Under what condition do vortex generators operate effectively at slow speeds?

Vortex generators are aerodynamic devices designed to manipulate the airflow over a surface, enhancing the lift characteristics and delaying airflow separation. They operate effectively at slow speeds when mounted on the horizontal stabilizer because this location optimizes airflow control over the tail surfaces, which is critical for maintaining stability and control of the aircraft during low-speed operations, such as takeoff and landing.

When located on the horizontal stabilizer, these generators help to maintain attached airflow over the control surfaces, improving their effectiveness and responsiveness. This is particularly important during slow flight conditions where there is a higher risk of stall and airflow separation.

The other options do not provide the same benefit. Vortex generators at the wingtip or vertical stabilizer may not effectively improve slow-speed performance, as they are positioned away from the areas where control is most needed during low-speed flight. Attaching them to the main landing gear is also ineffective since this location does not influence the critical airflow over the wings and control surfaces where lift is generated and where control issues are most likely to arise during slow speeds.