Quoted:
Quoted:
Sounds like loss of lift caused by the structure below and very low altitute. Called a dead mans hover and lucky it just turned into a hard landing with nobody hurt.
If you don't mind, can you give a Cliff Notes explanation as to why that happens?
Thanks.
Helicopter rotors provide lift by accelerating air downward. What matters is how much change in speed is done on the mass of air to produce the reaction lift force.
When near the ground, and/or wall-like structures, the downdraft can be deflected so it kind of circles around and goes back down through the rotors again. When this happens, the air is already coming down fast into the rotor, so it's like flying a helicopter in a strong downdraft, and it can get beyond the ability of the helicopter to overcome it.
ETA: it's much worse in a hover since a moving helicopter moves ahead of the recirculating air and is thus not affected as much or at all depending on airspeed.