During flight, a wing produces lift by accelerating airflow over its upper surface and, in some fixed wing flight, accelerated air can, and does, reach supersonic speeds, even though the airplane itself may be flying at a subsonic airspeed.

Helicopters stay up because, even though the body of the aircraft can be stationary, the wings rotate fast enough to provide lift in a manner similar to fixed wing aircraft.

Airborne insects can be likened to revolving propeller bladed aircraft because their wings generate lift by steadily pushing air downward.

Therefore, if, like helicopters, hovering insect flight is possible because of the high speed of their wings despite the static position of their bodies; then do really high speed wing flappers, e.g. hoverflies, also ever accelerate upper wing surface airflow to supersonic speeds?

If this is the case, then how do these insects avoid creating small sonic booms? Or if they don’t, then why don’t we hear them? Or maybe we do? Maybe the irritating whine we hear when listening closely to something like a hoverfly, is actually a series of really tiny sonic booms merging into one long hum?

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References:

https://en.wikipedia.org/wiki/High-speed_flight

‘Airplane Flying Handbook’ (2004). U.S. Department of Transportation Federal Aviation Administration Flight Standards Service

Sanjay P. Sane: ‘Induced airflow in flying insects I. A theoretical model of the induced flow’