1 c.f. E. Stresemann. “ Aves ” (in Handbuch d. Z.. 1931, p. 572). Beautiful pictures, particularly of the start, are also found in K. Lorenz's article (J.f. Ornithol. 81, p. 107, 1933). According to bis description, one must conclude that the first hand-pinion can also fulfil this function (c.f. Hermit Crow). Special mention should be made of Lorenz's comprehensive and unbiased observations. His work is certainly the best contribution to the study of bird-flight since Lilienthal, although many of the opinions he bases on it cannot be substantiated.

2 According to A. Magnan (“ Le Vol des Oiseaux et le Vol dps Avions,” Paris, 1931, p. 131), the frigate-bird is able to rise suddenly, without any perceptible motion of its wings, when its tail is spread wide out (“ tres etalee et grandement echancree ”).

3 Of course, many types of forked tail (for example, those which are loosely and lightly built) have biological, rather than aerodynamical, functions.

4 The conception of the boundary layer was originated by L. Prandtl and rests on the assumption that viscosity forces only come into play in the immediate neighbourhood of walls, whereas in the open air one can count upon an ideal non-turbulent flow.

5 c.f. Kuessner H. G. Luftfahrtforschung, V, XIII, p. 410 (1936), V, XVII, p. 370 (1940).

6 I/v=λ/s signifies the wave-length in relation to the semi-span s.

7 There are, of course, numerous measurements of speed and frequency of beat, but unfortunately no accompanying data concerning reduced frequency, which makes the figures quoted somewhat unreliable.

8 Vertical forces axe positive upwards, horizontal forces positive backwards. This gives a thrust with a negative sign.

9 va=as/λ is the amplitude of the beat, based on the frequency λ.

10 We will deal elsewhere with the flying technique of pterodactyls and its special and interesting problems, on the basis of exhaustive model experiments. Here, we will onlymention that one can even reproduce in a model the device (typical of this animal) by I which flight took place with the help of an elastic skin, under tension only at the J leading edge (the greatly lengthened fourth finger taking the place of a turn-buckle).

11 Several models have been demonstrated in public, e.g., at the National Competition for Indoor Flying Models organised by the National Socialist Flying Corps at Breslau (1940), where they obtained prizes; also before the Lilienthal-Gesellschaft für Luft-fahrtforschung in Berlin, on the occasion of Von Hoist's lecture to the Society (Jan. I 29th, 1941). The performances of the models shown at Breslau are described by W. Haas (Luftfahrt und Schule, 6, 22 (1940) ); in the same magazine is an article by Von Hoist on this subject; another by H. Winkler (Modellflug 6, 7, 1940), reproduces films of the Breslau models.

12 If we assume that the weights are in a ratio of x ³, the power loadings alter in the ratio x ^-l/2.

13 Lilienthal. Der Vogelflug als Grundlager der Fliegekunst. (3rd Ed.), Munich and Berlin, 1939.

14 Since, in the models so far made by us, the mechanically produced wing-movement is still modified by its elasticity, a completely accurate verification of the course of the movement is only possible hy means of very clear high-speed films.

15 In this connection we would point out that this is only valid for the theoretical case in which the identical movement takes place at each point of the span. For a quantitative statement one must, in our case, take into consideration the reciprocal influence of individual wing-sections carrying out different movements.

16 K. Lorenz describes how a tailless buzzard is clearly handicapped (by stabilising backand- forth wing movements): tailless crows, on the other hand, can fly without being visibly affected.

17 A flying model of this type, 0.5 m. in span, at the Breslau Competition, made a flight of 44 seconds duration, rising from 6 to 7 metres above its point of departure.