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Aviation History
1941
1941 - 2221.PDF
SEPTEMBER 25TH, 1941. FLIGHT 19T JET PROPULSION the drag of projecting wheels or half-wheels, a considerable saving of weight, complication and man-hours would beeffected. The added reliability thus gained would be definitely on the-creditside in offsetting speed losses due to the non-retractable wheels. However, onlycareful calculation and wind tunnel in- vestigation could determine the relative'merits of this conjecture, if and when all other problems are successfully solved. The pilot can be seated well forwardand given a wide field of view with no interference. In military aircraft thearmament, whether offensive or defen- sive, may be more conveniently arrangedand will command a less restricted field of fire. To continue .details of jet propulsiondesigns, I take first the 1917 scheme of 0. Morize, of Chateaudun, France. Inthis, liquid fuel is delivered to a combus- tion chamber, charged with air by anengine-driven compressor, and on com- bustion discharges through a nozzle intothe intake of a convergent-divergent "ejector." A complete plant is showndiagrammatically in the illustration. An 4 I I A Swiss design, the Eichelberg, has an independent engine-charging systemdriven by a separate exhaust gas turbine. Waste gas from the turbine is not mixed with the main air supply but discharged from a separate nozzle to assistpropulsion. On the right is a schematic layout of a complete plant having three compressor aggregates and two engine charging units. engine A drives a compressor B which delivers air throughan equalising chamber C to the combustion chamber D, the walls of which are lined with refractory material. The airinlet surrounds the jet E supplied with liquid fuel from tank F by an engine-driven pump G. A suitable meansof ignition, electric or incandescent, is provided and the combustion products are discharged through the com-bustion chamber nozzle into the convergent forward end of the "ejector" tube H. This creates a region of lowpressure and air is induced into the end of the tube H to join the rapidly moving gas stream. In the divergentportion of the tube H the velocity is diminished, thus im- parting increased pressure to the stream as it leaves thedischarge orifice. The stated aim is to securea negative pressure at the forward end and a positivepressure at the rear and thus exert a propulsive thrust ina direction opposite from the discharging gas-air stream.Control of the thrust is effected by varying the quan-tity and pressure of the fuel and the charging air whichsupports combustion. Modi- fied types of "ejectors," ofwhich two are shown, had a series of air intakes, a plural-ity of combustion chamber nozzles, as at J, or an annu-lar combustion chamber, as at K. In the same year H. S.Harris, of Esher, devised a propulsion plant of consider-able interest, although its fate remains for the timebeing obscure. He also sought to induce air into thedischarge tube in order to in- crease the mass of the jet.Instead, however, of achiev- es this aim by allowing thecombustion gases to expand in a tube of increasing cross- B The British Harris plantalso uses induced air, but the combustionchambers discharge into divergent-convergenttubes. sectional area, he inverted the process since adopted.The combustion gases enter the discharge tube at high velocity and low pressure (approximately atmosphericpressure) in the form of a cylindrical stream or jet. As this jet is cooled and contracted on its way to the dischargenozzle a "depression" is produced which induces the inflow of air at the open forward end of the discharge tube.The tube is of a considerable length in order to ensure that the stream of gas and air issuing from the discharge nozzleis completely cooled. Consequently, no rapid expansion will occur at the nozzle which might give a pressure dropand induce a reverse flow. To compensate for the con- tinued cooling and the corresponding decrease in volumeof the stream, the discharge tube is of decreasing cross- sectional area, thus maintaining the velocity. It is somewhat remarkable that the scheme contains ncsuggestion of utilising the effluents of the engine whict drives the blower supplying air to the combustion chainbers. On the othei hand, it appears ahead of its time in the proposal to employ powdered coal as a fuel, as analternative to liquid hydrocarbons, such as petrol or paraffin. Petrol Fuel Petrol is used tor the twin-jet plant shown in th(diagram. A twin-cylinder engine A drives directly a low pressure blower B, which draws air from a forward axiaintake and delivers to combustion chambers C. Fuel fron tank D is supplied to jets E and mingles with the air tcform a combustible mixture which is ignited by unspecified means. Presumably an electrical ignition system would beprovided. The combustion chambers are of an increasing cross-sectional area to accommodate the expansion of thecharge without increase of pressure and terminate in a slightly convergent nozzle to increase the velocity of thegas stream, which issues as a cylindrical jet. Each chamber is positioned axially in a long discharge tube F open atboth ends. The issuing gases, on cooling in this tube, produce a partial vacuum which draws air into the tubein the direction of flow and increases the mass of the rear- wardly flowing stream. A number of modified arrangements were suggested. In-stead of two jets, a single large discharge tube with a plurality of combustion chambers may be provided. Or aseries of combustion chambers may be fitted in each dis- charge tube and additional air be admitted at several pointsalong the tube at successive stages. The charging blower,
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