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Aviation History
1928
1928 - 0448.PDF
SUPPLEMENT TO FLIGHT 44 MAY 31, 1028 THE AIRCRAFT ENGINEER performance characteristics lor several methods of control of an over-com-pressed engine using gasoline and operating under sea-level conditions. For this work, a special single-cylinder test engine, 5-iu. bore by 7-in. stroke,and designed for ready adjustment of compression ratio, valve timing and valve lift while running, was used. This engine has been fully describedin N.A.C.A. Technical Report No. 250. Tests were made at an engine speed of 1,400 r.p.m. for compression ratiosranging from 4 to 7-6. The air-fuel ratios were on the rich side of the chemically correct mixture and were approximately those giving maximumpower. When using plain domestic aviation gasoline, detonation was con- trolled to a constant, predetermined amount (audible), such as would bepermissible for continuous operation, by (a) throttling the carburettor, (6) maintaining full throttle but gfcatly retarding the" ignition, and (c)varying the timing of the inlet valve to reduce the effective compression ratio. For the first and third methods, the throttle opening and the valvetiming, respectively, were adjusted so that the ignition timing could be advanced slightly beyond the advance, giving maximum power withoutexceeding the standard of permissible detonation. The optimum perform- ance for the engine when using a non-detonating fuel, consisting of 80 percent, of commercial benzol and "20 per cent, of aviation gasoline, was obtained as a basis for comparison. The following comparative results are based on the optimum performancefor the engine obtained with the non-detonating fuel at a compression ratio of 4-7. The power and fuel consumption with method (b) remained sub-stantially constant at the higher compression ratios, the order of the ignition timing permitting full throttle operation ranging from 30° at 4-7 to 3° at7-3; exhaust temperatures, heat loss to the coaling water, and explosion pressures at the higher ratios were normal. At a eo.npre^ion ratio of 7-5,the power obtained with method (nr) was about 39 par cent, less and the fuel consumption was considerably lower ; with method (b). time of inlet-valve opening constant and time of inlet-valve closing varied, the power was about 23 per cent, less and the fuel consumption was greatly increased ;with method (c), time of inlet opening and closing varied simultaneously, the power was about 29 per cent, lass ani the fuel con-sumption was greatiyiucreaupd. * From these remits, it miy h? conducted that rnethoi (c) gives the best all-round pgrformance and. being easily employed in service, appears to be the most practicable method for controlling an overeompressed engine usinggasoline at low altitudes. T.R. No. 273. " WIND TUNNEL TESTS ON AUTOROTATION AKD THE ' FLAT SPIN.' " By Montgomery Knight, N.A.C.A. This report deals with the autorotational characteristics of certain differingwing systems as determined from wind-tunnel tests made at the Langley Memorial Aeronautical Laboratory. The investigation was confined toautorotation about a fixed axis in the plane of symmetry and parallel to the wind direction. Analysis of the tests leads to the following conclusions :Autorotation below 30° angle of attack is governed chiefly by wing profile and above that angle by wing arrangement.The strip method of" autorotation analysis gives uncertain results between maximum CL and 35°. The polar curve of a wing system, and to a lower degree of accuracy thepolar of a complete airplane model are sufficient tor direct determination of the limits of rotary instability, subject to strip-method limitations. The results of the investigation indicate that in free flight a monoplane isincapable of flat spinning, whereas an unstaggered biplane has inherent flat- spinning tendencies.The difficulty of maintaining equilibrium in stalled flight is due primarily to rotary instability, a rapid change from stability to instability occurring asthe angle of maximum lift is exceeded. T.R. No. 274. " THE N.A.C.A. PHOTOGRAPHIC APPARATUS FOB STUDYING FUEL SPKAYS FEOM OIL ENGINE INJECTION VALVES AND TEST RESULTS FROM SEVERAL RESEARCHES." By Edward G. Beardsley, N.A.C.A. Apparatus for recording photographically the start, growth, and nut-offof oil sprays from injection valves has been developed at the Langley Memorial Aeronautical Laboratory. The apparatus consists of a high-tension trans-former by means of which a bank of condensers is charged to a high voltaqe. The controlled discharge of these condensers in sequence, at a rate of severalthousand per second, produce? electric sparks of sufficient intensity to illuminate the moving spray for photographing. The sprays are injectedfrom various types of valves into a chamber containing gases at pressures up to 600 lbs. per square inch.Several series of pictures are shown. The results give the effects of injection pressure, chamber pressure, specific gravity of the fuel oil used, and injection-valve design upon spray characteristics. T.R. No. 275. " THE EFFECT OF THE WALLS IN CLOSED- TYPE WIND TUNNELS." By George J. Higgins, N.A.C.A. A series of tests has been conducted during the period 1925-27 by theNational Advisory Committee for Aeronautics in the variable density wind tunnel on several airfoil models of different sizes and sections to determine theeffect of tunnel-wall Interference and to determine a Correction wniell can be applied to reduce the error caused thereby. The use of several empiricalcorrections was attempted with little success. The Prandtl theoretical corrections give the best results, and their use is recommended for correctingclosed wind-tunnel results to the conditions of free air. An appendix is attached wherein the experimentally determined effect ofthe walls on the tunnel velocity very close to their surface is given. This is of special interest because a " scale effect " was found in the boundary layer witha change in the density of the tunnel air. T.R. No. 276. " COMBUSTION TIME IN THE ENGINE CYLINDER AND ITS EFFECT ON ENGINE PERFORMANCE." By Charles F. Marvin, Jun., Bureau of Standards. As part of a general program to study combustion iu the engine cylinderand to correlate the phenomena of combustion with the observed performance of actual engines, this papsr, which wan outline-1 by S. W. Sparrow, and thework undertaken at the request of the National Advisors" Committee for Aeronautics, presents a sketchy outline of what may happen in the enginecylinder during the burning of a charge. Tt also suggests the type of informa- tion needed to supply the details of the picture and points out how combustion,time and rate affect the performance of the engine. A theoretical concept of a flame front which is assumed to advance radially from the point of ignition is presented, and calculations based on the area andvelocity of this flame and the density of the unburned gases are made to determine the mass rate of combustion. From this rate the mass which hasbeen burned and the pressure at any instant during combustion are computed. This process is then reversed iu an effort to determine actual rates ofcombustion and flame velocities from the pressures as recorded on indicator diagrams.The effects of different rates of combustion on engine performance are then discussed and the importance of proper spark advance is emphasised. T.R. No. 277. " THE COMPARATIVE PERFORMANCE OF AN AVIATION ENGINE AT NORMAL AND HIGH INLET AIR TEM- PERATURES." By Arthur W. Gardiner and Oscar W. Schey, N.A.C.A. This report presents some results obtained at the Langley Memorial Aero-nautical Laboratory during an investigation to determine the effect of high inlet air temperature on the performance of a Liberty-12 aviation engine.The purpose of this investigation was to ascertain, for normal service carburettor adjustments and a fixed ignition advance, the relation betweenpower and temperature for the range of carburettor air temperatures that may be encountered when supercharging to sea-level pressure at altitudes ofover 20,000 ft. and without intercooling when using plain aviation gasoline and mixtures of benzol and gasoline. Laboratory tests were made at full throttle over the speed range from1,400 to 1,800 r.p.m., in which the pressure at the carburettor and exhaust was maintained sensibly constant and the inlet air temperature varied from 45°to 180° F. The range of mixtures was that normally used in flight. Plain aviation gasoline, a mixture consisting of 30 per cent, (by volume) of com-mercial benzol and 70 per cent, gasoline, and a mixture of G"> per cent, benzol and 35 per cent, gasoline were'used. Additional tests were made with aWright E-4 aviation engine. The results ghow that for the conditions of test, both the brake and indicatedpower decrease with increase in air temperature at a faster rate than given by the theoretical assumption that power varies inversely as the square root ofthe absolute temperature. On a brake basis, the order of the difference in power for a temperature difference of 120° F. is 3 to 5 per cent. The observedrelation between power and temperature when using the 30 to 70 blend was found to be linear. But, although these differences are noted, the abovetheoretical assumption may be considered as generally applicable except where greater precision over a wide range of temperatures Is desired, in which case itappears necessary to test the particular engine under the given conditions. T.R. No. 278. " LIFT, DRAG, AND ELEVATOR HINGE MOMENTS OF HANDLEY-PAGE CONTROL SURFACES." By R. H. Smith, Construction Department, Washington Navy Yard. This report combines the wind-tunnel results of tests on four control surfacemodels made in the two wind tunnels of the Navy aerodynamical laboratory, Washington Navy Yard, during the years 1922 and 1924. The purpose ofthe tests was to compare, first, the lifts and the aerodynamic efficiencies of the control surfaces from which their relative effectiveness as tail planes could bedetermined : then the elevator hinge moments upon which their relative ease of operation depended. The lift and drag forces on the control surfacemodels were obtained for various stabilizer angles and elevator settings in the 8 by 8-ft. tunnel by the writer in 1922; the corresponding hingemoments were found in the 4 by 4-ft. tunnel by Mr. R. M. Bear in 1924. T.R. No. 279. " TESTS ON MODELS OF THREE BRITISH AIRPLANES IN THE VARIABLE DENSITY WIND TUNNEL." By George J. Higgins and George L. DeFoe, N.A.C.A., and W. S. Diehl, Bureau of Aeronautics, Navy Department. This report contains the results of tests made in the National AdvisoryCommittee for Aeronautics variable density wind tunnel on three aeroplane models supplied by the British Aeronautical Research Committee. Thesemodels, the BE-2E with K.A.F. 10 wines, the Bri3tol Fighter with R.A.F. 15 wings, and the Bristol Fighter with R.A.F. 30 wings, were tested over awide range in Reynolds Numbers iu order to supply data desired by the Aeronautical Research Committee for scale-effect studies.The maximum lifts obtained in these tests are in excellent agreement with the published results of British tests, both model and full scale. No attemptis made to cooipare drag data, owing to the omission of tail surfaces, r adiator, etc., from the model, but it is shown that the scale effect observed on the dragcoefficients in these tests is due to a large extent to the parts of the models other than the wings. T.R. No. 280. " THE GASEOUS EXPLOSIVE REACTION : THE EFFECT OF INERT GASES." By F. W. Stevens, Bureau of Standards. (1) Attention is called in this report to previous investigations of gaseousexplosive reactions carrier! out under constant volume conditions, where the effect of inert gases on the thermodynamic equilibrium was determined.The advantage of constant pressure methods over those of constant volume, as applied to studies of the gaseous explosive reaction, is pointed out, andthe possibility of realising for this purpose a constant pressure bomb men- tioned. (2) The application of constant-pressure methods to the study of gaseousexplosive reactions, made possible by the use of a constant-pressure bomb, led to the discovery of an important kinetic relation connecting the rate ofpropagation of the zone of explosive reaction within the active gases, with the initial concentrations of those gases : s = k\ [A]"l [B]"2 [C]"5. (3) By a method analogous to that followed in determining the effect ofinert gases on the equilibrium constant K, the present paper records an attempt to determine their kinetic effect upon the expression given above.It is found that this effect for the inert gases investigated—N2, He, and CO2— may be expressed as B -= *i [A]°l [B]"2 [Cl-s + ft Gi where Gk represents the initial concentration of the inert gas. From resultsobtained, it seems probable that the value of j8 depends upon the combined effect of the thermal properties of the inert gas on the heat distribution ofthe reaction, the property of heat conductivity being predominant. (4) An example of the utility of the constant-pressure bomb for the studyof the kinetics of the gaseous explosive reaction 13 offered in the results of the present paper.
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