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Aviation History
1917
1917 - 1023.PDF
> OCTOBER 4. 1917. ',; /;.- without having a forward angle, a backward travel of the centre of pressure would tend to raise the trailing edge of the top plane and lower the leading edge. This could only take place by swinging the point of the Vee backwards, which movement would necessitate a backward bending of the bottom plane. This would, of course, .have to be resisted by the internal drift wires of the bottom plane, but with a narrow chord and the single spar of the Nieuport it might not be an easy matter to provide an internal drift bracing adequate for resisting not only drift, but in addition the extra strain imposed by the backward travel of the centre of pressure of the upper plane. For large angles of incidence, however, the travel of the centre of pressure would have the reverse effect, that is to say it would tend to reduce the load on the lower drift wires. There can be little doubt, however, that it is the drift at low angles and high speed that is the more important, and hence the employment of double lift wires on the Nieuport. Even so it will be seen that the Nieuport type of wing bracing shows the lowest resistance factor of any of the types examined so far. Until now all the forms of wing bracing dealt with have had this in common that they embodied, to a smaller or larger extent, bracing wires in the con- struction. The next step in the evolution of a low resistance girder quite naturally took the form of a structure without any wires. An example of this These figures are admittedly quite arbitrary, but the values assumed appear to the writer to be reasonable. Any proof to the contrary will be welcomed. In measuring up the struts no account has been taken of the chassis struts, since these would have to be present with any wing bracing, and are simply made to perform an extra function. When coming to the question of structural strength the " wireless " bracing does not appear quite so good, at any rate for machines of so large areas as that chosen for purposes of comparison in these notes. The free length of spar in the upper plane is 10 ft. 6 ins. which is too large to be calculated to inspire confidence. As a friend of the writer's ex- pressed it once in discussing the merits of this par- ticular form of bracing " a pilot would have to have a child-like faith in the accuracy of the slide-rule wang- ler's stress diagram." For machines of smaller area, however, the " wireless " structure might be applic- able as it stands, although it would appear that certain modifications would greatly improve the strength without probably greatly increasing the resistance. Let us try to find out whether this would be the case. In the first instance, the chief objection to the arrangement illustrated is the excessive length of free spar. This could be very considerably reduced by employing, instead of the inverted Vee cabane, outward sloping struts carrying a centre section 32-OJ2I-Q 5-4"\ 5-4" AAEA 26 S 5-O" -rarra. LENGTH OF Fig. 5.—Diagram of Curtiss wireless type wing bracing. design is furnished by the Curtiss " wireless " scout, the bracing of which is shown in Fig. 5. As in pre- vious instances the area of 265 sq. ft. has been chosen for purposes of fair comparison, so that although the area is not the same as that of the Curtiss " wireless " the proportions are approximately correct. With this form of bracing, it will be seen, only a single inter- plane strut is employed. This is of a somewhat curious form, which has come to be known as a " K " strut, owing to its resemblance in side view to that letter. The centre of the top plane is supported from the body on two inverted Vee's. We now come to the most ingenious feature of the "wireless " design. Instead of the usual lift wires and landing wires there is on each side only a single Jstrut running from the bottom plane to the hub of the landing wheel. This arrangement is rendered possible by the employment of d la Sopwith. By making this centre section of 5 ft. span, which would not appear to be excessive, the free length of spar could be reduced to about 8 ft. 6 ins., while at the same time reducing the overhang of the top plane to about 5 ft. This arrangement is shown by the dotted lines in Fig. 5. On measuring the struts in the improved design it is found that the total length of these is actually a couple of inches less than in the original design, giving a resistance factor of 78.75, or slightly smaller than that of the original, while the load distribution would be better. As to the merits of the " K " struts for taking care of the travel of the centre of pressure on a somewhat large chord the writer is not prepared to argue. In his opinion a laminated I strut might be found a better proposition. On the other hand, as the inter- Ackerman landing wheels in which the hub is rigidly plane struts are doing the duty of lift wires it might secured to the undercarriage, the necessary resiliency be better to employ some form of steel strut, out- being provided by the leaf spring spokes of the wheels, wardly of I shape, but made up of two halves welded It will thus be seen that when the machine is in flight the two struts are performing the function of lift wires, while on landing they take on the duty of landing wires by working as struts in compression. From the table on the right of Fig. 5 it will be seen that the resistance factor is by far the loweskyet found. It has been arrived at by assuming that the resis- tance of the " K " struts is 2.5 times that of a stream- lined cable. For the ordinary strut it may be remembered a value of two times that of a cable was assumed. together along front and rear edges. Wood, although in itself quite strong in tension, is somewhat difficult to devise a terminal connection for, since there is always the danger of the layer outside the bolts of the terminal socket " pulling out " without the whole area of the strut section having taken its share of the tensile load. When the strut is working in compression this problem does not, of course, arise, as the socket simply acts as a guide to prevent the strut from slipping. (To be continued.) IO23 G 2
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