The purpose of a vortex generator on a horizontal stabilizer is to delay flow separation during which condition?

• A. High speed cruise
• B. Low speed flight
• C. Stall at high angle of attack
• D. Takeoff in crosswind

Answer: (B)

The main idea being tested is how boundary-layer energizing devices help keep flow attached on a tailplane. Vortex generators create tiny, energized vortices over the horizontal stabilizer. These vortices mix higher-energy air into the boundary layer, boosting its momentum near the surface. That keeps the flow attached longer and delays separation, which is especially important when the airplane is flying slowly.

In low-speed flight, the tailplane is more prone to becoming unstalled because the dynamic pressure is reduced and the tail can operate near conditions where the wake from the wing or flaps causes separation. By delaying separation, the horizontal stabilizer preserves lift and the elevator’s effectiveness, maintaining stable pitch control during approaches, gusty conditions, or other situations where speed is low.

At high speeds, the flow over the stabilizer is more likely to stay attached anyway, so there’s less need for vortices. The other scenarios mentioned involve different factors that aren’t the primary reason for adding vortex generators on the stabilizer.