Fig. 47.—Attitude of Wright Machine.
The nature of the aerocurve, its area, the angle of inclination of its chord to the line of flight, its altitude, etc., are not the only important matters one must consider in the case of the aerofoil, we must also consider—
§ 9. Its Aspect Ratio, i.e. the ratio of the span (length) of the aerofoil to the chord—usually expressed by span/chord. In the Farman machine this ratio is 5·4; Blériot, 4·3; Short, 6 to 7·5; Roe triplane, 7·5; a Clark flyer, 9·6.
Now the higher the aspect ratio the greater should be the efficiency. Air escaping by the sides represents loss, and the length of the sides should be kept short. A broader aerofoil means a steeper angle of inclination, less stability, unnecessary waste of power, and is totally unsuited for a model—to say nothing of a full-sized machine.
In models this aspect ratio may with advantage be given a higher value than in full-sized machines, where it is well known a practical safe constructional limit is reached long before theory suggests the limit. The difficulty consists in constructing models having a very high aspect ratio, and yet possessing sufficient strength and lightness for successful flight. It is in such a case as this where the skill and ingenuity of the designer and builder come in.
It is this very question of aspect ratio which has given us the monoplane, the biplane, and the triplane. A biplane has a higher aspect ratio than a monoplane, and a triplane (see above) a higher ratio still.
It will be noticed the Clark model given has a considerably higher aspect ratio, viz. 9·6. And even this can be exceeded.
An aspect ratio of 10:1 or even 12:1 should be used if possible.[37]
§ 10. Constant or Varying Camber.—Some model makers vary the camber of their aerofoils, making them almost flat in some parts, with considerable camber in others; the tendency in some cases being to flatten the central portions of the aerofoil, and with increasing camber towards the tips. In others the opposite is done. The writer has made a number of experiments on this subject, but cannot say he has arrived at any very decisive results, save that the camber should in all cases be (as stated before) very slight, and so far as his experiments do show anything, they incline towards the further flattening of the camber in the end portions of the aerofoil. It must not be forgotten that a flat-surfaced aerofoil, constructed as it is of more or less elastic materials, assumes a natural camber, more or less, when driven horizontally through the air. Reference has been made to a reversal of the—