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Aviation History
1955
1955 - 1115.PDF
12 August 1955 227 to select optimum r.p.m. for the compressorand airscrew independently. Compressor r.p.m.s and hence power, is under the con-trol of four large levers at the front of the main pedestal. In the original layout, air-screw reversal was effected by pulling the power levers straight through to the rear,past friction detents to give feel, after actuating a reversing selector; pitch reversalwas then obtained upon triggering of a nosewheel weight switch. The latter isnow eliminated, and aerodynamic braking is obtained simply by moving the reversingselector rearwards, lifting the power levers to clear stops and pulling through to therear. Symmetric braking is employed normally on the two inners. Although no separate engineer's station is provided, all the relevant instruments and controls areaccessible from the supernumerary seat. The navigator has a large chart table and a dais, normally flush with the floor, which canbe pulled up to a height suitable for operation of the Kelvin Hughes telescopic sextant. All crew members have exceptionallycomplete stowage space for technical publications, radio spares, airline pennants and similar accoutrements. All crew stationshave inter ;_.jm., the supernumerary position having a- choice of three jack-boxes in the circuits of the pilots, navigator and radioofficer. In all long-range Britannias provision is made for crew rest; fixed attachments for two bunks and two seats are incor-porated on die starboard side of the aircraft, immediately to the rear of die flight deck. Powerplants. All Britannias are powered by four BristolProteus turboprops. This engine is at present the only example in the world of a large and powerful unit developed solely forcommercial purposes, although the production engine has a back- ground of development which has already involved a total of40,000 hr. running. There is a great tendency, particularly in die U.S.A., to consider any engine undeveloped unless it has thebacking of an enormous period of military flying. Such an attitude is difficult to understand, and there is no doubt that the Proteus istoday the only transport turboprop in its power class ready for service, and its success is not a matter of luck, but of long years ofarduous making and breaking of engines. At this point, reference must be made to the forthcoming BristolB.E.25 turboprop (Flight, April 29th, 1955), which provides 75 per cent more power for cruising flight than does the Proteus.With such an engine the all-round performance of the Britannia would be enormously enhanced, details being given in the above-mentioned issue. It has been announced that B.O.A.C. are to purchase 60 B.E.25s for retrospective installation in some of theirBritannias, this being possible during major overhauls. During its long life, the Proteus has undergone changes soextensive that it would be invidious to single out any of the earlier design changes for special treatment. Instead, the basic productionengine is described briefly and related to the Britannia airframe. Air enters through an annular intake immediately behind thespinner, passes around the outside of the engine and then turns inward immediately ahead of the turbine section. The large light-alloy intake casting incorporates ducts which transmit die hot gases from combustion to the turbine, and the intake air passes betweenthese ducts and then flows forward through the 12-stage axial compressor and single centrifugal stage at the front, finally turningonce more to the rear and entering the eight separate combustion chambers. The latter are disposed around the casing of the axialcompressor, die diameter of the latter being small enough for the overall girth of the engine not to be increased thereby. The com-bustion chambers are conventional tubular assemblies, with anti- carbon burners injecting downstream. After traversing the ductsin the intake casting, die hot gases pass dirough die turbine section. The first two stages, both of which are shrouded, drive the com-pressor; the rear pair of stages, of which the first is shrouded, are mounted on a shaft which passes right down the centre of theengine and terminates at an input pinion to the airscrew reduction gear. The reduction gear design has been completely changed during development, and all Proteus now employ double-helical gears arranged to form a compound epicyclic unit with a ratio of 0.09:1. The reaction between the four planet gears and die stationary annulus ring is balanced by a torquemeter consisting of eight lap-fitted pistons in chambers supplied with a special high-pressure flow of oil. Not only does diis arrangement permit accurate measurement of engine output, but it maintains the annulus in accurate alignment. As the airscrew/reduction-gear/ppwer-turbine assembly is a mechanically independent system with very low resistance to rota- tion, a brake is fitted in the reduction gear to accelerate the run- down and prevent die airscrew from windmilling when the aircraft is parked. A Rotax linear electric actuator brings two shoes into contact with the inner surface of a drum machined integrally with the sun-gear-shaft drive-coupling. Power unit of the long range Britannias will be the Bristol Proteus 755. The chief distinguishing feature of this engine is the large alternator mounted under the main supporting ring. BRISTOL PROTEUS 7SS Free-turbine turboprop, with 12 axial stages of compression, plus one centrifugal, eight combustion chambers and two separate two-stage turbines. Dimensions and weight: Length (cone-fitting line to jet-pipe attachment), 101in; overall diameter, 40.1 in; dry weight, 3,0001b, including alternator drive and de-icing manifold. Performance:— SEA-LEVEL STATIC, I.C.A.N. Max. power Max. Proteus 705 (5-m/n) continuous continuous Compressor r.p.m. 100% 97.5% 11,700 Power turbine r.p.m 11,100 10,200/9,500 10.200/9.500 Airscrew r.pjn. „. ... 1,000 918/855 918/855 Airscrew s.h.p 3,650 3,300 2,920 Jet thrust (Ib) 1.220 1.120 1,100 Total equivalent h.p 4.120 3,730 3,345 Fuel flow (Ib/hr) 2,450 2,250 2,130 S.f.c. (Ib/hr/s.h.p.) 0.67 0.68 0.73 S.f.c. (Ib/hr/e.h.p.) 0 0.61 0.64 MAX. CONT. CRUISE AT 300 KT (R.P.M. AS ABOVE) Proteus 755 Proteus 70S at 30,000ft at 35,000ft Airscrew s.h.p 1,701 1,240 Jet thrust (Ib) 348 275 Total equivalent h.p 2,101 1,555 Fuel flow (Ib/hr) 1,000 770 S.f.c. (Ib/hr/s.h.p.) 0.59 0.62 S.f.c. (Ib/hr/e.h.p.) 0.48 0.495 During die starting cycle, only the compressor and its associatedturbine need be driven, and this is accomplished readily by a com- paratively small Rotax electric motor. The power turbine andairscrew do not begin to rotate until the gas flow has built up to a sufficient intensity. The fact that die gas flow is the only linkbetween the compressor and airscrew systems confers upon the Proteus unusual flexibility, and it is possible to maintain the speedof both parts of the power unit at die optimum value for each regime of flight. It may be noted that any failure in die airscrew drive takes thepower turbine completely off-load. Dangerous over-speeding could normally result unless some protective device were incor-porated to shut off the fuel. In the Proteus, such protection is provided by a Negretti and Zambra switch, which remains openso long as the pressure difference between torquemeter pressure and compressor delivery pressure exceeds a fixed margin. Anydrive failure almost immediately reduces torquemeter pressure to zero, and the pressure switch then energizes an English Electrichigh-speed actuator which closes the Lucas h.-p. fuel shut-off cock, the latter having been redesigned to close off all fuel very rapidlyupon receipt of the appropriate signal. The protection circuit is armed by an Isospeedic centrifugal switch originally designed tooperate at 10,000 r.p.m. and above. This system was tested on a Proteus 705 driving a regenerativedynamometer through a shear coupling. Power was increased at take-off speed of 11,100 r.p.m. until the shear coupling failed.Three tests were carried out, the power at failure being 3,450 s.h.p., or 130 s.h.p. above the normal take-off power for thisengine. The over-speed reached 15,000 r.p.m. on the power turbine (35 per cent above die take-off figure) and a Proteus 705has been run for five minutes at 16,100 turbine r.p.m. at the take-off jet-pipe temperature of 500 deg C. When the latter enginewas stripped, it was found to be completely unaffected by diis run. Before diese tests were carried out on die complete flight system,a total of 13 successful shear tests were carried out on Proteus 2s, and 15 on Proteus 705s, using prototype versions of the safetydevice. All Proteus are now protected by diis system, which is fullytype-tested. The pilot can disarm the circuit manually, and he is provided with a test panel with which all settings of the devicecan be checked. When the compressor r.p.m. are less than about 9,000, the powerturbine cannot over-speed dangerously, even when running free. Nevertheless, an additional over-speed switch, triggering an in-dependent circuit, is now being incorporated at die front end of die Proteus to cover the range down to 9,000 r.p.m., and so providecomplete protection at all compressor speeds. At present, this second circuit is additional to the first, and it is possible that thesetting of die existing Isospeedic arming switch may be raised to
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