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Aviation History
1938
1938 - 2156.PDF
SUPPLEMENT TO FLIGHT d 48 THE AIRCRAFT ENGINEER JULY 28, 1938 -50 C, a fall of 210 C, whilst owing to the short time elapsed the fuel temperature has only dropped 2 or 30 C. Hence the fuel temperature is approximately 18 or 19° C. higher than the surrounding air, a definite incentive to the release of dissolved air and natural vapour. In addition the surrounding air pressure has, in round figures, dropped from 30" Hg to 20" Hg, and this combination of reduced temperature and pressure causes at the slightest provoca tion a discharge of vapour in the same way as is evident when the stopper is removed from a charged mineral water bottle. The question of temperature difference can be minimised to a certain extent by having the piping exposed or near the skin of the aircraft, but one salient necessity remains, and that is the complete avoidance of suction heads any where in the fuel supply system. Location of Fuel Pumps Attempts have been made to isolate the pumps from the engines and at the same time reduce their suction heads by driving them with flexible shafts, but except in short lengths, straight runs and low duty conditions, they are by no means perfect. It is obvious that the only satisfactory .method is to place the pump at some level below the lowest tank so that the pump is what is commonly described as " drowned." There will then always be pressure throughout the fuel supply piping and the danger of air being drawn into the pump from an emptied tank is minimised. Remotely Operated Pumps Pumps operated at some distance from their prime source of power have been tried and can be classed as follows :— (I) Mechanical drive by a reciprocating or rotary flexible shaft. (II) Hydraulic drive from engine-driven pump. (III) Pumps driven by electric motors obtaining their motive power from generators, either driven direct by the engine or by a small separate power unit similar to a self-contained air compressor unit as in (V). \J-rn -CARBURETTER o 2s CONTROL VALVE EXHAUST AIR FUEL PUMP from AIR COMPRESSOR FUEL TANK J) m Fig. 6. Layout of fuel pump, control valve and carburetter. The Hobson compressed air operated fuel pump and its control valve is a remote-acting pump that has passed official tests, is self-regulating and can be installed anywhere in the aircraft, the only connection between it and the engine being a single low-pressure pipe line, the exhaust air being discharged overboard. It functions either in a vertical or horizontal position and if centrifugal force lowers the fuel pressure in the pump feed pipe, the control valve instantly speeds up the pump to regain the correct pressure. Air can be obtained free of cost in unlimited quantities, is non-inflammable and is, therefore, an ideal source of motive power, and if such a pump is placed below the lowest fuel Fig. 7. The Hobson fuel pump (left) and control valve. (IV) Pumps of the windmill type, i.e., driven by a small propeller, the pumps being mounted in such a position that they are in the propeller slip stream. (V) Compressed air drive from engine-driven air compressor or a small motor-driven air compressor. Method (I) is not satisfactory, especially in large aircraft. Method (II) has been tried and abandoned. Method (III) adds to the already complicated wiring system, is costly and heavy and the motors must be of the flame-proof type. Method (IV). These are reasonably satisfactory, but are subject to adverse climatic conditions and are projections which are unpopular on present-day high-speed aircraft. Method (V) is proved to be satisfactory, light in weight and with the increasing use of engine-driven air com pressors are particularly adaptable to aircraft, especially those of large dimensions. tank, nowhere in the system can a negative pressure exist in the fuel lines. Fig. 6 shows the layout of this pump and control valve. The latter controls the compressed air supply to the pump and is itself controlled by the fuel pressure below the float chamber needle seat in such a way that the greater the demand of fuel from the float chamber the faster the pump operates, the pressure at the needle seat being kept within a limit of T i lb. per sq. inch of the desired pressure. The volume of free air used per gallon of petrol pumped is slightly over 0.4 cubic foot, the pressure required is approximately 10 lb. per sq. inch above the pressure head to which the pump is delivering fuel and its capacity is 2,000 h.p. with some reserve for pumping from one tank to another at the same time. Fig. 7 shows the actual pump and control valve, the overall length being 10\ inches and the total weight 7 lb.
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