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Aviation History
1926
1926 - 0303.PDF
APRIL 29, 1926 4? THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT This investigation was made by the National Advisory Committee for Aeronautics at the request of the Bureau of Aeronautics, Navy Department, for the purpose of determin- ing the aerodyTnamic pressure distribution encountered on a "" C " class airship in flight. It was conducted in two parts : (a) Tests on the tail surfaces in which the pressures at 201 points were measured and (6) teats on the envelope in which 190 points were used, both tests being made under as nearly identical flight conditions as possible, so that the results could be combined and the pressure distribution over the entire airship obtained. The method of testing consisted of measuring the pressure by means of orifices located at the desired points connected to the tubes of a multiple liquid manometer. Simultaneous readings of all the pressures were obtained by photographing the manometer. The results as presented in this report are mainly in tabular form, and may be very briefly summarised as follows :— (1) The maximum local pressure encountered on a tail surface was 7-3 lb./sq. ft. (2) The maximum total normal force on a complete tail surface was 352 lbs. or a C-NF of 0-310 occurring on the bottom fin and rudder during a " reversal " of the rudder. (3) The maximum moment of the tail surface forces about the centre of buoyancy was 37,200 lb. ft. (4) The investigation of the envelope pressures, while show- ing the general distribution of pressure satisfactorily, is practically useless in the determination of total aerodynamic forces on the airship. (5) It is concluded that the pressures set up by a bump are larger than those obtained in manoeuvring. Report No. 224, entitled " An Investigation of the Coefficient of Discharge of Liquids through Small Round Orifices.'' by W. F. Joachim.—The work covered by this report was undertaken in connection with a general investigation of fuel injection engine principles as applied to engines for aircraft propulsion, the specific purpose being to obtain information on the coefficient of discharge of small round orifices suitable for use as fuel injection nozzles. Flow oi the liquids tested under high pressure was obtained with an intensifier consisting of a 5-in. piston driving a direct connected f-in. hydraulic plunger. The large piston was operated by compressed air and the time required for the displacement of a definite volume by the hydraulic plunger was measured with an electrically-operated stop watch. The- coefficients were determined as the ratio of the actual to the theoretical rate of flow where the theoretical flow was obtained by the usual simple formula for the discharge of liquids through orifices. Values for the coefficient were determined for the more important conditions of engine service such as discharge under pressures up to 8,000 lbs. per sq. in., at temperatures between 80° and 180° F., and into air compressed to pressures up to 1,000 lbs. per sq. in. The results show that the coefficient ranges between 0 • 62 and 0 • 88 for the different test conditions between 1,000 lbs. and 8,000 lbs. per sq. in. hydraulic pressure. At lower pressures the coefficient increases materially. It is concluded that within the range of these tests and for hydraulic pressures above 1,000 lbs. per sq. in. the coefficient does not change materially with pressure or temperature ; that it depends considerably upon the liquid, decreases with increase in orifice size, and increases in the case of discharge into compressed air until the compressed-air pressure equals approximately three-tenths of the hydraulic pressure, beyond which pressure ratio it remains practically constant. Report No. 225, entitled " The Air Forces on a Model of the Sperry Messenger Airplane Without Propeller," by MaxM. Munk, and Walter S. Diehl.—This is a report on a scale- effect research which was made in the variable-density wind tunnel of the National Advisory Committee for Aeronautics at the request of the Army Air Service. A -,1,-,-scale model of While this report is of a preliminary nature, the work lias progressed far enough to show that the scale effect is almost entirely confined to the drag. In the tests so far conducted, the drag at any given angle of attack within the normal VI \ "flying range is found to vary as I — j . The exponent n is constant for any one angle of attack, and ranges from — 0-045 at large angles of attack to — 0 17 at small angles. It was also found that the model should be geometrically similar to the full-scale airplane if the test data are to be directly applicable to full scale. If the condition of geo- metric similarity be fulfilled, the data obtained at a full- scale value of Reynolds number agree very closely with free-flight data. The variable-density wind tunnel, there- fore, appears to be a very promising instrument for procuring test data free from scale effect. It is also admirably suited for studying the scale effect and obtaining information which is necessary in an interpretation of the results obtained in atmospheric wind tunnels at low values of Reynolds number. Report No. 22fi. entitled " Characteristics of a Boat-Type Seaplane During Take-Off," by J. W. Crowley. Jim., and K. M. Ronan.—This report, on the planing and get-away characteristics of the F-5-L. gives the results of the second of a series of take-off tests on three different seaplanes con- ducted by the National Advisory Committee for Aeronautics at the suggestion of the Bureau of Aeronautics, Navy Department. The single-float seaplane was the first tested and the twin-float seaplane is to be the third. The characteristics of the boat type were found to be similar to the single float, the main difference being the increased sluggishness and the relatively larger planing resist- ance of the larger seaplane. At a water speed of 15 miles per hour the seaplane trims aft to about 12 deg. and remains in this angular position while plowing. At 2-25 miles per hour the planing stage is started and the planing angle is imme- diately lowered to about 10 deg. As the velocity increases, the longitudinal control becomes more effective, but over- control will produce instability. At the get-away the range of angle of attack is 19 to 11 deg., with velocities from the stalling speed through about 25 per cent, of the speed range. Report No. 227, entitled "The Variable-Density Wind Tunnel of the National Advisory Committee for Aeronautics." by Max M. Munk and Elton W. Miller.—This icport contains an exact description of the new wind tunnel of the National Advisory Committee for Aeronautics. This is the first American type wind tunnel. It differs from ordinary wind tunnels by its being surrounded by a strong steel shell, 35 ft. long and 15 ft. in diameter. A compressor system is provided to fill this shell—and hence the entire wind tunnel— with air compressed to a density up to 25 times the ordinary atmospheric density. It is demonstrated in the report that the increase ol the air density makes up for a corresponding decrease in the scale of the model. Hence such American type wind tunnel is free from scale effect. The report is illustrated by man ' drawings and photo- graphs. All construction details arc described, and many dimensions given. The method of conducting tests is also described and some preliminary results given in the reports. So far, the tests have confirmed the chief feature of this wind tunnel—absence of scale effect. Report No. 228. entitled "A Study of the Effect of a Diving Start on Airplane Speed," by Walter S. Diehl.- Equations for instantaneous velocity and distance flown are derived for an airplane which crosses the starting line of a speed course at a speed higher than that which can normally be maintained in horizontal flight. A specific case is assumed and calculations made for five initial velocities. Curves of velocity, average velocity, and distance flown areat the request of the Army Air service, A ,,,-scaie muuei oi viu»ra ^, ,^™:.,, r . shown the Sperry Messenger airplane with U.S.A.5 wings was tested plotted against time for eRch case and analysed It is sho* n without a propeller at various Reynolds numbers up to the full- that the increase in average velocity due to a dn ing start scale value. Two series of tests were made : The first on the original model which was of the usual simplified construction may be very large for short-speed courses. h»m™-t Nn 99Q entitled " Pressure Distribution overReport No. l£d, f1™1^ r Tand the second on a modified model embodying a great amount of detail. 260ft P p .f^ r . „ Modified Thick. Tapered Airfods.^.A.C£. »UU ^Modified.
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