Ground Effect

What is ground effect ?

Ground effect is an increase in rotor performance due to a reduced induced flow brought about by the presence of the ground (or a large restriction like a helipad or ship etc.) beneath a rotor system.
Ground effect means that less power is needed to hover near the ground that away from it.
Ground effect varies based upon the nature of the surface beneath the helicopter.
Hard surfaces like asphalt and concrete produce the most ground effect and give the greatest benefit.
Softer surfaces like grass produce less ground effect.
A sloping surface will produce less ground effect as the downwash will tend to flow down the hill.

Vector Diagrams

Hovering In Ground Effect (HIGE)

 
Vr = 192 m s-1
Vi = 6.42 m s-1
This is a blade element near the tip of a H300 in the hover In Ground Effect (IGE)
Example Content

Hovering Out Of Ground Effect (HOGE)

 
Vr = 192 m s-1
Vi = 8.42 m s-1
This is a blade element near the tip of a H300 in the hover Out of Ground Effect (OGE)
Example Content

Calculating The Induced Flow Reduction

 
Gareth Padfield's book Helicopter Flight Dynamics provides a method for calculating the reduction in induced flow velocity when in ground effect.

δVi = A Vi / 16 π Zg2
δVi
Difference In Induced Flow From OGE [m s-1]
A
Disc Area [m2]
Vi
Velocity of Induced Flow. [m s-1]
Zg
Height of Disc above the ground. [m]
Example Content

Web References

Paper on rotor wakes in ground effect