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Aviation History
1920
1920 - 0311.PDF
MARCH 18, 1920 u ' THE CASE FOR THE CANTILEVER WING The Question of Strength HITHERTO the cantilever wing has been studied from the aero-dynamical point of view only. Structurally the canti- lever is generally supposed to work out considerably heavier than the braced wing. With a carefully-graded section and suitable construction, there does not appear to be any reason why the cantilever should not be built for very little, if any, increase in weight. The fact that it has a high maximum lift coefficient is at once a point in its favour in this respect, since this enables a smaller wing to be employed. In the opinion of the writer, the cantilever wing should be built of metal, somewhat on the lines of the Boulton & Paul machine exhibited at the Paris Show, but having, of course, tapered spars, to conform to the tapering wing section. While on the subject of spars, the writer is inclined to think that for cantilever wings the usual two-spar arrangement is not necessarily the most economical. The reason for the general adoption of this arrangement in ordinary monoplane and biplane wings is that if more than two spars are employed either the lift bracing is increased, with a corresponding increase in resistance, or internal members of great strength —and weight—must be used for transmitting the load from BY " MARCO POLO " (Continued from page 288.) be more economical to employ more than two spars in a cantilever wing. This would have the result that the ribs could be built yery lightly, being in fact, nothing more than light longitudinal stringers. For some reason not imme- diately apparent, the position of the centre of pressure has not been dealt with in R. & M., No. 322, although one would have imagined that this is of some importance even in pro- peller design, and consequently, it has not been possible to determine exactly the maximum part of the total load which may have to be carried by each spar at the extreme positions of the centre of pressure. In Fig. g, I have plotted the bending moments for the complete wing. It will be seen that the maximum bending moment, which, of course, occurs at rib c, where the wing is supported, is slightly under 8,ooo lb.-ins. This is by no means high, and brings out very clearly the advantage of a graded wing. If the same load had been evenly distributed along the span, the bending moment at c, using the well-known cantilever beam formula, M = — would have been about 13,750 lb.-ins., which is very much in excess of that attained in the tapered wing.In order to obtain some slight idea of the spar size and spar m SECTION m 1 k > i h 9 ( e d c b a ANGLE OF INCIDENCE -A?O' - 3*40' - 3*80' -3*0' - 2'+0' - 2°20' -2'o' - lUo- - I°2O' -fo' - *0' - SO' - O'O' LIFTCOEFFICIENT • 0/8 O37 O6O • 080 O98 Hi • 130 • 160 • 178 • 190 • 2/0 225 • 256 LOAD CARRIED(lfe> 2 -42 ; » 8 -76 II -9O 15 -10 17-90 2.1 '10 26'60 30 so 33-30 36-80 4-1 -SO 4-7 SO BENDINS MOMENT 2 9O S66 8*0 1000 1088 1074- 1012 957 732. 4-00 O -6/O -J8O the braced main spars to the auxiliary spars. Now, in a cantilever wing, the first of these objections—that of added bracing and resistance—does not apply, while the provision of longitudinal members in the wing, capable of transmitting the load from main to auxiliary spars is a fairly simple matter, since it merely resolves itself into attaching all spars to the body, either direct or by a system of cabane legs -having horizontal top members to which all the spars are attached, It is, therefore, a question whether it would not weight which will obtain, I have made the bold assumption that for this particular wing the rearmost point reached by the centre of pressure is • 7 of the chord from the leading edge. For such a deeply-cambered section this may be somewhere near the mark, and if it is assumed that the rear spar be placed at this point, we may assume that for maximum load the rear spar may have to carry the entire weight of the machine. This, then, means that the maximum bending moment at c on the rear spar will be about 8,000 lb.-ins.
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