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Aviation History
1920
1920 - 0287.PDF
MARCH 11, 1920 zs IS 10 •9 2.0 IS IO made to allow for this. Presumably the effect would be to increase the lift on the inner ribs and to decrease that of the end ribs.] These are plotted in Fig. 7. It will be seen that the maxi- mum lift coefficient is • 71 at 180, while the maximum L/D (full size value) is 18-3. If the methods employed for arriving at these results are justified, and the writer does not imagine that they will bevery far out, the thick taper- ing wing is by no means soinefficient as many appear to think, considering that themaximum lift coefficient is high. In order to compare thewing with other and more orthodox aerofoils, it is notsufficient to compare the lift and L/D curves on abasis of angle of incidence, nor even the L/D valuesplotted as ordinates against / ivy max.^ ary aerofoil would requireexternal wing bracing. It is, therefore, proposed towork out, for some given size of wing and some definiteloading, the resistance of the cantilever wing and of RAF15 for various speeds, corre- sponding to the loading. As a basis for the compari-son I have chosen the plan form shown in Fig. 3, as-suming a chord of 5 ft. 6 ins. in the centre and 4 ft. at thetips, with a span of 25 ft. This gives an area of about120 sq. ft. I have assumed that it is intended to builda small, light monoplane with low-power engine, say,of about 25 h.p. Such a machine could, it has beenassumed, be built for a total' weight, " all up," of 600 lbs.In the case of the cantilever wing this would thus give aloading of 5 lbs. per sq. ft., and, as the maximum liftcoefficient is -71, the land- ing speed would be about37 m.p.h. In Fig. 8 is shown the wing resistance curve atvarious speeds for this area and loading. It will be seenthat the lowest wing re- sistance is 32 -5 lbs., at aspeed of 59 m.p.h. On the same graph is plotted thecurve of RAF 15, due regard having been paid to the factthat, for the same landing speed, the latter wing wouldhave to be of larger area, as its maximum lift coefficientis only -513, as against -71 for the cantilever wing. Inorder to make a fair com- parison, I have convertedthe L/D values of RAF 15 given in Report and Memoranda No. 248 to full-sizevalues in accordance with the method given in R. and M. No. 450, Fig. 20. Consequently, the L/D values for RAF 15are full-size ones (as far as it is possible to correct for scale effect), and the area of this wing is in proportion to the lowervalue of the maximum lift coefficient, i.e., 166 sq. ft., against the 120 sq. ft. of the cantilever wing. A monoplane of area -Jfc-—— 1 -~ J_J-**"* ^—— — FIG e • ————- e -—" •» • <• — 1 —•—*— — — i m k i £ e 287
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