"The primary reason a helicopter becomes less efficient as it shrinks is that the Reynolds number decreases as the scale decreases. The lower Reynolds number, a

"The primary reason a helicopter becomes less efficient as it shrinks is that the Reynolds number decreases as the scale decreases. The lower Reynolds number, a measure of the viscosity of air relative to the other forces acting on it, means the air appears thicker to whatever is passing through it, so the drag increases disproportionately compared to the lift." - http://www.ae.utexas.edu/faculty/422-dr-jayant-sirohis-research-scales-up-the-efficiency-of-miniature-aerial-vehicles http://www.ae.utexas.edu/faculty/422-dr-jayant-sirohis-resea... It's a scale-dependent effect of air.