Which two forces must balance for constant airspeed to be maintained in level flight?

Which two forces must balance for constant airspeed to be maintained in level flight?

• A. Thrust and Drag
• B. Lift and Weight
• C. Drag and Weight
• D. Thrust and Lift

Answer: (A)

In level, steady flight the speed stays constant because the forces along the direction of motion cancel each other out. The forward force is thrust from the engine, and the rearward force is drag from air resistance. When thrust equals drag, there’s no net force along the flight path, so the airspeed remains unchanged. If thrust exceeds drag, the aircraft speeds up; if drag exceeds thrust, it slows down. (Lift and weight balance to keep the aircraft at a constant altitude, but the question of constant airspeed specifically hinges on thrust balancing drag.)

In level, steady flight the speed stays constant because the forces along the direction of motion cancel each other out. The forward force is thrust from the engine, and the rearward force is drag from air resistance. When thrust equals drag, there’s no net force along the flight path, so the airspeed remains unchanged. If thrust exceeds drag, the aircraft speeds up; if drag exceeds thrust, it slows down. (Lift and weight balance to keep the aircraft at a constant altitude, but the question of constant airspeed specifically hinges on thrust balancing drag.)