Stability and Control

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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
TRADITIONAL AIRCRAFT CONTROLS
(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

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