of the plane in the vicinity of its leading edge, and again in part to the slowing of the flow over the remainder of the face (owing to its being a pressure region), it would appear that the net skin friction might even be less than that computed on the basis of a single surface.
§ 184. Some Consequences of the foregoing Aeroplane Theory.—The consequences of the peculiar behaviour of the aeroplane in respect of skin friction are of considerable moment.
The aeroplane, thanks to its power of evading a considerable portion of the resistance due to skin friction, is capable of being utilised for the support of the load without any very great loss of efficiency. Considered thus, and compared to an aerofoil of pterygoid form, it is found to give results that are really remark- able. Experimenting on a small scale, it is difficult to construct a model with a pterygoid aerofoil that, so far as gliding angle is concerned, will perform better than a ballasted aeroplane of the most crude description. An analogous example is found in the case of the screw propeller. Most of the theory relating to the aeroplane, wing form, and peripteral motion, finds its analogue in the theory of the screw propeller (Chap. IX.), and it is well known to designers of the latter that, so long as the pitch is rightly chosen in view of the torque and thrust, and provided that the angle, area, and proportions, of the 'blades are suitable, there is but a moderate gain in efficiency to be obtained by departure from the simple helical form of blade.
It is probable that the relative advantage of the pterygoid form becomes greater when the size of the aerodrome is increased, owing to the relations of weight and area discussed in § 196, and the relatively less importance of skin friction. If this should prove to be the case the present theory would account for the remarkable difference between the flight and wing form of birds and insects, showing in detail that which was anticipated in Chap. II. (Compare § 196.) In general the wings of flies, dragon flies, moths, etc., are approximately flat—they are in fact aeroplanes;
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