well, between the ground and 1/2 the rotor diameter is where it is most pronounced... The effects can still be felt at a height equal to the rotor diameter, but to a much lesser extent... For micros though, it doesn't take "much of an extent" to play hell with it though.. In general, I'd say about 1 1/2 times the rotor diameter will put you out of it clear and free... So if your hummingbird has a 7" blade (14" diameter), it should be barely noticable at 14" but completely eliminated at 21".