Ground effect

Ground Effect

Ground effect is the aerodynamic phenomenon that arises when an aircraft flies close to the ground — within about one wingspan in height.

Mechanism

The ground disturbs wingtip vortex formation:

1. Downwash reduced: less downward-directed flow under the wing.
2. Wingtip vortex weaker: vortices cannot fully develop because the ground inhibits them.
3. Effective angle of attack of the wing becomes larger (less downwash).
4. Induced drag drops drastically (typically 40-60 % at 0.1 spans height; Anderson Ch. 5).
5. Lift coefficient rises at the same α.

Height dependence

Effect strength depending on the ratio height/wingspan:

Cessna 172 with 11 m span: ground-effect range about 0 to 11 m AGL.

On take-off — pros and cons

Pros

• Aircraft "lifts off earlier" than the performance table suggests (less induced drag → less power required).
• Soft-field take-off: after lift-off, accelerate in ground effect to Vy → only then climb.

Cons / traps

• "Aircraft lifts off, can't climb": if you "leave" in ground effect but lack speed for climb → climb attempt results in stall, because drag returns out of ground effect.
• Hot and high: ground effect can deceive — aircraft feels okay on take-off but sinks once outside ground effect.

On landing

• Flare becomes easier: lift higher, sink rate naturally diminishes → softer touchdown.
• "Floating": with too high touchdown speed the aircraft floats long — runway may be insufficient.
• Crosswind landing: ground effect can amplify drift because aircraft "floats" longer.

Pilotage recommendations

Take-off

1. Performance from POH — ground effect already accounted for.
2. Don't rotate too early — wait until Vr.
3. Climb-out: at lift-off in ground effect, accelerate to Vx or Vy, then climb.

Landing

1. Stabilised approach at Vapp.
2. Over threshold: power idle, pitch up — flare begins.
3. Sink rate should fall to 0 in ground effect, smooth touchdown.
4. If floating: apply brakes, retract flaps (POH).

Special aspects

• WIG (Wing In Ground Effect) aircraft: specifically designed for efficient ground-effect use (e.g. ekranoplan, Caspian Sea Monster) — can only fly just above water.
• Birds use ground effect instinctively (gulls over water).
• Helicopters: similar effect in hover (IGE/OGE — In/Out of Ground Effect).

POH performance tables

Take-off distance and landing distance in POH tables are determined under standard conditions (ISA, MSL, no wind, hard runway). Ground effect is included — but actual wind, runway state, altitude, temperature must be accounted for separately.