Ground effect occurs when you are close to the surface which changes the airflow around the wing. When flying close to the surface, the airplane flies better because of less drag, more life and cannot fly at slower speed.
The three dimensional airflow pattern around the aircraft is altered by the ground during takeoff and landing. This restricts the vertical component of airflow around the wing which alters the wing’s upwash, downwash & wingtip vortices.
The cause of “ground effect” can be attributed to the differences in air pressure. When a aircraft is producing lift, high pressure below the wing tends to flow outward around the wing tips towards the low pressure on the top of the wing which reduces lift.
Some of this air does not make it to the top of the wing in time which creates a horizontal whirlpool of air in the wake of the aircraft. This whirlpool of air in the wake is called a vortex.
It is these horizontal whirlpools of air streaming from the wingtips that are called “wingtip vortices” or “wake turbulence”.
When an aircraft is low over a surface such as the ground or runway, the wingtip vortices are reduced because they are physically blocked by the ground from reaching the top of the wing. As a result, lift is increased and drag is decreased.
This greater lift and reduction of drag allows the aircraft to fly at lower speeds with less power.
Takeoff and Ground Effect
Ground effect can cause an aircraft to lift off at an airspeed that is not fast enough for flying OUT of ground effect. You will be out of ground effect once you climb more than one wingspan above the runway.
As lift decreases and drag increases, more thrust and a higher angle of attack is necessary which increases the stall speed.
Ground affect has the potential to make the aircraft lift prematurely which may result in settling back down to the runway and increases the distance needs to clear any obstacles on or at the end of the runway.
In order to best avoid premature liftoff, it is best to wait until you have reached the recommended speed for takeoff before allowing the aircraft to lift off.
Landing and Ground Effect
When landing, lift is increased and drag is decreased within one wingspan from the ground which causes the aircraft to have a tendency to float.
Excess speeds on landing approach can result in a greater “floating” distance so you will want to reduce the power as the airplane descends into ground effect to avoid overshooting the desired touchdown point on the landing strip or runway.
Floating on Landing
If you experience floating on landing, you have to take a few precautions and/or possibly take another course of action. Be sure to keep the aircraft on centerline and use the aircrafts lateral movement using the aircraft’s ailerons and rudder to prevent drifting from centerline.
Do not attempt to force a touchdown at a faster than normal airspeed as this could cause damage to the aircraft, especially if landing on the nose gear first.
If there isn’t enough runway to touchdown at normal speed and to stop without braking, you will have to make a go around pass and try again.