introduction to stability and control. stability summary axes, moments, velocities – definitions...
TRANSCRIPT
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INTRODUCTION TO STABILITY AND CONTROL
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STABILITY SUMMARY
Axes, Moments, Velocities – Definitions
Moments and Forces
Static Longitudinal StabilityTail EffectsWing Effects
Static Margin
Directional Static StabilityVertical TailWing/Body
Lateral Static StabilityVertical TailWing 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?
Ailerons
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AIRCRAFT MOTIONS - ROLL
Roll: Motion about the longitudinal (X) axis produced by the ailerons (l moment)
Ailerons
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AIRCRAFT MOTIONS - PITCH
Pitch:
Elevator
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AIRCRAFT MOTIONS - PITCH
Pitch: Motion about the lateral (Y) axis produced by the elevators (m moment)
Elevator
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AIRCRAFT MOTIONS - YAW
Yaw:
Rudder
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AIRCRAFT MOTIONS - YAW
Yaw: Motion about vertical (Z) axis produced by the rudder(s) (n moment)
Rudder
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STABILITY VS.MANEUVERABILITY (CONTROL)
Stable Aircraft—not very easy to move Not very maneuverableC-5, C-17, B-52, Passenger airplanes
Maneuverable Aircraft—very easy to moveNot 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 SMcg = 0
Straight and Level, Unaccelerated Flight (S.L.U.F.) SF = 0
L = W T = D
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CONVENTIONAL AIRPLANE
cg
Lw
Lt
Ma.c.
xcg
xac
xt
SMcg = 0 = Ma.c + Lw (xcg – xac) – Lt (xt)
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CRITERIA FOR LONGITUDINAL STATIC STABILITY
1. CM,0 > 02. ∂CM,cg / ∂ a < 0
Aircraft is not moving in pitch!
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LONGITUDINAL STABILITY—TAIL EFFECTSTail aft of cg is Stablizing
Canards are Destabilizing
Increase stability (more negative CMa) by
Lifttail Longer moment arm
St Larger tail
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LONGITUDINAL STABILITY—WING EFFECTSWing a.c. forward of c.g. is Unstable
Decrease instability (lower CMa)
↓ (hcg – hac) 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
+ Ncg
+b
Lv
V
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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
- Ncg
- Ncg
+b VLw/b
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Vertical Tail Contribution toLATERAL 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)
Rear View
y
z
-L
V
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Wing Sweep Contribution toLATERAL STATIC STABILITY
x
y
+ b
V
Rear View
y
z
Top View
V
Positive wing sweep is stabilizing
Less lift More lift