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Aviation History
1928
1928 - 1066.PDF
NOVEMBER 15, 1928 MACHINERY INSTALLATION OF R.101 "IT is machinery reliability which more than any other factor has limited the use of past airships. It is the use of petrol which . . . has constituted the greatest fire risk in connection with their use." With these two statements Wing-Comdr. T. R. Cave-Browne-Cave commenced his lecture to the joint meeting of the Royal Aeronautical Society and the Institution of Automobile Engineers on November 8, and in so doing he gave his raison d'etre for the use of heavy fuel-oil engines and their installation in the new rigid airship, R.101, which is being erected at the Royal Airship Works at Cardington. Safety had, the lecturer said, been sought by the use of Beardmore heavy-oil engines, and reliability, although possibly prejudiced by the use of such new engines, had been sought in the design of the installation and in the preparations made for testing. Each engine was carried in a self-contained unit, which could be easily changed for another complete unit while the airship was at the mast. A unit could be mounted on a gantry and tested under conditions essentially similar to those obtaining during flight. It had been hoped that the engines might have been available in time to allow a unit to undergo such thorough tests and development that by the time the airship itself was ready for flight the engines would cause no uncertainty in the mind of the captain. Owing to difficulties experienced with torsional resonance this hope had not been realised, but he submitted that the delay, although unfortunate, had not justified turning aside from what must be accepted as the safest system, and eventually, perhaps, the most economical also. After describing the placing of the five power cars on R.101—a single car aft, on the centre line, to give slipstream on the rudder for approaching the mooring mast, and two pairs " outboard," ahead of and aft of the passengers' quarters respectively—the lecturer gave a brief outline of the design of the Beardmore " Tornado " engines. Originally it was hoped that each engine would develop 700 b.h.p. at 1,000 r.p.m., but the engines were now to develop a continuous full power of 585 b.h.p., with a maximum of 650 b.h.p. The paramount merit of the engine was that it burned oil of such a high flashpoint that the fuel had to be heated to the temperature of boiling water before it would give off any inflammable vapour at all. The " Tornado " engines were of the eight-cylinder " in-line " type, with a bore of 8-25 in. and a stroke of 12 in. Such an engine gave a long crankshaft, which had been found to be subject to torsional vibration. Diagrams obtained with the torsio- graph showed a variation of plus and minus 30 times the mean torque at a speed of 950 r.p.m., but this was exagge- rated by the instrument, and after improvements had been made half that variation was recorded. Maj. Carter had now evolved a formula whereby the dangerous resonant speed could be predicted from the drawings of the shaft. New shafts had now been designed which were so much stiffer that the major critical speed fell outside the normal running range, but it was not possible to keep clear of the minor critical speeds, and a damper flywheel was being fitted at the end farthest from the airscrew. Further, a spring coupling was being fitted between the shaft and the airscrew so calculated as to bring the major critical speed fax below the normal running range. The method of fuel control was then explained. The consumption of the engine at its continuous full power was 0-385 lb. per b.h.p./hour. This was a saving of 30 per cent, as compared with the petrol engine, and, moreover, the consumption decreased slightly for a decrease in power. The engine weighed about 8 lbs. per h.p., but there was no reasonable doubt that this weight could be halved when the design had undergone refinements. A 40-h.p. auxiliary engine started the main engines through a reduction gear of 20 : 1, and it had been found that less than 10 h.p. was required to run a main engine up to the 100-120 r.p.m. at which it started. The lecturer then described the hub of the variable-pitch airscrews to be used, and which were capable of being turned about their axis so as to give ahead or astern thrust or to be in a neutral position. Hollow-steel blades were used in the tests, but owing to the abnormal torque fluctuations due to resonance it had now been decided to use solid alloy blades, the root strength of which could be greatly increased. To provide for proper support of the power cars it had been necessary to lead thrust wires to a bearing carried on the outer face of the airscrew hub. Evaporative engine cooling was being used, and it was hoped ultimately to pass the steam from the separator to fabric duct condensers formed in the outer cover and so avoid extra drag. Until the ducts could be thoroughly tried out, however, triangular honeycomb radiators were being provided. The waste steam from the two amidships engines was led to a large retractable radiator, which could be drawn up into the discharge of the ventilating fan of the passengers' quarters. Originally it had been intended to use waste steam for cooking, and a boiler evolved for this purpose was described, but it had now been decided to do the cooking by electricity. A constant-speed windmill for driving auxiliaries when the air-speed was above 40 m.p.h. was described. The power developed by this windmill was 12 to 15 h.p. The lubricating oil was carried in two shaped tanks in the car structure of each engine, thus avoiding the necessity for drawing oil from storage tanks in the hull. The larger of these tanks was fitted with a baffle, so that a comparatively small quantity of oil was in general circulation, and would, therefore, warm up quickly. The heat given to the oil was large in the " Tornado " engine, presumably because of the large surface of the crank-case kept at almost 100° C. by proximity to the cylinder jackets. The oil-wetted surface of the crank-case was, however, correspondingly large. Violent circulation of air through the crank-case was a very effective method of oil cooling. It was proposed to draw the whole of the induction air through the crank-case, and a satisfactory apparatus was made. Some doubt was raised as to the safety from fire, and as the risk could not be wholly disproved the system was abandoned in favour of the con- ventional Potts oil coolers. Accessibility had been carefully studied, and the engineer in the power car was able to pass along the full length of both sides. It would be possible, in flight, to remove a cylinder head, piston and connecting rod. The fuel storage system in the hull had not only to provide a supply of fuel to every engine car, but must make it possible to move large quantities fore and aft for trimming purposes. The storage tank unit was one of 224 gallons, with a clearance volume of 10 per cent. From the storage tanks the fuel drained by gravity to a transfer tank, from which it could be blown by compressed air to feed tanks situated above each engine car. At the bottom of each feed tank there was a steam box by which the oil could be heated to flow more easily to the engine car. The capacity of the fuel system was 29 tons, which left a considerable margin above the 22 tons required to reach Egypt in 40 hours at an airspeed of 76 m.p.h. against an average head wind of 15 m.p.h. Compressed air was also used for trimming water ballast. Certain selected fuel tanks were fitted with special jettison valves which consisted in wells in the bottom, sealed by discs of 28-gauge aluminium. Cutters operated by hand and travelling around this disc, somewhat after the style of the familiar tobacco tin opener, cut away the disc, and the fuel issued as a smooth stream of 12 in. diameter, which did not break up until well below the level of the airship. The fuel pipes in the hull totalled a length of over 1,500 ft., and the water pipes over 500 ft. There were over 100 cocks, and the cocks and pipe joints had presented some difficulty. A development of the " olive " joint had been adopted, in which duralumin flanges with bolts took the place of screwed nipples. For a commercial airship it did not appear economically sound to fit water recovery apparatus solely for reducing the quantity of hydrogen that must be replaced after each voyage. If the surplus of hydrogen could be burnt as fuel it would reduce the quantity of oil carried and made water recovery easier from the engines in which it was burnt, and apparatus of reasonable size was likely to be satisfactory. Experiments made had shown that hydrogen introduced with the air could be burnt with fuel oil in so high a propor- tion that all the gas which became available from the use of oil in five engines could be burnt, with oil, in two engines. This would effect a saving of about 20 per cent, of the total oil required for a given power. This was convenient, because onlv two engines need be fitted with recovery apparatus. Wing-Commander Cave-Browne-Cave concluded by giving a demonstration of the non-inflammable nature of the heavy- oil fuel. A quantity of petrol was poured into a vessel and ignited. Some of the fuel oil was then poured on the burning petrol and extinguished the fire ! 988
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