Private Pilot License (PPL) Aerodynamics Practice Exam

What causes an airplane (except T-tail) to pitch nose down when power is reduced and controls are not adjusted?

1. The downwash on the elevator from the propeller slipstream is increased

2. The downwash on the elevator from the propeller slipstream is reduced

3. The elevator effectiveness is increased with reduced power

4. The aircraft's weight shifts forward upon power reduction

The correct answer is: The downwash on the elevator from the propeller slipstream is reduced

When power is reduced on an airplane, particularly on designs that are not T-tails, the airflow around the aircraft changes significantly. One of the key effects is the reduction of the downwash on the elevator caused by the propeller slipstream. Under normal operating conditions, the rotating propeller creates a downward flow of air (downwash) that enhances the lift on the horizontal stabilizer and, consequently, the effectiveness of the elevator. When power is decreased, the thrust from the propeller decreases, leading to a reduction in the intensity of this downwash. With less downwash, the airflow over the elevator is less effective at providing the necessary lift to maintain the aircraft's pitch attitude. This reduction in the elevator's aerodynamic effectiveness ultimately leads to a nose-down pitch because the tail loses some of its lifting capability without the enhanced downwash, resulting in a natural tendency for the nose to drop. Understanding this interaction between the propeller slipstream and elevator effectiveness is crucial for pilots, as it emphasizes the importance of maintaining control inputs during changes in power settings.