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Aviation History
1949
1949 - 1427.PDF
August 18th, 1949 FLIGHT 191 BRISTOL PROTEUS A High-compression Turboprop for Wing Mounting WHEN a new aircraft power unit is projected mostcareful examination of a number of long-range con-siderations must be conducted before a decision to proceed is taken. Some years must elapse before the unit is developed for commercial production and when that stage is reached it must be able to exceed or at least meet current requirements. The mechanical and thermodynamic design must be sufficiently advanced to avoid the risk of being out-moded and flexible enough to permit full advan- tage to be taken of any new materials or fuels which may become available. Further, the type, size and operating conditions of aircraft for the period ahead must be envisaged with sufficient accuracy to ensure acceptance of the new unit. A firm building both aircraft and engines is advantageously placed in this respect, but, even so, a decision is not easily arrived at. The Bristol Proteus project, now approaching fruition, bears evidence of admirable foresight and appears to be a happy solution of the many conflicting factors. At the inception it was reasonable to expect that for the ensuing decade the bulk of civil and transport flying would be accomplished at medium speed and medium altitude. The Theseus turboprop, particularly intended for civil opera- tion, was undergoing development for an output of 2,500 equivalent shaft horse-power. Aimed to meet the require- ments of somewhat larger craft of improved performance, the Prcteus was projected for an initial output of 3,500 equiv. s.h.p. For higher outputs two Proteus units could be coupled by gearing to drive counter-rotating airscrews, as planned for the Brabazon 2 installation. The Proteus was conceived on somewhat bolder lines than the Theseus, with emphasis on minimum overall diameter to make it suitable for buried installation in the wings. To this end it is de- signed for lateral air intakes, although a peripheral intake can be arranged if desired. It would be erroneous to regard the Proteus as merely a scaled-up version of the Theseus. Certainly there are mechanical similarities, such as the axial compressor with a terminal centrifugal stage and the independence of com- pressor and airscrew turbines. (The so-called "free tur- bine " system, incidentally, promises to characterize Bristol designs.) Fundamentally, however, the two units represent different methods of tackling the same problem. As both were intended primarily, but not exclusively, for civil appli- cations a relatively low specific fuel consumption was of major importance. With the Theseus the aim-was to achieve this end by utilizing a medium compression ratio, actually about 4.25:1, and eventually a heat-exchanger. For the Proteus no heat-exchanger is contemplated but, instead, a compression ratio "very substantially" higher than that of the Theseus is the thermodynamic basis of the design. With the unit rotating at the designed maximum speed of 10,000 r.p.m., the high ratio of compression is achieved by means of twelve axial stages followed by a single centri- fugal stage. Air enters by way of eight smoothly swept radial passages in a fabricated steel casing which bridges the structure between the compressor and turbine casing. The passages are defined between tubular members which enshroud the ends of the combustion chambers leading to the turbine entry. The air mass flow is approximately 45 lb/sec. On account of the high rotational speed the axial rotor is of disc construction. Twelve discs of forged R.R.58 light alloy are clamped between the flanges of hollow S.96 steel end shafts by eight long bolts. Torque is transmitted from disc to disc and shaft flange by eight rows of dowel pins, pitched between the clamping bolts which are thus relieved of shear stress. Rotor blades, of low camber and high speed airfoil section, are forged in L.42 light alloy, coined, machined and finally anodized as a pre- caution against corrosion. Blade roots are of "fir-tree" shape, fitting in axial grooves in the rotor and spaced be- tween stages by distance pieces. It is of interest that the blades are produced to the same profile and to common The complete unit is' suspended, with freedom for expansion, in a cylindrical mounting structure.
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