What are the four forces that act on an aircraft?

• A. Gravity, Inertia, Lift, Drag
• B. Lift, Drag, Thrust, Weight
• C. Lift, Inertia, Thrust, Weight
• D. Lift, Drag, Pressure, Velocity

Answer: (B)

The four forces acting on an aircraft are Lift, Weight, Thrust, and Drag. Lift is the upward force created by the wings as air flows over them; Weight is the downward pull of gravity on the aircraft; Thrust is the forward push produced by the engines; and Drag is the backward resistance the air offers to the aircraft’s forward motion. These external forces interact to determine the airplane’s motion, and in steady, level flight lift balances weight while thrust balances drag. The other terms listed aren’t the four external forces: inertia and velocity aren’t forces, and pressure is part of how lift and drag arise but isn’t counted as a separate force on the free-body diagram.