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Aviation History
1925
1925 - 0014.PDF
and is, sent out in this manner, it may be mentioned that Technical Note No. 209 is dated December, 1924, and that thus the tests are of quite recent date, and yet here we have the results on this side of the Atlantic early in January. In this country we should probably have had to wait at least six months before information of this sort was issued. However, after this slight digression, let us return to the subject of rotating cyUnders. That the fundamental conception of an air stream acting on a rotating cylinder is not new will be realised when it is stated that as long ago as 1852 Professor Magnus Carried out, at the request of the Royal Prussian Artillery Commission, some experi- ments with a rotating cylinder placed so as to be in a current of air produced by a hand-driven centri- fugal blower. Magnus was not, of course, in a posi- tion to measure the differences in pressure on the high-velocity and low-velocity sides of the cylinder, but by means of vanes placed at the sides of the cylinder, close to its walls, he was able to demonstrate that such a pressure-difference did exist, or at any rate that the streamlines were altered by the rotation of the cylinder. The results served to explain certain phenomena in connection with rotating projectiles, but the effect was looked upon as detrimental, as, indeed, it was, for the particular purpose investigated, and it does not appear to be until quite recently that the idea of applying the Magnus effect has occurred to anyone, Herr Flettner having been the first to put it to practical use. The American experiments on rotating cylinders, although possibly not the first to be carried out, the Gottingen laboratory having made certain tests for Herr Flettner, are the first of which the results have been published, at any rate in this particular form, and are, therefore, of special value. On examining the tables and graphs, one is at once struck by the fact that the drag coefficient of the rotating c rlinder is smaller than that of the stationary cylinder between r =0*5 and r = 2-0. When the value of r is in- creased beyond the value of 2, the drag again increases, and the maximum observed value of L D (7 • 8) occurs at r—2-5, the drag coefficient then having very nearly the same value as that of a stationary cylinder. Thus, from the point of view of efficiency, it would appear that r should be kepi at approximately 2-5, i.e. the ratio of circumferential to translational speed should have about this value. There is, however, no indication that the actual lift is limited in this •way, the lift coefficient, or Ccw, as it is expressed in the report, increasing with increased value of r. If it be imagined that the rotating cylinder is employed to give lift, this means, of course, that even at very low airspeeds the cylinder can, if driven by the engine, support the machine. The cyhnder (we are referring to the smooth cylinder) tested in the wind tunnel was 4J inches in diameter, and about 5 feet long, so that the area was 1 -875 sq. ft. At 3,6(X) r.p.m. and a wind speed of 7 metres per second, the " lift " or cross-wind force, was 2-21 kgs., or 2-6 lbs./sq. ft. This at a horizontal speed of about 15| m.p.h., and there is no sign of a limiting value to the lift co- efficient having been reached. . .I7.,"""'- | ANT ANY 8, 1-925 The experiments further indicate, in the case of the smooth cylinder, that less power is required to rotate the cylinder in a wind than in still air, so that evidently there is a reduction of friction. In this connection it is of interest to note that the power consumption is roughly proportional to the speed only. For instance, at 1,500 r.p.m. the power con- sumption was 23-8 Watts, and at 3,000 r.p.m. the consumption was 44-8 Watts only. It .therefore appears as if the main obstacle to employing the rotating cylinder as an aircraft " wing " is likely to be a mechanical difficulty in running it at high speeds, and not one of power consumption. Unfortunately the tables given in the report do not include power consumption at 7 m. /s. and 3,600 r.p.m,, otherwise it would have been possible to find how much power was, under those conditions, being expended in order to lift 4-86 lb's. It is, however, illuminating to examine the case for which figures are given. At 15 metres per second and 2,700 r.p.m., i.e. when the value of r was very low (1 -08) and conse- quently the L/D ratio poor (2-22), the cross-wind force was 1 -578 kgs. (3-47 lbs.) ; the power consump- tion was 37-2 Watts (0-05 h.p.). To this figure, however, should obviously be added that for the power necessary to drive the rotor through the air—in other words, to overcome the drag. As the drag force was 0-71 kg. (1-56 lb.), and the speed 15 m./s. (49-2 ft./sec), the drag horse-power was 1 -56 x 49-2 : 550 = 015 h.p. Thus the total power expended in lifting 3-47 lbs. was 0-2 h.p., or a lift of 17 -3 lbs./h.p. In view of the fact that this figure was attained under unfavourable conditions, there is every reason to expect that had it been possible to run the rotor at higher speeds the lift per horse-power would have far exceeded anything hitherto attained with ordinary aerofoil wings. At any rate it seems fairly clear that at low speeds at least quite exceptional lift per horse-power can be obtained with a rotor. At high speeds, of course, considerations connected with the strength and balance of the cylinder soon place a limit on the values of r attainable. The tests of a compound strut were, it will be seen, somewhat disappointing, but in this connection we would refei briefly to experiments made in Holland, and of which we hope to give details next week. These experiments were made with the rotor built into the leading edge of an aerofoil, and it was found that although the section used was not a very efficient one, the addition of the rotor resulted in a very considerable increase in lift coefficient. There thus seems to be a possibility of combining the rotating cylinder and the ordinary aerofoil in such a way as to increase the lift. In the Dutch experiment the rotor was placed centrally in the leading edge, but it is at any rate conceivable that other positions may be found to give even better results. Although it is always dangerous to prophesy in these matters, it appears, from what has been pub- lished, that the application of tho rotating cylinder principle to helicopters, or direct-lift machines, in which the blades of the lifting screw are in the form of rotors, might be simpler of solution than the corresponding application to aeroplane wings. Replacing R.A.F. Personnel ' THE naval ratings and marines who are to replace certain of the R.A.F. personnel hitherto employed in aircraft carriers are ordered to be drafted gradually to units of the Fleet Air arm in these vessels. The airmen thus rendered surplus will hi' disembarked at the nearest R.A.F. base at each stage. as arranged between the commanding officer of the carrier and the R.A.F. officer commanding the base concerned. The men for each flight or unit will he detailed by the com- manding officer of the carrier from the total numbers borne on the ship's books, replacements in each flight or unit being made gradually as additional ratings arc drafted to the ship. 14
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