Downwash from the propeller increases elevator and rudder effectiveness at what speed?

• A. Yes
• B. No
• C. Only at high speed
• D. Only in a stall

Answer: (A)

Propeller slipstream adds energy to the flow over the tail, so the elevator and rudder see higher dynamic pressure and become more effective. This improvement is most noticeable at lower airspeeds, where the aircraft’s normal flow is weaker and the extra flow from the propeller represents a larger change. As speed increases, the free-stream flow dominates, so the relative gain from the propeller wash is smaller. In a stall, the flow is disturbed and the downwash effect becomes less reliable. So, downwash from the propeller does increase elevator and rudder effectiveness, especially at low speeds.