In some machines the top plane is supported from the fuselage by struts which are formed integrally with a horizontal compression member, as in [Fig. 16]; the section of the vertical struts being shown by [Fig. 17]. The ply-wood is cut to the shape of the complete component, and forms a tie for the spruce layers, which are jointed at the junction of the vertical and horizontal members.
Strut Materials.
Referring again to the material generally employed for struts, i.e. silver spruce, it is perhaps necessary to explain further the reasons for its predominance over ash, as on a strength-for-weight ratio the latter wood is slightly the better material. The points already detailed, indicate that an interplane strut is stressed essentially in compression, and therefore the chief characteristic of ash, great tensile strength, is of but secondary importance. There is also the fact that, for the same weight, spruce would be thicker, and correspondingly more able to resist collapse. However, in machines of the flying-boat class, where the engine is invariably mounted between the four central plane struts, and consequently subjected to an amount of vibration varying with the type of engine used, ash forms the material.
Tapering of Interplane Struts.
The correct shaping of struts longitudinally, particularly those for interplane use, is apparently a rather controversial subject. Taking the case of an untapered strut, it is evident that the greatest stress will be located at or near the centre, so that if at this point the section is strong enough, clearly there must be an amount of superfluous material at the ends. By suitably reducing or tapering the strut from the centre one can obtain the same degree of strength for less weight. Conversely, for the same weight a much stronger strut is possible. So it has always appeared to the writer. It is, however, admittedly possible that unless carefully done, the operation of tapering a strut may actually diminish the strength. One method of tapering, that of making the maximum cross-section at the centre, and from this point diminishing in a straight line to the ends, is undoubtedly open to criticism, and a way more nearly approximating to the correct method of shaping is to reduce the cross-section at various points so that the finished contour is curvilinear, as in [Fig. 18]. In this connection it is pertinent to emphasize the importance of ensuring that all strut ends are cut to the correct bevels, and this is particularly applicable to those struts which seat directly in a socket. The slightest irregularity will cause considerable distortion when assembled under the tension of the bracing wires, and frequently the writer has seen an ostensibly perfect strut assume the most hopeless lines directly the operation of truing up is commenced.
Fig. 18.—Tapering of interplane struts.
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.