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Fig. 2 gives the approximate section through a streamline strut such as used in the interplane bracing of a biplane. The length is (L) and the width is (d), the latter being measured at the widest point. The relation of the length to the width is known as the "fineness ratio" and in interplane struts this may vary from 2.5 to 4.5, that is, the length of the section ranges from 2.5 to 4.5 times the width. The ideal streamline form has a ratio of from 5. to 5.75. Such large ratios are difficult to obtain with economy on practical struts as the small width would result in a weak strut unless the weight were unduly increased. Interplane struts reach a maximum fineness ratio at about 3.5 to 4.5. Fig. 3 shows the result of a small fineness ratio, the short, stubby body causing the stream to break away near the front and form a large turbulent region in the rear.

An approximate formula showing the relation of fineness ratio and resistance (curvature equal) was developed by A. E. Berriman, and published in "Flight" Nov. 12, 1915. Let D = resistance of a flat plate at a given speed, and R = resistance of a strut at the same speed and of the same area, then the relation between the resistance of the flat plate, and the strut will be expressed by the formula R/D=4L/300d, where L = length of section and d = width as in Fig. 2. This can be transposed for convenience, by assuming the drag of a flat plate as D = 0.003AV², where A = area in square feet, and V = velocity in miles per hour. The ratio of the strut resistance to the flat plate resistance, given by Berriman's formula, can now be multiplied by the flat plate resistance, or strut resistance = R = 0.003AV² X 4L/300d. = 0.012LAV²/300d. It should be understood that the area mentioned above is the greatest area presented to the wind in square feet, and hence is equal to the length of the strut (not section) multiplied by the width (d).