Which of the following is NOT one of the four forces acting on an aircraft?

The four primary forces acting on an aircraft during flight are lift, drag, thrust, and weight. Lift is the upward force generated by the wings as air flows over them, allowing the aircraft to rise. Drag is the resistance an aircraft encounters as it moves through the air; it opposes thrust and is largely influenced by the aircraft's shape and speed. Thrust is the forward force produced by the engines, propelling the aircraft through the air.

Torque, while a relevant concept in aviation, particularly in discussions about the dynamics of rotary-wing aircraft or engine operations, is not classified as one of the four fundamental forces acting on an aircraft. Instead, torque refers to the rotational force around an axis which can lead to a yawing moment in multi-engine aircraft; however, it does not directly contribute as one of the primary forces that keep the aircraft in flight like lift, drag, and thrust do. This distinction clarifies why torque is not included among the four main forces acting on an aircraft.