I've heard people say "unload the controls" or "unload the wing/propeller."
What does this mean exactly? It is usually followed by releasing back pressure on the elevators or starting/ending a turn.
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When maneuvering the aircraft by deflecting the control surfaces with the stick, the wing with the downward aileron and the tail will generate additional lift, which increases the load on that wing (using the ailerons) and on the tailplane (using the elevator).
Unload the controls is a request to neutralize the pressure on the controls. Usually this means bringing the stick back to the center position, reducing the controls surface deflection, the amount of lift generated, and therefore reducing the aerodynamic load on the wings and tailplane.
Unload means immediately reduce the lift being produced by a lifting surface. You:
The typical reason to "unload" is to provide greater a margin from a stall or to reduce torque on an engine.
The term unload is easily understood from a bicyclist's perspective. When a steep hill flattens or a bike is shifted into lower gear, the bicyclist experiences unloading.
In conventional aircraft, control force (called stick force gradient) exists to inhibit pilot over-control. Control force is proportionate to the lift being produced and to aircraft speed. Therefore, to say "unload the controls", is the same as saying to "unload the lift on that surface".
When you are manipulating the airplane, for example in a turn, you're increasing the aerodynamic loading of the wings. In a 30-degree level turn, the loading is 1.3G, i.e. 1.3 times the weight of the aircraft in straight-and-level flight. In a 60 degrees level turn, the loading is 2G.
If the plane is flying with excessive aerodynamic loading for a continued amount of time, it can end up in a stall. This is because to achieve a higher loading, the angle of attack must be increased; eventually it may exceed the critical angle of attack. With a higher angle of attack, the airframe will also produce more drag, with slows the aircraft down.
Excessive loading can also lead to structural failure, as you are asking the wing / elevator structure to provide more lift, increasing the stress of the airframe.
"Unload the wings" is a general term meaning "to reduce the lift produced by the wings". You're asking too much from the airplane. The action to execute depends on the state of the aircraft. If you're in a level turn, reducing the bank angle reduces wing loading. If you're pulling up, easing back pressure on the yoke / stick reduces wing loading.
In aerial dogfighting, "unloading" means to ease back on the stick and allow the airplane to pick up speed. High-G loading bleeds off energy very quickly. A pilot must manage his energy during a fight. If he keeps making very sharp turns, the high aerodynamic loading of the airframe will slow the aircraft down; and slow moving targets are easy targets.