What effect does propeller slipstream have on tailplane effectiveness at high power?

• A. It increases tailplane downwash, improving elevator effectiveness.
• B. It has no effect on tailplane downwash.
• C. It causes tailplane stall at high power.
• D. It reduces elevator effectiveness due to altered tailplane downwash.

Answer: (D)

Propeller slipstream changes the flow field over the tailplane, which alters how effective the elevator is at producing a pitching moment. When power is high, the slipstream is strong and accelerates air as it flows across the horizontal stabilizer. This changes the tailplane’s downwash and the local angle of attack for any given elevator deflection. Because the tailplane is now in this modified, faster flow with increased downwash, the change in tailplane lift produced by moving the elevator is smaller, so the elevator’s ability to trim or pitch the airplane is reduced. In short, the stronger propeller slipstream at high power reduces elevator effectiveness through altered tailplane downwash.