Lift always acts perpendicular to the relative wind and to the lateral axis of the aircraft. Because lift is referenced to the wing rather than to the Earth’s surface, it is a frequent source of misunderstanding during flight training. Lift is not always directed upward relative to the ground; its direction changes as the aircraft changes attitude or flightpath.
The magnitude of lift depends directly on air density, wing area, and airspeed, and also on the wing design and angle of attack (AOA). Lift increases as AOA increases up to the critical or stalling angle. Beyond this point, lift decreases even if AOA continues to increase. In conventional aircraft, lift is therefore controlled primarily through changes in AOA and airspeed.
During climbs and descents, lift is no longer directly opposite weight. Instead, the lift vector tilts along the flightpath, so only part of it acts vertically. In a climb, a portion of lift acts rearward, increasing effective drag and requiring more thrust to maintain airspeed. In a descent, a component of weight acts forward along the flightpath, reducing the thrust required. For this reason, pitch changes primarily control the flightpath, while power adjustments compensate for the energy change produced by the altered lift distribution.
In banked flight, lift is divided into vertical and horizontal components. The vertical component supports the aircraft’s weight and the horizontal component produces the turn. Because only part of the lift acts upward, the pilot must increase AOA—and usually power—to maintain altitude. Without this correction the aircraft descends even if airspeed remains constant. This is why coordinated turns in instrument flight require simultaneous pitch and power adjustments to hold both altitude and airspeed.
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.