What is meant by thrust?

• A. The forward aerodynamic force produced by a propeller, fan, or turbojet engine as it forces a mass of air to the rear, behind the airplane.
• B. The vertical lifting force produced by wings.
• C. The resistance due to friction on the airframe.
• D. The downward force from gravity.

Answer: (A)

Thrust is the forward force produced by the propulsion system as it accelerates air rearward. It’s the reaction force, per Newton’s third law, that pushes the airplane forward when a propeller, fan, or turbojet pushes air toward the back. The engine or propeller transfers momentum to the air, and the airplane experiences an equal and opposite forward push. The basic idea can be thought of in terms of mass flow and velocity change: the propulsion device moves air and increases its backward speed, creating a forward force on the aircraft.

This is distinct from lift (the upward force from wings), drag (resistance along the surface), and weight (gravity pulling downward). In steady flight, thrust must overcome drag to maintain speed.