Which force is NOT one of the four forces acting on an airplane?

The correct answer is momentum. In the context of aviation and the forces acting on an airplane, the four primary forces are lift, drag, thrust, and weight (gravity). These forces are fundamental to the flight dynamics of an aircraft.

Lift is the force that acts perpendicular to the relative wind and allows an airplane to rise off the ground. Drag is the resistance force that opposes an aircraft's forward motion through the air. Thrust is the forward force produced by the aircraft's engines to overcome drag and propel the aircraft.

Momentum, while an important concept in physics, refers to the quantity of motion an object possesses and is related to an object's mass and velocity. It is not considered one of the fundamental forces acting on an airplane during flight. Instead, momentum is a result of the interactions of the forces when an airplane is in motion, but it does not act on the aircraft in the same way as lift, drag, and thrust do.