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For additional information on flight control systems, refer to the appropriate handbook for information related to the flight control systems and characteristics of specific types of aircraft.
Flight Control Systems Flight Controls
Aircraft flight control systems consist of primary and secondary systems. The ailerons, elevator (or stabilator), and rudder constitute the primary control system and are required to control an aircraft safely during flight. Wing flaps, leading edge devices, spoilers, and trim systems constitute the secondary control system and improve the performance characteristics of the airplane or relieve the pilot of excessive control forces.Primary Flight Controls
Aircraft control systems are carefully designed to provide adequate responsiveness to control inputs while allowing a
5-3
F
igure 5-1. Airplane controls, movement, axes of rotation, and type of stability.Lateral axis (longitudinalstability)Aileron—RollRudder—YawElevator—PitchLongitudinal axis (lateral stability)Vertical axis(directional stability)AileronRollLongitudinalLateralRudderYawVerticalDirectionalElevator/StabilatorPitchLateralLongitudinalPrimaryControlSurfaceAirplane MovementAxes of RotationType of Stability
Figure 5-4. Airplane controls, movement, axes of rotation, and type of stability.
Figure 5-5. Adverse yaw is caused by higher drag on the outside wing, which is producing more lift.
Lif
tDragLiftDragFigure 5-2. Adverse yaw is caused by higher drag on the outside wing, which is producing more lift.Adverseyaw
natural feel. At low airspeeds, the controls usually feel soft and sluggish, and the aircraft responds slowly to control applications. At higher airspeeds, the controls become increasingly firm and aircraft response is more rapid.
Movement of any of the three primary flight control surfaces (ailerons, elevator or stabilator, or rudder), changes the airflow and pressure distribution over and around the airfoil. These changes affect the lift and drag produced by the airfoil/control surface combination, and allow a pilot to control the aircraft about its three axes of rotation.
Design features limit the amount of deflection of flight control surfaces. For example, control-stop mechanisms may be incorporated into the flight control linkages, or movement of the control column and/or rudder pedals may be limited. The purpose of these design limits is to prevent the pilot from inadvertently overcontrolling and overstressing the aircraft during normal maneuvers.
A properly designed airplane is stable and easily controlled during normal maneuvering. Control surface inputs cause movement about the three axes of rotation. The types of stability an airplane exhibits also relate to the three axes of rotation. [Figure 5-4]
AileronsAilerons control roll about the longitudinal axis. The ailerons are attached to the outboard trailing edge of each wing and move in the opposite direction from each other. Ailerons are connected by cables, bellcranks, pulleys and/or push-pull tubes to a control wheel or control stick.
Moving the control wheel or control stick to the right causes the right aileron to deflect upward and the left aileron to deflect downward. The upward deflection of the right aileron decreases the camber resulting in decreased lift on the right wing. The corresponding downward deflection of the left aileron increases the camber resulting in increased lift on the left wing. Thus, the increased lift on the left wing and the decreased lift on the right wing causes the airplane to roll to the right.Adverse Yaw
Since the downward deflected aileron produces more lift as evidenced by the wing raising, it also produces more drag. This added drag causes the wing to slow down slightly. This results in the aircraft yawing toward the wing which had experienced an increase in lift (and drag). From the pilot’s perspective, the yaw is opposite the direction of the bank. The adverse yaw is a result of differential drag and the slight difference in the velocity of the left and right wings. [Figure 5-5]
Adverse yaw becomes more pronounced at low airspeeds. At these slower airspeeds aerodynamic pressure on control surfaces are low and larger control inputs are required to effectively maneuver the airplane. As a result, the increase in aileron deflection causes an increase in adverse yaw. The yaw is especially evident in aircraft with long wing spans.
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Differential aileron
Aileron deflected up Aileron deflected down
Figure 5-6. Differential ailerons.
F
igure 5-4. Frise-type ailerons.LoweredNeutralRaisedDrag
Figure 5-7. Frise-type ailerons.
Application of rudder is used to counteract adverse yaw. The amount of rudder control required is greatest at low airspeeds, high angles of attack, and with large aileron deflections. Like all control surfaces at lower airspeeds, the vertical stabilizer/rudder becomes less effective, and magnifies the control problems associated with adverse yaw.
　
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