In this lesson, we expand upon our previous discussion about induced flow and its effect on angle of attack to explain ground effect.

Ground effect is the increased rotor efficiency and lift performance that we feel when within roughly one rotor diameter of the ground. The maximum possible ground effect in a given situation will be reduced by a variety of factors such as:

Distance from the ground, even within the one rotor diameter threshold
Uneven surface features; rough terrain, tall grass, etc.
Ground obstructions; trees, bushes, cargo, buildings, etc.
Bodies of water

Ground effect is one of two major factors taken into account when determining helicopter hover performance (the other factor being density altitude). Pretty much any RFM (rotorcraft flight manual) you look at will have two separate charts, one for IGE hover (in ground effect) and another for OGE hover (out of ground effect).

Ground effect is also important in maximum performance takeoffs. You will hover as close as you can to the ground to maximize ground effect, set the throttle to the top of the green arc, and pull maximum allowable manifold pressure with the collective. Once sufficient altitude has been achieved (obstacles have been cleared, pinnacle updrafts have been avoided, etc) then you can resume a normal climb to cruise.

This video talks more about why ground effect exists, how its effect is calculated, and what its relationship is to induced flow. Thanks for watching!

Disclaimer: Although I am training to become an instructor, I have not yet been certified. The contents of this video are not intended to be used as a substitute for actual instruction.

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