From Geir Johannesson
Regarding the “magic number” that describes the locomotion involved in flying and swimming (18 October, p 14), I seem to recall having read somewhere that momentum transfer for rotating devices in liquids, such as ships’ propellers or steam turbines, is theoretically maximal when the angle of attack is 45 degrees.
With a little algebra, this is precisely equivalent to a Strouhal number, which certainly falls between 0.2 and 0.4.
Reykjavik, Iceland
