Which physical principle explains the reactive torque on an aircraft when a propeller spins?

• A. Newton's Third Law
• B. Bernoulli's Principle
• C. Conservation of Energy
• D. Coanda Effect

Answer: (A)

The main idea is action–reaction. To spin the propeller, the engine applies a torque to the propeller shaft. The propeller then pushes on the air (and the air pushes back on the propeller and engine) with an equal and opposite torque. That opposite torque twists the aircraft about its longitudinal axis, producing the reactive torque you feel as the airplane tends to roll in the direction opposite to the propeller’s rotation. The stronger the power and the larger the propeller, the greater this reaction.

Bernoulli’s principle explains lift from pressure differences, not this rotational reaction. Coanda effect describes fluid flow following a surface, not a torque on the airframe. Conservation of energy is a broad principle, not the specific mechanism here.