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Aviation History
1923
1923 - 0309.PDF
JUNE 7, 1923 It was evident that real knowledge could be obtained onlyby extensive experimentation on actual airships. What was needed primarily was a series of measurements of pressuresover the envelope and surfaces of an airship when in steady flight and when making manoeuvres. For this purpose anon-rigid airship, Navy type C, was placed at the disposal of the Committee. It is 200 ft. long, 40 ft. in diameter, and has200,000 cubic ft. capacity. Pads were specially designed for the measurement of pressure. These lie practically flushwith the envelope of the airship, and each consists essentially of a metal box whose top and bottom surfaces are pear-shaped, roughly 2 ins. by 4 ins., and held a distance of one-hundredth of an inch apart by means of studs ; in thetop plate there are grouped in a comparatively small circle There are the three problems referred to at the beginning ofthis paper as requiring an elaboration of the methods for the study of pressure distribution ; and no one can question theimportance of the results obtained in the proper design of aircraft. Quite a different set of questions has been asked ourCommittee, which lead in the end to an investigation of the so-called scale effect. Certain questions can, of course, beanswered on theoretical grounds, and answered definitely ; but the great majority cannot. Any aircraft is a complicatedmechanism made up of many parts ; all of these have definite aerodynamical characteristics ; but from a knowledge of thesewe cannot pass to that of the machine as a whole. The question as to the changes in forces and moments with scale, Americ a n N.A.C.A. trailing Kymograph air- speed meter. 22 holes each three-hundredths of an inch in diameter; a brasstube £ in. in diameter serves as an outlet from the box. This is connected by rubber or aluminium tubing to a liquidmanometer in the car of the airship. There are about 400 of these pads on the envelope andsurfaces of the airship, 36 being in the bottom fin and rudder. Simultaneous reading of 260 manometers may be madephotographically. This investigation of the aerodynamics of an airship is not yet completed, but I can show you certain observations which indicate the importance and novel character of the results being obtained. One illustration shows the pressure distri- bution over the bottom fin and rudder in circling flight, and the other when the airship while in steady flight has its helm put hard down. The drawings do not require much explanation, but emphasis especially in manoeuvres, is exceedingly difficult. The firstinvestigation which should be made on scale effect is to determine which aerodynamic properties are most susceptibleto the effect ; after that, the number of problems to be under- taken is practically infinite. At Langley Field our Committee has facilities for studyingscale effect by four different methods, two of which are, I believe, unique. We have an ordinary wind tunnel, having a5 ft. throat and fitted with fans so that an air-speed of 100 m.p.h. (147 ft. per second) may be used ; this gives acertain Reynolds number, not very large. A larger number may be obtained by a free flight method in which a largemodel is suspended below an aeroplane in steady flight; we have perfected methods for suspension and measurement, andthe results are, on the whole, satisfactory. To secure a still larger Reynolds number, the Committee has had constructed Section through the American N.A.C.A. variable density wind tunnel, designed by Dr. Max Munk. may be placed upon the results shown when circling flight is begun. When the helm is suddenly applied, and before the airship attains an appreciable angular velocity, the angular acceleration creates such a large force on the vertical fins in the opposite direction to the force on the rudder that the net force on the stern of the airship is much smaller than has been supposed hitherto. It follows that the condition of the sudden application of the rudder is not a serious one from the point of view of the stresses in the hull of the airship. JPre- sumably the reversal of the helm, when the airship is in a steady turn, does not cause a large increase of the bending moments beyond those already existing in that condition. during the past year a wind tunnel to operate with air com- pressed to 20 atmospheres or more. The tunn'el proper is 5 ft. in diameter at the experimental chamber, and is enclosed in a cylindrical tank with hemispherical ends. The walls of the tunnel are hollow, providing an annular dead air space in which the balance mechanism is installed. This may be controlled automatically, or settings may be made by small electric motors, operated from outside, which attach or release heavy balancing weights by means of cams, or shift lighter weights along balance arms. The model is attached to the balance by wires, there being three balance arms for measuring lift, drag and pitching moments. The tank is 35 ft. long and 309 .«..*.
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