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Aviation History
1939
1939 - 1725.PDF
568 u®^ JUNE I, 1939 I Fig. 4 (left) shows plan forms and sections of wing models tested by the Wright Brothers in 1901. Fig. 5 (above) is a reproduction of an original sketch and photograph of the Wright Brothers' lift-measuring balance of 1901. known and the angle measurable, the lift and drag forces could be calculated. The relationship between the tunnel tests and glider tests gave the "scale effect," which was assumed by the Wrights to be the same for all their model wings. By these steps they reduced the critical assumptions to one: that the order of merit of the aerofoils tested would not be seriously affected by Reynolds Number, although the absolute values of the coefficients might change. Another astounding fact was revealed by the lecturer: From their 1901 and 1902 tests the Wright Brothers calculated that the unit drag of a square flat plate of large area was equal to the speed, in m.p.h., multiplied by the coefficient 0.0033. The figure accepted in modern times, based upon a vast amount of research, is 0.00328 ! Tunnel Results in 1901 As an example of the sort of results obtained by the Wrights in their works is published Fig. 7, which is reproduced from the 1901 original. Lift is plotted against angle of attack as a percentage of the resistance of a square fiat plate, and drag (or drift as it was called in those days) as a percentage of the lift. Dr. Lewis pointed out that in present non-dimensional terms the drag coefficient of a square flat plate is about i|, so that the maximum lift coefficients obtained by the Wrights with circular arc aerofoils varied from about 1.3 to about 1.5, values clearly in agreement with modern knowledge of aerofoils. "With this information," Dr. Lewis concluded this section of his lecture, "it is amply evident that the Wright Brothers, through their investigations in 1901, gained a clear physical conception of the behaviour of wings on which to base the design of their successful ' flying machine.' " Turning to trends of modern development, Dr. Lewis said that this had been mainly towards increase in size and air speed. He recalled that since its early days the N.A.C.A. had endeavoured to follow the policy of anticipating the future needs of aeroplane design by the construction of wind tunnels having special characteristics. By way of examples he quoted the following equipment at Langley Field: The variable-density wind tunnel; the 20ft. propeller research tunnel; the full-scale tunnel; and the high-speed jet tunnels for aerofoil research under airscrew-operating conditions. More recently, the lecturer continued, anticipation of further increases in aeroplane performance led to the construction of the 8ft. 500 m.p.h. tunnel, which had been found effective in providing design information for the development of high speed aeroplanes. The most recent effort in connection with the policy of the N.A.C.A. was the 19ft. pressure tunnel. Designed in 1937. this tunnel was now being placed in operation. In it large models can be tested at pressures from sub-atmospheric up to several atmospheres, and at airspeeds in excess of 250 m.p.h. The tendency was for wind tunnels to become more specialised, and Dr. Lewis described three types, each of which had application to a special field of aerodynamics directly related to the fundamental problems of aeroplane design. Reduction of drag was important to speed, range and efficiency. For the interpretation of wind tunnel results to free atmosphere conditions a helpful simplification resulted from the concept that turbulence in the atmosphere was of such a nature that its effect on the transition from laminar to turbulent flow in the boundary layer might vanish under free flight conditions. Recent investigations had indicated that there might be some possibility of more extensive laminar boundary layers and consequent lower drags. The N.A.C.A. low-turbulence tunnel, designed in accordance with Eastman «. Jacobs' suggestions, was completed in the spring of 1938- an(^ is in manv ways the most highly specialised tunnel yet con structed by the X.A.C.A. The test section is 3ft. by 7.5ft., and is arranged for test ing models with a chord of several feet and a span of only 3ft., the flow being essentially two-dimensional. In this tunnel values of the critical Reynolds Number for transition had been found to exceed 6,500,000. The maximum values of the equiva lent flat plate Reynolds Number were those given by Professor B. Melvill Jones as approaching 5,000,000. Aerofoils designed to take advantage of true low-drag laminar boundary layers over a major portion of their surfaces showed spectacular results in drag reduction. The lecturer was not able to report results of the latest investigations, but said the American studies were directed towards the actual mechanism of transition under the simplified conditions which appeared susceptible of almost com plete experimental control. . , In describing the vertical gust tunnel and its working, tne lecturer pointed out that these gust loads were the most im portant conditions for which the structure should J» designee because (1) the relative structure weight increased with size, making it necessary to reduce the design loads of large ae - planes to a minimum; (2) increased size was accompany
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