Ground Effect

Mechanism

When within approximately one wingspan height of the surface:

• The ground interferes with the formation of wing tip vortices.
• The vortices are weaker.
• Downwash is reduced.
• The effective Angle of Attack increases.

Effects

1. Reduced Induced Drag: The clearer lift vector (less backward tilt) means less induced drag.
2. Increased Lift: For a fixed angle of attack, lift increases ($C_L$ increases).
3. Pitch Change: Downwash change alters the airflow over the tailplane, typically causing a nose-down moment (reduction in downwash on tail).
4. Local Static Pressure: Increases below the wing (cushion effect).

Consequences for Flight

• Landing: The aircraft tends to "float". If speed is not reduced, the aircraft will glide further than expected.
• Take-off: The aircraft may become airborne before reaching the safe climb speed ($V_2$). If it climbs out of ground effect too early, induced drag will suddenly increase, potentially causing it to sink back down.