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Aviation History
1950
1950 - 0799.PDF
FLIGHT, 27 April 1950 3*9 be employed and conseqoeirtly make possible a lower specificrate of fuel consumption (say, 12 to 15 per cent lower). Also, its relatively smaller diameter permits tighter installation andreduced frontal area, which diminishes aerodynamic drag, an :all-important consideration at high Mach numbers which maydominate the choice. The centrifugal compressor is cheaper than the axial type to develop, to produce, to maintain andis certainly of more robust construction and less vulnerable to foreign bodies or icing. Its efficiency can be put at 79 or80 per cent. Although much effort is being lavished on the axial type,it must be acknowledged that the civil airliners now flying and doing real work—the de Havilland Comet, Avro CanadaJetliner and Vickers Viscount—are fitted with centrifugal compressor units. High-energy Ignition System.—Typical of our detailedresearch into particular problems affecting aircraft turbine operation is the high-energy ignition system developed by theRoyal Aircraft Establishment. This has enabled us to dispense with the familiar torch igniter and its auxiliary fuel spray, asthe electric discharges it produces at the rate of about one per second are of such high energy and short duration that eachcan by itself create ignitable conditions in the combustion chamber. Each discharge can vaporize a sufficient quantity ofthe main fuel spray and then ignite a sufficient volume of the resulting mixture to produce a general " light-up." " Light-up " Problems It was soon found that erosion of the electrodes of the ortho- dox high-voltage igniter plug was too rapid and it was decided to use a surface-discharge type of plug with a large electrode area. This consists of two circular electrodes of heat-resisting steel separated by an annular gap of 0.02m in which mica is tightly compressed and made flush with the electrode surfaces. A thin layer of carbon is deposited on the mica surface and the plug is positioned in the combustion chamber so as to maintain this deposit during normal operation. In the ignition system an induction coil, operated by a vibra- tor from a 24-volt d.c. supply, repeatedly charges a condenser through a sealed spark gap until the condenser voltage increases to about 2 kV. The condenser then discharges through a 2 kV sealed spark gap, an inductance, and the surface discharge plug, all in series. The duration of the discharge is of the order of 50 micro- seconds and the peak current is about 1,500 amp. The total weight of the equipment, including a screened plug lead, is about 10 lb. The system has successfully ignited aircraft-turbine combus- tion chambers in the test house over a range of limiting com- bustion conditions, from high pressures equivalent to those at high flying speed at low altitudes to low pressures encountered at high altitudes. In actual flight, successful relights have been obtained on several turbine units at altitudes up to about 35,oooft. This system is not yet in general Service use, Thrust Augmentation.—America seems to be somewhat ahead of us in the application of afterburning to stajKlard aircraft. In Britain we are developing afterburning—or reheat, as we are urged to term it—as a temporary power-boost system for fight- ers and bombers as demonstrated at the last Farnborough Display. With increased temperature it gives greater percent- age efficiency—a theoretical 34 per cent at 1,500 deg K. For civil aircraft, particularly at take-off lxjost, we incline towards water-methanol coolant for the compressor, as after- burning equipment is calculated to impose a penalty of 2 to 3per cent on the crnise consumption. We estimate a maximum benefit of from 10-12 per cent can be gained from water-methanol injection. If more is obtained it is regarded as an indication that the characteristics of the compressor andturbine are poorly matched. Both water-methanol injection and liquid-fuel rocket motorshave been developed to assist the take-off of the dc Havilland Comet when fully loaded from tropical or high-altitude run-ways. During the various tests throughout the winter, tire recoid-breaking Comet lias been successfully flown for 5$ hoursfvt a stretch with standard tankage. The next specimen will have more tarikag? in the wings and, incidentally, a bogk-undercarriage. With improvements in the Ghost turbines, Major F. B. H.tMord estimates another five per cent reductionon published figures of specific fuel consumption. Present tests have been carried out at an altitude of 15,000ft to 40,000ft butthe Comet will go to 45,000ft. Turboprops.—British faith, in relatively low-powered turbo-props for civil use—typified by the Armstrong Siddeley Maroba and Rolls-Royce Dart—is unshaken,. We are pleased with theresult of the icing trials of the Dart and the performance of this unit in the Vickers Viscount as adopted by British Euro-pean Airways. For our largest civil aircraft, the Bristol Brabazon landptaneand the Saunders-Roe Princess flying-boat, we are to use the Bristol coupled multi-stage Proteus, which has, unfortunately,been delayed by improvements now being introduced. It is of interest to note that in America you are working on somewhatsimilar lines. I refer to the installation of Allison XT-40 units in the Convair XP5Y-1 long-range flying-boat. The first turboprop likely to be in regular military use is thelarger Armstrong Siddeley Python now in production for carrier-borne strike aircraft. Valuable experience is also beingaccumulated with the coupled Mamba in new anti-submarine aircraft. For this duty the coupled turboprop, offering cruisingeconomy and high maximum power for closing on the target, appears ideally suited. We expect a progressively stronger challenge from Americain the turboprop fields. Technical Co-operation.—Enormous benefit is to be obtainedby technical co-operation between our countries. It existed even l>efore Pearl Harbour, when our first Whittle jet unit wasflown over to America for the General Electric Company. The Rolls-Royce and Pratt and Whitney arrangement is anotherexcellent example of mutual aid. It was recently stated by a Pratt and Whitney executive that some members of their tech-nical staff knew their way around the Rolls-Royce plants better than around their own ! We might, I think, consider the experimental installation ofour most advanced turbine power units in your equally advanced airframes. Security considerations, admittedly, do not make technicalliaison easy, but they must not be allowed to stifle it. As turbine power units become more complicated, the incuba-tion period between initial inception and delivery for service tends to lengthen. Continued technical liaison can materiallyreduce this critical period, and have the combined effect of strengthening our positions in the van of engineering progressand provide added security in a troubled world. Standardization of all minor parts and accessories should bepursued so long as it is not permitted to impede design, but we should guard against any attempt to standardize ideas. COMETS LATEST RECORD London-Cairo at 425 m.pk as Preliminary to Tropical Trials ON Monday last, April 24th, the de HavilTand Comet leftEngland for tropicalization tests in Africa and broke the London to Cairo record in doing so. Leaving Hatfield at 6.47 a.m., the aircraft reached Farouk Airport 5 hr 10 min later, achieving a speed of 425 m.p.h. over the distance of 2,TQ6 miles. This stage-length is slightly longer than that between Prestwick and Gander—2.116 miles. The actual inter-city distance, considered for record purposes, is 2,183 mHes. The new performance—which, of course, is subject to F.A.I, homologation—breaks the 6 hr 35 min 40 sec (334.3 m.p.h.) record set up by S/L Neville Duke in a Hawker Fury last February. By comparison "with its previous high-speed long-distance nights (to Caste! Benito, Rome and Copenhagen), the Comet carried a much heavier load ot 8,000 1b. In airline service, with the cabin fully furnished, this would represent 34 pas- sengers and their baggage, two stewardesses, and all the necessary amenities for a 2,200-mile stage-length with stand-off allowances. Although the Comet was thus carrying a very useful equivalent payload, its weight on take-off from Hat- field's single runway—short by modern airport standards— was below the normal figure of 105,000 lb. The pilot was G/C. John Cunningham, with Mr. Peter Bugge as second officer and a flight crew of four. Also on board were Sir Geoffrey de Havilland and other executives of the enterprise—Mr. R. M. Clarkson, Mr. C. T. Wilkins and Mr. A. T. Widgery—together with Capt. Alderson (a senior pilot of B.O.A.C ) and a ground crew of five who will service the machine during its trials. In addition, a larger-than-usual load of test equipment was carried. During the trials, which are expected to last two or three weeks, the Comet is operating from Eastleigh (Nairobi) and Khartoum to assess its performance in tropical heat and at high altitudes. It flew on to Nairobi on Tuesday, covering the 2,200 miles in 6 hr 9 min. The airfield at Nairobi is 5,370ft above sea level and 1 deg 18 min south of the equator. Since its first flight last July the Comet has logged some 250 hr on T73 flights. . , / . . ... . .
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