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Aviation History
1923
1923 - 0283.PDF
MAY 24, 1923 FIVE NEW AMERICAN AEROFOILS Some Useful Wings for Light 'Planes The American Natio'nal Advisory Committee for Aeronautics early this year issued a report on a series of tests on thick aerofoils (N.A.C.A. Report No. 152, "The Aerodynamic Properties of Thick Aerofoils, II") which were carried out in the 5-ft. wind tunnel at the Langley Memorial Aeronautical Laboratory. In order to reduce scale corrections, the speeds at which the different wings were tested was kept high, 30 to 50 metres per second (98-4 to 164 ft. per second). The models all measured 18 ins. in span, and were kept to a uniform aspect ratio of 6, i.e., a 3-in. chord for the parallel wings and a mean chord of 3 ins. for the tapered wings. As some of these aerofoils appear to be rather promising for gliders and light 'planes, we have thought that it might be of interest to select from the large number of aerofoils of which data are given in the report a few which seem parti- cularly suitable. Consequently in the following tables and figures particulars are given of five aerofoils which, with the exception of one, No. 55, lend themselves to cantilever construction. The five aerofoils are shown in plan view in Fig. 1, in which the sections at centre and tips are indicated in solid black. These sections are not to be regarded as absolutely to scale, as it was difficult to plot them with any degree of I PIVE AMERICAN centre and tip AEROFOILS : Plan views, with sections shown in silhouette. exactness on such a small scale, but they do indicate in a general way the peculiarities of the sections. Thus No. 55, which is rectangular in plan view, has a deeply cambered centre section and a fairly deeply cambered tip section. This means that there is not room for very thick spars, and that probably external bracing would be necessary. The other wing of rectangular plan form. No. 73, also tapers in thickness, but both the centre section and the tip section are bi-convex. Of the three tapered wings, No. 59 has the same section at the tip as at the centre, the tip section being merely a geometrical reduction of the centre section. The bottom surface is flat and the upper fairly deeply cambered. In all three tapered wings the chord at the tip is one-half the chord at the centre of the span. No. 79 has the same centre section as No. 59, but the tip section is different, being not geometrically similar to the centre section. From the curves it will be observed that the result is a very great increase in the maximum L/D, but a reduction in the maximum lift coefficient. No. 81, like No. 73, is of bi-convex section, but whereas No. 73 is rectangular in plan form, No. 81 is tapered, withthe chord at the tip one-half of that in the centre. In this case tapering has led to a slight decrease in maximum L/D,but a slight increase in maximum lift coefficient. As originally plotted hi the American report, the Germanmethod is followed, i e., the lift coefficients are twice as large as ours, and the characteristics are plotted as polardiagrams, with K L plotted on a base of KD. In order tomake the results more easily accessible to British readers, especially those who are not very familiar with the differentsystems employed by the various nations in presenting the results of wind-tunnel tests, we have re-plotted the curvesfor these five wings in the manner usually adopted in this country. Thus in Fig. 2 the " absolute " lift coefficients are plotted on a base of angle of incidence, while in Fig. 3 L/D ratios are plotted on a base of " absolute " lift coefficient. This last method, of course, compares the different wings at various speeds for the same wing loading. As, however, it is usually desired to compare various wings on a basis of same landing speed (which necessitates different wing loadings), Fig. 3 has been re-plotted in Fig. 4 on a base of A, the various values of A corresponding to various values of the ratio KL/KL max. For the benefit of those of our readers who are not very familiar with graphs showing wing characteristics it may be explained that, to take an example, A = 0-5 means that this point on the curve corresponds to a lift coefficient of one-half of the maximum lift coefficient, i.e., KX/KL max. =0-5. In other words, if the maximum lift coefficient is 0-5, the lift coefficient corresponding to A = 0-5 is 0-25. Thus the curves in Fig. 4 show the efficiency, or, in other words, the value of lift/drag ratios, at fractions of the maximm lift coefficient ranging from 0 • 1 to 1. In order to form an even clearer picture of how the different wings compare, it is obviously possible, as the speed varies as the square root of the lift coefficient, to obtain a scale of speeds by calculating the value of =. corresponding to the various VAvalues of A itself. This has been done in Fig. 4, where the lower scale is one of =. while the upper is one of A. With these introductory remarks for the benefit of our younger readers, let us turn to the curves and the results which they show. From Fig. 2 it will be seen that the section which gives the highest lift is No. 59. The resistance is, however, also high, and the maximum L/D is only about 15-7. Thus, in spite of the fact that No. 59 gives a high lift, it will not be a very economical section. No. 55 is also a fairly high lift section, the maximum lift coefficient being / 5 4 3 •* 31 < • S' / / V / y' y' y y ,y y' y y' y' : 4\. y y ^./;' y y ' y_/y\y yy • i y' ^ y' y y y _ — > y y y'\^ "* .y ^, y ^~ 0 y'' _^. — — 55 — 79-- 81 2 4. * Fig. 2 —FIVE AMERICAN AEROFOILS : Lift co- efficients plotted on base of angle of incidence. 283
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