for Light Weight, Strength and Rigidity
The name of the game for model aircraft airframes, at least the ones meant to fly, is high
strength to weight. That means making engineering choices that result in an airframe that is
only as strong as necessary to withstand flight stresses and ground handling.
Anyone can easily build a model that is strong. We see it all the time in models that are not only
strong, but also much heavier than they need to be. This pitfall is avoidable if you understand
how loads are distributed through the airframe and you target those specific loads and avoid the
temptation to add more structure "just to be safe."
Engineering Choices
Heavy models are almost always poorly engineered. A well engineered model can still end up
heavy due to poor material and equipment selection or poor building techniques. Even so, a
properly engineered model at least gives you a fighting chance. A poorly engineered model is
usually a lost cause unless you take it back to the drawing board and redesign the entire model.
Poor engineering usually is due to one or more of the following:

The designer doesn't understand model aircraft structures and over-designs.
The designer copies poor engineering concepts of other bad designers or he comes up
with all new poor concepts of his own.

The design is compromised to ease building at the expense of additional weight.

Poor strength-to-weight materials are used to decrease costs to the manufacturer.
Additional cheap and heavy material is added to compensate for lack of strength which in
turn adds more weight.
Any model having lite-plywood fuselage sides is weaker and heavier than the same model would
be if it had properly engineered balsa wood sides. Even if the plywood sides have large cut-outs,
Warren truss fuselage construction is lighter while attaining a higher strength-to-weight ratio.
Builders have become so lazy over the years that any time they discuss a model having built-up
fuselage sides, the kit is called a builder's kit meaning that the kit is only for "true builders."
Call it what you want, but one fact will never change — lighter models fly better.
If you aren't willing to do the work to build a lightweight aircraft then don't be surprised when
your models don't fly as well as those built by builders who are willing to make the effort.
What I can't figure out is how the new generation of 3D models that have just a handful of ribs
having large cut-outs and a profile fuselage weighs as much or more than a "real" airplane that I
build that has 20+ ribs in a larger wing. Actually I do know why and here's a clue. Contest balsa
ribs don't weigh anything. You could put 50 of them in a wing and it wouldn't make but (at
most) an ounce of difference in the finished weight.
The lack of ribs in 3D aircraft is only to give the illusion of light weight. More ribs make a more
durable wing having a more accurate airfoil. You get all of this at no weight penalty.
To counter this supposedly lightweight wing, the designers take a thick (heavy) slab of balsa,
slap on some plywood (heavier) around the nose and call it a fuselage. So much for the weight
savings of having only 4 ribs in the wing!
And by the way, take a look at the size of the leading edge and spars on some of these 3D
planks. They are much larger and heavier than what is normally used on more traditional wings.
Heavy spars weigh more than light ribs.
Shown to the left is a rib set for a
biplane having a 7" chord including
ailerons. The set includes 42 full ribs
and 32 half ribs. I cut more ribs than
necessary because the design is not
finalized.
Approximately 3/4 of the ribs are cut
from contest balsa. The rest are
medium or hard balsa to be used in the
wing center sections.
I intend to space the full ribs 2" apart
with one half rib between full ribs.
Therefore there will be a rib every 1" at
the leading edge.
The set of 74 ribs weighs a total of 30
grams (approximately 1.06 ounces).
What that means is that if the design
could somehow use no ribs at all the
weight savings would be only a little
over an ounce.
Nevertheless I will remove the interior
of the ribs for a few reasons. First, I
am paying attention to grams in this
model. By doing so at every juncture,
the overall weight savings will be more
significant.
Additionally, the cutouts will make it
easier to pass servo leads. Lastly, the
ribs will look more attractive under
transparent covering.
The weight has dropped to 21 grams
(approximately 0.74 ounces) after
removing the interior areas. The
overall
weight
savings
is
approximately 1/4 ounce. This savings
is insignificant when taken on its own.
More ribs provide a more accurate
airfoil as well as a stronger and more
durable wing.
Skimping on ribs doesn't make any
sense at all — especially when the
design includes other components that
are heavier and weaker than necessary
such as a profile fuselage built from
slabs of plywood epoxied to a balsa
plank.
Here's the wing almost completed. It
still needs ailerons and a few small
pieces of plywood for the cabane and
interplane strut mounts.
The spars are Sitka Spruce which is
probably the best spar material there is
short of carbon fiber. They are light,
strong yet will flex significantly before
breaking.
The spars coupled with full span shear
webs make an extremely strong beam.
This model is being built for up to a
.30 glow engine, but could easily
handle a .40 four-stroke which is a
heavier engine than I would use.
If you want your models to have stellar
performance, then high strength to
weight is the name of the game.
That means a lot of strength and very
little weight. It can be done. Attaining
this goal is more work, but it's worth
it.
By the way, the wing ended up using
17 full ribs and 16 half-ribs. I'll have a
bunch of full ribs left over.
Designing a Lightweight Model Airplane
The most important thing to keep in mind when designing a model is to learn to use lightweight
materials arranged such that they spread loads rather than using plates and sheets to over-build a
structure. My guess is that this poor technique is mostly used by designers who don't really
understand the loads on a model so they just make sure there's lots of material in there to ensure
nothing breaks. It works, but adds lots of dead weight.
Wood has grain in only one direction. More often than not, loads come from multiple
directions. That's why a lot of designers use a lot plywood. The ply's in the sheet are arranged
such that the grain of each ply is 90 degrees to the adjacent ply. Plywood is great stuff — for
building houses and other structures that are not supposed to fly.
Plywood in models should be used only as a last resort when nothing else will work. It should
not be relied on as a crutch simply to ensure something is strong enough that could have been
strengthened through significantly lighter means.
Always ensure that joints are a good fit. They are stronger and lighter than an ill-fitting joint
that uses excessive glue to fill the gap. Make it a habit to use gussets and other small,
lightweight reinforcements when necessary rather than slapping on plywood plates.
Clamp joints or use weight whenever possible. You would be surprised how little glue is needed
to hold a joint together if it is under pressure while it dries.
When laminating, for example, you can coat both parts and squeegee as much glue back off as
you can get. I'm not exaggerating. Put the parts under a lot of weight while the glue dries and
there will be no separating them.
I often make my own plywood so that the part will have structural integrity while being smaller
or having large openings.
Wing Design
Wings vary wildly in weight for comparable areas. Properly engineered, any wing can be very
light and very strong. Use contest balsa for everything but the spars and leading edge. The
leading edge can be contest balsa, but it's prone to dings and dents so harder wood will help with
durability.
When I design a wing, I always start by drawing the airfoil to determine the thickness of the
wing. The next thing I do is determine how far apart the spars can be. The farther apart the
spars are, the stronger the wing. What I mean is the vertical distance between a pair of spars
(upper and lower).
Once I have the distance, I design a beam that can support the entire load of the wing while
keeping in mind any sheeting used will reinforce the wing somewhat. For example, leading and
trailing edge sheeting do add to the strength of the wing assuming the center is glassed.
After the spar system is designed, I build the wing around it. This approach keeps me from
continually adding more and more weight to the wing to strengthen it "just in case." I already
know the spar system is going to be strong enough so everything else added to the wing is just to
provide shape or anchor points and can be as light as possible. These items do not need to do
anything to keep the wing from breaking. That's the job of the spars.
The following wing example is fully sheeted and has four servos, yet is very light due to
engineering choices and especially due to wood selection.
Fuselage Design
People who think fuselage sides should be made from lite-ply shouldn't be designing model
airplanes.
If the fuselage side needs to be sheeted, then a much better choice is lightweight balsa.
However, there are loads that go across the grain. There are two ways to go about supporting
these loads. The heavier way is to build the sides from thicker balsa. The lighter way is to build
the sides from thinner balsa and reinforce the inside with vertical supports and, in some cases,
diagonal bracing between the verticals. Essentially you're building a truss that's sheeted on the
outside.
The lightest way to build a fuselage
(which also gives the best strength to
weight) is to build truss-work sides with
gussets at all joints and no sheeting. It
is more work, but lighter, stronger and
more rigid (for their weight) than any
other method in use.
Thick plywood doublers inside a
fuselage don't do anything useful. Often
it is a good idea to have a plywood
doubler, but it doesn't need to be 1/16"
plywood. That's the quick way to add several ounces of weight to the model. Use 1/64" ply
instead and keep it as small as possible. I normally extend it just past the tank compartment into
the radio compartment at most. On smaller models, I don't use any doublers at all.
Most slab-sided fuselages have cross-grain sheeting on the bottom. This sheeting does not need
to be very thick because of the way the grain is arranged. For the same reason, it can normally
be contest balsa or slightly heavier weight. But it does not need to be hard, heavy balsa. That's
overkill and unnecessary weight.
The following fuselage construction example looks robust and it is very strong. However, it is
also very light due to engineering choices and wood selection. A typical kit fuselage that is
generally identical is usually much heavier due to the wood provided which is not hand selected
and graded for best strength to weight.

Fuselage Construction Example
Tail and Flying Surface Design
The tail is often a slab which is heavier
and has a lower strength to weight ratio
than a built up tail. For example, the
horizontal stabilizer on Rustik is 3/4"
thick, has about (20) 1/16" ribs, (2) 1/8"
square hard balsa spars and 1/32" shear
webs for the entire span. It is sheeted
with 1/32" contest balsa and has solid block tips.
The entire assembly including the elevator weighed 2.1 ounces prior to finishing. It is strong,
very rigid, has an actual airfoil and probably weighs about the same as a 1/4" slab of contest
balsa having the same area.
Additionally, a flat slab can easily bow or warp - even after the model is completed. The builtup and sheeted assembly will not. If you want to build a slab stabilizer, then use contest balsa
and cap the ends with medium balsa to help prevent it from cupping.
Add a firm balsa trailing edge about
1/4" wide to put hinges in. These pieces
will strengthen the stabilizer while
keeping the bulk of the part light. Do
the same for the rudder and fin.
If you really want to build flat plates for
the tail surfaces, but want them even lighter than slabs, then build truss work. You can leave
them open or sheet them. For example, if the tail is 1/4" thick then you can build it from 3/16"
square sticks (contest balsa) and then sheet it with 1/32" contest balsa. The sheeting will add
tremendous rigidity and strength which is why the whole thing can be contest balsa.
Use a glue that is not water based to apply the sheeting and sheet both sides at the same time. I
use slow-drying epoxy smeared on to the stick work in a thin film. Put the assembly between 2
sheets of wax paper and put a lot of weight on it while the glue cures. Let it sit for at least a day
- longer is better. It should be very flat and stay flat when the glue is cured.
Epoxy is heavy? I guarantee I can build a tail the same size and thickness this way lighter than a
slab tail can be built.
Selecting Lightweight Equipment for Radio Control Model Aircraft
Now you know two ways to build a heavy airplane — use too much of the wrong materials or
select a poorly engineered design. Combine those two areas for a double-whammy, weight-gain
bonus.
But why stop adding weight there? There is a another area that can add lots of useless weight —
equipment selection.
A Cat Chasing his Tail (Wermacht not withstanding)
A power plant has a snowball effect in both directions. Using a heavier, more powerful engine
requires a more robust structure to handle it. Now we not only have to beef up the structure to
handle the engine but we have to beef up the structure to handle the beefing.
All of the sudden the landing gear is heavier, the landing gear mounting system is heavier, the
wing spars are heavier, the wing mounting system is heavier, the fuselage needs to be more
robust, the tail surfaces need to be stronger, heavier servos are needed, a bigger onboard battery
is needed, etc. The list goes on.
If we go the opposite route and use a lighter, less powerful engine, then we can lighten all the
above. This is why some guys can build monster airplanes that fly on a .40 and other guys build
the same size airplane and stuff a chainsaw engine in it. I'd rather fly the plane with the .40 in it.
A lighter aircraft has less mass which means there is less inertia which means the structure is
more likely to survive impacts. The adage, "Build to fly, not to crash" is absolutely true.
Learn to live by it.
A lighter airplane can maintain flight at lower airspeeds, will climb better, will have better stall
characteristics and will perform better aerobatics.
A light airplane will begin and end maneuvers much more crisply than a heavy airplane. Swing
a baseball bat and try to stop it. Now strap a couple bricks on it and do the same thing. Which
way was it easier to start and stop the swing? Physics is physics — airplanes behave in exactly
the same way.
The "Right" Material in Every Circumstance
Many designers get stuck using the same material in an application regardless of the model size,
intended power plant or flight envelope.
For example, a designer may use 1/8" light ply fuselage sides for any model from .20 to .90. If
the sides can handle a .90, then they are certainly overkill for a .20 size model.
Why is it that 1/4 x 20 nylon bolts are used to retain a wing on a .40 size model if they are also
used to hold the wing on a 1.20 size model? If a standard servo can handle a .40 size model, then
why would you use it in a .15 model? Take the time to think things like this through.
Check the specs of equipment and use nothing larger and heavier than necessary. Don't install
heavy duty servos unless they're needed. So called "standard servos" are the most useless servos
made. They were good when they first came out, but they have become obsolete.
There are now mini servos that have more power, more speed and sometimes include a ballbearing. They're also half the size and weight of a standard servo. If a standard servo will work,
then so will a Hitec HS-85 and there are plenty of others.
Don't use a larger onboard battery pack than necessary. A small 3-channel model can get several
flights from a 250 - 350 mAh pack. Having a field charger will allow you to top off the battery
between flights.
Landing gear can add a lot of weight in a hurry. Landing gear weights vary greatly. You don't
have to buy expensive carbon fiber gear, but if you do you'll save more weight. Practice learning
how to fly your airplane to a landing and you can get away with a lighter landing gear.
A way to save a lot of weight inexpensively is carefully selecting wheels for the model. Take a
postage scale to the hobby shop to weigh different brands and styles.
Aluminum spinners are pretty, but heavy. If you need the nose weight to balance the plane then
fine, but if it's nose heavy, don't put lead in the tail before you switch to a plastic spinner.
Aluminum engine mounts are also heavy. For most engines, a fiberglass-filled nylon mount is
strong enough and much lighter than aluminum.
Always choose the lightest radio gear, hardware, control systems and other installed equipment
that will do the job and be reliable.
The point to all of this is that it is possible to have strength and durability where needed and still
build a light model. If you have a poor design and make cut-outs in the ribs, then you've wasted
your time for no real improvement. If you pay attention to every detail to ensure the lightest
material is used that will provide adequate strength and rigidity, then you'll see a big difference.
In general, always look for ways to use the lightest wood possible. Normally that means
building up a structure rather than cutting out slabs. Again, it's engineering and material
selection - not glue and not lack of ribs - that determines the weight of the finished model.
About Airfoils for Flying Model Aircraft
Airfoils come in several flavors. The most commonly used airfoils for flying model aircraft are:






Symmetrical
Semi-Symmetrical
Flat Bottom
Modified Flat Bottom
Under-cambered
Reflexed
Each family encompasses a large selection of airfoils but we use very few of them due to habit or
not having a clue about how to properly select a real airfoil. By "real" I mean an airfoil that has
been designed and tested by the aerospace industry.
Data on real airfoils won't apply in our realm anyway. The airfoils are tested at larger sizes than
the average model and things change as size changes. However, the airfoils should scale down
comparatively.
For example, if one airfoil is tested to stall sooner than another airfoil, then the stall speeds may
be much different when scaled down but both airfoils should still stall in the same order. That
may not always be true, but it's a good rule of thumb.
If you are purpose-building something competitive that requires the best possible airfoil for the
application then I can't help you. However, if you enroll at a school of Aerospace then you can
probably get help there.
Secrets that shouldn't be secret
Almost nobody who designs model airplanes would have a clue how to pick an airfoil for their
design based on real airfoil data. We learn from experience knowing that the subtleties between
one airfoil and another close to the same shape will make a very small difference — one that
would only be noticed by an expert pilot. These behaviors are not different enough to cause any
problems in your design unless you do something like change a round leading edge to one that is
razor sharp.
If a designer is agonizing over whether to use a 14% or 16% symmetrical NACA section he can
choose either and the plane will still fly well. The difference may be that the 14% version flies
and lands a little faster, but is smoother whereas the 16% version may perform aerobatics in a
slightly smaller volume.
The difference won't be something like one airfoil is "right" and the other makes your plane loop
back into the ground on take-off. Again, I'm comparing airfoils that are basically the same shape
within a family. Note that there will be a huge difference between any under-cambered vs. any
symmetrical airfoil.
If you want to design unique planes that fly for fun, then stay in the realm of reason and make
your airfoil close to the shape of airfoils used by other planes of roughly the same type. Don't
get hung up about it. If you can't decide then copy an airfoil in use on a model and scale it to the
right size for your model.
I almost never use a real airfoil in my designs. I have airfoils that I've used on previous models
so I know how they behave. For a new design I adjust the airfoil to behave more how I want it to
on the new model. For example, I may thicken it to slow the model or make the leading edge
radius smaller to allow sharper stall maneuvers.
If I need an airfoil of a type I have never used before then I pull out my airfoil books and look at
drawings. The data is meaningless to me. I imagine the airfoil in the application and fly it in my
mind. Watch the videos in my gallery and you'll see that it works well and I'm not seeing sport
designs that fly better than mine.
Aircraft Design Comes First
These are the decisions I make before selecting a specific airfoil:
1. Specify desired flight characteristics (airspeed envelope, aerobatic capabilities, etc.).
2. Specify the wing-loading and power loading ranges. Be disciplined about designing to
those goals.
3. Decide on a wing planform (chord(s), span, taper and sweep).
4. Determine the most appropriate airfoil family.
Because all designs represent numerous compromises you'll have to use the above to decide
which characteristics are more important than others. Select a specific airfoil using whatever
information you have.
Like Dogs, All Airfoils are Good
If the plane has undesirable traits it's probably not the fault of the airfoil assuming you choose
one within the realm of reason. An airplane with a razor thin wing should fly well, albeit very
fast. If the pilot tries to slow the plane down to Cub speeds for landing, it will snap over on it's
back and probably be destroyed.
I have watched several web videos of scale planes snap-rolling into terra firma. In nearly every
case the plane had a high wing loading which required the aircraft to maintain a higher airspeed
to remain in flight.
Most of these videos provided clues to the problem such as the plane flying around with its tail
dragging through turns (tail heavy) or the pilot trying to yank it around like a Kaos with
aileron/elevator turns. The Kaos has a 20/oz ft2 wing loading. A precision scale SBD Dauntless
might have a 60/oz ft2 wing loading.
Too many pilots believe themselves to be expert pilots but they're kidding themselves. They
know how to yank and bank an over-powered, lightweight sport model. When they step outside
the realm of lightly loaded, over-powered aircraft and into the realm of heavily loaded aircraft
they often find that they get into trouble fast.
Aerodynamic Stalling
One of the main concerns of fledgling model airplane designers is how to avoid choosing an
airfoil having wicked stall characteristics. All airfoils have a stall angle. This is the angle of
the chord line of the wing to the direction of flight. When this angle is at or beyond the stall
angle the air breaks away from the wing and the wing stops producing lift. In other words, the
aircraft isn't flying any more. It's falling from the sky.
The leading edge radius takes the lead role in stall characteristics. A sharp (small radius)
leading edge typically has a shallow stall angle. That means it will stall sooner than a blunt
leading edge.
There are other factors as well, but they become too technical and less practical. Just know that
if you want your plane to have gentle stall characteristics you should use a larger radius leading
edge. The smaller the radius you use the more you risk having a plane that will stall suddenly
and sharply.
A tip stall occurs when a wing tip stalls before the wing root. In most cases this causes the
aircraft to roll over. If the plane is close to the ground it's usually a total loss.
There are several ways to avoid or delay tip stalls.

Build the wing with washout.
Washout simply means the wing is built with a twist so that the wing tips are at a lower
angle of incidence than the wing root. Washout also limits aerobatic capabilities.

Sand the leading edge such that it becomes more blunt toward the tip.

Avoid high aspect ratio wings having a high taper ratio.
Taper ratio is the length of the tip chord divided by the length of the root chord. Aspect
ratio is the wing span divided by average wing chord. High aspect ratio wings, such as
sailplanes, with high taper ratios tend to be more prone to tip stalls than low aspect ratio
wings, such as deltas.
Some CAP aerobatic planes tend to tip stall easily due to the taper ratio and sharp leading
edge.
My Thumb's Rule of Airfoil Selection
Choosing an appropriate airfoil family for any given design is usually simple. If the plane is to
be a precision aerobat then a symmetrical airfoil is most appropriate because it flies the same in
any given attitude.
If the plane is to fly slowly or carry a load but is not intended to do aerobatics then a flat-bottom
or under-cambered airfoil should be considered.
By the way, when I say flat-bottom I don't mean a true flat-bottom airfoil. Some airfoils are
called "modified flat bottom." This is an airfoil having a straight line from the main spar to the
trailing edge but curves up to the leading edge from the spar.
A modified flat-bottom airfoil is actually a semi-symmetrical airfoil, but most modelers consider
it to be a flat bottom airfoil because most of the underside isn't curved. I'm right, they're wrong,
but if you say semi-symmetrical then they'll picture something other than what you're talking
about.
Note: Any airfoil that is not symmetrical is a cambered airfoil.
The terms "flat-bottom" and "semi-symmetrical" are not used by the aerospace industry and they
probably laugh at us when they hear us use those terms. Nevertheless, this article is for us, not
them, so I will continue using these incorrect pretend names so you aren't shunned by your
fellow modelers who don't like it when you talk too fancy.
Symmetrical Airfoils

Use for aerobatic airplanes - particularly monoplanes.
A design intended to be aerobatic should always have symmetrical flight surfaces (wing,
horizontal stabilizer and vertical stabilizer). Flat surfaces (which are symmetrical sections) work
well for tail surfaces to a point but aren't as good as a true airfoiled section.
Semi-Symmetrical Airfoils

Use for secondary trainers, sailplanes and sport aerobatic biplanes. If the biplane is
intended to do precision aerobatics then a fully symmetrical airfoil should be used.
Secondary trainer manufacturers make a big deal out of semi-symmetrical airfoils but they are
over-rated. If a beginner moves up too quickly and hasn't mastered his primary trainer yet then a
secondary trainer with this type of airfoil is probably the best bad choice. Otherwise, a
lightweight, well behaved model with a symmetrical airfoil makes a good secondary trainer.
Sailplanes often use highly refined and tested airfoils that provide the best lift to drag so that
they can scoot across the sky quickly in their search for thermals and then climb easily in the lift.
Sailplane designers tend to take a lot of care in their airfoil selection. They have to because the
airfoil is the only thing making their plane fly. They don't have an engine to fall back on.
Flat-Bottom Airfoils


Don't use true flat-bottom airfoils for anything.
So called "modified" flat bottom airfoils are excellent for slow, gentle flight.
True flat-bottom airfoils are a poor choice for any design. They are next to impossible to trim
properly because they are extremely speed sensitive. It may be possible to trim this trait out, but
it means spending hours tweaking the wing incidence, decalage and engine thrust.
I've never flown a model with a flat-bottom airfoil that could even come close to being trimmed
as it was built. I don't particularly enjoy cutting the tail off my planes numerous times
attempting to get it right.
The rest of this discussion refers to modified flat bottom airfoils.
Flat bottom airfoils are used for powered aircraft that are willing to make the compromise of
having more drag in exchange for slow flight or high lift capabilities. They do not penetrate the
air well but can stay aloft at very low speeds. I have built a handful of models having flat bottom
airfoils that can hover right in front of me because the aircraft's minimum flight speed was below
the wind speed.
For example, if the model can fly at 10 MPH and the wind is blowing 15 MPH then the model
can fly backward (relative to the ground) at 5 MPH. As far as the air is concerned (which is the
only thing the airplane cares about) the aircraft is flying forward at 10 MPH.
An aircraft that is identical except for having a symmetrical airfoil will have a higher minimum
flight speed.
Under-Cambered Airfoils

Use for scale models, sailplanes and some high-lift situations.
I don't know much about under-cambered airfoils. They are mostly used for scale aircraft from
the dawn of aviation. They tend to have high lift and are sometimes used in free flight models
and some very small radio control aircraft.
Reflexed Airfoils

Use for flying wings.
A reflexed airfoil has a trailing edge that is turned up slightly. The airfoil shown above is
extremely exaggerated to get the point across. I haven't tested it but it probably has five times
the amount of reflex it needs. If you print the image and scale it to your design, then don't hand
launch the model — it will probably loop, hit you in the back of the head and kill you instantly.
Most airfoils have a natural tendency to pitch forward. If you were to push a wing forward or
just drop it, it would rotate or tumble forward all the way to the ground.
The horizontal stabilizer prevents an aircraft from doing the same thing. Flying wings don't have
a stabilizer so the wing must be self-stabilizing. The reflex provides this stabilization.
Note that a true reflexed airfoil isn't necessary and often isn't used with flying wing model
aircraft. A lot of designers fake it by adjusting the elevons so they are slightly up. Thwing! and
my JGRC Aggressor both use faked "reflex" and fly very well.
Airfoil Thickness
Airfoil thickness is simply the percentage of the wing chord that the airfoil is deep at it's
thickest point. For example a wing having a chord of 15" that has a 10% thick airfoil will be 11/2" (1.5") thick.
How thick should the airfoil be? I find that wing thickness is a compromise between speed and
lift. A thicker wing has more drag but more lift and is capable of slower flight. Thicker wings
also tend to "bounce" around more in the air because they can't cut through it as easily.
A thinner wing has less lift but is faster. The shape of the leading edge plays a part in this as
well.
One other thing to note is that as wings get thicker they also become stronger. If a wing is thick
it is easy to build it strong using conventional construction techniques. If the wing is thin then
more exotic techniques are required to prevent the wing from breaking in flight.
Of course there are limits to everything. I've seen airfoil listings that are thicker than 30%. The
thickest wing I have built was about 20% and I didn't like anything about it in flight.
From as far back as I can remember through the 1980's, most sport designs had airfoils in the
range of 14% to 16% thick. These airfoils have proven to be safe with few or no bad habits at
reasonable wing loadings and can slow down nicely to land. I normally use airfoils from 12% to
18% depending on the airplane. For an extremely fast model I may use an airfoil around 10%
thick.
In the 1980's several things happened that changed the way we design model airplanes. Pilots
came to desire aerobatic models that fly at speeds below Mach 1, four-stroke engines became
widely available and the new Turnaround Pattern competition required planes to fly at a more
constant airspeed.
A thin airfoil simply isn't going to slow down when the airplane is diving toward the ground
even with the engine at idle. More drag was needed, but it had to be smooth, clean (nonturbulent) drag. In other words, airfoil shaped. The easiest way to create this drag was to build a
thicker wing which also creates more lift at slower speeds. These models also had to revert to
old-time, lightweight construction techniques because lighter planes maneuver better and fly
slower.
Drag increases exponentially with airspeed. Frontal area, drag and airspeed are inseparable so
you need to have a feel for how they work together to decide how thick the wing should be. This
is an area where I really can't speak scientifically. I have a good feel for how it works and do
pretty well with that knowledge.
Carefully match the power plant and propeller to the airframe instead of matching the propeller
to the power plant alone. All airplanes have a maximum airspeed at which they will fly
smoothly. If the engine has more power available after this speed is reached you won't see more
speed, but the model will begin to buffet or worse - something might flutter off.
Airplane
Engine
Average weight Stik .45
Lightweight
model
3D
.45
Lightweight Floater .40
Sport-Aerobatic
Biplane
.60
Propeller Top
Airfoil
Pitch
Speed
6"-7"
4"-5"
4"-5"
6"-7"
Flight Characteristics
15%
symmetrical
Smooth flying, medium to
large aerobatics, reasonable
landing speed.
50
MPH
18%
symmetrical
Slow flight, aerobatics in small
area, very slow landing speed,
buffeting at high speeds and
susceptible to gusts at low
speeds.
45
MPH
Hovers in steady winds, very
low flight speeds, minimal
16% semiaerobatics,
difficult
or
symmetrical
impossible inverted flight,
landing at a crawl.
65
MPH
Very aerobatic in a smaller
13% semi- area. Tumbles well. Requires
symmetrical more "down" for inverted
flight.
80
MPH
Speed Demon
.40
8"-9"
(piped)
100+
MPH
<12%
symmetrical
Flies fast, lands fast, extremely
large aerobatics.
Styles of Wing Construction for Flying Model Aircraft
There are a number of types of wing construction for model airplanes. Methods range from
simple outlines covered in tissue to complex geodetic structures.
The examples on this page are the most typical construction methods used to build wings for
radio control, control line and free-flight model airplanes.
Non-Sheeted Rib and Spar Construction
Wings that are not sheeted are built with ribs that have an outline identical to the airfoil outline
with whatever cut-outs are necessary. This type of wing is arguably the simplest to build of
wings that have ribs as well as the lightest.
Typically this type of wing is very flexible. The covering will stiffen the wing somewhat. Note
that shrinking the covering, regardless of type, can cause the wing to warp or bow.
Semi-Sheeted Wings
Often wings such as the ones above have some sheeting added to the center section to strengthen
it and provide more support where the wing mounts to the fuselage. The remainder of the wing
has no sheeting. The unsheeted sections of the wing have a rib pattern that is the same as the
airfoil.
The ribs in the sheeted section of the wing must have the thickness of the sheeting cut from the
perimeter of the ribs.
I usually cut all the ribs using the master pattern. The ribs that will be sheeted are stacked and
pinned together so that the thickness of the sheeting can be cut away using a scroll saw.
D-Tube Wing
A D-Tube style wing is sheeted from
the main spar to the leading edge on
both the top and the bottom of the wing.
Aft of the main spar may be cap strips.
The rib pattern is created by subtracting
the thickness of the sheeting from the
outline of the entire airfoil if cap strips
are to be used.
If cap strips are not used then the
thickness of the sheeting is subtracted
from behind the leading edge to the rear of the main spar.
A true D-Tube has shear webbing. The webbing along with the leading edge sheeting gives it
the "D" shape when viewing a rib cross-section from which the wing derives its name.
Many people believe a D-Tube wing is significantly lighter than a fully sheeted wing. That is
not true.
A D-Tube wing uses the same amount of glue to hold on the skins and cap-strips as a fully
sheeted wing does. The entire perimeter of each rib is glued. The difference in the amount of
sheeting used is minimal.
If you were to weigh the amount of sheeting that would be used to fill in the empty areas of a DTube wing you would find that it only weighs a few grams on a typical .40 size model assuming
contest balsa is used.
Fully Sheeted Wing
Fully sheeted wings can either have a
built-up rib and spar structure or a
hot-wire cut foam core.
Built-up versions have a rib pattern that is identical to that of a D-Tube wing. The thickness of
the sheeting is subtracted from around airfoil to make the pattern.
Most builders have difficulty with their first built up sheeted wing, but the experience is valuable
and a poor initial result should not dissuade you from trying it again.
You will not see too many fully sheeted, built-up wings on sport models for several reasons.
As I mentioned already, many builders are afraid to even attempt it because significantly more
skill is required to build a sheeted wing than other types. The ribs must be sanded to flow
smoothly from one rib to the next in order for the sheeting to lay smoothly without undulations.
Additionally, it can be difficult to sand the skin smooth at the seams. This is mostly a problem
with poor technique or using the wrong glue. The skins should be sanded after joining them but
prior to gluing them to the rest of the structure.
Excessive sanding after the skin is glued to the ribs will cause it to have a "starved horse" look.
This is because the sheeting sands away at a faster rate where it is supported by the ribs which
results in ripples in the skin. In the worst case it is possible to sand through the sheeting at the
ribs. Sheeted wings also weigh more than a non-sheeted wing, but only marginally more than a
D-Tube wing.
most builders do not build fully sheeted wings for the above reasons and the fact that a fully
sheeted wing does not fly significantly better than a D-Tube wing.
Sport models having fully sheeted wings most commonly have a foam core which are
exponentially easier to build. The main drawback of a foam core wing is weight. A properly
designed and well built balsa wing will always weigh less than an otherwise identical foam core
wing.
Some people will claim that they can build a foam core wing that weighs less than a balsa wing.
Without exception, every person who has made this claim to me could not produce a wing to
back their claim. I'll believe it when I see it.
Jedelsky Wing
A Jedelsky wing is created by edge-joining two sheets of balsa wood at an angle. The angle on
the bottom of the wing formed by the two sheets is left intact which creates an undercambered
airfoil.
The upper surface of the wing is carved, planed and sanded to an airfoil shape. Jedelsky wings
are most commonly used for free-flight gliders.
Hybrid Wings
The above are the most common types
of wing construction, but there are many
other ways a wing can be built. One
method that is used from time to time is
a series of spars that have cap strips
overlaid to create the airfoil. No ribs are
used.
Each spar is the height that an airfoil
would be at the percent chord at which
the spar is located. The caps form the
curve of an airfoil over the spars. As
few as one spar can be used. A sheet
may also be laid over the spar rather
than ribs.
This type of wing structure is most commonly used on very small models where cutting a set of
consistent and accurate ribs is most difficult.
I used a variation of this construction method for my Thwing! design. Rather than using cap
strips, I created a lattice skin made from balsa.
How to Calculate Airfoil Ordinates
Part 1 of this series provides some background and provides sources for obtaining airfoil
ordinates.
Some airfoils have a large number of ordinates. I've seen sets of ordinates having over 1,000
points. If you are manually plotting an airfoil you do not have to plot every ordinate. You only
need enough so that you can draw the airfoil with reasonable accuracy.
I would say plotting 20 points each top and bottom is accurate enough for most airfoils. In areas
where there are tighter curves you should plot points closer together. Examples are near the
leading edge or the reflexed portion of that type airfoil.
Coordinate Standards
Airfoil ordinates are simply points that define the shape of the airfoil. The numbers are given in
percentage of the wing chord. There is more than one standard, but they are all easy to figure
out.
The standards I know of are as follows:
1. Stations from 0% to 100% chord. In this case, multiply the chord of the airfoil you are
plotting times percent of the station/ordinate pairs in percent. In other words, if the
number given is 1.25 then multiply times 1.25%. If your calculator does not have a
percent key, then multiply times 1.25 and then divide by 100.
Ordinates of this type are presented in two sets of ordinate pairs - one for the upper portion of the
airfoil and one for the lower.
2. Stations from 0 to 1. In this case it is straight multiplication of the chord times each of
the station/ordinate pairs. This standard also differentiates between the top and the
bottom of the airfoil.
3. The last example is the style used for computer programs. This is listing of ordinate pairs
with no differentiation between the top and bottom of the airfoil. Numbers are from 0 to
1. The listing starts at the trailing edge of the airfoil and moves forward defining the
underside of the airfoil and then the leading edge, the top of the wing and back to the
trailing edge again.
It sounds more complicated than it is - again, it is simple multiplication.
Calculating the Ordinates to be Plotted
For this example I will be plotting a NACA 2412 airfoil. The NACA 2412 is a semisymmetrical airfoil (cambered) that is stable and somewhat fast although it would not be the best
choice for an extreme speed aircraft. It would be a good choice for a one-design club racer
because it has no bad habits and will not get to speeds that the average pilot can't handle.
The first table below is the set of ordinates for the NACA 2412. The listing uses standard (1)
above.
I will be calculating ordinates for and plotting an airfoil having a 9" chord. Multiply all stations
and ordinates by the chord. Again, the numbers given in the ordinate listing are percentages.
That means you multiply the chord by the station or ordinate in percent.
To find the second station for example, multiply 9" x 1.25%.
The leading edge (L.E.) radius is also multiplied by the chord to get the actual radius. This is
also a percentage.
The second table contains the resulting numbers after multiplying them by the wing chord. All
numbers are in inches for this example. Calculating and plotting works the same regardless of
your number system.
NACA 2412 Ordinates
NACA 2412 (9" Chord)
Upper surface
Upper surface
Lower surface
Station Ordinate Station Ordinate
Lower surface
Station Ordinate Station Ordinate
0
0
0
0
0.000
0.000
0.000
0.000
1.25
2.15
1.25
1.65
0.113
0.194
0.113
-0.149
2.5
2.99
2.5
- 2.27
0.225
0.269
0.225
-0.204
5.0
4.13
5.0
- 3.01
0.450
0.372
0.450
-0.271
7.5
4.96
7.5
- 3.46
0.675
0.446
0.675
-0.311
10
5.63
10
- 3.75
0.900
0.507
0.900
-0.338
15
6.61
15
- 4.10
1.350
0.595
1.350
-0.369
20
7.26
20
- 4.23
1.800
0.653
1.800
-0.381
25
7.67
25
- 4.22
2.250
0.690
2.250
-0.380
30
7.88
30
- 4.12
2.700
0.709
2.700
-0.371
40
7.80
40
- 3.80
3.600
0.702
3.600
-0.342
50
7.24
50
- 3.34
4.500
0.652
4.500
-0.301
60
6.36
60
- 2.76
5.400
0.570
5.400
-0.248
70
5.18
70
- 2.14
6.300
0.466
6.300
-0.193
80
3.75
80
- 1.50
7.200
0.338
7.200
-0.135
90
2.08
90
- 0.82
8.100
0.187
8.100
-0.074
95
1.14
95
- 0.48
8.550
0.103
8.550
-0.043
100
0
100
0
9.000
0.000
9.000
0.000
L.E. radius: 1.58
Slope of radius through L.E.: 0.10
L.E. Radius = 0.142
This particular airfoil has stations that are identical for both the upper and lower surfaces but that
is not always true. Be sure to pay attention to what you are doing. I have made the mistake of
assuming the stations were the same when they weren't which resulted in some strange airfoil
plots.
Now that you have the numbers they need to be plotted on paper. The ordinate/station pairs are
simply (x, y) coordinates. The Station is X and the Ordinate is Y.
Plotting and Drawing an Airfoil
This is the third and final installment of this series.
Part 1 of this series provides some background, explains coordinate standards and provides
sources for obtaining airfoil ordinates.
Part 2 explains how to calculate the ordinates that are absolutely required before the airfoil can
be plotted.
This part provides a details how to plot the ordinates on paper and then draw the airfoil.
Plotting the Airfoil
An airfoil can be drawn with a
minimum of drafting instruments.
You will need a sharp pencil, accurate
scale (ruler), and a good curve. I use
ship curves because they better match
the shape of an airfoil. French curves
are more common, but tend to have
curves that are too sharp.
If you do not want to buy ship curves
then an adjustable curve might work.
I've tried few different types of
adjustable curves and none of them
were satisfactory to me. Your results
may vary.
If you must use French Curves, try to
find one that is at least twice the length
of the airfoil you are drawing. You can
also bend a stick of wood which is
surprisingly accurate. I use a piece of
1/8" x 1/4" spruce to draw long curves,
such as fuselages, when I draw plans.
The calculator only needs to be able to
multiply, so any calculator will work.
Draw a centerline slightly longer than
the airfoil chord.
Draw lines to represent the front of the
leading edge and the rear of the
trailing edge.
The chord of this airfoil is 9" so that is
the distance the lines are spaced.
Make tick marks along the centerline
to indicate the station locations.
The intersection of the leading edge
and centerline is point (0, 0) for this
ordinate standard.
Some ordinate standards have the
trailing edge as point (0, 0). If you
aren't sure what standard you're using,
just plot the points. If you are using
the same standard as I am here, the
airfoil will point to the left.
If you plot the points backwards, the
airfoil will point to the right. Either
way you end up with the same airfoil.
Draw vertical station lines through the
ticks you made in the previous step.
If the stations are different for the
upper and lower portions of the airfoil
then you should probably make ticks
for one side. Then draw the lines.
Repeat for the underside of the airfoil.
Tick off the ordinate locations at each
station.
The trailing edge of this airfoil tapers
to 0" thickness. However, I will sheet
this wing with 1/16" balsa. That means
I will have to fudge the airfoil
somewhat to account for the sheeting.
The next image actually represents two steps combined into one. I neglected to scan the drawing
between steps.
Draw the slope of radius through the
leading edge.
Slope = Rise over Run = y divided by
x. In this case the Slope is 0.1.
To draw the slope line, start at point (0,
0). Measure back 1" (x) along the
airfoil centerline and from there
measure up .1 inch (y). Draw a line
through point (0, 0) and the point you
just marked.
The center of the circle representing
the leading edge is found on the slope
line by measuring from point (0, 0) to a
distance equal to the radius of the
leading edge.
For example, if the diameter of the
leading edge is 1", then measure back
1/2" (radius) along the slope line. That
is the center of the circle that
represents the leading edge. Draw the
circle.
Using curves that match the point best,
draw the airfoil outline. Normally I
use several different curves by
selecting the curve that best matches
the airfoil in any given section.
The airfoil is tangent to the leading
edge.
Because of the thickness of the sheeting, it is not possible to draw the exact outline through the
plotted ordinates. However, the finished product will be close enough that in our realm, nobody
would notice the difference in the flight characteristics.
Be as accurate as you can but do what needs to be done to make the wing something that can
actually be built and not just a theoretical ideal.
Establishing the Rib Pattern
The airfoil outline is complete but it can't be used as is. The actual outline will not be a part of
the pattern unless the wing has no sheeting.
What we need to do is subtract the thickness of the sheeting from the pattern and draw the
location of structural details such as the Leading Edge, Sub-Leading Edge, Spars, Ailerons
and Trailing edge.
The order in which you do these things in does not matter as long as you know what's what.
In this example, the Leading and
Trailing Edges are 1/4" wide. The
Sub-Leading edge is 1/8" wide and the
Main Spars are 3/8" wide. The Aileron
is 1-1/4" wide.
Main spars are located where the wing
is thickest for maximum strength.
Work your way around the perimeter
of the airfoil and make tick marks
inside the airfoil outline to indicate the
thickness of the sheeting.
Arrange the scale so that it is
perpendicular to the airfoil at the
point from which your are measuring
for best accuracy. I normally eyeball
this, but better would be to use an
adjustable triangle that is adjusted to be
perfectly tangent to the airfoil at each
point.
Make enough tick marks to draw an
accurate outline.
Draw the rib pattern using the tick
marks. The rib pattern outline should
be parallel to the airfoil.
Finish any other details necessary. If
some ribs are different than others,
which is usual, then you should
probably cut two patterns at the same
time.
For example, you may want to add
landing gear cut-outs to the second
pattern. I usually draw unique cut-outs
directly on the ribs after cutting them
out using a single master pattern.
Matching ribs from each wing panel
are stacked and cut at the same time.
I tend to draw my pattern and glue it directly to whatever will be used for the template. If you
think you might want to save the original drawing then make copies.
Most copiers do not make exact size reproductions
Copiers either enlarge or reduce from the original to a small degree. Usually it is by such a small
difference that it is not a problem. Be sure to check before you start cutting patterns and ribs. If
the pattern off by an unacceptable amount you'll have a really bad day if you find out that the
wing you just built doesn't fit.
How to Make a Set of Ribs for a Constant Chord Wing
The simplest type of wing to build from scratch is one having a constant chord planform.
A rib template can be made from a hard material such as thin plywood or Formica. A hobby
knife is used to cut around the pattern to make as many ribs as needed.
Alternatively, rib blanks can also be stacked and cut using a power tool such as a scroll or band
saw. The latter methods are usually the fastest, but not always the best.
The rib blanks for the stack are usually rectangular which means a lot of wood goes to waste.
Additionally the stack of blanks is tall and narrow and spongy which means it can flex or distort.
I have run into this problem more often than not. The end result is a lot of wasted wood and an
unusable set of ribs. My preferred method is to cut the ribs around the pattern. It takes more
time, but has given me a better result.
I will use a scroll saw to cut short stacks of ribs. I usually stack the ribs for the wing center
section to trim off some of the rib to account for the sheeting after cutting and sanding the rib
blanks. I don't stack ribs more than 1/2" high to cut with the saw for the reason mentioned above.
Modify individual ribs or matched pairs as necessary to include cut-outs for landing gear blocks,
holes for servo leads and pushrods, etc.
The first step is to plot an airfoil from which the rib pattern is created. Add details to the
drawing such as the leading edge, trailing edge, spars and sheeting. It is helpful to draw in other
details such as the holes for servo leads even if they will not be in every rib.
It is generally wise to place the main spars at the thickest section of the airfoil. The farther the
spars are spaced apart, the stronger the wing will be.
The set of ribs presented here were used to build the wing for Rustik. I had only a rough idea
what the wingspan would be at the time I cut these ribs. I cut some extra rib blanks to give
myself flexibility.
I do not go along with the current design philosophy made popular by 3D aircraft of spacing ribs
7" apart. I like ribs spaced no more than 2-1/2" apart and even closer when the wing will be fully
sheeted.
This wing will be fully sheeted, have no dihedral and be constructed in one piece. The spars and
sheeting will be full span and shear webs will be used. No dihedral bracing will be necessary as
the wing will be adequately strong without it. Four servos will be used in the wing (2 ea. flaps
and ailerons). The control surfaces will be built up separately.
These ribs have large internal cut-outs to save weight and allow passage of servo leads. The cutouts are optional if you build a wing using torque-rod driven ailerons.
Always drill a hole in every rib to allow the wing to vent. The more ribs there are the more
venting there should be. You may need to drill additional holes if you can't make the hole larger.
Drill a hole in both the forward and aft portions of the rib if the wing has shear webs. Venting is
important to prevent your wing from exploding from sudden changes in atmospheric pressure
while flying. I have never seen it happen but I have heard of it happening — particularly among
sailplanes that fly at higher altitudes.
1/16" aircraft plywood is used for the
master pattern. Any thin, hard
material will work as long as you can
shape it.
Draw a centerline on the hard pattern
using an ink pen. Attach the paper
pattern to the plywood using spray glue
being sure to align the centerlines.
The pattern is cut slightly over-size and
then carefully sanded to shape. The
pattern should be as stable as possible.
Interior cut-outs will be made after the
ribs are cut.
Note that the spars are located at the
thickest point of the wing.
Sandpaper is spray glued to the back of
the pattern to prevent it from slipping
when cutting the ribs.
The ribs are cut from several sheets of
lightweight contest balsa. I used a
hobby knife to cut around the plywood
pattern.
The spar cut-outs are deliberately
undersize so that they can be sanded to
a perfect fit.
The ribs are more flexible and fragile
with the internal cut-outs so I will
make those after the ribs are sanded to
shape.
Square the stack of ribs and pin or
nail them together so they can be gangsanded.
T-Pins are not long enough in this case
so I used small nails from both sides of
the stack. The nails are located within
the cut-outs that will be made later. Be
sure to use enough pins or nails so the
stack can not shift while sanding.
The ribs have been sanded to a uniform
shape using a small sanding block (not
shown). The small hardwood sanding
block shown here was used to finish
the spar cut-outs. Scrap pieces of spar
material are used to check the progress
during sanding.
It should be obvious that the spar
notches in the ribs should be sanded to
match the spars. Don't sand the scrap
spars to match the spar notches.
Take careful measurements to ensure
the stack of ribs is as uniform as
possible. A diagonal line drawn across
the ribs indicates the order in which the
ribs are stacked as well as which side is
up.
Do not remove the pins until you are
absolutely satisfied with the rib set.
Optional — Make the internal cutouts. This operation does not need to
be as precise as the outline so the ribs
are divided into two stacks. A scroll
saw speeds things up.
Make the cut-out in the plywood
pattern. The outline of the cut-out is
traced on the topmost rib. Holes are
drilled to allow passage of the scroll
saw blade.
If you do not own a scroll saw you can
use the plywood pattern and a hobby
knife to make the cut-outs. Normally I
drill the corners with the ribs stacked
and then use a knife and straight-edge
to connect the corners of each rib
individually.
The completed rib set. The three ribs
at the bottom are cut from thicker
stock. The center of these three ribs is
1/4" balsa and will be in the center of
the wing.
The ribs on either side of the center rib
are 3/16" balsa and will be positioned
directly over the wing saddles. These
two ribs must fully support the wing so
the cut-out is smaller to ensure the ribs
will have adequate strength.
On the right is the plywood pattern.
In some cases additional holes or cut-outs will be needed. For example, the center ribs may need
to be cut in half to allow the use of dihedral braces. Select the appropriate ribs from their
position in the stack taking right and left wing panels into account. Make any modifications
necessary. When the ribs are completed put them in a safe place until you need them.
Always save your templates.
Lightening Wing Ribs
I received an e-mail from a person who wanted more information about the strength of a rib that
had large internal sections removed. It is a legitimate concern simply because nobody wants to
build a model that self-destructs in flight.
I have used ribs similar to this in several of my models and have had no problems. To date not a
single rib has broken in flight or from handling. However, you do have to be careful when
handling the wing. For example the ribs used for Great Gonzo have a 1/4" outline. The wing
must be lifted by grasping the spars or the center section sheeting. There is a good possibility
that the rib would break if the wing were lifted by supporting it under a rib.
I have had to repair Great Gonzo three times due to damage that occurred on the ground on
exceptionally windy days. The first damage was caused when Great Gonzo blew off a table at
the field and fell several feet to the ground. It flipped over in the air and landed upside-down
directly on the wing which flexed and tossed the model back into the air. Damage incurred was
a broken leading edge near the wing root.
The other two times both occurred during take-off. Great Gonzo was blown over when it turned
cross-wind (pilot error) and cart-wheeled several times. The first time this happened a main spar
was broken as well as the leading edge. The second time the tail end of the fuselage had a minor
crack and the leading edge of the wing broke again. Notably, no wing ribs were broken in any of
these incidents.
My Stik 30 has smaller cut-outs because the model was designed for higher performance. As I
mentioned on the My Stik 30 page, the ailerons can deflect up to 45° in each direction on high
rates. The roll rate is so fast I can not count them. Additionally, when the Webra .32 was
mounted on it I put the model in several full-throttle, terminal-velocity dives of several hundred
feet. The airframe had no problems with this.
My Stik 30 uses the same wing that was on the first version that I crashed on take-off at full
throttle. That crash was due to putting a wing tip in the ground and cart-wheeling the plane (my
favorite way to break airplanes apparently) across the field.
The wing was subjected to a significant torsion load but the only damage to it were a couple
dents in the wing tips and a broken turbulator spar. None of the ribs were damaged. The
fuselage was totaled.
As far as weight savings goes, it is all relative. If you are trying to shave every ounce of weight
from your aircraft, then the effort to remove weight from the ribs is worth it. On the other hand,
the weight savings of the ribs alone probably is only an ounce or so for a .40 size ship assuming
the use of contest balsa. If the ribs are made from heavier wood then the weight-savings will be
greater.
The following images show the wing construction of My Stik 30. Again, this wing survived a
full throttle cartwheel across the field and is still in use.
My Stik 30's wing has turbulators on
the top and bottom of the forward
portion of the wing and cap strips on
the aft portion of the wing.
Note the shallow grooves in the spars
(a little deeper than 1/64"). These
make it very easy to align the shear
webs between the spars and provide
additional gluing area. The grooves
can be cut on a table saw or router
table.
The black items you see on the T-pins
are called Pin Clamps and are
manufactured by Rocket City.
Another view of
construction. The
strong. There are
full-span wood
construction:





the wing under
wing is light and
twelve one-piece,
strips in the
3/8" x 3/4" Leading edge (1)
1/8" square Turbulators (6)
1/4" x 3/8" Main Spars (2)
1/16" x 1-1/2" Trailing edge
sheet (2)
3/8" square Trailing edge (1)
The completed wing.
How to Make a Set of Ribs for a Tapered Wing
There are several ways to fabricate a rib sets for tapered wings. The method shown here is the
fastest and one of the most accurate. The only drawback is that the ribs must be evenly spaced
assuming all the ribs are the same thickness.
If ribs are needed that do not space equally then you will have to interpolate those ribs from the
surrounding ribs or glue an over-sized blank in place and then sand it to shape with the wing
panel.
Another way to make ribs for a tapered wing is manually plot or use rib-plotting software to
draw each rib pattern individually. The ribs are then cut in pairs. This is my least favorite way
as it is the most time-consuming, tedious and presents more opportunities to make mistakes.
The method being presented requires two airfoil patterns for an entire wing panel instead of one
for each rib. The idea is to shape all the ribs between the two patterns.
The number of blanks can be determined one of two ways:
Method 1: Determine the wing span. Determine the rib spacing. Divide the span by the rib
spacing and add two.
For example, if you are building a 56" wing having ribs spaced 2" apart then you would need 30
ribs (15 per panel).

Number of ribs = (wing span ÷ rib spacing) +2
Method 2: Cut a random number of blanks and then space them equally to make a wing of the
desired span.
Preparing the templates
Make copies of your drawings and
spray glue them to a hard template
material. Aircraft plywood, Formica or
aluminum are good choices.
In this case, I used 1/8" aircraft
plywood.
Take your time shaping the templates
to make them as accurate as possible.
Depending on how you do things, it
may or may not be important that the
centerlines of the ribs are aligned to
one another as they will be in the
finished wing.
For example, if you cut spar notches
after the ribs are sanded to shape and
separated from the blanks then the
templates do not have to be perfectly
aligned now.
I will be cutting the spar notches while
the ribs are clamped between the
blanks and am incorporating a slight
degree of washout in the wing, so I
aligned the tip rib over the root rib
accordingly.
This wing will also be swept, but the
templates do not need to be aligned to
reflect that. I aligned the ribs so that
the 33% lines are aligned.
With the templates screwed or doubletaped together, drill holes for clamping
bolts.
Always save your templates.
Making the Ribs
If you want to save wood you can
estimate the size of each blank. I find
that to be too much trouble and simply
cut each blank to the maximum size.
The blanks were drilled in batches
using the template as a guide.
Remove four blanks from the stack.
Cut around the templates to make two
root and two tip ribs. These will be the
actual root and tip ribs.
I usually make them from a harder
balsa and they become built-in sanding
templates when sanding the wing using
a long block.
Put these ribs aside. They do not get
clamped between the templates with
the rest of the blanks.
Separate the remaining blanks into two
stacks. Bolt one stack together
between the plywood templates.
There are twelve 1/16" blanks in this
stack.
When you make the matching set, turn
the templates to be mirror images and
bolt them together so they taper the
opposite way.
This helps ensure that any material
removed from the templates when
sanding the first set doesn't affect the
second set.
Begin by planing and carving to rough
shape. All I've done to this point is use
a razor plane.
Use a good sanding block to finish the
set. Avoid sanding the templates.
I blocked up the trailing edge to level
the centerline and then ran the set
through my table saw to cut the spar
notches.
In the past I used a razor saw to cut the
sides of the notches and a hobby chisel
to chip out the waste.
The ribs have a taper around the edge
that must be removed.
There are two ways that I might do it.


Pull matching ribs from both
sets and sand them together.
Sand the taper off after the
wing panel is built which is
probably more accurate. Any
spars on the building board will
have to be blocked up to clear
the tapered aR.
Introduction to Building a Model Aircraft Wing
This series of articles show one way to build a wing for a flying model aircraft. The construction
is simpler than many wings in that it is constructed in one piece with no dihedral or joiners of
any type. However, the principles demonstrated in this series apply to almost any wing
construction.
The primary difference between this wing and many others is that a wing is most commonly built
in two panels. The panels are joined together using fiberglass tape, plywood or hardwood
dihedral braces or using a removable tube system.
Think of this construction as building one panel of a two-panel wing if that helps.
Primary Goals


Demonstrate construction techniques that ensure a wing is built straight, strong and light.
Describe how to mount a wing square to a fuselage in all respects.
It does not matter if you are building from a kit, plans or your own design — these principles still
apply.
Secondary Goals


Show how to scratch build a wing without plans.
Demonstrate ways of adding individual style to make an airplane stand out.
Many kit instructions simply tell you what glues to what and in what order, but the instructions
do not teach you how to build a straight wing or explain what each part does and why it is
important.
Wing Description
As you are reading through this article,
keep in mind that I have taken the
construction to the extreme for my
Rustik project.
The point is the details of how to
prepare the parts and turn them into a
straight wing — not how to go
overboard in embellishments (but this project is a great example of that too).
Rustik is a scratch-built aircraft meaning it is being built without plans. The wing planform is
fairly standard. The wing span will be roughly 50" and the chord is 12" giving the wing 600
square inches of area and approximately a 4:1 aspect ratio.
I plotted a "real" airfoil and promptly forgot which one I chose. All I remember is that it is a
15% symmetrical airfoil. I believe it is a NACA 0015.
The wing will be built in one piece. All strip wood and sheeting is one piece running the full
span. Shear Webs will be used along the entire span. The wing is fully sheeted and will have
built up ailerons rather than tapered aileron stock which is heavier and more flexible.
I used a lot of closely spaced ribs (23 in total). On a D-tube wing I normally space the ribs
slightly farther apart: 2-1/4" to 2-3/4".
Controls
Four servos will be used to control flaps and ailerons. They will be mixed using a computer
transmitter to allow flaperons, crow-mixing, etc. The servos will be mounted so that the servo
arms and pushrods exit the bottom of the wing.
The entire wing will receive a natural finish beginning with lightweight (0.5 to 0.75 ounce/square
yard) fiberglass cloth. It will then be painted with clear polyurethane.
Reality Check
Some of the decisions I made for aesthetic purposes carry a weight penalty. This wing could be
closely the same aerodynamically, but lighter than what is presented here.





D-Tube construction instead of fully sheeted.
Fewer ribs spaced further apart.
No inlays.
Built-up wing tips instead of solid wing tips.
Cover with plastic film instead of fiberglass and resin. Note that my fiberglassing
technique probably weighs the same or less than film covering. Adding primer and paint
would add more weight, however.
Additionally, the wing could be built having one aileron per panel instead of a flap and an
aileron. Servo weight is cut in half as well as removing four servo extensions (two in the wing
and two from the receiver), hatches, hardware, etc.
Overall, the several ounces of potential weight savings will be noticeable in flight.
If you are building from a kit, then some parts of this series will not apply to the model you are
building. You can read just the parts that apply to your project, although each part contains tips
that may be helpful to you.
Preparing to Build a Model Aircraft Wing
The example I am presenting here is typical of almost all sport wings as far as order of
construction goes. However, some models may have unique items that must be installed in a
certain order.
Study the instructions and plans to thoroughly familiarize yourself with the assembly sequence.
If you are building from a kit then I suggest that you sand the sheets holding the parts before
removing them. They will look nicer under a transparent covering.
If the ribs are die-cut, then it is almost always beneficial to sand the back of the sheet. That's the
side that the die did not enter from. This practice helps the parts fall out easier. Never force die-
cut parts from the sheet. Unless a part falls out on its own, I almost always run a sharp X-Acto
around the perimeter of every part to ensure they do not get damaged when they are removed.
Identify all the parts and ensure everything is present. If you are building from scratch or from
plans then make a "kit" by cutting all the parts for the wing before you start building. Having
everything ready speeds up construction and helps it proceed smoothly. Having a kit also helps
prevent construction errors.
All modelers have built themselves into a corner at one time or another. As a consequence we
all have performed surgery on our models even though we really try to avoid it. The best
defense against this is familiarizing yourself with the construction and being patient.
Often mistakes are made because we want to get something glued in place so we can move along
and skip a step in our rush.
Useful Formulas used with Flying Model Aircraft
Trigonometry for Model Aircraft Builders
Trigonometry is the tool of choice when working with angles. A baffling problem can often be
simplified by illustrating it with a triangle.
Building Surface
For example, wing dihedral is easily represented by a triangle. The building surface is one side
of the triangle, the wing panel is the second side and the amount the panel is propped up at the
tip is the third and final side of the triangle.
About Triangles
A triangle has six parts — three angles and three sides. The three angles always add up to
180°. If you know two of the angles, add them together and subtract that total from 180 to find
the third angle.
If you are a builder you will sometimes find that you are provided with a useless piece of
information, such as the dihedral angle. All that you really need to know is how far to prop up
the wing tip when you glue the wing halves together.
This is where trigonometry comes to the rescue. If you design your own models you will find
that you use trigonometry even more frequently. Again, it is not difficult. It is just a matter of
understanding how to use it. You don't have to know why it works, but if you care to you can
read the proofs in a high school geometry book.
By the time I was in high school I had already designed many models and I would say the two
most useful classes I took in regards to model-building are Geometry and Algebra.
You probably remember sitting in class asking yourself, "What will I ever use this for???"
I was fortunate enough to already have the answer to that question by the time I got there so I
found the classes interesting and very useful. I have used the knowledge I gained ever since.
Geometry is extremely useful at the drafting table. Almost everything you learn is practical
when it comes to drawing. If you want to design your own planes, then you'll be doing a lot of
trial and error work in your shop if you don't know how to do the math.
Algebra is excellent for finding the missing answer. For example, if you scale a plane with a
wingspan of 60" and a chord of 9-3/8" to have a span of 72" then Algebra is the most efficient
way to find the new chord.
Terminology

Theta (Ø) represents the unknown angle you are solving for. If you know the angle and
are solving for sides of the triangle, then write the angle on your diagram rather than the
symbol.

A Right Triangle is a triangle having one 90° angle. This is the best triangle to use to
represent various problems we encounter. The other two angles can be anything that add
up to 90°, such as 45° and 45° or 60° and 30°. Note that no triangle can have two 90°
angles which would be a straight line and not a triangle at all.

Hypotenuse is the name of the longest side of a right triangle.
The Three Primary Trigonometric Functions
Tip: Windows 95 and later come with a Calculator applet that can do everything we need. The
calculator is normally found in the Programs -> Accessories folder of the Start Menu.

After starting the Calculator applet, select Scientific from the View menu to make the
trigonometry functions available.
Everything that follows assumes that we will be working with Right Triangles.
There are three primary trig functions, Sine, Cosine and Tangent (SIN, COS and TAN on most
calculators). These terms were created to scare school children back in the day when educators
thought that fear was an effective teaching tool. If you are still reading, then you're about to find
out how simple these functions are.
The above three functions each do essentially the same thing — divide the length of one side of a
triangle by the length of another side. That's all there is to it.
Sine Ø = the length of the opposite side of the triangle
divided by the length of the hypotenuse.
Sine Ø = Opposite ÷ Hypotenuse
Tangent Ø = the length of the opposite side of the
triangle divided by the length of the adjacent side.
Tangent Ø = Opposite ÷ Adjacent
Your average right triangle.
Cosine Ø = the length of the adjacent side of the triangle
divided by the length of the hypotenuse.
Cosine Ø = Adjacent ÷ Hypotenuse
Confused? All you need is a calculator that has trigonometry functions and it is one key press.
Enter the angle, press the appropriate function and out
pops the number.
How
to
Remember
Trigonometric Functions
Thanks to Graham P. for providing
the following method to help
remember
the
three
primary
trigonometric functions:
Some
Sine =
Old
Opposite ÷
For example, if you want to know the Sine
enter 5 into the calculator and press the SIN
calculator should display 0.0871557 give or
decimal places. Basic algebra solves the
equation, one unknown).
of 5° then Horse Hypotenuse
key. Your
take some
Caught Cosine =
rest (one
Another Adjacent ÷
The above works only when you know the angle. If you
know the lengths of sides of the triangle but not the angle Horse
itself then you can use the Arc functions to find it.
Trotting
Arc functions turn the number from the trigonometric
function (Sine, Cosine or Tangent) back into the angle. On
These functions are Arcsin, Arctan and Arccos (noted as
SIN-1, TAN-1 or COS-1 on most calculators).
Asphalt
Hypotenuse
Tangent =
Opposite ÷
Adjacent
For example, lets say you are given the measurement to
prop up the wing tips for the correct dihedral.
You know the wing span and the amount the tips are propped up which are two sides of the
triangle. Divide the height the wing tip is propped up by half the wing span, and that is the Sine
of the dihedral angle. Now put that number in your calculator, press Arcsin (SIN-1) and the
calculator will display the angle.
Calculating the Aspect Ratio of a Wing or Flying Surface
The Aspect Ratio is the ratio of wingspan to average wing chord.
Average wing chord for a tapered wing is the root chord plus the tip chord divided by two. For a
constant chord wing, the average chord is the chord anywhere along the wing panel.
Why Aspect Ratio is Important

The Aspect Ratio of a wing is an indicator of the aircraft's roll response.
All else being equal, high aspect ratio wings (narrow chord to span) will have a slower
roll response than a low aspect ratio wing.

The Aspect Ratio of a flying surface largely determines the lift to drag ratio of the
surface.
High aspect ratio wings, such as on sailplanes, are more efficient and have a higher lift to
drag ratio.

High aspect ratio wings are more easily broken and are less tolerant of poor engineering,
poor building and flight outside design parameters.
To Calculate the Aspect Ratio
There are two ways to calculate the Aspect Ratio of a flight surface.
Method 1
Divide the wing span by the average wing chord. For example, if the root chord is 12" and the
tip chord is 8", then the average chord is 10" assuming a straight tapered wing.
Let's say the wing span is 50". Divide the span by the average chord to determine the aspect
ratio:
50" ÷ 10" = 5:1 aspect ratio
Method 2
Square the wing span and divide by the wing area. This is helpful for wings where determining
the average chord would be difficult such as elliptical wings.
502 ÷ 500 = 2500 ÷ 500 = 5:1 aspect ratio
You can also trace this information backwards to find the average chord of a wing. Simply
divide the wing area by the wing span.
500 ÷ 50 = 10" average chord
alculating Wing Dihedral
Dihedral is an angle raising the centerline of the wing tip above the centerline of the wing root.
It can also be expressed as a measurement of length.
Wings having the tip centerline below the root centerline have anhedral.
Note that many plans show the dihedral measured from the bottom of the root rib to the bottom
of the tip rib. This is incorrect. Dihedral is correctly measured from centerline to centerline.
Follow the plan regardless of this. The designer should have worked out the correct
measurement even if the plan does not express it correctly. In other words, block up the wing at
the location shown on the plan by the distance shown on the plan.
Why Dihedral is Important

The dihedral of a wing determines the amount of self-correction capability an aircraft
has in the roll axis.
More dihedral allows more self-correction at the expense of less lift, more drag and less
axial rolls. Nevertheless, dihedral is sometimes necessary and is especially important for
trainer aircraft.

Aircraft designs having no ailerons need dihedral in order to turn using only the rudder.
An aircraft that has no ailerons and no dihedral will tend to yaw and skid through rudder
turns. Some dihedral is always required for rudder only aircraft models.

It is sometimes necessary to change the dihedral of an aerobatic design to rid the aircraft
of undesirable traits such as rolling when rudder is applied (roll coupling).
Converting the Dihedral Angle to a Measurement
In most cases you can disregard this section because the amount of dihedral is usually given as a
ready-to-use measurement. However, if the dihedral is given as an angle you will need to
convert the angle into a number you can use.
This article discusses how to convert angles to measurements. It does not cover how to
determine the correct dihedral angle for your design.
In these examples, the Sine function is the most appropriate. See the Trigonometry page for
more details.
The Sine of an angle = the length of the opposite side of the triangle divided by the hypotenuse
of the triangle. You will either need a table that gives the values of Sine for various angles or a
calculator that can determine the Sine of an angle.
For example, let's say you have a 60" wing that has 5° of dihedral. The dihedral is per wing
panel, not the total amount. In other words, the included angle between the wing panels is 170°
(a straight wing having no dihedral is 180°).
Sketch a small diagram to help you out. The hypotenuse of the triangle is half the wingspan (30"
in this case).
Sine Ø = Dihedral ÷ Half Wing Span
For this example:
Wingspan =
60"
(note that
we
Dihedral
angle
Dihedral measurement = Unknown
will
be
using
=
half
the
wingspan)
5°
1) find the Sine of the angle:
Ø = 5°
Sine 5° = 0.087156
2) Plug in the answer above to find the opposite side of the triangle.
0.087156 = Dihedral ÷ 30"
Dihedral = 30" x 0.087156
Dihedral = 2.6" (under each tip)
Calculating the Wing Area for Constant Chord, Tapered and Delta Wings
In order to determine the wing loading, you must know the wing area. Wing area for model
aircraft is always given in square inches (in2) with the exception of European kits that give
area in metric units such as square centimeters or square decimeters.
Why Wing Area is Important

Taken on its own, wing area is not important. However, the wing area must be calculated
to determine wing loading which is very important.
Calculating Wing Area of Multi-Wing Aircraft

Calculate the wing area for each wing individually. Add these areas together to find the
total wing area.
Notes


If you are building from a kit or a plan then the wing area should be somewhere on the
plan, in the instructions or on the kit box.
When calculating the wing or stabilizer area, the area that crosses the fuselage is included
even if that area does not provide lift.
Calculating the Wing Area for Constant Chord Wings
The area of a simple rectangular, constant chord wing is found by multiplying the width x the
height. In aircraft terms that is:
Wing Area = Wing Span x Wing Chord
Calculating the Wing Area for Tapered Wings
To find the area of a tapered wing, use the formula for a Trapezoid. Find the average chord
and multiply it times the wing span:
Average Chord = ( Root Chord + Tip Chord ) ÷ 2
Wing Area = Wing Span x Average Chord
Note: It does not matter if a wing sweeps or not. The formula for a tapered wing is used with no
regard for the sweep.
If the wing has multiple tapers then calculate each trapezoidal area and add them together.
Calculating the Wing Area for Delta Wings
If a Delta Wing has a Tip Chord greater than 0 inches, then use the formula for a tapered wing
as above.
Delta Wing having a Tip Chord greater than 0"
If the Tip Chord is 0" then use the formula for a triangle:
Wing Area = 1/2 x Wing Span x Root Chord
Delta Wing having a Tip Chord of 0"
Calculating the Wing Area for Elliptical Wings
Why Wing Area is Important

Taken on its own, wing area is not important. However, the wing area must be calculated
to determine wing loading which is very important.
Calculating Wing Area of Multi-Wing Aircraft

Calculate the wing area for each wing individually. Add these areas together to find the
total wing area.
Notes


If you are building from a kit or a plan then the wing area should be somewhere on the
plan, in the instructions or on the kit box.
When calculating the wing or stabilizer area, the area that crosses the fuselage is included
even if that area does not provide lift.
Is it a True Elliptical Wing?
Finding the area of an ellipse is not difficult. Unfortunately, determining if a shape actually is an
ellipse is another story.
There is no way to tell if a wing is a true ellipse just by looking at it. The only way to know for
sure is to actually use the Root Chord and Wing Span and plug them into the formula to plot an
ellipse and start plotting points.
If the points fall on the outline of the wing, then you have a true ellipse. Otherwise you have to
use other ways of determining the wing area.


Calculating the Area of a True Ellipse
Calculating the Area of a Shape that is not a True Ellipse
How to Plot the Outline of an Ellipse
If you want to plot an ellipse, then the formula is:
(x2 ÷ a2) + (y2 ÷ b2) = 1
Where
a
=
b = Root chord ÷ 2
Wing
Span
÷
2
Basically what you do is determine the wing span and the root chord and then start plugging in
numbers along the span to find the chord at those locations.
There is also a method using two points (thumb tacks) and a string, but I do not know it off the
top of my head. It is fairly simple and I am sure you can find it on the net someplace.
Finding the Area of a True Ellipse
You can use the formula for an ellipse to calculate the area IF the wing is a true ellipse (or close
enough).
Area of an Ellipse = a x b x PI
Where
a
=
b = Root chord ÷ 2
Wing
Span
÷
2
(In case you've forgotten, PI = 3.14)
Therefore (without showing all the steps that got me here)...
Wing Area = ( 3.14 x Span x Chord ) ÷ 4
Finding the Area of a Shape that is not a True Ellipse
There is a strong possibility that the forward portion of the wing is not the same ellipse as the aft
portion. You can split the wing into two parts by drawing a spanwise line.
Calculate the elliptical area for the forward portion and the elliptical area for the rearward
portion and add these areas together. Divide the result in half to find the actual area.
The other option is to draw the best fitting trapezoidal sections inside the ellipse, add all their
areas together and then make an attempt at counting how many square inches did not get
included.
This should get you close enough for practical purposes although the figure should be as exact as
possible if you plan to release plans or a kit.
How to Calculate the Wing Loading of a Flying Model Aircraft
The wing loading of an aircraft is the measure of weight carried by each given unit of area.
For model aircraft, wing loading is expressed as ounces per square foot (oz./ft2). Experience
with different models will make this figure more meaningful to you.
Why Wing Loading is Important

Wing loading is the only indicator of how "heavy" an aircraft is. The actual weight of an
aircraft is meaningless.
A 50 lb model having as many square feet of wing area is a lightweight. A 6 lb model
having 2 square feet of wing is very heavy and will fly like a sledgehammer (or maybe
not quite that well).

The lighter the wing loading, the slower the aircraft can take-off, fly and land. It will also
have a better climb.

A larger model can have a higher wing loading and fly comparably to a smaller aircraft
having a lower wing loading due to differences in the aerodynamics of different size
aircraft.
For example, let's say we have two aircraft that are absolutely identical except for
physical size. The smaller model has a 36" wing span while the larger aircraft has a 108"
wing span.
The smaller model may have a wing loading of 8 oz./ft2 and the larger aircraft may have a
wing loading of 35 oz./ft2. Both of these aircraft may perform nearly identically at
substantially different wing loadings due to the difference in size. Note that these figures
are off the top of my head and not meant to be taken literally.
It is a good idea to inform the person who is test flying your model as to the wing loading so they
have an idea of how long of a take off run it will need to build air speed. This is something that
comes with experience because there are no stall warning indicators in model aircraft as there are
in full-scale aircraft.
How to Calculate Wing Loading
In this example, we will use an aircraft weighing 5-1/2 lbs (5 lbs 8 oz.) with 600 square inches of
wing area. Calculating the wing loading requires that the wing area be converted to square feet
(ft2) and pounds to ounces.
1) Convert the area to square feet. There are 144 (12 x 12) square inches in a square foot.
600 in2 ÷ 144 = 4.17 ft2
2) Convert the total empty weight (ready-to-fly with no fuel) to ounces. There are 16 ounces in
a pound.
5.5 lbs x 16 = 88 oz.
3) Divide the weight by the area:
88 oz. ÷ 4.17 ft2 = 21.1 oz./ft2
Using round numbers, this gives the aircraft a wing-loading of 21 oz./ft2 or
You can perform the entire calculation in one shot using simple substitution:
(Weight x 2304) ÷ Wing Area
Where weight is in pounds and wing area is in square inches
Plugging the numbers from this example into the above formula gives us this:
( 5.5 x 2304 ) ÷ 600 = 21.1 oz./ft2
For multi-wing aircraft, divide the overall weight of the aircraft by the total wing area for all
wings.
Finding the Mean Aerodynamic Chord (MAC)
Many kits and plans indicate that the Center of Gravity (CG) should be located at a certain point
of the Mean Aerodynamic Chord. This figure is usually given in percentage, but is sometimes a
measurement.
The CG can be measured from any point along the span from the leading edge of the wing if the
wing is a rectangular (constant chord with no sweep) planform.
If the wing is tapered then you must locate the MAC before you can locate the CG.
The Mean Aerodynamic Chord is not the average chord.
To Locate the Mean Aerodynamic Chord on a Tapered or Delta Wing
Measure the root and tip chord. Then draw the following lines on the plans:




At the root of the wing, draw a line parallel to the centerline of the fuselage extending
forward from the leading edge and rearward from the trailing edge. Both lines should be
the length of the tip chord.
Do the same thing at the tip but drawing the lines the length of the root chord.
Connect the ends of the lines so that they create an "X" over the wing panel. Where the
two lines intersect is the spanwise location of the Mean Aerodynamic Chord.
If the plan indicates that the CG should be located at some percentage of the MAC, then
measure the MAC and put the CG the given percentage back from the leading edge along
the MAC. For example, if the MAC is 10" and the plan indicates the CG should be 25%
back from the leading edge, then the CG is 2-1/2" back from the leading edge at the
MAC.
This drawing should help you visualize what you need to do:
Note: The lines cross at the spanwise location of the MAC. It is not the fore/aft CG location
(unless the CG happens to be located at 50% MAC).
The following formula will give the measurement (chord) of the MAC. It does not give the span
wise location of the MAC.
rc
=
t = Taper Ratio = (Tip Chord ÷ Root Chord)
Root
Chord
MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t ))
Using the drawing above, let's say the root chord is 11" and the tip chord is 6"
t = 6 ÷ 11 = .5455
Now plug t into the formula to find the MAC. Note that the wingspan and sweep do not matter.
No matter what the span or how much the wing is swept, the MAC will always be the same
length.
MAC = 11 x 2/3 x (( 1 + .5455 + .54552 ) ÷ ( 1 + .5455 ))
MAC = 22/3 x ( 1.8431 ÷ 1.5455 )
MAC = 7.3333 x ( 1.8431 ÷ 1.5455)
MAC = 7.3333 x 1.19254
MAC = 8.7453"
To Locate the Mean Aerodynamic Chord on an Elliptical Wing
My thanks to Alasdair Sutherland, author of Basic Aeronautics for Modelers (Traplet
Publications), for providing us with this information.
From Alasdair Sutherland
"The MAC of an elliptical wing panel is 85% of its root chord, and you will find it 53% of the
panel's span from its root chord. The panel Area = 0.785 x span x root chord. This also works
for semi-circular panels, by the way, as they are just special ellipses."
a
=
b = Root Chord / 2
Wing
Span
/
2
Calculating Engine Offset
Most model aircraft designs have a certain amount of down and/or right thrust. The back of the
propeller should be on the centerline of the fuselage when viewed from above and on the thrust
line when viewed from the side.
If the model you are building has a thrust adjustment and you center the engine mount on the
intersection of these two lines then the propeller will be in the wrong place.
See the Trigonometry page for more details.
This problem is very similar to solving for dihedral.
In this example we are going to say that the back of the propeller is five inches from the firewall
and the right thrust is 4°. The next image is exaggerated to make it easier to explain the concept.
Ø = 4°
Sine 4° = 0.069756
Plug in the number you just calculated to find the opposite side of the triangle.
0.069756 = Offset ÷ 5"
Offset = 5" x 0.069756
Offset = 0.34878 from center or approximately 11/32"
How to Calculate Speed and Propeller Efficiency
These formulas are simply presented for your amusement. You do not need to know this math or
ever work these problems unless you are a speed freak or just like playing with numbers.
Calculating Theoretical Speed
Theoretical speed means that the propeller is 100% efficient and that there is no loss due to
aerodynamic drag, etc. A perfect airplane flying in a perfect world. That's not going to happen
here on earth, but this still gives you a starting point.
For this example we'll use an engine turning a 7" pitch propeller at 15,000 RPM.
Convert Revolutions Per Minute (RPM) to Revolutions Per Hour (RPH):
RPM x 60 = RPH
15,000 x 60 = 900,000 RPH
Find Inches Per Hour assuming 100% efficiency:
RPH x Propeller Pitch = Inches per Hour
900,000 x 7 = 6300000 inches per hour
Convert to Miles Per Hour (12" x 5280' = inches in a mile):
6300000 ÷ (12 x 5280) = 99.4 MPH
The bottom line (assuming 100% propeller efficiency and zero airframe drag):
Speed = ( RPM x Pitch ) ÷ 1056
In reality the average sport model with this combination might do 75-80 MPH on a good day.
Calculating Propeller Efficiency
Going a little farther, we can actually set up a speed trial to determine how fast an aircraft is
going and then determine propeller efficiency using those numbers (time over distance).
So let's say you time your aircraft on a 100 yard (300 feet) course (upwind and downwind to
make it even). The average time is 2.7 seconds.
Convert the distance covered to miles by dividing distance covered in feet by number of feet in a
mile. There are 5,280 feet in a mile.
300 ÷ 5280 = .0568 miles
Convert elapsed time to hours by dividing time in seconds by seconds in an hour. There are
3600 seconds in an hour.
2.7 ÷ 3600 = .00075 hours
Find Miles Per Hour:
.0568 ÷ .00075 = 75.7 mph
If our timer was accurate and the distance is accurate then that speed will be accurate. An easier
way is to use a radar gun, but then you don't get to do all this fun math.
Going back to the previous example, let's determine the overall loss of efficiency and then, for
convenience, blame it all on the propeller.
Divide actual speed by the theoretical speed using a 100% efficient propeller and an aircraft
having zero drag:
75.7 ÷ 99.4 = 76.16% efficiency
Unless we have an onboard tachometer, we do not really know what the RPM of the engine is.
Also, the lack of efficiency could very easily be attributed to the airframe design - not
necessarily the fault of the propeller. Still, it is something to play around with if you are so
inclined.
The Academy of Model Building
These pages comprise the overwhelming majority of content on the site. Articles are filled with
tips, ideas, how-to's as well as information about engines, construction materials, tools and many
other topics.
Presented here are techniques and philosophies I use to address and resolve a variety of common
and not-so-common problems in my shop and at my drafting table.
Unless specifically stated, there are no "air tips" on this site that I have heard about but have not
actually tried.
Although most of this information applies specifically to flying model aircraft, much of it can be
applied to model building of any genre.
The Basis of Everything
There are many attitudes that will guarantee better built, better flying aircraft. Hold yourself to a
standard that is as high as possible but doesn't make you give up. A good project has subprojects. If you're doing it right, then the main and sub-projects are a lot of fun.

Building an airplane that isn't straight is not acceptable.
When you adopt this attitude you'll have a better understanding why I am so anal-retentive with
some of my building techniques. If a former isn't flat then how do I know where on which edge
to measure from?
An aircraft that isn't straight will never trim properly until it is. It doesn't matter how high end
your computer radio is. The trim of a crooked model will always be speed-sensitive.
I hope I have just burdened you with a lot more work — devising ways of making sure all your
work is straight. You're welcome.
Note for beginning builders: A crooked airplane will fly. Do the best you can. Your building
skills will improve with experience and motivation. Your first few airplanes won't be perfect.
That's ok and normal.

Building a model having anything that doesn't work properly is not acceptable.
Don't jury rig your planes. Make sure each part works as intended before installing it. Fix any
problem you find. Don't convince yourself that the problem will work itself out on its own.

Building an airplane that is unnecessarily heavy is not acceptable.
You have to understand the properties of the materials you work with in order to use them
efficiently.
Spend some time destroying some balsa (or whatever material) so you can learn what it can
take. Bend it, drop it, twist it, hit it with a hammer. Do whatever you want to it so you can
observe the results.
A nose dive into the ground is a lot more force than a soft blow with a hammer and many balsa
aircraft survive the impact with nothing more than an easily removed dent or maybe not even a
scratch.

How long it takes doesn't matter.
It may take six months or more to build a quality model aircraft. A well-built, easily maintained
model requires more discipline and planning but if you take the time to build it right you can
realistically expect the model to last until you just don't want to fly it any more. And if you
change your mind you can put it back in the air with a reasonable amount of maintenance such as
a new fuel lines, clevises and an onboard battery pack.
There's not good reason for a model aircraft to just deteriorate to the point where you're forced to
ground and retire it.
Build your plane well and fly it to its limits