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Aviation History
1949
1949 - 0167.PDF
JANUARY 27TH, 1949 FLIGHT REFUELLING IN FLIGHT Development and Economics of a System which Increases Payloads and Reduces Maintenance ^ NOTWITHSTANDING the con-siderable accumulated experi-ence in the technical aspects of refuelling aircraft in flight, neither the Ministry of Civil Aviation nor the Cor- porations has come to any decision as to the adoption of the system on the long-distance air routes. The reports on the B.S.A.A. trials over the South Atlantic and those of B.O.A.C. over, the North Atlantic are apparently still being - analysed. In the meantime the U.S. Air Force has been making good progress with this method of extending bomber range, and there is little doubt that it will be rapidly developed in America. The main principles of refuelling in flight have been described from time to time in this journal, but in the paper which he'read before the annual meeting of the Society of Automotive Engineers in Datroit, on January 13th, Mr. C. H. Latimer-Needham, chief engineer of Flight Refuelling, Ltd., described the system in great detail, and examined the economics of refuelling in flight. On the assumption that our readers are already familiar With the main prin- ciples of the system, we refrain from quoting those parts of the paper which describe its features and outline the history of its development. It is, how- ever, surprising to find that although the system is in this country always linked with the name of Sir Alan Cob- ham, the Americans have done a great deal of work in the past, before Sir Alan founded Flight Refuelling, Ltd. No one has, however, continued development work as persistently as he. Hitherto the flow of fuel from tanker toreceiver aircraft has been by gravity. This means that the tanker has to fly at a higher •_.level than the receiver. Pumping would have the advantage of giving a greater rangeof formating positions and might make pos- sible a reduction of hose length and drag,and increased rates of flow. Mr. Latimer- Needham outlined a pump system which,for the expenditure of one kilowatt, enabled formating height to be reduced from 75 to50ft with the same rate of flow through the same length of hose. Jet-propelled aircraftwill require a much higher rate of flow than the present 100 to 120 gallons per minute,and developments are in hand aimed at rates of 250 to 300 gallons per minute. A large part of Mr. Latimer-Needham'spaper was devoted to the economics of flight refuelling. There is, he said, a basic still-air range of approximately 3,000 miles,vyhich is sufficient to enable all air- liners to operate on flight-refuelledsen-ices over most of the world's air routes, irrespective of total routelength. Three vital corollaries fol- lowed from this fact: — ^ (i) For any given payload, the re- -1 quirements of all long-distance routes Q can be met by a single aircraft specifi- g cation, leading to standardization of -3 design aiid of air fleet. <t (ii) The relatively short-range re- quirement exercises a profound in- fluence on the size of airliner, enabling it to be considerably smaller than one designed to cover the route non- stop without flight refuelling. (iii) By restricting the fuel load to a practical proportion of the aircraft gross weight, a high payload factor is ensured. Mr. Latimer-Needham then ex- amined the world's air routes, and outlined possible arrangements of refuelling points on some of them. London-New York is one of the most important, and it was found that an airliner should ideally have u fuel capacity of rather less than half the disposable load. The accommodation for passengers and freight should be such that its own weight plus the payload accounted for the remainder of the disposable toad. This was, he said, a fundamental considera- tion. It might be affected by frequency of service, but the fifty-fifty fuel/payload rule gave maximum efficiency. This study, Mr. Latimer-Needham said, led to the conclusion that aircraft should be designed in ranges of progressively stepped-up sizes so as to cater for, say, 50, 5O.OOO 40.000! 30,000 20,000 1O.OOO 3OO A/C GROSS WTXIO' Fig. 2. Payload of flight-refuelled passenger airliners on the London-New York route. the problems associated with the design and construction of giant aircraft were avoided.Another benefit arising out of flight re- fuelling was the reduction iri airport size andrunway lengths, as the machine takes off relatively light. Mr. Masefield had shownin his lecture how cost of aircraft per pound of equipped weight rose with increase in size.This was avoided with flight refuelling. Headwinds were another bugbear of long-distance non-refuelled flights. With flight refuelling, it would often be possible toachieve full capacity payload on both west- ward and eastward flights. Intermediate landings for taking on fuelcaused delays. With the greater speeds ex- pected, this became more serious. At an LONDON - NEW \ORK UNREFUELLED 100, 150 passengers, etc. The fuel load, , average ground speed of 450 m.p.h., a two- hour delay (one hour for each intermediatestop) in a journey time of 10 hours would reduce the effective speed to 375.5 m.p.h.,a loss of nearly 100 m.p.h. Every take- off and landing also represented an extraelement of risk, and the greater part of engine wear and tear took place duringtake-off. During the B.S.A.A. and B.O.A.C. experiments, it was found that the receiveraircraft required a remarkably small amount of maintenance. In order to get the maximum possiblebenefit from flight refuelling, it was neces- sary to design the aircraft specially for thejob. A flight-refuelled transatlantic airliner to cam- a passenger payload of 50 wouldweigh about 100,000 lb. If 100 passengers were to be carried, the gross weight wouldbe about 160,000 lb. Payload percentages for the two cases would be about 13 percent W and 16 per cent W respectively. Mr. l.atimer-Xeedham argued that three generictypes of airliner would meet the require- ments of air transportation on practicallyall the world's air routes. Thev would have payload capacities of 50, 75 and 100 passen-gers respectively. Their main particulars would be: — 1000 3.OOO 4.OOO 5.OOC RANGE - STATUTE MILES Fig. I. Effect of range on aircraft size for constant payload. being roughly equal to the useful load, wouldbe found to give an unassisted range of approximately 3,000 still-air miles, and thiscould be extended to any desired distance by means of flight refuelling. ' -. Range and Size The effect of range on aircraft size wasdealt with next. It is shown graphically in Fig. 1, in which payload is assumed con-stant. It is seen that the rate of increase in size beyond the " ideal " becomes in-creasingly steeper. Thus.the aircraft of 5,000 miles' range designed for the London-New York route, unrefuelled, is three times as large as the ideal airliner. It was, hesaid, now easy to see how the quest for long range was likely to lead to large-sizeuneconomical aircraft. Fig. 2 is based on a number of designsprepared for the flight-refuelled North At- lantic route. It is seen that payload, ex-pressed as a percentage of the gross weight, increases as the weight in-creases, and reaches a figure of 15 per cent of W at a gross weight of about 125,000 lb. At 250,000 lb the payloadamounts to as much as 17 per cent \V. The payload also increases withgross weight for flight-refuelled air- liners, in contradistinction to thediminishing payload of unrefuelled aircraft designed for long-distanceroutes. Mr. Latimer-Needham calculatedthat for the North Atlantic route a flight-refuelled aircraft need be onlybetween one-half and one-third the size of one designed to take off withsufficient fuel for a 5,000-mile flight. This was important because theinitial cost of an aircraft, the cost of insurance, fuel and oil, and mainten-ance, were all roughly directly pro- portional to the size of aircraft. Also No. ofpassengers Gross wt. (lb) . .Pavload (lb) .... Payload % \V ....Wing area— (65 lb/sq ft) .. Wing span (ft) (A.R.10) 50 100,000 13,000 13 1.540 124 75 130,000 19,500 15 2,000 142 100 160,000 '26,000 ]6 2.4*'5 157For North Atlantic traffic it was estimated that five airliners, each carrying 50 passen-gers and making two daily trips, or ten daily departures in all, would meet requirements.A flight-refuelled airliner for 50 passengers would be relatively small, and would pre-sent no new difficulties or experimental fea- tures. Such an airliner had been made thesubject of a design study, and Mr. Latimer- B 10
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