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Aviation History
1925
1925 - 0030.PDF
JANUARY 15, 1925 PRELIMINARY STUDY OF THE INFLUENCE OF A ROTATING CYLINDER ON A WING UNDER above heading a most interesting article, by Dr. Ir.E. B. "Wolff, appears in the Dutch engineering jot :nal De Ingenieur of December 6, 1924. Dr. Wolff is Direct r of theEijks-Studiendienst voor de Luchtvaart at Amsterram, and has been kind enough, upon seeing the interest I ,.IGHT istaking in the subject of the application of the rotor principle to aerodynamics, to call our attention to the article andinform us of further tests that are to be made at Amsterdam. The experiments described were, Dr. Wolff static, under-taken mainly as a result of certain works by J>ukowsky, Bjerkness and Ackeret, which gave Dr. Wolff the idea, afterthe Delft Conference on Applied Mechanics in April, to make certain tests at the Rijks-Studiendienst. which corresponds to DUTCH EXPERIMENTS WITH ROTORS : Arrange-ment and dimensions of the rotor and aerofoil. our National Physical Laboratory. Aa the main problem tobe solved was whether or not it would be possible to obtain increased lift from an aeroplane wing by building into itsleading edge a rotating cylinder, no attempt was made, during the preliminary experiments, to obtain drag or centreof pressure measurements, the lift only being measured. Incidentally, it should be mentioned that when Dr. Wolffundertook the tests he had no knowledge of Herr Flettner's work in Germany, so that it appears that Germany, Hollandand the United States have almost simultaneouslv attacked the problem of the rotor. (An account of the Americanexperiments was published in FLIGHT last week.) The wing section used in the Dutch experiments does notappear to have been a particularly efficient one, but was employed because templates were already in existence, andone section appeared as good as another for the purpose of finding out whether there was a possibility of obtaining in-creased lift. The overall dimensions of the wing are given in the accompanying diagram, which also shows the method ofmounting the rotating cylinder on a spindle. The aspect ratio of the wing itself was low (5-4), so that the low value ofthe maximum lift coefficient is scarcely astonishing It is of interest to note that the ratio of cylinder diameter to wingchord is fairly high, i.e. 20 per cent. It seems likely that future tests may show this ratio to be of considerable import-ance, and for the sake of efficiency it is to be hoped that a smaller ratio will be found to give as good results. Theaspect ratio of the cylinder itself was approximately 27, or roughly twice that of the cylinder tested in America, whichhad a diameter of 4£ ins. and a length of approximately 5 ft. The model was suspended in the wind tunnel " upsidedown," and, as already stated, measurements were made of lift only. The rotor was driven by an electromotor of \ h.p.and was geared upjn the ratio 7 to 1. The experiments included readings taken at various air speeds and rotor speeds,at angles of incidence ranging from — 2° to -+• 16°. As it was thought that the opening between cylinder and wingsection might cause a certain leakage of air, the gap was closed with paraffin wax on the upper surface and the contoursmoothed down. This proved to effect a considerable increase in lift over that obtained with the slot open, as will be seenfrom the accompanying graph, which shows the results at a wind snfed of 16-7 m. per second (54-8 ft./sec). In thearticle m De Ingenieut the results of the tests are tabulated, but as the graph shows with sufficient accuracy the figuresobtained we have not thought it necessary to publish the table. It will be seen that with the cylinder stationary and the slotopen the maximum lift coefficient occurs at an angle of incidence of 0° and has a value of 0-3 (absolute) only. Withthe upper end of the slot closed this figure is increased to 0-413, and the upper critical angle is increased from 0° to 4C.The other two curves show the lift coefficient at a channel air speed of 16-7 m./s., and with the cylinder revolving at3,000 r.p.m. and 17,000 r.p.m., respectively. The rotational speed of 3,000 r.p.m. corresponds to a peripheral speed of5-9 m./s. (19-35 ft./sec), and that of 17,000 r.p.m. to 33-5 m./s. (110 ft./sec), at which the ratios of peripheral totranslational speed are 0 • 353 and 2 respectively. It may be recollected that in the American tests with the plain rotor it V1 was found that the best L/D was obtained when -^ =2-5approximately, but that the value of the lift coefficient seemed to increase with increase in this ratio, up to thehighest measured, where the ratio was 4-32 and the lift coefficient 9'48. or 4-74 in " absolute " units.Whereas the American tests on a plain rotor showed the lift coefficient to be very nearly proportional to the value of r 10 DUTCH EXPERIMENTS WITH ROTORS : Results of wind-tunnel tests. With slot closed and rotor stationary the maximum lift coefficient is 0-42. With the rotor running a"t 17,000 r.p.m. this value is increased to 0 564, and it is interesting to note that very nearly as good results are obtained with the rotor running at only 3,000 r.p.m. (0 52). (ratio of peripheral to translational speed). the Dutch experi-ments with a wing section provided with a rotor in its leading fldge seem to indicate that this ratio is not critical. Thus withthe cylinder rotating at 3,000 r.p.m., the lift coefficient is 0-522, which is only increased to 0-527 when the rotor speedis 17,000 r.p.m. Compared with the section with rotor stationary and upper end of slot or gap closed, the increasein lift is considerable. Below the critical angle of what may be termed the normal wing section this increase is in the neigh-bourhood of 10 per cent., but when the critical angle has been passed the increase is much greater. Thus the maximum liftcoefficient of the " normal " section is 0-413. while with the rotor running at 3,000 r.p.m. the value is increased to 0*522.The upper critical angle is raised from 4° to 8°. When the rotor speed is increased to 17,000 r.p.m., the maximum liftcoefficient: becomes 0-564 at an angle of 9-6", an increase in lift of 73-3 percent. It is interesting to note that the general shapes of the curvesare very similar to those obtained with the Handley Page 30
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