STATIC LONGITUDINAL STABILITY

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PURPOSE OF CONTROLS IN AIRCRAFT:

Ailerons

Ailerons are mounted on the trailing edge of each wing near the wingtips and move in opposite directions. When the pilot moves the stick left, or turns the wheel counter-clockwise, the left aileron goes up and the right aileron goes down. A raised aileron reduces lift on that wing and a lowered one increases lift, so moving the stick left causes the left wing to drop and the right wing to rise. This causes the aircraft to roll to the left and begin to turn to the left. Centering the stick returns the ailerons to neutral maintaining the bank angle. The aircraft will continue to turn until opposite aileron motion returns the bank angle to zero to fly straight.

Elevator

An elevator is a moveable part of the horizontal stabilizer, hinged to the back of the fixed part of the horizontal tail. They move up and down together. When the pilot pulls the stick backward, the elevators go up. Pushing the stick forward causes the elevators to go down. Raised elevators push down on the tail and cause the nose to pitch up. This makes the wings fly at a higher angle of attack, which generates more lift and more drag. Centering the stick returns the elevators to neutral and stops the change of pitch. Many aircraft use a fully moveable horizontal stabilizer called stabilator or all-moving tail.

Rudder:

The rudder is typically mounted on the trailing edge of the vertical stabilizer, part of the empennage. When the pilot pushes the left pedal, the rudder deflects left. Pushing the right pedal causes the rudder to deflect right. Deflecting the rudder right pushes the tail left and causes the nose to yaw to the right. Centering the rudder pedals returns the rudder to neutral and stops the yaw.

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WEATHER COCKING EFFECT:

Application of the static stability principle to rotation about the z axis suggests that a stable airplane should have “weathercock stability. That is, when the airplane is at an angle of sideslip β relative to its flight path (see Fig. 3.1 I), the yawing moment produced should be such as to tend to restore it to symmetric flight. The yawing moment N is positive as shown. Hence the requirement for yaw stiffness is that ∂N/∂β must be positive. The non-dimensional coefficient of N is

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