Principles of Flight Stability and Control
Airplane axis
Static stability Degree of Stability
Dynamic Stability
Static Longitudinal Stability
Factors affecting Static Longitudinal Stability Position of the Center of Gravity
Factors affecting Static Longitudinal Stability Position of the Wing’s Center of Pressure Design of the Tailplane Wing Downwash
Graphical presentation of Static Longitudinal Stability
Effect of elevator deflection
Control Force Stability • A pull force must be present to obtain and maintain airspeeds below specified trim speed, and a push force – above the specified trim speed; • The airspeed must return to within 10% of the original trim speed during the climb, approach and landing conditions, and must return to within 7. 5% during cruise; • The average gradient of the stable slope of the stick force curve vs speed curve may not be less than 1 lb for each 6 kt.
Maneuvering Stability
Tailoring the Control Forces Increase maneuvering stick force stability Preloaded spring Experiences the same forces as the aircraft
Directional Static Stability
Factors affecting Static Directional Stability
Lateral Static Stability
Wing Dihedral
Wing Sweepback
High Wing and Low Center of Gravity
High Keel and Low Center of Gravity
Interaction between Lateral and Directional Static Stability • Amount of dihedral • Amount of sweepback • Keel surface area DUTCH ROLL Directional stability is less than is lateral stability
Yaw Damper Systems
Speed Stability
Piston-engine aircrafts are less susceptible to speed instability because: • Response to throttle movement is almost immediate • Propeller slipstream produces additional lift without any change in airspeed Deploying the flaps and lowering the undercarriage help to delay the onset of speed instability by increasing profile drag, thus reducing the minimum drag speed
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