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Aviation History
1961
1961 - 0662.PDF
FLIGHT, 18 Ma •ess 1961 SIX DERBY WINNERS... high flame temperature, to obtain the best possible specific weightand specific consumption. The engine is a two-shaft unit, the propeller being driven by the 1-p system. All Tynes have air-cooled no;zl: guide vanes and h-p turbine rotor blading. The engine is at present in the early stages of commercial and militaryservice, and its development stretches ahead to powers far exceed- ing anything previously obtainable from any long-life prime moverof comparable size and weight. Before discussing this programme of development, it is con-venient to explain the metallurgical problem which smote the engine last year. Last May, when hours on complete enginesexceeded 20,000, a Tyne running on the bench at Derby suffered catastrophic failure of an h-p compressor disc. Although thisappeared to be an isolated occurrence, every engine which had flown was examined, and in one of these powerplants another h-pdisc was found to have a radial crack with an origin near that of the disc which broke up. Rolls-Royce requested Tyne users—Vickers-Armstrongs and Canadair—to suspend flying while a wide- spread investigation sought the cause of the trouble. It was soon determined that the failure stemmed from an obscuremetallurgical condition affecting less than 0.5 per cent of all the wheels concerned, and restricted flying with cleared discs wasresumed in July. Extreme difficulty was experienced in reproducing the failure on the bench, but eventually it was concluded that,although Rex 535—high-tensile, creep-resistant steel—was giving perfect service in the Conway and the latest Avons, trivial differ-ences in the manufacture of the Tyne wheels could lead to an undetectable loss of properties. Accordingly, Rolls-Royce scrappedthe 3,500 discs previously made, and started a crash programme to produce discs in vacuum-melted material, using modified forgingdies to increase the working of the material and heat-treating according to a revised schedule. This cure is complete and permanent, and the Vickers Vanguardultimately received its C of A on December 2. The Vanguard 952 for BEA has the RTy.l Mk 506, with a minimum rating of 4,785e.h.p. The Vanguard 953 aircraft for TCA have the RTy.l 1 Mk 512, incorporating improved material in the h-p turbine rotor blades topermit an increase in flame temperature. Cruise power is even further advanced, by raising the recommended r.p.m. from 12,500to 13,5CO. Both engines are now in intensive airline service, and an approved overhaul life is at present being negotiated. Theonly major trouble experienced affects the turbine bearings and the h-p compressor shaft oil seal, which was found to wear and leadto excessive oil consumption. This problem was cured by the introduction of gramophone (spiral) seals. The problem at the rearend has been more serious. Some vibration had been noted during bench running, but more severe vibration,| some clearly propeller-induced, has been suffered during Vanguard services. TCA have been particularly prone to the trouble, and voluntarily reduced theircruise r.p.m. to 13,000. The vibration has affected both the h-p and 1-p turbine bearings, and a complete cure is expected by re-designed housings. Generally, however, the performance of this complex and powerful unit has been most encouraging. To match the engine to aircraft demanding increased take-offthrust at the expense of reduced jet thrust, Rolls-Royce have evolved the RTy.12 Mk 515, in which an increase in jetpipe nozzlearea raises the power fed into the propeller. Engines of this type are soon to enter service in the Canadair CC-109 and CL-44D-4,and are specified for the Short Belfast. Both the Canadair and Short aircraft have a 16ft de Havilland propeller, the Vanguard propellerbeing a 14ft 6in unit. Stage 3 in the Tyne's development is a military rating, and this isa direct stretch of the RTy.12, with practically no mechanical changes. Designated RTy.20, it has a still higher flame tempera-ture, obtained at the expense of falling below airline-type overhaul periods. A water/methanol system is also available for the RTy.20,to restore power up to ISA + 17°C. The Tyne Mk 21 of this family powers the Breguet 1150 Atlantic, with a 16ft propeller, and the 672 Tyne RTy.20 Tyne Mk 22, driving an 18ft propeller, has been chosen f r «•»Transall C-160. Having requirements unlike those of any other aircraf theWestland Rotodyne is scheduled to be powered by a Tyne ci rived from the RTy.12, and employing w/m boost to give a : Smincontingency rating of more than 7,000 s.h.p. Investigation: have shown that further development of the Tyne is feasible ;/p to10,000 h.p. within the existing frame-size of the engine. VTOL Rolls-Royce have for many years pioneered the techniqueof jet-lift VTOL (vertical take-off and landing) by the employment of separate lift and propulsion engines. The subject is a vas'i oneand many manufacturers have tended to prove that their own particular solution is the optimum. Rolls-Royce thinking is basedon considerable flight and laboratory experience, and both separate lift and propulsion engines—the so-called composite powerpiant--and swivelling-nozzle or tilting engines are active projects. The first Rolls-Royce lift engine was the Nene, which poweredthe "bedstead" which flew in 1953. Thrust/weight ratio of the Nene was 3.2. The axial Soar of the same era had a t/w ratio of 6.6,and the RB.108, powerpiant of the Short SC.l, improves this ratio to8.1. The RB.108 first ran in 1955. Since that time development ofspecialized lift engines has been intensive, and a unit entering the development phase achieves the outstanding t/w ratio of 16. Although there is always one size of engine at which t/w ratio is amaximum, this is not a critical factor, and thrust can be placed any- where within the bracket 2,OOO-8,OOOlb without more than 10 percent penalty. Specific consumption is not important in pure lift engines, for they are rarely called upon to run for more than aminute at a time; their control system can be simplified, for they need run only at less than 5,000ft and 250kt; and the low forwardspeed and ram pressure allows a substantial reduction in carcase weight. Moreover, the combustion system has to operate only overa restricted range of conditions, and can be designed for enormous heat-release per unit volume. Drastic simplification can be effectedthroughout the fuel and accessory systems, and the main rotating assembly can be hung between only two bearings and incorporatenew materials for minimum weight. VTOL Techniques Most missions—by any type of flying machine—can be dividedinto a cruise regime and an entirely different phase of getting off the ground and back again. The latter demands a total lift greater thanthe weight while the cruise regime demands propulsion by the smallest possible engine(s), operating at a substantial throttle open-ing, if acceptable range is to be achieved. Although the use of a single engine with swivelling nozzles is ideally simple, and minimizesthe propulsion dead-weight that has to be carried around in cruising flight, the thrust available for cruising is greatly in excess ofthat required, and the resultant throttling raises specific consumption and curtails available range. On the score of weight there is littleto chose between the two sytems; much depends on the skill of the designer. One of the most direct and instructive methods of comparing theseveral competing systems is to examine all the possible methods of making a supersonic type of aircraft clear a 50ft obstacle 500 ftaway, and of being fully wing-supported at a given distance and height. Rolls-Royce have carried out many detailed studies intooptimum powerpiant arrangements and take-off techniques, using this type of comparative basis. Failure of a lift engine at any given point can be shown to be notcatastrophic, if the autostabilization and lift-engine system is designed to hold any lift asymmetry. The question of how theautostabilization should work is fundamental, and the SC! has undoubtedly provided most of the answers. It is worth noting thatsuch a system is also demanded by an STOL combat machine, and the latter's overall t/w ratio must in any case exceed unity. More-over, the VTOL machine can be overloaded just as can the STOL, and the most important factor is to decide whether the overall t/wshall be 1.2, 1.4 or some other value. Failure of the propulsion engine is actually the most critical casein a composite-powerplant type of machine. Other problems con- cern the ingestion of debris and hot gas at take-off, which could notonly shorten engine life but could critically reduce thrust at an embarrassing moment. The hot-gas problem is not severe, andRolls-Royce have found that it is possible to fly upwards out i-S the centre of a doughnut-like vortex ring of hot gas without ing> ?nnglift air more than 2° or 3° above ambient. The debris prob; -rn is more serious, and can play havoc with the ground, and an; 'iiwgstanding on it, and well as with the aircraft. Again, the "ro; ;ng technique takes the aircraft clear before the point of unst k isreached; and, it will be remembered, the SC.l has taken off ver cauy and operated from plain grass.
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