Ground effect on take-off and landing

What ground effect is

Just before touchdown, the aeroplane seems not to want to go lower and keeps floating. Pilots have known this feeling since the early days of flying. It is ground effect. When an aeroplane comes within several feet of the ground or water, the surface restricts the vertical airflow around the wing. This changes the upwash, the downwash and the wingtip vortices, the swirls of air at the wing tips.

EASA’s PPL(A) syllabus lists ground effect under principles of flight, with the focus on its effect on the take-off and landing characteristics of an aeroplane. The explanations and figures below come from the FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK, chapter 5). It explains the physics of the effect.

Why induced drag falls

Induced drag is unavoidable whenever a wing produces lift. It is the result of the work the wing does to carry the aeroplane: the wing accelerates a mass of air downwards. The more downwash there is, the harder the wing pushes the air down. The angle of attack is the angle between the chord line of the wing and the oncoming air. At a high angle of attack, induced drag is high. Because that corresponds to low speeds, induced drag predominates at low speed.

In ground effect the wingtip vortices become weaker. This changes the lift distribution along the span and reduces the induced angle of attack and the induced drag. The wing therefore needs a lower angle of attack to produce the same lift coefficient. The lift coefficient (CL) is a dimensionless number that relates the lift to the dynamic pressure of the airflow. If the angle of attack stays the same, the lift coefficient increases. At low speed, where induced drag predominates, the thrust required also falls significantly.

The effect is large only very close to the ground. At constant lift coefficient, induced drag falls by only 1.4% at a wing height of one wingspan, by 23.5% at a quarter of the span and by 47.6% at a tenth of the span:

Wing height above the ground	Reduction in induced drag
one wingspan	1.4%
one quarter of the span	23.5%
one tenth of the span	47.6%

A large reduction therefore occurs only when the wing is very close to the ground. That is why you notice ground effect mainly at lift-off and just before touchdown.

On take-off

Because of the reduced drag, the aeroplane may seem capable of taking off well below the recommended speed while it is in ground effect. There is also an instrument error: in most cases ground effect raises the pressure at the static source, so airspeed and altitude read too low. The aeroplane can therefore be airborne at a lower indicated airspeed than is normally required.

If it climbs out of ground effect with a deficiency of speed, the greater induced drag may leave it with marginal initial climb performance. The FAA handbook lists four changes when an aeroplane leaves ground effect:

• You need a higher angle of attack to keep the same lift coefficient.
• Induced drag and the thrust required increase.
• Stability decreases and the pitching moment changes nose-up.
• The static source pressure falls and the indicated airspeed rises.

In extreme conditions, for example high gross weight, high density altitude and high temperature, the aeroplane can become airborne but be unable to keep flying out of ground effect. It then settles back onto the runway. The handbook warns against forcing an aeroplane into the air with too little speed: the manufacturer’s recommended take-off speed is needed for adequate initial climb performance.

On landing

If you bring the aeroplane into ground effect at a constant angle of attack, the lift coefficient rises, the thrust required falls and the aeroplane floats. Because drag is low and there is no deceleration when power is reduced, any excess speed in the flare turns into a considerable float distance.

Ground effect is strongest at heights below the wingspan. On the final approach the power must be reduced to offset the extra lift. Otherwise the aeroplane tends to climb above the desired glidepath.