Strouhal Number

A simple number called the Strouhal number describes locomotion produced by the flapping of wings. It equals the frequency of flapping multiplied by its amplitude, divided by forward speed. Theory suggests that in most cases peak efficiency is reached when this number lies between 0.2 and 0.4.

'Nature clearly has a rule that if you want to cruise efficiently, you need to operate at this Strouhal number,' says Thomas. 'It's astonishing that you get the same result for a moth and a whale - one's in air, the other's in water, and there's such a colossal difference in size.'

Contribution by Banannah