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Aviation History
1955
1955 - 0100.PDF
100 FLIGHT CHOICE OF TURBOPROPS A Preliminary Appraisal of Three Very Efficient British Power Units WHATEVER causes there may be for uneasiness in theBritish aeronautical scene as a whole, the industry andits customers can derive solid satisfaction from the achievements of our engine manufacturers. Our prowess in this field is no flash in the pan, neither is it founded on one engine or company. Such firms as Armstrong Siddeley, Bristol, de Havilland and Rolls-Royce have consistently pro- duced gas turbines of the first quality during a dozen competi- tive years. They have now been joined by Napier and Blackburn and form an unmatched source of all types of modern gas turbine aero engine. The three units, the Bristol BE.25, the Napier Eland and theRolls-Royce R.B.109—all turboprops for transport aircraft—with which this account is concerned are, in spite of increasing "know-how" and effort abroad, likely to place our engine industry in an even stronger position in the years to come. Apart from theEland, which has been fully described, these engines are still in a very early stage of development, and very little specific informa-tion has been issued about them by their manufacturers, Bristol and Rolls-Royce.Nevertheless, over a period of weeks—and particularly from foreign sources—it has been possible to gather together quite a lotof information and data about these two later units. The draw- ings presented here are also of a provisional nature, having beenbased upon previously published outlines, and in the case of the Rolls-Royce R.B.109, an examination of a mock-up.Certain comments are applicable to all three engines. Each is descended from, but bears litde resemblance to, an earlier turbo-prop of comparable size but very much lower output and efficiency. The new units can, dierefore, take advantage ofmany years of painstaking research; and, although they are very much more advanced man their predecessors, it is likely that—such is the improved understanding of such things as seals, com- bustion systems, gear teeth, bearings, and the like—their develop-ment will prove to be surer and certainly no slower than has been customary in the past. Each of the new engines employs a direct ram intake, thediameter of which has clearly been determined by the minimum size of spinner for an airscrew hub capable of transmitting thepower—and quite a large spinner it is seen to be. The air is drawn in around the full periphery of the spinner and then passesdown a tapering intake duct surrounding the reduction gear before passing to die axial compressor.Each of the new compressors is remarkably small in relation to the output of the engine—an indication of high efficiency and alsoof fairly high rotational speed. In the basic design of a turboprop one has to strike a balance between optimum r.p.m. for both theengine and airscrew: if the engine is designed to run at a slow speed, the size of the compressor/turbine assembly becomes ex-cessive, while too high a rotational speed brings trouble in the design of the reduction gearing. The Eland is known to have asingularly efficient single-stage reduction gear. All ouier things being equal, a high pressure-ratio results in ahigh overall efficiency. Unfortunately, the characteristics of axial compressors are such that, the greater the pressure ratio, thegreater the difficulty of achieving ease of handling and flexibility in operation. There are various ways of overcoming the tendency.The basic method is sheer sound design, so that the compressor operating line is as far as possible from the surge line (or, rather,vice versa). Another is to fit variable-incidence inlet guide-vanes and blow-off valves, to prevent stalling during starting. Anodierapproach is to split the compressor into mechanically independent "spools" which automatically match themselves when the engine is running steadily. This last arrangement is used on die Bristolengine and is probably also a feature of the Rolls-Royce. The three engines all have, or will have, automatic controlsystems to prevent damage to the engine through mishandling. The familiar Dart is now being fitted with a turbine-inlet tempera-ture control Flight, January 14th), and one might expect its successor to have a similar arrangement. The Eland has a veryeffective variable-datum control in which turbine-inlet tempera- ture is governed by a mercury-vapour system. The new Bristolengine retains, in an advanced form, die electronic temperature- controller developed by Ultra and first used on the Proteus. Each of the new engines is being developed as a complete powerunit, with comprehensive accessories—many specially developed for the engine concerned. Another feature common to the trio isa bleeding-off point for compressed air suitable for a cabin-air system. In the case of die Rolls-Royce engine this air is extractedbetween the two compressors, a fact which may indicate an ex- ceptionally high overall pressure ratio. All three engines should cowl superbly, although it might atfirst be considered that, were it not for the large-diameter intake, a very much slimmer cowling would be possible. It may surprisesome readers to find that, in fact, a comparatively bulbous cowling based on the intake diameter has a drag scarcely greater than thatof a pencil-slim cowling similar to that of the Viscount. Further- more, it offers considerable space for accessories without becom-ing inaccessible and without breaking the exterior profile. Bristol BE.2S. Although this turboprop owes a very greatdeal to the Bristol engine division's work upon the Proteus and Olympus, it is a wholly new engine. In one very important respectit differs from every other gas turbine yet built: it is not a "full throttle" engine but, to employ piston-engine parlance, is "super-charged." The general idea is that, although die engine is potentiallycapable of a sea-level output of something in excess of 7,000 h.p., it is throttled to a maximum of 4,000 s.h.p. This surprising pro-cedure pays several dividends. It means that the 4,000 s.h.p. will always be available, even in the most adverse conditions of airfieldaltitude, temperature and humidity; this would permit unpre- cedented simplification of flight-planning for airliners and wouldalso, of course, allow full payloads to be carried from high, tropical airfields. Again, a very high power will be available for cruising—possibly not much less than the maximum continuous power at sea level. Finally, and a point which is probably proving most usefulto Bristols at the present time, the BE.25 can be fully developed without going to the truly immense trouble of designing anddeveloping a reduction gear and airscrew system capable of accept- ing almost twice the power. On the other hand, of course, it could be argued that the BE.25will be larger than it need be for the s.h.p. which it provides. There are two turbines, the high-pressure stage driving the h.p.(rear) compressor and the rear turbine (which has three stages) driving both the low-pressure (front) compressor and the reduc-tion gear to the airscrew. This arrangement overcomes a short- coming of the free-turbine turboprop. The power turbine of thelatter type of engine tends to "run wild" and overspeed if it is taken off-load, such as might occur were die reduction gear to fail.The BE.25 power turbine also drives a compressor—die com- pressor doing most of the work, in fact—which forms a fool-proof safety device. The combustion system of the BE.25 is described as being of"tubo-annular" form, with ten flame-tubes mounted in an annular casing. Fuel is supplied by a manifold surrounding the diffusersection of the engine, and the main fuel pump lies under the rear compressor case. Other features are shown in the drawing. The Bristol BE.25 "supercharged turboprop," showing its wasp-waisted layout, which facili- tates the installation of accessories. _ 1, Saddle oil tank to starboard; 2, electric starter; 3, 50-kVA alternator; 4, Ultra electronic control box; 5, starting pump; 6, main air bleeds; 7, fuel filter; 8, four dynamic suspension units; 9, mounting ring; 10, barometric pressure control, torque limiter and shut-off cock; 11, oil cooler (port and starboard) fed by pipe from individual ram intake; 12, oil-pressure transmitter; 13, oil pump; 14, scavenge filter; 15, main air intake; 16, airscrew reduction gear. 10
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