Insects fly by flapping their wings. Small flying robots are often powered by motor driven spinning wings based on the high efficiency of a circular motor movement. These robots are based on the principle that flapping insect wings generate lift by creating a tornado-like leading edge vortex.
For robots the size of a hummingbird, flapping wings are less efficient due to the loss of inertia when wings stop and reverse courses. Rotation can produce high aerodynamic efficiency…
Source: Living With Insects Blog