Design of Strut Sections.
Although, strictly speaking, the design of strut shapes is outside the scope of this book, a few remarks anent the development of streamline may emphasize the advances made, and also the need for careful construction. The resistance of a body is generally considered to increase as the square of the speed, i.e. double the speed and head resistance is doubled, and while this is true for a moderate range of speeds, experiment has proved that for high speeds, exceeding say 100 miles per hour, resistance increases at rather less than as the square of the speed. However, it is certain that the correct shaping or otherwise of the struts and other exposed members, affects generally the performance in flight of the aeroplane. The accepted feature of all streamline forms is an easy curve, having a fairly bluff entrance and gradually tapering to a fine edge. The ratio of length to diameter, called the fineness ratio, varies in modern machines, being in some instances 3 to 1 and in others 5 to 1, a good average being 4 to 1. Considering only the point of head resistance, it would be better to choose a section of high fineness ratio, but constructionally such a strut would buckle sideways under a moderate load, and therefore the cross section must be sufficient to resist this. The strut section used on the earliest aeroplanes, such as the Wright biplane, shown by [Fig. 19], is
Fig. 19. Fig. 20.
Fig. 21. Fig. 22.
Figs. 19–22.—Strut sections.
nothing more than a rectangle with the corners rounded off. [Fig. 20] shows a development of [Fig. 19] consisting of a semi-circular head with a cone-shaped tail, which by gradual evolution has resulted in the section [Fig. 21]. Some experiments carried out a considerable time ago by Lieut.-Col. Alec Ogilvy, revealed the rather interesting point that a strut shaped as in [Fig. 22] gave the same results as a similar strut taken to a fine edge. The reasons for the non-suitability of a sharp-pointed section are apparent from a consideration of [Fig. 23], showing the action of a side wind with the resultant dead air region.
Fig. 23.—Showing inefficiency of pointed section in a side wind.