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Aviation History
1934
1934 - 1420.PDF
SUPPLEMENT TOFLIGHT APRIL 26, 1934 THE AIRCRAFT ENGINEER arrangement in the form shown in the diagram is applic- able only to engines in which the overhead valve mechanism is totally enclosed. With the open type of valve gear found on many engines the oil returning from the valve head and stem would be flung out and, apart from the loss of oil, would make the aircraft behind the engine very dirty. The oil supply is, of course, intended to be taken from the general supply, and in some cases it might be necessary to fit an oil pump of slightly greater capacity than normally pro- vided. Apart from arranging for the gallery oil pipe, it would be necessary, in converting an existing aero engine to take oil-cooled valves, to bore out the valve guides to take the larger valve stems. It might have been feared that carbonisation of the oil inside the valve might occur and might block the passages. Experience with a car engine indicates that this does not take place, the heat transference via the quickly-flowing oil stream being so rapid that the valve never gets really hot. The temperature of the oil after passing through the valve is raised by a few degrees only. BENCH AND FLIGHT TESTS OF A ROOTS TYPE AIRCRAFT ENGINE SUPERCHARGER. By the Staff of the Engine Experimental Department, R.A.E. Communicated by the D.S.R., Air Ministry. R. & M. No. 1558. (23 pages and 16 diagrams.) September, 1933. The purpose of the investigation was to investigate the application of theBoots class of displacement supercharger to an aircraft engine, bench tests of a unit of this type, built to N.A.C.A. specification, were carried out toexamine its performance characteristics, and subsequently flight tests were undertaken to determine the capabilities of a Lion V.S. engine fitted withthis supercharger. The bench calibration tests covered a range of rotor speeds from 2,750 r.p.m.to 3,900 r.p.m., and the necessary quantities were measured to enable the power absorbed by the supercharger and the volumetric and adiabaticefficiencies to be calculated. From this information the probable rated altitude in combination with the Lion engine was deduced. The flight tests, which were carried out in a D.H. 9A aircraft, consistedof determinations of (1) the rated altitude on climb and in level flight; (2) the ceiling ; and (3) level speeds at the rated boost pressure of the enginebelow the rated altitude and at the maximum obtainable boost pressure above it. During the several flights, additional observations were recordedrelating to the individual performances of the aircraft, engine and supercharger. The Roots supercharger wag found to be especially suitable for engineswith rated heights in the vicinity of 20,000 ft. At pressure ratios corres- ponding to altitudes of this order a gatlsfaetory volumetric efficiency ismaintained, and the adiabatic efficiency compares favourably with that of centrifugal superchargers. The power wasted below the rated altitude bypermitting a proportion of the compressed air to escape to atmosphere is not excessive, and at ground level the total power absorbed by the super-charger can be reduced to a negligible amount. TECHNICAL LITERATURE SUMMARIES OP AERONAUTICAL RESEARCH COMMITTEE REPORTS These Reports are published by His Majesty's Stationery Office, London, and may be purchased directly from H.M. Stationery Office at the following addresses: Adastral House, Kingsway, W.C.2; ISO, George Street, Edinburgh; York Street, Manchester; 1, St. Andrew's Crescent, Cardiff; 15, Donegall Square West, Belfast; or through any Bookseller. THE EXPERIMENTAL DETERMINATION OF PITCHING MOMENT OP AN AEROPLANE DUE TO ROTATION IN PITCH. By A. S. Halliday, B.Sc., Ph.D., D.I.C., L. W. Bryant, B.Sc., A.R.C.Sc, and C. H. Burge. R. & M. No. 1556. (27 pages and 28 diagrams.) March 30, 1933. All previous measurements of the damping moment of an aeroplane inpitch have been made by the method of oscillation in the wind tunnel. Unfortunately, as pointed out in R. & M. 718, this method does not determinethe true value of M^ directly, but is subject to a correction due to the fact that the incidence changes during the experiment. In consequence of the appreciable time taken by the downwash fromthe wings to reach the tail, the effective incidence of the tail does not at a given instant correspond to the incidence of the winss at the same instant.It has been shown in R. & St. 826 that at the incidences of normal flight the correction for this effect can be applied with some confidence, particularlyif the rate of change of downwash at the tail is actually measured on the model. But the simple theory of R. & M. 718 can hardly be accepted withoutquestion for application to attitudes at the stall and above, and any method of experiment which avoids the necessity for making the correction is ofgreat value in determining M^ at large incidences. The whirling arm provides «uch a method, since the incidence remains constant throughout the motionThere are large differences in the values of the derivative K Bm- at angles.of incidence above the critical angle obtained by the two methods. Below the critical angle they give the isame result to an average accuracy of about10 per cent, provided the oscillation measurements are corrected for rate of change of wing downwash in accordance with the principle laid down inR. & M. 718.* In view of the uncertainty of this downwash correction it is considered that the whirling arm measurements are to be preferred for thenormal flying range of incidences. For ordinary calculations of longitudinal stability it is probably sufficiently accurate to compute KB»»- from the tailforces alone making no allowance for wings and body. The derivative Z? for the wings may equal or considerably exceed Z for the tail. The experimental measurements of Z? and KB«»j for a sing •aerofoil agree reasonably with the theoretical values. M. 718. The effect of the lag of downwash on thejongitudinal NOTE ON THE USE OP NETWORKS TO INTRODUCE TURBU- LENCE INTO A WIND TUNNEL. By E. Ower, B.Sc.. A.C.G.I., and R. Warden, Ph.D., M.Eng. R. & M. No. 1559. (8 pages and 4 diagrams.) July 31, 1933. Variations of turbulence in the wind-tunnel stream are known to havea marked effect on the results of certain types of experiments, and in attempts to study this effect wire or cord networks have been used on various occasionsto introduce artificial turbulence into the tunnel. Experience at the N.P.L. suggested that this method might be unsuitable on account of the fluctuationsin the time average of the velocity across a section introduced as a result of the "shadows" of the cords. Some measurements of the distributionof mean velocity and static dsestyse at different distances downstream of two networks were therefore made and the results confirmed the impressionpreviously formed. While the transverse static pressure distribution was uniform at a distance of only 10 in. behind a network of J-in. cords withli-in. mesh, a violently-disturbed distribution of mean velocity was observed at this distance, and even at 3 ft. behind the network the time mean ofthe velocity at different points in a section was found to vary by over i per cent. Similar explorations were made with a streamline body in the tunnel,and it was found that the network seriously modified the velocity fleld near the body. It is concluded that networks are not suitable agents to employ for anyquantitative investigations of the effect of turbulence on wind-tunnel data. THE FLOW NEAR A WING WHICH STARTS SUDDENLY FROM REST AND THEN STALLS. Aeronautics Laboratory, Cambridge. R. & M. No. 1561. (9 pages and 12 dia- grams.) August 8, 1933. When an aerofoil is started suddenly from rest into steady motion, thefluid flow in its neighbourhood passes through a series of different forms which approach asymptotically to the steady form which occurs in thewind tunnel and In the steady flight of aeroplanes. During this transition period a vortex sheet, or surface across which the velocity Of the flow isdiscontinuous, is shed from the trailing edge of the aerofoil and remains behind in the fluid, the strength of the discontinuity at the trailing edgefalling asymptotically to zero as the steady state is approached. H. Wagner* has advanced the hypothesis that, if the wake is infinitelythin and the trailing edge sharp, the form of the flow and the magnitude of the lift during this transition stage should be approximately calgulable onthe assumption that the flow everywhere outside the wake is irrotational, and that the strength of the discontinuity across the wake at the trailingedge has that unique value which is necessary to avoid infinite velocity at the edge. On this hypothesis, he calculated the distribution of vorticityin the wake and the lift at various distances from an impulsive start of an aerofoil idealised to the form of a thin flat plate of infinite span, set at aninfinitesimal angle of incidence to the direction of motion. P. B. Walker, t using the same apparatus as was used for the experimentsnow to be discussed, photographed the two-dimensional fields of flow at various distances from a sudden start of an aerofoil of R.A.P. 30 profile setat 7'5° incidence. These photographs showed the flow changes to be sub- stantially of the form supposed by Wagner, and measurements of the velocityfields near the aerofoil gave a curve of circulation around the profile against distance from the Btart, which was in remarkably close agreement withWagner's theoretical curve, provided that, in each instance, the circulations were expressed as fractions of the circulation ultimately reached when themotion had become steady. The first signs of the stall were observed when the distance travelledwas 2-8 times the chord of the aerofoil and, at that instant, it is deduced from the measured circulations that the lift coefficient was more than oneand a half times the maximum which can act steadily upon this profile at the Reynolds number (1-3 x 10 ) of tho experiment. Photographstaken after the stall had begun show the shedding of vorticity which must occur as the lift falls to the steady value. * *v.,K,i * J1- 718- T,he effeS* Of J?6 laF °f J0™";?811 °n the longitudinal fa Angewand'teltathematik und Mechanik. Vol. 5, No. 1,"February I92Tstability of an aeroplane and on the rotary derivative M ?.-W. L. Cowley f ••Growth of Circulation about a Wing and an Apparatus for Measuring and H. Glauert f Growth of Circ g ppFluid Motion." P. B. Walker. Reports and Memoranda N». 1402. 410 k
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