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

• A. Centripetal force
• B. Lift
• C. Weight
• D. Drag

Answer: (A)

The airplane is acted on by four forces: lift pushing upward, weight pulling downward, thrust pushing forward, and drag pulling backward. Centripetal force isn’t an extra, separate force on the aircraft. It’s the inward acceleration required to make the path curved, which comes from the existing forces. In a turn, the lift vector tilts and its horizontal component points toward the center of the turn, providing the centripetal force. In straight-and-level flight, there’s no turning, so there’s no separate centripetal force. That’s why centripetal force isn’t considered one of the four primary forces.