Ground Effect

What is Ground Effect?

Ground effect in aviation refers to the changes in aerodynamic forces that occur when an aircraft operates near the ground, usually within a height of one wingspan above the surface. The presence of the ground interrupts the normal downward airflow (downwash) behind the wing, suppressing the formation of wingtip vortices and altering the pressure distribution around the wing.

Effects on Lift and Drag

When an aircraft enters ground effect:

• Lift increases due to a higher effective angle of attack as downwash is reduced.
• Induced drag decreases because weaker wingtip vortices form, and less energy is lost to the air.
• The coefficient of induced drag (CDi) drops, making the wing more efficient.

Changes to Angle of Attack

• The induced angle of attack (the angle by which downwash reduces the effective angle) decreases in ground effect.
• The effective angle of attack increases, so the wing produces more lift at the same geometric angle.
• The critical angle of attack (alphaCRIT) is effectively reduced, meaning the wing can stall at a lower geometric angle when close to the ground.

Impact on Takeoff and Landing

• On takeoff, as the aircraft leaves ground effect, lift decreases and induced drag increases, requiring careful speed management to avoid premature liftoff or settling back onto the runway.
• On landing, increased lift and reduced drag can cause the aircraft to float, potentially extending the landing roll.
• Low-wing aircraft experience stronger ground effect due to their proximity to the surface.

Ground Effect Symptoms and Operational Considerations

• Aircraft may feel more buoyant or reluctant to settle during landing.
• On takeoff, the aircraft may lift off at a lower speed but struggle to climb once clear of ground effect if not enough speed is achieved.
• Pilots must be aware of these changes to avoid misjudging landing distance or takeoff performance.