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Aviation History
1952
1952 - 1218.PDF
532 FLIGHT, 2 May 1952 MODERN FUEL SYSTEMS... shallow tanks with many baffles. Such gauges were useful in their present state, but would only begin to fulfil their true function when the maximum overall tolerance of accuracy under flight conditions could be kept within ±2 per cent, with ±£ per cent as the eventual aim. The lecturer then discussed the mechanical details of fuel-system fittings, and pointed out that unions and fuel cocks should be provided with the full nominal bore. There also appeared to be great scope for the design of a light, flexible, fire-resistant hose which would obviate the necessity of making special end-fittings to suit a particular angle of take-away. The difficulty of making joints leak-proof against keiosine was well-known. In the lecturer's experience, considerable efforts had been made to find a flat-face seal remaining tight down to —40 deg C, and yet which did not squeeze out under the pressure of the joint when warm. Several solutions appeared possible based upon the principle of dropping a rubber sealing ring into a space into which it would fit so as just to prevent the rubber flowing. 16,000 § 12,000 5 8,000 8 z 4.000 N . as. *JTR t 1 AN! ^ ,-- /P Off ' Ol JTBC STA u s * 3AR RBO JTB D ARC JA^ 1 U 40 SO 120 160 200 240 280 320 360 TIME AFTER TAKE-OFF(min) Fig. 3. Fuel in Comet G-ALVG during a flight on February 21st, 1950. Mr. Walker then examined the factors governing the design of a suitable fuel system for a turbine-engined aircraft. The necessity of finding space in an aircraft for ever-greater quantities of fuel while reducing weight had led to the present practice of using the aircraft structure to contain the fuel, either directly by making spaces in the structure sealed compartments, or by lining these spaces with a thin- walled flexible bag, the self-contained rigid tank having been relegated to the outside of the aircraft in the form of wing-tip or pylon tanks. Much disappointment had been experienced with integral tanks, but if the right approach was made to the design, they could possibly give less trouble than any other form of tank. In particular, it was important that the basic structure should be dictated by the tank and the sealing problem in minute detail be given full consideration from the beginning of design. As a general principle, the designer should aim at achieving sealing without the aid of plastic sealant, such a sealant being a second line of defence only. While bag tanks might be fitted where structural problems prohibited the use of integral tanks, they had further advantages in being unaffected by normal structural deflections and were possibly more resistant to failure in the case of a crash. But great care was necessary in the storage, handling and fitting of such bags, most leakage trouble being due to lack of care in such respects. Consideration had also to be given to venting in order to prevent pressure inside the ba^ falling below that in the tank compartment, to avoid the bag sucking in and putting heavy local strain on the attachments and forcing fuel out of the vent pipes. Equal care had to be accorded the design of the bag compartment to provide complete support for the tank within a smooth interior free from cutting edges. Methods of attachment of the bag had presented some difficulty in the past; Mr. Walker thought positive fixing studs attached to the structure from inside the bag provided the best solution. In laying out the fuel system a principal point to be borne in mind was that the turbine was more particular than a piston engine with regard to the amount of air or vapour which could be delivered with the fuel. A suction feed could be dangerous, particularly at low levels, while a second cause of air in the feed line was either low pressure, or high line velocity and local restrictions in the pipe. Such effects could usually be eliminated by provision of tank booster pumps. The most difficult condition to be designed against was that of negative g, although the Comets, throughout their flying, had shown no need for such con sideration, and on ground test the engines had regained normal running after the feed pipe had been opened to full air bleed for eight seconds. The difficulty of feeding fuel from thin swept-back wings without incurring undue weight and complication needed careful consideration. In addition to providing at least two feeds from each tank to deal with different flight attitudes, complicated baffling was required to control the travel of fuel in the tanks. Venting and fuel pressure then became correspondingly complicated, and high loads due to fuel head could be imposed on the tank walls. It might be that a new method of feed, such as "squeezing" the fuel from a tank might be possible. The approach to the design of the fuel system for transport aircraft differed from that for fighters. The former should be as flexible in operation as possible, while the latter type of aircraft required a system fully automatic in all respects. This latter point unfortunately led to a growth of complication on an aircraft least able to take it, and the designer might well ask whether the steadily growing requirements were not defeating the object of producing a useful fighting unit. Although vents should be simple stacking pipes they did, in fact, present the most difficult problem of the whole fuel system. Mr. Walker enumerated the conditions under which the vent had to operate, and said that the aim should be to achieve all the requirements with a simple pipe run, resorting to more complicated devices only when these were essential. Swept-back wing tank-systems could become extremely com plicated due to the lack of any consistently high place in the tank from which the vent outlet could be taken. In considering tank supercharging, it would be seen that while kerosine required no such pressure-rise to prevent boiling, wide-cut gasoline required up to 2J lb/sq in to meet the conditions of reduced pressure at 50,000ft altitude and 45 deg C temperature. Civil aircraft could well do without the additional complication of a pressurized vent system unless it was justified as a means of emergency fuel feed or to increase jettison rate. On the other hand, military aircraft with high rates of descent demanded such rates of inward venting to the tanks; with non- supercharging, vent sizes became extremely difficult to accommodate without producing water traps and restrictions. Supercharge by air or inert gas was necessary in such cases where depression could not be permitted. The wing design, however, had to allow for the supercharge pressure, since this was additive to the normal wing surface suction pressure. Although Mr. Walker admitted having little experience of the relative effectiveness of the various means of preventing tank explosion, he discussed the subject because, he said, previous efforts to find reasonable accommodation for such systems had been rather discouraging; but there was no case as yet to justify the fitting of tank protection on civil aircraft, particularly if kerosine was the fuel. After discussing several known methods of tank explosion prevention, the lecturer spoke of the R.A.E.-developed suppression system, which appeared to be a brilliantly conceived solution to a difficult problem. Much development was required, however, before it could be effectively installed in civil aircraft. Refuelling the Comet [see Flight, February 1st, 1952] was controlled by valves developed by Flight Refuelling, Ltd., placed as low in the tank as possible, both so that the ingoing fuel did not agitate the suiface of the tank when the tank was nearly full, and also so that the same valves could be used for off-loading. Two automatic cut-Off levels were provided in this aircraft, the first operating when the tanks were nearly full, and the second after topping-up at a reduced fuelling rate; blow-off valves were also provided to guard against unserviceability of the cut-off switches. The greatest difficulty with automatic cut-off systems was to position the switch high enough in the tank. As with vents and fuel feeds, this problem was accentuated with shallow flat tanks and swept- •30 SOOi 40 eo 120 160 200 240 TIME AFTER TAKE-OFF (mlp) 220 320 Fig. 4. Fuel temperatures during the fight to which Fig. 3 refers. A=Port outer tank, 4yin above chord- D—Feed pipe to port engines. line. B = Port outer tank, bottom. C = Starboard outer tank, bottom. E=True air speed. F—Ambient air temperature. back wings. A pressure-operated cut-off might be better than the float-operated switch, but it would be more difficult to control the air space at varying fuelling rates. . The final section of Mr. Walker's paper dealt with the very com prehensive testing which modern fuel systems required. It paid to do as much rig-testing as possible before the first flight. With the Comet, a small quarter-scale model was first made of the wing, with accurately positioned tank compartments roofed with Perspex. This should be supplemented with a full-scale rig rotating in the pitching and rolling planes. If the rig testing was thoroughly carried out, little time needed to be spent on the prototype aircraft before flight. It was unlikely, however, that all flight conditions could have been covered on the ground, and some flight testing was essential. Much work had been done by the R.A.E. and Rolls-Royce, Ltd. in establishing the minimum temperature to which fuel would fall when operating at high altitudes. With respect to the warning shown by the results of these tests, no trouble had been experienced with Comets during their many hundreds of hours at high altitude. Fig. 3 showed the way in which fuel had been used during a flight in which careful measurements had been made of the wing skin and fuel temperature on the first Comet, the results of the fuel-temperature measurements being shown in Fig. 4. It was to be hoped that these results would shortly be supplemented by those of other aircraft; it could be said that these tests were most carefully conducted, and could be taken as representing the conditions prevailing on this aircraft. Taking the absolute minimum permissible fuel temperature as —45 deg C, the Comet could fly in temperatures of down to —70 deg C. A lowering of the specification freeze-point of kerosine by 5 deg C or more would be a valuable contribution to operation at low temperatures.
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