"I think I understand why. Here's my theory:
Winds near the ground (generally below 20 feet) are naturally turbulent. In essence, this means that the aerodynamic forces change quickly and randomly. The single-rotor/tail-rotor…"
"Yes, but there are also other elements at play. One thing is that helicopters also have better aerodynamics that quads. They are simply less affected by the wind. Also, i think the rotor disk offers better control reactions for…"
"@Paul G.: In order to be efficient, you need a large rotor disk area. Full-size helicopters would be extremely hazardous on the ground if the rotor was not at least 7 feet high (for obvious reasons). This does make them less stable, but…"