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 due to limiting the loss of inertia. However, motors decrease in efficiency with decreasing size. At small sizes, piezo actuators can be more efficient than tiny motors. For this reason, a robot the size of a Drosophila fly, that flaps its wings like a fruit fly could theoretically be more efficient and hover for a longer time than a similar robot with rotating wings. Expect insect inspired robots to become and integral part of our future.