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 Ailerons (All moving) Elevators 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?

AIRCRAFT MOTIONS - ROLL Roll: Motion about the longitudinal (X) axis produced by the ailerons (l moment)

AIRCRAFT MOTIONS - PITCH Pitch:

AIRCRAFT MOTIONS - PITCH Pitch: Motion about the lateral (Y) axis produced by the elevators (m moment)

AIRCRAFT MOTIONS - YAW Yaw:

AIRCRAFT MOTIONS - YAW Yaw: Motion about vertical (Z) axis produced by the rudder(s) (n moment)

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  M cg = 0 Straight and Level, Unaccelerated Flight (S.L.U.F.)  F = 0  L = W T = D

CONVENTIONAL AIRPLANE cg LwLw LtLt M a.c. x cg x ac xtxt  M cg = 0 = M a.c + L w (x cg – x ac ) – L t (x t )

CRITERIA FOR LONGITUDINAL STATIC STABILITY 1. C M,0 > 0 2. ∂C M,cg / ∂  < 0 Aircraft is not moving in pitch!

LONGITUDINAL STABILITY—TAIL EFFECTS Tail aft of cg is Stablizing Canards are Destabilizing Increase stability (more negative C M  ) by  Lift tail Longer moment arm  S t Larger tail

LONGITUDINAL STABILITY—WING EFFECTS Wing a.c. forward of c.g. is Unstable Decrease instability (lower C M  )  ↓ (h cg – h ac ) 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 Design Considerations (Main Contributor) Vertical tail aft of c.g. is stabilizing To increase directional stability -- Vert. tail further aft -- Vert. tail bigger (or add another) Top View x y + N cg  LvLv VV

Wing/Body Contribution to DIRECTIONAL STATIC STABILITY Design Considerations - Fuselage area forward of the cg is directionally destabilizing - That’s why aircraft have tails! Top View x y - N cg  VV L w/b

Vertical Tail Contribution to LATERAL STATIC STABILITY Design Considerations - 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 AR vt and/or Increase e vt ) Rear View y z -L VV

Wing Sweep Contribution to LATERAL STATIC STABILITY x y +  VV Rear View y z Top View VV Positive wing sweep is stabilizing Less lift More lift