Lift always acts in a direction perpendicular to the relative wind and to the lateral axis of the aircraft. The fact that lift is referenced to the wing, not to the Earth’s surface, is the source of many errors in learning flight control. Lift is not always “up.” Its direction relative to the Earth’s surface changes as the pilot maneuvers the aircraft.
The magnitude of the force of lift is directly proportional to the density of the air, the area of the wings, and the airspeed. It also depends upon the type of wing and the AOA. Lift increases with an increase in AOA up to the stalling angle, at which point it decreases with any further increase in AOA. In conventional aircraft, lift is therefore controlled by varying the AOA and speed.
Pitch/Power Relationship
An examination of [Figure] illustrates the relationship between pitch and power when controlling flightpath and airspeed. To maintain constant lift as airspeed decreases, pitch must be increased. The pilot changes pitch through elevator control, which alters the AOA. When back pressure is applied to the control, the tail moves downward and the nose rises, increasing the wing’s AOA and lift.
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| Relationship of lift to AOA |
Under most conditions the elevator produces a downward force on the tail. This force requires energy that is taken from aircraft performance (speed). When the CG is located farther aft, less downward tail force is required. As a result, less energy is used for tail downforce and more remains available for aircraft performance.
Thrust is controlled with the throttle to establish or maintain the desired airspeed. The most precise method of controlling the flightpath is to adjust pitch while simultaneously using power to control airspeed. To maintain constant lift, any change in pitch requires a corresponding change in power, and vice versa.
If the pilot wants the aircraft to accelerate while maintaining altitude, thrust must be increased to overcome drag. As speed increases, lift increases, so pitch must be lowered to reduce AOA and maintain altitude. To decelerate while maintaining altitude, thrust is reduced below drag. As speed decreases and lift is reduced, pitch must be increased to maintain the required AOA and altitude.