In steady, unaccelerated flight, which set of forces correctly represents the four fundamental forces acting on an aircraft?

• A. Lift, weight, thrust, and drag.
• B. Drag, weight, thrust, and center of gravity.
• C. Lift, gravity, thrust, and drag.
• D. Lift, weight, thrust, and inertia.

Answer: (A)

In steady, unaccelerated flight, the aircraft must have zero net force, so the forces acting on it balance in pairs. The four forces involved are lift, weight, thrust, and drag. Lift acts upward to support the aircraft, weight acts downward due to gravity, thrust pushes forward, and drag acts backward opposite the motion. When speed and altitude are constant, lift equals weight and thrust equals drag, keeping the aircraft from speeding up, slowing down, climbing, or descending.

This is why the set of forces that includes lift, weight, thrust, and drag is correct. The other options mix in things that aren’t forces (center of gravity is a location, inertia is not a separate force) or swap weight for gravity as if it were a separate force, which isn’t how the aircraft’s forces are described in flight dynamics.