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Aviation History
1920
1920 - 0286.PDF
Junkers, etc., if the wing was very inferior aerodynamically to other and more orthodox sections. What further appeared to the writer to indicate that these wings were not chosen without a good and sufficient reason was the fact that the Gottingen Institute (which corresponds to our National Physical Laboratory) has tested a bagatelle of between 300 and 400 different wing sections, and that it is, therefore, scarcely likely that, with such data to choose from, a very inferior wing would have been allowed to get as far as the fighting front in any numbers. The writer had hoped that either the National Physical Laboratory or one of our private establishments possessing a wind tunnel would have considered the subject worthy of a test, but up to the present this does not appear to have been the case. In the absence of results of model tests I have, therefore, attempted to make an estimate of the data of such a wing, and although there are admittedly too many un- known or uncertain factors in the estimate to make it as absolutely conclusive as would be a model test, it is hoped that the results arrived at may cause sufficient interest to lead to thorough wind tunnel tests being made on such wings, either by the National Physical Laboratory or by private firms. As a basis on which to build up an estimate of the charac- teristics of a thick tapering wing, use has been made of the results of tests at high speeds on six aerofoils suitable for airscrew design (Reports and Memoranda No. 322). The reason for choosing these sections is partly that some of them are thick sections which lend themselves to cantilever wing construction, and partly that they were tested at high speeds, and the data relating to them may, consequently, be used without scale correction. It is not necessarily suggested that these sections are the best possible for a cantilever wing, MARCH II, 192© ^ TABEE II.—Aerofoil No. 4. Flat Under-Surface ~ Maximum thickness •.:;. Length of chord = o>127°- . ^ Radius of the nose = 0-02 of chord. Distance of maximum ordinate from nose = o -j of chord. Distance of ordinate No. of ordinate. 1 2 ." 3 4 5 6 78 9> 10 from leading edge, expressed as a fraction of chord. °'°5O-IO O-2Q 0-30 0-40 0-50 o-6o0-70 o-8o ' 0-90 Length of ordinate. expressed as a fraction of chord. 0-0794 0•1020 01218 0-1270 0-1244 0-1151 0•1020 o-Oj86o 00668 - 00423 in the centre rib, I have placed section No. 4 a short distance out, i.e., as rib c in Fig. 3. Section No. 1 I have placed "at*/. Fig. 3- .'•.•• For structural reasons it is an advantage to have tapered chord, and as a basis I have, therefore, chosen the taper shown in Fig. 3. This is, of course, quite arbitrary, and could be varied to any extent desired. To arrive at an estimate of the data of each of the ribs or sections, I have assumed that the rate of change in characteristics is proportionate to the distance of any section from the two sections of which the* data are given. In other words, the data for each rib are^arrivedfat by interpolation. In the case of ribsjzjand b F/C.3 m /'--»• 5 0 0 k- 42 50 - - I -4-1 Z5 • •• TTT3&7S - - 6O-S 8 3 SQ. FT but they will serve as a very useful indication of what one may hope to obtain from a thick tapering wing. Of the six aerofoils included in Reports and Memoranda No. 322, I have made use of sections No. 1 and No. 4, shown with their performance curves in Figs. 1 and 2, their dimen- sions being given in Tables I and II. Sections No. 5 and No. 6. are thicker than No. 4, but although section 5 has a slightly higher maximum lift coefficient, neither 5 nor 6 have so good an L/D ratio as No. 4. Also for the small machine I have in mind as being most suitable for cantilever wings, the extreme thickness of 5 or 6 is not required for the necessary strength, section No. 4 having a maximum ordinate of 0-127 of the chord. In order slightly to increase the wing depth TABLE I.—Aerofoil No. 1. Flat Maximum thickness Length of chord ~ ° Radius at the nose = 0-0133 Distance of i No. of ordinate. I • r 2 . 3 4 5 6 7 8 9 10 Under-Surface •Oo7 7- _. ... ~ of chord. maximum ordinate from nose = 0-3 of chord. Distance of ordinate from leading edge, expressed as afraction of chord. 0-05 0 • 10 0 -20 030 0-40 0'5O o-6o 0-70 o-8o 0 -90 Length of ordinate, expressed as a fraction of chord. •- 0-0510 0-0651 0-0775 0-0817 0-0806 0-0761 0-0694 0 0593 00451 00273 the values thus obtained may not be strictly accurate, since these lie outside the given section, c, but it is thought that the inaccuracy will not be of any great importance to the general result. On the assumption, therefore, that this method is permissible, we arrive at the values of Lc and L/D shown in Figs. 4 and 5. The reason for plotting the results on two charts is that the curves would otherwise lie so close together that, in reproduction, they would "close up." Having now assumed data for each of the ribs or sections shown in Fig. 3, account must be taken, before figures of the complete wing can be arrived at, of the relative size of each section, since this will influence the final result. Also, for purposes of stability it is an advantage to have a " wash- out " of the angle of incidence. From considerations of the data of the inner and outer sections I have chosen a wash- out of 40. That is to say, when the centre rib is at an angle of incidence of 40 the end rib has an angle of o°. This amount of " wash-out " gives a wing like that shown in Fig. 6, in which alternate ribs only are plotted. The " wash-out " is obtained by rotating the ribs around the centre line of the plane con- taining the maximum ordinate. The dihedral is also deter- mined quite arbitrarily, on this basis, the plane containing the maximum ordinates having a symmetrical taper in front elevation. By preparing a series of tables which take into account the wash-out and the difference in chord of the various sections, figures of the complete wing are obtained. [It should be pointed out that while these figures take into account the shaj e and relative size of each section, they do not allow for location. That is to say, no account is taken of their distance out from the centre line. In the ordinary uniform wing it is known that the centre section is subject to greater lift than are the end ribs, but as no figures are available on which to base an estimate of the manner in which the pressure changes in a tapered wing, no attempt has been 286
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