Ground Effect
Ground effect is a purely aerodynamic effect seen in planes, helicopters, marine transport and in motorsports like Formula 1 cars. It can be simply defined as the enhanced performance of a lift producing device in close proximity to ground.
When a plane comes in for landing, a few seconds before actual touch down on the runway, it feels like the plane is floating just above the runway. This floating occurs due to ground effect.
Now, to fully understand what ground effect is, you must first familiarise yourself with what is induced drag, downwash, cushion effect and how it affects an aircraft. Let us cover these topics one after another.
Induced Drag occurs due to the pressure difference between the 2 sides of the wing. This difference causes vortices at the wing tips called wingtip vortices, which causes induced drag. Induced drag also effects downwash, but we’ll get to that sortly.
This period of floating above the runway is mainly because the ground or runway interferes with the effective formation of these vortices and thus their strength reduces, leading to reduced induced drag. Hence, for the same thrust, there is an increase of velocity which drives an exceptionaly increase in lift. This effect is more pronounced on low winged aircrafts because of the proximity of the wing to the ground.
Let us crunch some numbers!
When the wing is at a height equal to its span, the reduction in induced drag is only 1.4 percent. When the wing is at a height equal to one-fourth its span, the reduction in induced drag is 23.5 percent and, when the wing is at a height equal to one-tenth its span, the reduction in induced drag is 47.6 percent. Due to this variation, this effect is most usually observed, when the plane is very close to the ground i.e during take-off and landing.
Now, let’s talk about Cushion effect.
As the plane flies very close to ground, the air flow between the wing and ground is compressed to form an air cushion. This leads to an increase in pressure on the underside of the wing, generating more lift.
Third is the downwash.
Due to the profile of an aerofoil or due to the presence of wing tip vortices at the wingtips, flow over the wing is directed downwards. This downward flow of air is stronger near the wing tips because of wing tip vortices and gets weaker as we go towards the root of the wing.
Consider a wing making an angle with the free stream airflow known as geometric angle of attack. Due to downwash, the local airflow changes its direction slightly downwards and the angle between this local airflow and wing is known as effective angle of attack, which is, keep in mind, always less than the geometric angle of attack.
We all know that lift acts perpendicular to the airflow. Hence, resolving this lift vector, we get two components, lift and drag. Increase in the strength of downwash, increases the drag. Near the ground, the strength of downwash decreases as most of the local airflow remains as that of the free stream, which decreases drag. In-turn this decrease in drag increases the velocity of the plane and hence the lift.
It is observed that lift is increased in all the three cases either due to decrease in drag leading to increase in velocity and hence lift or due to increase in pressure because of cushion effect, when the plane is near the ground. Hence the term “Ground Effect”.
So the consequences of this effect is that,
· For a given velocity and angle of attack, the wing generates more lift due to cushion effect.
· For given thrust, the velocity is greater due to reduction in drag.
· It also reduces the stall speed and stall angle.
This effect has been exploited not only by the aviation industry but also in the marine transportation and automotive industry (sports cars) to increase efficiency.
Vehicles called GEV (Ground Effect Vehicles) such as the ‘ekranoplane’, are more like ship with wings. These ships float above the water surface that is once they gain required velocity, the ship loses contact with water hence there is remarkable decrease in the drag and increase in efficiency.
