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I understand the ground effect and the air cushion of approximately 3-6 inches when the helicopter is close to the ground. With that in mind, my questions are:

  1. Why does the helicopter become inherently unstable when close to the ground?

  2. What are some methods to keep it smooth when operating close to the ground?

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  • $\begingroup$ A helicopter is constantly inherently unstable, and requires constant input from a pilot to maintain stability in flight $\endgroup$ – Carlo Felicione Dec 2 '17 at 19:07
  • $\begingroup$ @CarloFelicione: … except for Kamov-type helicopters. They are inherently stable. $\endgroup$ – Peter Kämpf Dec 2 '17 at 20:43
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The ground effect can be seen as an increase in air pressure underneath the lifting surface because the down-flowing air cannot stream through the ground. An image from this book::

enter image description here

In the hover, the down streaming airflow is pretty docile, with turbulent whirls only occurring in the transition to forward flight in ground effect. Full scale helicopters are inherently unstable with an eigenfrequency period in the order of a couple of seconds, and the instability gain is not really affected by the ground effect: amount of lift is kept relatively constant by pilot action, and ground effect mainly reveals itself as a reduction in induced drag.

So for full scale helicopters, pitch/roll controllability of the helicopter does not change (much) in ground effect. Depending on objects on the ground and the slope of the terrain there can be changes in trim as the pressure increase influences the horizontal tail for instance, but these do not fundamentally change eigenfrequency period.

For model helicopters, hovering in ground effect is a much more difficult task: the recurring blade period brings on a temporary pressure increase from lift creation. From Prouty:

The wake in ground effect is not really a steady flow, as is assumed by the theory, but a flow with large-scale fluctuations that can be felt as gusts by an observer standing near the helicopter. Reference 1.21 speculates that these fluctuations are associated with the vortex that is made up of the individual vortices from the blade roots. This vortex apparently writhes like a pinned snake along the ground, causing the entire wake to shift and wobble.

Since scale models have a higher revolution than full scale helicopters, the frequency and distribution of these vortices may be what causes higher instability for hovering.

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  • $\begingroup$ @ koyovis Is there anyway the lessen the effects of this? Such as small circles with the cyclic to create a decreased performance of the blades lift therefore reducing th special effects? Just thinking out loud $\endgroup$ – Wes Dec 2 '17 at 18:33
  • $\begingroup$ @Wes With full scale hell's or with scale models? Have updated the answer, full scale hell's don't seem to have much of a problem with decreased controllability. The moments of inertia are of course a lot higher but I don't know how the effect of turbulence scales with model copters. Smaller rotors have a lower Reynolds number which scales all effects differently. $\endgroup$ – Koyovis Dec 3 '17 at 4:11
  • $\begingroup$ you are correct in the assumption that I was referring to full scale helicopters I.e the bell 206. $\endgroup$ – Wes Dec 3 '17 at 13:56

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