Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/442

App. IV. surface of the road, showing that the periods of active flight become very short in comparison with the length of the "leap."

Most of the smaller birds are able, by adopting the leaping mode of flight, to attain speeds of about thirty or forty miles per hour.

The probable reason for the leaping mode of flight being confined to the smaller birds is to be found in the considerations discussed in §§ 195, 196. The influence of aerofoil weight (wing weight) is less important in the case of a small aerodrome or bird than of a large one. Consequently nature can endow a small bird relatively with an extent of wing surface not "commercially " possible in the case of a larger bird, so that the smaller bird can, in normal active flight, fly slower than a large one, but by adopting the leaping mode it can, in effect, divest itself of its superfluous surface, and can then rival the larger birds in velocity. The leaping mode is, in fact, a means of adjustment, by means of which the conditions of least resistance can be approximated under considerable variations of velocity. If one of the larger birds, with its limited relative area, were to force its velocity up to the point at which leaping flight would pay, it would require an amount of energy per second far beyond its actual horse-power capacity.