### Stability and Control

```INTRODUCTION TO STABILITY AND
CONTROL
STABILITY SUMMARY
Axes, Moments, Velocities – Definitions
Moments and Forces
Static Longitudinal Stability
 Tail Effects
 Wing Effects
Static Margin
Directional Static Stability
 Vertical Tail
 Wing/Body
Lateral Static Stability
 Vertical Tail
 Wing Sweep
(All moving)
Elevators
Ailerons
Rudder
ALTERNATE CONTROL METHODS
Canards
Spoilers (T1) and Speed Brakes
Wing Warping
Center of Gravity Shift
V-tail (combines pitch and yaw control)
Thrust Vectoring or Asymmetric Thrust
Flaperons (flap and aileron)
Elevons (elevator and aileron)
Ruddervators (rudder and elevator)
AIRCRAFT MOTIONS - ROLL
Roll: what is it?
Ailerons
AIRCRAFT MOTIONS - ROLL
Roll: Motion about the longitudinal (X) axis produced by the ailerons (l moment)
Ailerons
AIRCRAFT MOTIONS - PITCH
Pitch:
Elevator
AIRCRAFT MOTIONS - PITCH
Pitch: Motion about the lateral (Y) axis produced by the elevators (m moment)
Elevator
AIRCRAFT MOTIONS - YAW
Yaw:
Rudder
AIRCRAFT MOTIONS - YAW
Yaw: Motion about vertical (Z) axis produced by the rudder(s) (n moment)
Rudder
STABILITY VS.
MANEUVERABILITY (CONTROL)
Stable Aircraft—not very easy to move
 Not very maneuverable
 C-5, C-17, B-52, Passenger airplanes
Maneuverable Aircraft—very easy to move
 Not very stable (unstable in many cases)
 Require Flight Control Systems to keep aircraft pointy
end forward
 F-16, F-22
MOMENTS AND FORCES
Trimmed Flight
SMcg = 0
Straight and Level, Unaccelerated
Flight (S.L.U.F.)
SF = 0

L=W
T=D
CONVENTIONAL AIRPLANE
Lw
Lt
cg
Ma.c.
xac
xt
xcg
SMcg = 0 = Ma.c + Lw (xcg – xac) – Lt (xt)
CRITERIA FOR LONGITUDINAL
STATIC STABILITY
Aircraft is not moving
in pitch!
1. CM,0 > 0
2. ∂CM,cg / ∂ a < 0
LONGITUDINAL STABILITY—TAIL
EFFECTS
Tail aft of cg is Stablizing
Canards are Destabilizing
Increase stability (more negative CMa) by
⁭ Lifttail
⁭ St
Longer moment arm
Larger tail
LONGITUDINAL STABILITY—WING
EFFECTS
Wing a.c. forward of c.g. is Unstable
Decrease instability (lower CMa)
 ↓ (hcg – hac) Shorter Moment Arm or move
c.g. forward
LONG.-STATIC STABILITY - TOTAL
AIRCRAFT
Most parameters are fixed once the aircraft is built
C.G. can be moved
 Cargo location
 Fuel location
 Weapons, Stores, etc.
Variable Geometry wings—change cg
CONVENTIONAL TAIL - STABILIZING
F-22
F-16
CANARDS I - DESTABILIZING
Su-35
Long-Eze
CANARDS II - EUROFIGHTER
Vertical Tail Contribution to
DIRECTIONAL STATIC STABILITY
+b
x
V
Design Considerations (Main Contributor)
y
+ Ncg
Lv
Top View
• Vertical tail aft of c.g. is stabilizing
• To increase directional stability
-- Vert. tail further aft
-- Vert. tail bigger (or add another)
Wing/Body Contribution to
DIRECTIONAL STATIC STABILITY
x
+b
Lw/b
V
Design Considerations
y
- Ncg
- Ncg
Top View
- Fuselage area forward of the cg
is directionally destabilizing
- That’s why aircraft have tails!
Vertical Tail Contribution to
LATERAL STATIC STABILITY
Design Considerations
V
-L
z
Rear View
y
- Vertical tail above c.g. is stabilizing
- To increase lateral stability
-- Vert. tail taller”
-- Vert. tail “bigger” (more area)
-- Increase Vert. tail lift curve slope
(Increase ARvt and/or Increase evt)
Wing Sweep Contribution to
LATERAL STATIC STABILITY
x
Positive wing sweep is stabilizing
+b
V
V
y
Less lift
y
More lift
z
Top View
Rear View
```