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Published byKyleigh MacKay Modified over 2 years ago

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INTRODUCTION TO STABILITY AND CONTROL

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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

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TRADITIONAL AIRCRAFT CONTROLS Ailerons (All moving) Elevators Rudder

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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)

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AIRCRAFT MOTIONS - ROLL Roll: what is it?

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

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AIRCRAFT MOTIONS - PITCH Pitch:

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

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AIRCRAFT MOTIONS - YAW Yaw:

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

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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

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MOMENTS AND FORCES Trimmed Flight M cg = 0 Straight and Level, Unaccelerated Flight (S.L.U.F.) F = 0 L = W T = D

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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 )

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

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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

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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

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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

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CONVENTIONAL TAIL - STABILIZING F-22 F-16

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CANARDS I - DESTABILIZING Su-35 Long-Eze

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CANARDS II - EUROFIGHTER

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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 VV

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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 VV L w/b

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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 VV

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

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