As I've stated previously, according to one estimate, average airspeed velocity of a European swallow is roughly 11 meters per second, which amounts for 39.6 kilometers per hour or 24.6 miles per hour. This airspeed is estimated through a Strouhal number, which is calculated with the frequency of wing strokes multiplied by the amplitude of the wing, and then divided by the animal’s forward speed through the air. As we want to calculate the airspeed, we need to pick a reasonable Strouhal number and find out a frequency and an amplitude. In a calculation done by Jonathan Corum, science graphic editor at New York Times, the Strouhal number was assumed at 0.13, frequency was referenced at 7 beats per second and amplitude at 0.18 meters per beat, which yields the aforementioned number.
Source: http://style.org/unladenswallow/
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