AOE 2104--Aerospace and Ocean Engineering
Fall 2008
AOE 2104
Introduction to Aerospace
Engineering
Lecture 9
Stability and Control
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Stability and Control
Stability
Static
Longitudinal
Criteria
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Control
Dynamic
Lateral
Directional
Adverse Yaw
Adverse Roll
Lecture 9
Coordinate Systems
Control Surfaces
Coordinated Turn
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Coordinate Systems
•
The movement of the center of mass can be in three directions: x, y, or z. These
directions can either be related to the body of the airplane (body axis system) or
to the relative wind (wind axis system) .
Control perspective – body axis system
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Note: Neither of
the above
mentioned axis
systems are
necessary
perpendicular or
parallel to the
ground.
Aerodynamic perspective – wind axis system
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Degrees of freedom (motion)
Six degrees of freedom
 Axial
 Normal
 Transverse
 Roll
 Pitch
 Yaw
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Roll, Pitch, and Yaw
Nomenclature!
Axial: A
Normal: N
Side: Y
Roll: L
Pitch: M
Yaw: N
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Aft, Port, Starboard, Fore
•
Often in Stability and Control, people refer to front, rear, left and right as
port, fore, aft, and starboard.
Rear = Aft
Left = Port
Right = Starboard
Front = Fore
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Control Surfaces
•
How do you control or induce the 6 motions we described earlier?
Other methods
than can be used:
-Vectored Thrust
-Aerodynamic
Forces
If ailerons are designed to control the roll motion, why are they
located that far on the wing?
Why do they deflect in opposite directions?
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Ailerons and Rolling Moment
Ailerons deflect differentially (one moves up while the other one moves
down). The increased camber on one of the wing results in increased lift on that
wing, resulting in the roll motion.
Why do you need a vertical stabilizer?
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Roll Control through Wing Twist
We saw earlier the benefits of wing twist…
Instead of deflecting a flap-like control surface, studies have looked into having
variable twist wings, on which ailerons are called twisterons.
http://www.centennialofflight.gov/wbh/wr_experience/1903b/html/warp.htm#
Utah State
Utah State
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Motion Control Advanced Technology
Morphing Wings change their shape to reach optimal flying efficiency.
Active Aeroelastic Wing
Courtesy of NASA
Courtesy of Benini
and Marques
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Ailerons and Pure Control Reaction
The resulting geometry differential can sometimes lead to a drag differential, that will
end up in yawing moment.
Of course, if the desire is to turn, the induced yaw may be just fine.
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Rudder and Yawing Moment
The rudder is essentially a flap on the vertical stabilizer.
It is used to control the motion about the aircraft’s vertical axis. How ?
When the rudder is deflected, the increased camber on the vertical tail leads to
an increase in lift causing the yawing moment.
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Rudder and Pure Control Reaction
How ???the aircraft around its vertical axis results in a velocity differential
Rotating
between the two wings, resulting in a lift differential, leading to adverse rolling.
Higher
Velocity
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Lower
Velocity
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Rudder-Aileron Cross-Control
Occasionally a pilot will “cross-control” an airplane to keep the wings level in yaw.
This requires applying the rudder in one direction while using the ailerons in
the other direction to prevent the rolling tendency that comes from yaw.
This can be a very effective way to descend quickly without building up a lot of
speed. It can also be a technique used to keep the airplane aligned with a runway
while making a landing approach in a cross wind.
Cross-Wind
Crosswind force
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Elevators and Pitching Moment
The elevators are essentially flaps on the horizontal stabilizer.
They are used to control the motion about the aircraft’s transverse axis (axis
“passing through the wings”). How ?
Why do you need a horizontal stabilizer?
When the elevators are deflected, the increased camber on the horizontal tail
leads to an increase in lift causing the pitching moment.
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Elevators and Pure Control Reaction
How ???
No adverse motion, other than drag.
Elevators are used to increase or decrease the lifting capability or lift
coefficient of the wing.
They are used, along with the engine power or thrust setting, to enable
the plane to climb or descend at the desired rate and to help provide the
extra lift needed in a turn.
Why would you need extra lift in a turn?
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Control – Coordinated Turn
Any vehicle that is turning must produce a force along
the turn radius in the direction of the turn to overcome
the centripetal acceleration.
You could use rudder to produce side force…But wings
have much greater area, and can therefore produce
much greater force!
The wing is rolled into the turn and part of the lift
produced by the wing is used to provide the force to
pull the plane through the turn.
But, by using some of the lift to make the turn there is
no longer enough lift to balance the weight!
The result is that to keep everything balanced or
coordinated in a turn all the controls on the plane must In a coordinated turn, altitude and
turning radius are constant.
be involved:
1. The rudder is used to begin the turn.
2.The ailerons are used to give the
needed amount of roll.
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Lecture 9
3.The elevator is used to increase the
angle of attack and, thus, the lift.
4.Finally, power or thrust is increased to
overcome the added drag.
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Stability
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Static versus Dynamic Stability
Static-vehicles initial tendency following a disturbance
Dynamic- time history of the vehicles motion after its responds to its static stability
Aircraft Stability – Static Stability
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Aircraft Stability – Static Stability
Stable
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Neutral
Lecture 9
Unstable
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
LONGITUDINAL STABILITY
Assume lift acts behind CG and the gust increases AoA, what happens?
For
a stable
corrects
we want the
lift to act
Now
assume
liftaircraft,
acts in one
frontthat
of CG,
what itself,
happens?
behind the CG.
What if lift acts at CG?
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
STABLE OR UNSTABLE?
LONGITUDINALLY
STABLE!
Remember, by convention positive pitching moment = nose up.
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
STABLE OR UNSTABLE
LONGITUDINALLY
UNSTABLE!
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
CONVENTIONAL TAIL SURFACE
For stability, we will always have a
nose down pitching moment. How do
we keep the plane from flipping nose
down onto its back?
One way is to use a horizontal
stabilizer.
Since the horizontal stabilizer is aft of
the aerodynamic center, to counteract
the nose up motion induced by the
lift, we need a down force from the
stabilizer.
This means we want the stabilizer at a
negative AoA.
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Advantage of CONVENTIONAL TAIL SURFACE
An increase in angle of attack
on the wing will be
accompanied by a decrease in
This wing
/ stabilizer
arrangement
the negative
angle
of attack of is
also self
correcting.
Why?
the horizontal
stabilizer,
reducing the download on the
tail so that the α of the wing
decreases
.
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
DIRECTIONAL STABILITY
Airplanes achieve yaw stability the
same way an arrow does. The
feathers (really small fins) at the aft
end of an arrow aerodynamically
correct any tendency to yaw by
producing a yaw-countering side
force.
The fins on a missile work the same
way and an airplane does the same
with a vertical stabilizer.
The rudder allows to overcome this inherent stability when yaw is desired.
Virginia Tech
Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
LATERAL STABILITY
Roll stability is more complicated.
We want stable roll, however we want to
be able to roll on demand (for
maneuvers like coordinated turn for
example).
Two ways to reach stable roll:
- High Wings
- Wing Dihedral
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Lateral Stability – High Wings
Pendulum effect = stable roll.
Higher
Pressure
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
LATERAL STABILITY AND SWEPT WING
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
LATERAL CONTROL AND LARGE WINGS
At high speeds, large wings (and therefore large ailerons) tend to have complex
dynamic motions that reduce their efficiency.
Using spoilers to create a lift differential is usually then preferred.
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Lateral Stability – Roll Induced by propellers
How do we fix this?
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Pulling it all together…
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Lecture 9
21 October 2008
AOE 2104--Aerospace and Ocean Engineering
Fall 2008
Assignments: Read the relevant sections of Chapter 7
Homework 6: Due 10 November
Timeline for the rest of the semester: (based on your midterm feedback)
Nov 3-Stability and Control
Nov 5-Aircraft and Spacecraft Propulsion
Nov 10-Helicopter Aero-special guest presentation
Nov 12-structures quiz
Nov 17-performance and stability quiz
Nov 19-intro to space
Nov 24/26-break
Dec 1-orbital mechanics
Dec 3-take home space quiz due
Dec 8-review for final
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Lecture 9
21 October 2008