and we have as yet no means of ascertaining same, either for any particular point in the length of the aerofoil or generally for all points. The partial solution of this problem is reserved for a later chapter. It is probable that in nature the conformability of the trailing edge is substantially ensured by the extreme flexibility of feathered construction, an incidental advantage of this method being undoubtedly an automatic adaptability to variation of velocity and load.
§ 119. On the Plan-form of the Aerofoil: Aspect Ratio.—In the experiments of Professor Langley and others, planes of long, narrow plan-form, in pterygoid aspect, and at moderate angles, have always been found to give a greater lifting effort, ceteris paribus, than other forms, or than the same form moving end on. The reason of this is at once evident when it is considered that the amount of the fluid traversing the regions o o o o, Fig. 67, or "stray field," is relatively much less when planes of great lateral extent are employed, and every increase in the lateral extension of the plane makes the relative loss of field still smaller, the behaviour of the plane approaching more and more nearly to the ideal case in which the conservation is complete, and the plane reaps the benefit of the whole up-current generated.
Wherever flight has been successfully achieved, advantage has been taken of the influence of aspect; the aspect ratio varies amongst birds from about 4:1 (as in the lark, also scops owl) to about 14 or 15:1 (in the albatros). The wing spread with which Lilienthal successfully experimented had an aspect ratio of about 8:1, similar proportions being adopted in gliding machines subsequently by Pilcher, Chanute, and others. The author, experimenting in 1894, successfully employed a ratio of 13:1, and Phillips in his captive flying machine, about 1893, succeeded, by his "Venetian blind" method of construction, in employing a ratio of more extreme proportion still.
§ 120. On Plan-form (continued): Form of Extremities.—The form of the extremities of an aerofoil exerts a considerable
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