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Aviation History
1946
1946 - 0919.PDF
MAY 9TH, 1946 FLIGHT 473 Jet Propulsion for Civil Aircraft PAPER BY MAJOR F. B. HALFORD, F.R.Ae.S., M.S.A.E. Hypothetical Air Liner for High-speed, High~altitude Operation : Axial'flow Power Units : Performance and Costs ^N Wednesday, May ist before an audience of tech-nical and non-technical members and guests of the Royal Society of Arts, Major Frank Halford, de-signer of the D.H. Goblin and Ghost engines, read his paper on Jet Propulsion. He commenced by reviewingvery briefly the history and working principles of jet power units and then turned his attention to pure jet propulsionfor civil airliners. Major Halford dealt simply and concisely with the advan-tages, requirements and problems associated with aircraft as a means of transportation under headings of speed, drag,inefficiency of airscrews at very high speeds, and the effi- ciency, almost limitless power and simplicity of jet units.His explanation, in simple terms, of compressibility and its kindred problems was excellent. " Aircraft and Engines—One Problem In more detail, Major Halford dealt with the need forair-conditioning for high-altitude flying and then described the features of an imaginary airliner that could be builtduring the next ten years, the time depending upon the men and money devoted to such development. "Let usconsider a machine of igo,ooolb all-up weight," he said. "First of all it is quite impossible to split the aircraft andits power plant into two separate problems. If we are to achieve the optimum overall result the two problems mustbe regarded as one." In choosing four power units for the hypothetical aircraft he added, "If we adopt more thanfour it would entail more complication and maintenance than is necessary. If we adopt less than four (for long-range work) we should not be in a happy position in the event of one unit failing in the air: Four, therefore, appearsto be the ideal number, providing the units can be installed Nickel or Complex Iron 6ase Alloys 600 1935 1940 I94S 1950 YEAR 1952 I960 TRANSATLANTIC TYPE General Characteristics Total weight 190.000 Ib Wing loading 58 Ib/sq ft Engine static thrust (nominal rating) 4 X 12.000 Ib Payload—non-stop \ , c nnn ik / 50 passengers and luggage at 230 Ib New York-London / I3)UUU 1D — \ +3,500 Ib mail and/or freight Cruising speed 615 m.p.h. at 40,000 ft (M=0.925) Take-off run, still air, hottest day anticipated in New York (104 deg. F.) 2.000 yd to 50 ft Journey Times and Costs (Allowing for average westerly wind of 50 m.p.h.) York S.oTp.ntlE.S.T. lapsed time 7 hr 6 min AXffiJ0* If.Rfl'a^.^ ^^ed time 6hr .4 min Single fare ; Approx. £60 The trend of development of blade materials for gas turbines.The curve is based on the assumption of 300 hours' life at full duty with a blade stress of U tons/sq in. without adding appreciable external drag, and we willplace them in the wings as we shall need all the fuselage space for passengers, mail and other purposes." With regard to the selection of the jet units he said."Now at, say, 40,000ft, the aircraft in view needs a total thrust of io,2oolb to propel it at its selected cruisingspeed. This means that each of the four units must be designed to give a thrust, at 40,000ft of 2,5501b for theminimum possible fuel consumption. Bearing in mind what has been accomplished to date, and allowing for im-provements which can reasonably be expected in the near future, I suggest that the following design target figuresneed not be considered unreasonably optimistic for a jet engine cruising, say, at 40,000ft at 600 m.p.h. Expansion ratio . . .. 15 :1Compressor efficiency . . 85 per cent Combustion chamber "drag" efficiency 95 per centSpecific fuel consumption 0.88 lb/hr/lb thrust. On the subject of the take-off possibilities of such a unitMajor Halford said: "Here we come up against a design 'critical.' It has become common knowledge that one ofthe main difficulties which laced Air Commodore Whittle and pioneers of the gas turbine was the lack of suitable-material for the turbine blades, which have not only to operate in gas temperatures of over 600 deg C, but whichare, at the same time, subjected to centrifugal and bending stresses of a high order." (The progress made in turbineblade material is graphically illustrated, left.) Turbine Blade Temperatures "For take-off purposes we do not mind if the specificfuel consumption increases; all we want is the highest thrust in order to reduce the take-off run to the minimum.In-a given turbine this would depend on what temperature we dare to run the turbine blades, and for the turbine wehave in mind I think a maximum blade temperature of 850 deg C—corresponding to a gas temperature of1,000 deg C at entry to the turbine—should be obtainable by 1950. Perhaps one ought to add that there is an alter-native to this continuous chase after higher temperature- resisting materials for turbine blades, namely to cool theblades internally with air or water. In practically all their jet units the Germans adopted air-cooled turbine blades,owing to their wartime shortage of suitable heat-resisting materials. The temperatures mentioned," said MajorHalford, "would ensure a maximum take-off thrust from
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