When an aircraft flies over a surface at an altitude of less than half its wingspan, it is under ground effect.
The main reason ground effect occurs is because, as pressure builds up on the underside of the wing, a ‘cushion’ of air is formed which causes the aircraft to tend to float, or even bounce, on the runway.
Ground effect is common during take-off, landing or low passes; basically, whenever the aircraft is very close to the ground.
Another reason behind the ground effect is the change in induced resistance, which we will explain in detail in another post so that we don’t go into too much detail in this one.
In aircraft wings, we distinguish between the upper surface, or extrados, and the lower surface, called intrados.
Well, when the airflow circulates through the wings in flight, pressures drop on the extrados, while they increase on the intrados. This is what generates lift and keeps the aircraft in the air.
The variations in air pressure are due to changes in velocity in the different parts of the wing, and it is a large, complex and interesting subject.
So, if you want to know more, we recommend you read our post on Bernoulli’s Principle and this one on lift in aircraft.
In their first flights, all student pilots must learn how to control the aircraft when it is under ground effect.
In this scenario, the aircraft becomes much more sensitive to control stick movements, especially pitch and roll, and will travel more distance on the runway.
To control the ground effect generated, the best option is to make very slight corrections to the controls and wait for the aircraft to slow down. As this happens, simply pull back on the stick to achieve the perfect landing.
Although ground effect makes it difficult to control the aircraft, it also has a positive side: induced drag is significantly reduced, making the aircraft much more efficient. It’s not all bad, is it?
During landing, if there is ground effect and an excessive pitch correction is made, the aircraft will tend to climb, resulting in what we colloquially call ‘ballooning’.
In these situations, the best thing to do is re-take altitude and abort the landing, as the aircraft is flying at low speed and in landing configuration, so there is a risk of a stall or a very hard take-off.
As we have already mentioned, ground effect brings with it a great advantage: efficiency. So it is not surprising that this advantage was exploited in a more conscious way.
This is why, during the Cold War, Russia developed the ekranoplans, which were large seaplanes that flew at very low altitude over water using ground effect.
These aircraft became very popular at the time. In fact, Russia built a 544-tonne, 42-metre aircraft that travelled at speeds of over 400 km/h, hovering between 30 cm and 3 m above sea level. Such was its size that it became known as the ‘Monster of the Caspian Sea’.
Unfortunately, the project failed when the aircraft was completely destabilised by a gust of wind, resulting in a major safety breach.
Today, this technology is still used in the design of smaller seaplanes, mainly for transporting small groups of passengers.
