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Aviation History
1926
1926 - 0168.PDF
MARCH 11, 1926 AMERICAN AERONAUTICS" (Concluded horn page 98) OWING to lack of space it has not been found possible to publish until now the concluding instalment of our article on " American Aeronautics," which was commenced in our issue of February 18, 1926. The first article dealt with the general constitution and organisation of the American National Advisory Committee for Aeronautics, and a commencement was made by referring briefly to the work done during 1925 by and on behalf of the Sub-Committee on Aerodynamics at the Langley Memorial Aeronautical Laboratory, where most of the aerodynamic research work is carried out, at Stanford University, at the Washington Navy Yard, at the Bureau of Standards, at the Massachusetts institute of Technology, and at McCook Field. The following notes deal briefly with the work of two other sub-committees, viz., that on power plants for aircraft, and that for aircraft materials. The Committee on Power Plants for Aircraft, it might be mentioned, has direct control of the power-plant research conducted at Langley Field, and also of special investigations authorised by the Committee and conducted at the Bureau of Standards. At the Langley Memorial Aeronautical Laboratory the study of the application of fuel injection to the two-cycle engine with spark ignition has been continued and broughtto a conclusion. The induction system of the Liberty engine cylinder was further modified, which gave improved results. The study did not include design development necessary to promote satisfactory operation at low speeds and loads. Ii was found that 53 b.h.p. could be consistently developed with the modified Liberty cylinder at 1,300 r.p.m. (116 lb. 'sq. in. B.M.E.P.) using a scavenging air pressure of 5J lbs. per square inch. Only 28 b.h.p. is obtained with the standard Liberty engine at the same speed. The application of fuel injection to four-cycle aero engines operating with Diesel engine fuel oil and using the heat of compression for ignition, has been further studied. A special steel cylinder has been used to stud ' combustion chamber forms. A pre-combustion or bulb type cylinder head has been tested at speeds up to 1,800 r.p.m., using an eccentric- operated pump, and an automatic diaphragm type injection valve. Brake mean effective pressures up to 88 lbs. per square inch and fuel economies comparable to those of present- day aircraft engines have been obtained with an injection advanced angle of approximately 11' before top dead centre. A limited number of tests have been made to determine the effect of variation of the injection rate on the power output and fuel economy. A second head of slightly concave cross- section and arranged for injection of fuel directly into the cylinder is being tested. A cam-operated fuel pump and a spring-loaded type of injection valve are being used in this work. As a result of the tests it is thought that the pre- combustion chamber type of cylinder head is the more pro- mising of the two for application to aero engines. A good deal of research work has also been carried out at this labora- tory on fuel injection pumps and valves, on characteristics of fuel sprays, and on fuel characteristics. Super-charging is a subject which has for a good many years received attention in America, and the following reference to tests from the report of the Power Plants Sub-Committee is of interest. Comparative climb performance tests with and without super- charging of a D.T.2 seaplane, carrying equivalent military loads up to 2,000 lbs., have been completed, the super- charger fitted being of the Roots type. The results have shown that a material improvement in the performance of this type of aeroplane could be obtained by supercharging, even when heavily loaded and operating to moderate alti- tudes. It was found that when maintaining practical sea- level pressure at the carburettor at all times when super- charging the absolute ceiling was increased 90 per cent, when operating without military load, and 50 per cent, when operating with load. The service ceiling was increased about 80 per cent, for all loads, while the average rate of climb to the service ceiling was the same. The climb when super- charging was inferior at low altitudes, owing to the use of large propellers. What is claimed to be the first successful supercharging of an air-cooled engine at high altitude has been accomplished at the Langley Memorial Laboratory, using the Roots type supercharger. Further tests with the Lawrance (Wright) J-l engine with Roots supercharger in a T.S. land 'plane have been completed. When using the same propeller and * Extracts from the Eleventh Annual Report of the Anrerican National Advisory Committee for Aeronautics. maintaining full supercharging to 16,000 ft., the original service ceiling of 16,100 ft. was increased 65 per cent. The absolute ceiling was increased 56 per cent., the time to 16,100 ft. was reduced 59 per cent., and the average rate of climb to the new service ceiling was 43 per cent, greater than to the original ceiling without supercharging. Additional information of the effect of supercharging on the cylinder head temperatures of each of the nine cylinders has been obtained, giving maximum recorded temperatures for the various cylinders ranging between 500° F. and 560c F. Close examination of the engine revealed no undue wear or other ill-effects as a result of supercharging. Other experiments with a Roots type supercharger on an engine having somewhat different cylinder construction have also shown a considerable increase in performance, appar- ently obtained with no detriment to the engine and without encountering excessive cylinder head temperatures. A study has been continued of the relative performance of the normal compression engine, the high compression engine, and the supercharged normal compression engine. The results have shown that the maximum power output of the high compression engine operating on domestic aviation petrol at sea-level is obtained by maintaining full throttle and retarding the ignition timing sufficiently to suppress detonation, although the fuel consumption with this method is high. The results of these tests will be used in connection with a programme of flight tests which is being conducted to deter- mine the relative performance of a service type seaplane of the load carrying type, equipped with a normal com- pression engine, a high compression engine, and a super- charged normal compression engine, all having the same displacement and being used both with direct and geared propeller drives. At the Bureau of Standards engine tests under approximate altitude conditions have been carried out by reducing the pressure at the entrance to the carburettor and at the exhaust port, exactly as in an altitude laboratory test, but allowing the air surrounding the engine to remain at sea-level pressure. Similar tests were made under " true " altitude conditions, i.e., with the air surrounding the engine reduced to a pressure corresponding to the altitude. It was concluded that pro- vided certain precautions were taken, satisfactory results could ordinarily be expected with the approximate type of tests. New Engine Types Both the Bureau of Aeronautics of the Navy Department and the Engineering Division of the Army Air Service have continued their efforts towards an increase in the reliability of aircraft engines, and the two organisations have co-operated closely in this development. A striking piece of work on the part of the Air Service is the new air-cooled Liberty. The report states that this engine has demonstrated on test that the air-cooled in-line engine will be one of the important developments of the future. A description of this engine was published in FLIGHT of January 21, 1926. In this report is found the interesting statement that the Wright model P-2, the 400-h.p static radial air-cooled engine, which incorporates the fan type supercharger for rotary induction purposes, has passed its acceptance tests with very excellent performance. Twelve of these engines have been ordered for flight-testing purposes. In view of the keen rivalry between air-cooled and water- cooled engines, the following paragraph from the report of the Power Plant Sub-Committee is of interest : " The rise of the air-cooled engine with important reduction in power plant weights, due to the elimination of the cooling system, has forced the water-cooled engine to new endeavours. Increased power has brought about a balance on the basis of specific power plant weights between the two engines. This will undoubtedly force the air-cooled engine into the higher speeds and reduction gearing. Since results indicate that a large percentage of power plant failures are due to faults in gasoline, oil and water lines, the air-cooled engine still has important advantages. This fact accounts for the energy which is now being put into development of the three air- cooled engines for the Navy." The reference in the last sentence is to three Wright engines, the J-4 A of 800 cub. in. capacity, which will be used as a training engine, the new R-1,200 cub. in. engine which will be used in the observation and fighter class, and the 1,600 cub. in. P-2 intended for 148
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