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Aviation History
1961
1961 - 0658.PDF
668 Conway RCo.42jl mock-up FLIGHT, 18 May SIX DERBY WINNERS... Engines for the Saab-35B Draken are made under licence by SFA (Svenska Flygmotor), whose RM-6C is the Avon Mk 60 with an SFA afterburner. The RM-6C is in production at Trollhatten, and compares closely for performance with the British Avon 301. Some of these advanced Avons, including that flying in the Mirage, have a convergent/divergent nozzle, which confers a significant improve- ment in performance at high altitudes and Mach numbers. CONWAY A full description of this pioneer powerplant was published in Flight for January 15, 1960, when its evolution from the early concepts of Sir Frank Whittle was noted. The present Conway stems from as recently as 1956, when Trans-Canada Air Lines asked for a by-pass tuibjjet (turbofan) of some 15,0001b thrust for their DC-8 aircraft, while the Ministry of Supply con- tracted for a 17,2501b engine for the Victor B.2. It is now common knowledge that the need to fit the engine into the wing of the Victor resulted in the by-pass ratio being fixed at the low value of 0.3; yet the present engines are superior to their American rivals in thrust, installed weight and specific fuel consumption. Bench running of the first engines of this type started in November 1957. Production deliveries for the Victor B.2, Boeing 707-420 and Douglas DC-8-40 started in 1959, and commercial service with both the latter aircraft began on April 1, 1960. It is worth noting that the 707-420 has an all-British installation, incorporating a Rolls- Royce reverser and silencer, whereas the DC-8-40 retains the Douglas-designed nacelle, partly built by Ryan, using the charac- teristic ejector nozzle and target-type reverser. All the 300-odd engines so far despatched to Boeing and Douglas are of the RCo.12 standard of build, the 707 engine being the Mk 508 and the DC-8 engine the Mk 509. Even today these are very advanced engines, having a high pressure ratio, two major rotating assemblies, a relatively high flame temperature and two propulsive flows. Ideally, Rolls-Royce would have preferred to start the engine off in military use, and then enter airline service with one major operator. Instead, the Conway has been plunged into intensive commercial service in two types of aircraft built 6,000 miles away, and with a whole range of operators simultaneously. All things considered, teething troubles have been surprisingly few, and the engineers at Derby now express themselves well pleased. Service Experience In general terms, trouble with the Boeing installation has been less than with DC-8 engines, and the incidence of snags of all kinds has fallen encouragingly from the high levels suffered during the first months of service. For example, BOAC, who have the largest fleet of Conway-engined aircraft (707-436), experienced 0.67 un- scheduled engine removals per l,000hr flying in June 1960. The corresponding figures for the subsequent months are: 0.6, 0.61, 0.67,0.35,0.3,0.34,0.29,0.21, 0.12 and (for the 14,000hr last month) 0.00. On April 30, total hours in scheduled service were 286,400, of which 188,360 were flown by Boeings and the remainder by DC- 8-40s. Initial approved life was l,000hr; the figure has now climbed to l,600hr in the case of Lufthansa (Boeing), and 2,000hr should be reached by most operators in the first quarter of 1962. Between overhauls, nothing need be done to the engine, or to the Rolls- Royce reverser on the 707, apart from routine external servicing. Apart from isolated problems peculiar to the Douglas installation, more than two-thirds of all unscheduled removals have resulted from four clearly denned causes. In addition, some h-p compressor- blade trouble was experienced during early operations with the DC-8-40, until a thrust-reverser drill to eliminate gas re-ingestion was evolved and adopted. This is now satisfactory and the reverser trouble has ceased. Development has also produced solutions for the four causes of most of the trouble in 1960, which were:— L-p compressor front bearing Originally, clearances in this bearing were exceedingly fine, and at high altitude the cold casing contracted enough to pinch the rollers and either break up the track or promote creep of the inner race. Practically all this trouble was confined to the first 50,000hr, and it has been almost unknown since the summer of last year. The immediate palliative was to raise the clearance between the housing and the race by 0.003in, and to make the inner race more of an interference fit on the shaft to remove the possibility of its rotating. These measures, carried out inthefield.havebeenmost satisfactory, and at the first overhaul the bearing housing is mach- ined out to restore the original bore dimensions. New production housings are stabilized by being carefully heat-treated. be da ing de- nial Centre bearing Plucking of centre-bearing cages was found restricted to components from one particular manufacturer, ; satisfactory alternative was quickly introduced. H-p compressor bidding Failures of h-p compressor rotor bl; were at one time a major source of trouble, and its solutio; manded protracted investigation. It was found that at rotat speeds well below the maximum a slight stall developed arour>..; the outer radius of the h-p compressor, leading to vibration in a fo; n of flapping mode which, if continued for a sufficient time, led to fa-;gue failures originating at points of local debris damage. It wi••, an infrequently encountered condition, and one which proved elusive when attempts were made to simulate it on the ground, but b\ the late autumn it was decided to minimize running between 57 and 70 per cent r.p.m., and the trouble virtually ceased overnight. Later action to eliminate the rotating-stall problem entirely will probably involve decreasing the blade twist at the front of the rotor and then slightly increasing the twist on later stages. This should result in a compressor with improved idling characteristics, without penalty. At the same time, the opportunity will be taken to increase the axial clearance between the inlet guide vanes and first h-p rotor stage, to prevent contact between the two sets of blades in the event of bird ingestion. H-p turbine blading Although Conway flame temperatures are some 200°C higher than those of the Dart, the use of air-cooled blades restricts metal temperatures in the h-p stage to within very few degrees of those in the earlier engine. Nevertheless, for obvious reasons, the introduction of new materials in cooled blades has always lagged behind that in solid blades. Thus, the Conway started off with h-p blading in the older Nimonic 95A, and, not- withstanding Rolls-Royce's painstaking research over many years into all aspects of cooling blades from the inside, a few of these blades suffered failure resulting from thermal cracking. The intro- duction of cooled blades in Nimonic 105 has provided the answer; but, at the same time, it is worth noting that the life of the entire hot end of the engine is closely related to both the total time spent at peak temperature and the number of occasions at which this temperature is reached. Thus, long-haul operations facilitate the achievement of higher component lives, and much also depends on an operator's take-off technique. Inevitably, the hottest parts of all gas turbines suffer when take-off power is held for protracted periods, and modern anti-noise techniques are having a very bene- ficial effect. At some airfields, for example, an initial steep climb is made at full power to gain as much height as possible before reach- ing a populated area; power is then reduced and the aircraft held level for a time which may be as great as 15min, before opening up to climbing r.p.m. Sufficient experience has been gained for Rolls-Royce to know that in the matter of overhaul life and component life the Conway will match the world-beating standards set by the Avon and Dart. This is the more remarkable when it is remembered that Avons are already running to a 2,900hr trial overhaul cycle, and have many components which have reached lives of over 5,5OOhr. This being the case, Rolls-Royce have with confidence embarked upon a programme of Conway development. The obvious steps to take in such development are: increased flame temperature (to give more power); add a zero-stage (to give much greater power, and improve specific consumption); and reduce clearances, and effect detail refinements throughout the engine (to minimize leakage and further improve specific consumption). During the early design of the Conway, a zero-stage was added to the l-p compressor, so that in all production engines the first com- pressor stage is overhung ahead of the front bearing. Advantage has been taken of this fact in the design of a new front end to the engine, fabricated in steel sheet, in which increased taper on the first rotor stage allows the diameter of the intake to be opened out Dart RDa.10/1
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