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Aviation History
1917
1917 - 0995.PDF
SEPTEMBER 27, 1917. CHT WING BRACING AND HEAD RESISTANCE. By MARCO POLO. IN comparing the performances of modern aero- planes with those of pre-war days, one cannot but be impressed by the enormous progress made during the few intervening years. This progress has not been in one direction only, but is distinctly notice- able in every aspect of the capabilities of an aero- plane, such as climb, manoeuverability, reliability, duration and speed. To what can this great im- provement be ascribed ? Not to any single thing by itself, but to general improvement in detail design. As far as the speed and climb are concerned, progress has been to a very large extent due to engines of greater reliability and higher power. It In Fig. 1 is shown diagrammatically the front spars with their wire bracing of what may be termed a standard scouting biplane. The span is 28 ft., and the chord and gap are 5 ft. respectively. Although a stagger is almost universally employed in modern scouts, we have chosen the vertical type as being simpler to deal with, and as this does not materially affect our argument, it is, we think, permissible. The "free length " of spar is about 8 ft. 3 ins., a somewhat excessive distance for great strength, and calling for a fair size spar, but the proportions, if not the actual dimensions, conform approximately to standard practice. ^:The attachment of the top Zg'-o- 2S'-o" 140 */t\?25aft TOTAL A*EA 265 ajt TOTAL LENGTH OF Z8'~O" f 2 8' -O BEH&TH.NCE FACTOX Fig. 1.—Diagram of the wing bracing of a standard scout. is doubtful, however, whether this increased power is the sole, or even the chief, cause of the progress made. Very great improvements have been made in the aeroplanes themselves, notably as regards wing sections, of which there are now in existence examples far exceeding in efficiency those available before the war. Another point that has received the closest attention of designers and constructors is the reduction of head resistance. This applies not only to the body of the aeroplane, but also to the wing bracing and strutting, and it is the latter aspect with which it is intended to deal in the present notes. In order to provide a foundation on which to base a comparison of various forms of wing bracing it will be assumed that a designer is faced with the problem of designing a machine to fulfil certain given conditions, and that in order to do this he plane is of the type termed " canopy " mounting by the Germans, i.e., there is a short top plane centre section, supported on four vertical struts from the body. From the small tables of dimensions attached to each diagram it will be seen that the last figure in the table is called " Resistance factor." This expression, which is employed for want of a better term, had better be explained before proceeding any farther. Before it is possible to make a fair com- parison between the various forms of wing bracing in a machine of the scout type, it is necessary to assign to each of the component parts of that bracing a value for resistance. Now, as the purpose of the present notes is not to go into calculations of the actual resistance in pounds atfa certain speed, but merely to compare one form of wing bracing with another, it will not be necessary, for the purposes SMN CHORD- AXEA TOTAL. I'*•*»*•« S'-C" /¥o'/t Z&'-O' 5-0 J /25'lt. CAP: S'"0" TOTAL LEN0TH Of STRUTS . $0 -0" WIK£ REitSTf \NCE FACTOR 2&5 Fig. 2.—Diagram of the wing bracing of a scout of the Halberstadt type. finds that, with the power and wing sections at his disposal, he requires an area of 265 sq. ft. This is, perhaps, a somewhat larger area than that used on the latest fighting machines, but as this fact will not affect the argument qualitatively, it has been chosen for the sake of convenience. Now, in order to get the best possible results out of his machine, with the area, wing section and power given, assuming that the form of body has already been decided upon, the designer will naturally ask himself what is the form of wing bracing that will give the least resistance, conforming, of course, with the question of proper structural strength. of the argument, to know the actual resistance per foot run of the wires and struts employed, but it will be required to know how the resistance of one unit length of strut compares with that of one unit length of wire. From the reports published by the various institu- tions carrying out wind-tunnel experiments, it would appear that a 10-gauge smooth round wire offers approximately the same resistance per foot run as a good section strut of the dimensions employed on a machine of the scout size and type. Since, how- ever, in modern aeroplanes, at any rate on high speed ones, round wires are never employed, it will 995
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