U5AEA15
AIRCRAFT STRUCTURES-II
PREPARED BY
Mr.S.Karthikeyan
DEPARTMENT OF AERONAUTICALENGINEERING
ASSISTANT PROFESSOR
We have looked at..
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Airfoil Nomenclature
Lift and Drag forces
Lift, Drag and Pressure Coefficients
The Three Sources of Drag:
– skin friction drag in laminar and turbulent flow
– form drag
– wave drag
Airfoil Drag Polar
Cd vs. Cl
Rough airfoils
have turbulent flow
over them, high drag.
Smooth airfoils have
laminar flow over
at least a portion
of the surface.
Low Drag.
Form Drag
Form drag may be reduced
by proper design, and
streamlining the shape.
Source: http://www.allstar.fiu.edu/aerojava/flight46.htm
Supersonic wave Drag
For a given airfoil or wing or aircraft, as the Mach number
is increased, the drag begins to increase above a
freestream Mach number of 0.8 or so due to shock waves that
form around the configuration.
Shock waves
How can shock waves be
minimized?
• Use wing sweep.
• Use supercritical airfoils, which keep the
flow velocity over the airfoil and the local
Mach number from exceeding Mach 1.1 or
so.
• Use area rule- the practice of making the
aircraft cross section area (from nose to tail,
including the wing) vary as smoothly as
possible.
How can shock waves be
minimized?
Use sweep.
30  sweep
M= 0.8
In your design...
• The Maximum Mach number is 0.85
• Wings for supersonic fighters are designed to
reduce wave drag up to 80% of the
Maximum speed.
• In our case, 80% of 0.85 is 0.68.
• If we use a wing leading edge sweep angle of
30 degrees or so, the Mach number normal to
the leading edge is 0.68 cos 30° ~ 0.6
Effect of Thickness and Sweep
on Wave Drag
Source:
http://www.hq.nasa.gov/office/pao/History/SP-468/ch10-4.htm
Supercritical Airfoils
Their shape is modified to keep the Mach number on the airfoils
from exceeding 1.1 or so, under cruise conditions.
Conventional vs. Supercritical
Airfoils
Wing Drag
• Since a wing is made up of airfoils, it has
– skin friction drag
– profile drag
– wave drag at high speeds,
and
– Induced drag due to tip vortices
TIP VORTICES
Effect of Tip Vortices
Downwash
Induced Drag
Induced drag is caused by the downward rotation of the
freestream velocity, which causes a clockwise rotation of the lift
force.
From AE 2020 theory,
C D ,i
C L2

AR e
e= Oswald efficiency
factor
Variation of Drag with Speed
Induced drag decreases as V
increases, because we need less
values of CL at high speeds.
Other drag forces (form,
skin friction , interference)
increase.
Result: Drag first drops, then rises.
At High Values of a Wings Stall
We need high CL to take-off and land at low speeds.
http://www.zenithair.com/stolch801/design/design.html
Achieving High Lift
One form of flaps, called Fowler
flaps increase the chord length as
the flap is deployed.
How do slats and flaps help?
1. They increase the camber as and when needed- during
take-off and landing.
High energy air from the bottom side of the airfoil
flows through the gap to the upper side, energizes slow speed
molecules, and keeps the flow from stalling.
Leading Edge Slats
Help avoid stall near the leading
edge
High Lift also Causes High Drag
We have looked at..
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Airfoil aerodynamics (Chapter 5)
Sources of Drag (Chapter 5)
Induced Drag on finite wings (Chapter 5)
Wave Drag, Profile Drag, Form drag
Airfoil and Aircraft Drag Polar
High Lift Devices
AERODYNAMIC PERFORMACE
• Performance is a study to see if the aircraft meets all the
requirements.
• Level Flight (Is there enough thrust and/or power?)
• Climb Performance (Will it meet the requirement that the
aircraft can gain altitude at a required rate given in feet/sec?)
• Range (How far can it fly without refueling?)
• Takeoff and Landing Requirements
• Others… (e.g. Turn radius, Maneuverability…)
• You will learn to evaluate aircraft performance in AE 3310.
• Performance engineers are hired by airlines, buyers, and aircraft
companies.
Your Fighter Has Certain Requirements
• Level Flight at a Maximum Speed of Mach
2 at 30,000 feet altitude.
• Range (1500 Nautical Mile Radius with 45
Minutes of Fuel Reserve)
• Takeoff (6000 foot Runway with a 50 foot
obstacle at the end)
• Landing (6000 foot Runway)
• Will your fighter do the job?
Your transport aircraft has certain
requirements, say..
– Payload:150 passengers weighing 205 lb. each
including baggage.
– Range:1600 nautical miles, with 1 hour reserve.
– Cruise Speed:
M=0.82 at 35,000 feet.
– Takeoff/Landing: FAR 25 field length
– 5000 feet at an altitude of 5,000 feet on a 95 degrees F day.
– Aircraft should be able to land at 85% of Take-off weight
• Performance calculation is the process where
you determine if your design will do the job.
Level Flight Performance
• We assume that the gross weight GW is available. You will know this for your
aircraft after Homework Set #4. An estimate of wing area S is assumed to be
known (Homework, later in the course).
a  RT where   1.4
• Select a cruise altitude. Compute the speed of sound
• Select a set of M : 0.4, 0.6, 0.8….
• Find Aircraft Speed = M  times a
• Find CL = GW / (1/2 * r * V2 * S)
• Find CD = CD,0 + CL2/( AR e) (this info is given in our course)
• Find Thrust required T = CD * (1/2) * r * V2 * S
• Plot Power Required (T times V) or thrust required vs. Speed
• Plot Power Available for your Engine (number of engines times T times V) or
thrust available at this altitude and Speed (Supplied by Engine Manufacturer)
• Where these two curves cross determines maximum and minimum cruise
speeds.
Level Flight Performance
Power Required
Power
HP
Power Available
with all engines
Excess Power
Aircraft Speed (Knots)
Best speed for longest endurance flights
since the least amount of fuel is burned
Maximum Rate of Climb
• Find Excess Power from
previous figure.
• This power can be used to
increase aircraft potential
energy or altitude
• Rate of Climb=Excess
Power/GW
Power
HP
Excess Power
Aircraft Speed (Knots)
Absolute Ceiling
• Absolute ceiling is the
altitude at which
Power available equals
power required only at
a single speed, and no
excess power is
available at this speed.
• Rate of climb is zero.
Power
HP
Power
required
Power available
Aircraft Speed (Knots)
Equilibrium Gliding Flight
L
Glide Angle, q
q
W
W cosq = L
W sinq = D
Gliding Distance
Glide Angle, q
Altitude h
Gliding Distance = h/tanq  h * L/D
Ground
Gliding Flight
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D=W sinq where q is the equilibrium glide angle.
L= W cosq
Tanq = D/L
Glide distance = h/ tanq = h ( L/D).
Cruise Speed for Maximum Range
V L/D
Speed for maximum range
Aircraft Speed (Knots)
From your level flight performance data plot V L/D vs. V
As will be seen later, the speed at which V L/D is maximum
gives maximum range.
Calculation of Range
We have selected a cruise V.
Over a small period of time dt, the vehicle will travel a distance
equal to V dt
The aircraft weight will decrease by dW as fuel is burned.
If we know the engine we use, we know the fuel burn rate
per pound of thrust T. This ratio is called thrust-specific
fuel consumption (Symbol used: sfc or just c).
dt
= Change in the aircraft weight dW/(fuel burn rate)
= dW / (Thrust times c)
= dW/(Tc)
Distance Traveled during dt=VdW/(Tc) =V [W/T](1/c) dW/W
Calculation of Range (Contd…)
• From previous slide:
– Distance Traveled during dt=V[W/T](1/c) dW/W
• Since T=D and W=L, W/T = L/D
• The aircraft is usually flown at a fixed L/D.
• The L/D is kept as high as possible during
cruise.
– Distance Traveled during dt= V[L/D](1/c) dW/W
Calculation of Range (Contd…)
• From previous slide:
– Distance Traveled during dt= V[L/D](1/c) dW/W
• Integrate between start of cruise phase, and end
of cruise phase. The aircraft weight changes
from Wi to Wf.
• Integral of dx/x = log (x) where natural log is
used.
• Range = V[L/D](1/c) log(Wi/Wf)
Breguet Range Equation
 Winitial 
1 V L

Range  
 log e 
W

c D
final


Structures & Weights
Group/
Designer Responsibility
to keep Wfinal small.
Propulsion Group/
Designer Responsibility
to choose an engine
Aerodynamics Group/
with a low specific
Designer Responsibility
fuel consumption c
to maximize this factor.