Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/440



of the smaller birds habitually fly at a considerably greater velocity than would be computed from the pressure-velocity tables (Tables IX, and X.) on the lines of § 187.

The means by which this is accomplished is instructive. The bird flies briskly for a short distance and then closes its wings, continuing its flight as a simple projectile, so that the total flight consists of alternations of active flight and projectile flight. The flight path under these conditions consists of a series of leaps, as given in Fig.1 62, in which the thick lines represent the periods of active flight and the fine lines the periods when the wings are closed.

It is evident both from the form of the flight path and from the behaviour of the bird that the whole of the sustentation takes place while the wings are spread, and that during this period the wings actually sustain both the weight of the bird and the centrifugal component due to its curvilinear flight path, and the sum of these is the effective load on the wing area in the sense of §§ 185—187.

The present note is based on visual observation. The largest bird witnessed by the author as employing the leaping mode of flight is the green woodpecker {Picus viridis); the weight of this bird averages about six to seven ounces (180 grams). Larger birds, as, for instance, the partridge, glide with wings outstretched when not in active flight. The greatest length of "leap" in proportion to the corresponding active period, noted by the author, is about 3 : 1 (Fig. 162, c).