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Aviation History
1956
1956 - 1107.PDF
17 August 1956 253 sive efficiency in a jet-type engine. The basic thinking behind the by-pass turbojet is quite straightforward. If one assumes that the component efficiency is 100 per cent throughout, then the power developed in any jet-type engine depends on the pressure ratio, the top temperature and the mass flow. The cycle efficiency, i.e., specific fuel consumption, depends only on the pressure ratio. In practice, of course, the component efficiencies are something less than 100 per cent and, when this is taken into consideration, the cycle efficiency is found also to be a function of top temperature. For shaft-driving engines it is therefore advantageous, from both the specific-power and efficiency points of view, to increase the combustion temperature to the practicable limit. Where the thrust is derived from a propulsive jet, however, one must also consider the propulsive, or Froude, efficiency, which is a function of the difference between the speed of the jet and the speed of the aircraft, both measured with respect to still air. As combus- tion temperature is increased the jet velocity also rises and the propulsive efficiency falls. Eventually an optimum temperature is reached above which, although the thrust will continue to rise, the specific fuel consumption will begin to worsen. During the past few years progressive improvements in hot-end construction, and in particular the use of air-cooled turbine blading, have allowed increases in combustion temperature to a point at which specific fuel consumption is being seriously affected. In the supsrsonic aeroplane this is of secondary importance, but in the high-subsonic bomber or transport it is desirable to suppress the maximum temperature. In fact, for a turbojet with a high pressure-ratio cruising at 550 m.p.h. at 40,000ft, the combustion temperature for the best specific fuel consumption is only about 600 deg C, which is very much less than the values at which the latest military engines are now operating. In the by-pass engine the blades of the forward (low-pressure) compressor are made very long so that they can handle a tre- mendous mass flow. Part of this flow is ducted around the remainder of the engine and expelled as a relatively cool propulsive jet of atmospheric air. The air from the inner portions of the low-pressure compressor is taken through the second (high-pres- sure) compressor, through the combustion chamber and turbine section and finally emerges as a fast-moving hot core surrounded by the by-pass jet. The jet velocity is no more than moderate, the outer by-pass jet being inherently slow-moving and the inner hot jet being partly de-energized by the energy extracted through the turbines to drive the by-pass flow. On the debit side is the increased size of low-pressure compressor (compared with an equivalent simple turbojet) and the fact that there must be losses in the transfer of energy from the hot gas to the ducted air. By-pass investigation has been going on at Derby for some eight years. With a development contract from the Ministry of Supply, design progressed on various engines named Conway and the first of these ran in August 1952 at the RCo.2 rating of 9,500 lb thrust. This was equal to the rating of what was then the latest member of the corresponding family of pure turbojets, the RA.14 series of Avon. One of the chief factors to be settled in the design of an engine like the Conway is the by-pass ratio, i.e., the mass flow in the by-pass flow expressed as a proportion of that going through the high-pressure section of the engine. During the development of the Conway this factor has by no means remained constant, and much of the future development of the engine will be devoted to increasing the ratio from the present average of 0.4-0.5 to some- thing like 0.8. This has been made possible largely by the "podded" installation used in the American jet airliners—which the managing director of Vickers-Armstrongs (Aircraft) recently referred to as "God's gift to the by-pass engine." Design investi- gations show that this will not raise the engine weight although specific fuel consumption steadily improves as the by-pass ratio is increased. The gain expected is of the order of an additional One of the earlier types of Conway by-pass turbojet about to start bench-running. Note the great length of upstream blading. By-pass characteristics: curves showing the effect of variation in by-pass ratio on a unit rated at 20000 Ib thrust. The full lines repre- sent a hot engine weighing 5J000 Ib and the broken lines a cooler-running unit weigh- ing 5Ji00 Ib. The consump- tion ratio is the fuel consumption at 475 knots (stratosphere) expressed as a ratio of that of a simple cool-running turbojet. 5O c 5 45 t, 2 o 4O \ / O (PURE JET) HOT <, COOl ENGINE CARCASE / S IV \ x' TAKEAIRF ^" -OFF LOW 4 -6 -8 10 1 BY-PASS RATIO 2 1 y •o* 2° 5 96 | , 92 5 j •Ml O UU Q 5OO 4OO fi u.a 3OO < 2OO4 1-6 six per cent, which Rolls-Royce calculate is equivalent to a range improvement of rather more than ten per cent compared with a simple turbojet. Development to the RCo.3 rating in July 1953 produced a sub- stantial increase in thrust to a value of 11,250 lb. An engine of this type was successfully put through a simulated 150-hr type- test in July 1954. Full type-test approval was gained in July 1955 by the RCo.5 which incorporated detail changes. The type-tested thrust was 13,000 lb, and it was then announced that the recorded specific fuel consumption was lower than that for any other type- tested jet engine. Last year the RCo.5 was mounted in an external pod beneath the fuselage of an Avro Ashton test bed converted by D. Napier and Son at Luton. With this aircraft full investiga- tion of flight performance has already been made. The first order for the Conway was announced on May 9 of this year, when Trans-Canada Airlines publicized their purchase of four Douglas DC-8 Model 1910 long-range transports, with an option on two more. Delivery to the airline was to begin in December 1959. This order marked the first major change in a buying pattern which had been established for 200 aircraft for similar applications. Explaining their departure from accepted practice, T.C.A. commented not only on their happy past associa- tion with Rolls-Royce, but also on the fact that, compared with its American counterpart, the Conway promised to be appreci- ably lighter, cheaper and more efficient. Aircraft of the calibre of the "inter-continental" DC-8 and Boeing 707 need engines with greater thrust than any yet announced for the Conway or any other commercial engine. Estimates of the actual thrust required have been made in the bracket 15,000-17,000 lb, and to meet this ambitious target Rolls- Royce are currently producing a new family of exceedingly advanced Conways capable of delivering approximately twice the thrust of the first engine. In order to establish an economic pro- gramme and also as a means of improving the striking power of Bomber Command, the Ministry of Supply are to purchase a considerable number of production engines of this new high-thrust family for installation in one or more types of V-bomber. Rolls- Royce already have a firm commitment to produce such an engine in numbers. The civil "advanced Conway" is the RCo.10 and it promises to be the most efficient powerplant for long-range jet transport yet developed. Flight development with the Ashton has permitted existing Conways to be demonstrated at the altitude, thrust and specific fuel consumption which have been guaranteed for the RCo.10, although of course die present engine has to run at higher r.p.m. owing to its smaller diameter. Compared with the future military powerplant, the RCo.10 will be somewhat derated by running at lower gas temperatures. As a result, no special development problems are foreseen. Military orders will pro- vide a substantial background of flying during the 40 months available before delivery to T.C.A. It is expected that full A.R.B. and C.A.A. clearance will be granted within three years and that the RCo.10 will be put on a 1,000-hr basis within 18 months of first service. The latter should be easier to achieve than it has been with short-range engines, owing to die fact that the utiliza- tion of the DC-8s should be of the order of 11 to 13 hr per day. Features of the DC-8 installation are electric starting, a fuel/oil heat exchanger, reverse thrust and a silent-running nozzle. For Boeing, Rolls-Royce have undertaken to supply complete power nacelles incorporating the reverse-dirust unit and silencing nozzle. These nacelles would be shipped to Seatde, and Rohr Aircraft of Chula Vista, California, would supply installation components and the pylon strut. Rolls-Royce are confident that other operators will follow the example of T.C.A. in specifying the Conway for future orders of long-range jet transports.
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