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Aviation History
1942
1942 - 1588.PDF
n8 FLIGHT JULY 30TH, 1942 STEAM IN THE AIR The Bechard Steam Power Plant (France). Bechard developed a rotary boiler type consisting of three casings in which a shaft with three discs is made to rotate. The three casings are externally heated and partly filled with water, before starting, which partici pates in the rotary movement owing to friction against the wheels, during rotation. In operation a constant flow of water is produced, thus eliminating the need for a sepa rate feeder pump. The rotation of the boiler ensures even heating of all parts. In steady running steam generation proceeds (as shown in the diagram) in three stages. Fresh 'teed-water enters the first casing, is brought into rotation, pre-heated and partly evaporated. In the second casing this evaporation is completed. The third casing serves as a super-heater, and the steam issues through nozzles and is axially deflected in a ring of guide blades. The effective performance of the turbine, which appa rently works with 100 per cent, reaction, is produced by the reactive shock of the steam issuing from the nozzles. According to details communicated by Bechard in January, 1938, tests with his No. 5 steam power plant produced the following results: — 150 kg. of dry steam at a pressure of 4.5 kg. The con sumption of residual oil fuel was 13.5 kg., showing an efficiency of 0.6, at a speed of 950 revolutions. In another test Bechard obtained a speed.of 1,200 revo lutions. These results have been achieved with a machine of a total heating surface of 0.96 sq.m. and without re heating the water. Relying on his experience, Bechard maintained that there should be no difficulty in producing a plant operating at 1,800 r.p.m. with a.steam output of 200 kg./sq.m./hr. at an atmospheric pressure of 30 to 35 kg., superheated to 500 deg. C. and with a boiler efficiency of 90 per cent. Problems and Possibilities In order to appreciate correctly the possibilities of the employment of steam power for the propulsion of aircraft, one has to bear in mind, first of all, the inherent require ments of such power plants. Broadly speaking, they can be summarised as follows: — (a) Light weight (low weight per unit power). (b) Reliability in service. (c) Economy (high thermal efficiency). •(d) Durability. (e) Compactness (less drag). (f) Flexibility. (g) Good behaviour at high altitudes. The report on the investigation on steam propulsion for aircraft published by the American National Advisory Committee on Aeronautics concluded that "on the basis of the weight of the power plant alone steam power plants for aircraft are precluded. On the basis of economy alone, they are again precluded. On the basis of the resistance of the cooling surface required alone, they are precluded. On the basis of the sum of these three considerations they are absolutely impossible." While this verdict, pronounced by such eminent experts, appears to wipe out. any further possibilities for steam power propulsion, it should not be taken as final. The trend of aircraft design which calls for more powerful power units opens, perhaps, new vistas for the employment of steam. Large flying boats, some recently designed and some already on construction, necessitate power plants which in power output and size surpass everything hitherto known. The American Martin XPB2M-1 "Mars," the French Potez-SCANi6i and the Latecoere 631 are some where in the region of 65 tons or more. The Breguet Com pany is engaged on projects of flying boats of a weight of 75. 25° and even 950 tons, with twin decks in the wings. Such large size aircraft will permit, perhaps, the employ ment of steam propulsion if the necessary improvements are made and existing drawbacks are abolished. COMMENCEMENT OF RUNNING AT STEAM GENERATION FULL SPEE-D The diagram shows the stages of operation of the Bechard steam aircraft engine. In the view of different experts a successful develop- «#i* ment of steam propulsion will embody the following fea- * tures:—The unit weight of steam plants will have to be considerably reduced. This is possible if the durability of the steam plant is reduced to that of internal combus tion aircraft engines. Two-stage turbines will make it possible to make use of the entire available gradient between the working pressure of 100-200 atmospheres and the exhaust back pressure of 0.4 atmospheres. A low specific fuel consumption is necessary which in mercury-vapour plants has already attained 170 g./h.p./hr. Such improvements already challenge Diesel installations. While the flexibility of steam power plants is as yet far behind that of internal combustion engines, a further pro gress in this direction is possible. Moreover, since they may be mainly destined for larger types of aircraft, it is not necessary to achieve that high flexibility which is required for fighter aircraft. The necessity of regulation and maintaining a substan tial power output at any altitude is evident. A suggestion to supercharge the boiler as a whole and, similar to the practice adopted for supercharged internal' combustion engines, expand the combustion gases issuing from the boiler in an exhaust turbine, thus recovering the power required to operate the boiler-supercharging blower, appears to go a long way towards improvement. Air heaters utilising the waste heat can also be coupled in series behind the boiler, and their exhaust further used in reaction ^ nozzles to produce supplementary thrust. (Knoernschild). Finally, there is the problem of compactness. This is an important factor not only from the weight aspect but also because compactness is an important factor in aircraft design. Here the main problem of a suitable condenser so designed and placed as to produce a minimum of drag is urgent. While American experiments tried to solve it by a "wing radiator" it was suggested (Meredith) that com pletely enclosed condensers with forced draught are prefer able. It was further suggested that in such arrangement and with a suitably adapted cooling-air intake and discharge orifices, a portion of the condenser heat could be turned to produce extra thrust. While it is not attempted here to advocate the employ ment of steam propulsion for aircraft or to indulge in pre dictions, it should be realised that considerable progress has been achieved in this direction. During the past two decades the internal combustion engine has acquired a monopoly in the propulsion of aircraft. Experiments and researches with steam propulsion shown briefly in this summary were rather limited to single efforts without that mighty impetus which is derived from production on a larger scale. The results in steam power development achieved) despite such handicaps seem to point out that this devel opment has not reached its final stage.
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