Is flapping flight aerodynamically efficient?

Flapping flight is much less efficient for some insects than previously thought. The induced power factor for flapping wings is discussed in terms of a number of non-ideal effects for hovering and for forward flight. These effects include losses due to non-uniform downwash, wake periodicity and effective disk area. For some insects, the induced power can be twice the ideal value in hover, and can be even higher than this in forward flight. The figure of merit is evaluated for eight hovering insects giving values between 15% and 28%. These low efficiency levels are attributed to the contribution of pressure drag resulting from operation at high angles of attack. The effects of aspect ratio and stroke plane angle on efficiency are also discussed. Efficiency of flapping flight is compared with efficiency of other non-flapping flight modes including rotary and fixed wing. Flapping flight usually goes beyond other flight modes with respect to comparable types of loss, and it has further ways of being less efficient.