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Introduction to 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

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Introduction to Stability and Control

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  1. Introduction to Stability and Control

  2. 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

  3. (All moving) Elevators Traditional Aircraft Controls Ailerons Rudder

  4. 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)

  5. Aircraft Motions - Roll • Roll: what is it?

  6. Aircraft Motions - Roll • Roll: Motion about the longitudinal (X) axis produced by the ailerons (l moment)

  7. Aircraft Motions - Pitch • Pitch:

  8. Aircraft Motions - Pitch • Pitch: Motion about the lateral (Y) axis produced by the elevators (m moment)

  9. Aircraft Motions - Yaw • Yaw:

  10. Aircraft Motions - Yaw • Yaw: Motion about vertical (Z) axis produced by the rudder(s) (n moment)

  11. 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

  12. Moments and Forces • Trimmed Flight SMcg = 0 • Straight and Level, Unaccelerated Flight (S.L.U.F.) SF = 0 • L = W T = D

  13. Conventional Airplane Lw Lt cg Ma.c. xac xt xcg SMcg = 0 = Ma.c + Lw (xcg – xac) – Lt (xt)

  14. Criteria for Longitudinal Static Stability Aircraft is not moving in pitch! 1. CM,0 > 0 2. ∂CM,cg / ∂ a < 0

  15. Longitudinal Stability—Tail Effects • Tail aft of cg is Stablizing • Canards are Destabilizing • Increase stability (more negative CMa) by Lifttail Longer moment arm St Larger tail

  16. 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

  17. 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

  18. Conventional Tail - Stabilizing • F-22 • F-16

  19. Canards I - Destabilizing • Su-35 • Long-Eze

  20. Canards II - Eurofighter

  21. +b V + Ncg Lv Top View x y 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)

  22. +b V Lw/b - Ncg - Ncg Top View x y Wing/Body Contribution to DIRECTIONAL STATIC STABILITY Design Considerations - Fuselage area forward of the cg is directionally destabilizing - That’s why aircraft have tails!

  23. 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 ARvt and/or Increase evt) V y -L z Rear View

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

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