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Aviation History
1958
1958 - 0886.PDF
904 FLIGHT, 27 June 1958 THE TURBINE HELICOPTER . . . imagined that the power turbine would always be leaning upagainst the freewheel, and that some form of ground test equip- ment might have to be introduced to make sure that the freewheelwas functioning. Tests proved this to be very nearly the case, as we have found under ground idling conditions that the freewheelneedle can only just be split from the rotor needle." In view of the position of the engine under the cockpit floorit was expected that a large amount of heat would be generated and would have to be dispersed. This was certainly the case atthe start, but it yielded to normal treatment by increasing the flow of ventilating air and by insulating where necessary. In general, the whole development had been quite rapid andapart from other advantages the turbine had justified itself on one outstanding point, that of the quick start. In the case of the pistonengine the warm-up period took a very considerable time- something over ten minutes from cold. With the turbine thistime was under a minute, apart from cockpit checks. In fact, with the pilot in his seat the helicopter had been demonstrated tobecome airborne in under 30 seconds from cold. The process was so quick that the planeguard helicopter for an aircraft carriercould now be contemplated as sitting at readiness on an outrigger platform, and not continually airborne during operations.By using an American-built S-58 for the initial flight testing of the Gazelle, one year's development of the turbine installationhad been carried out; and when the first Westland-built Wessex arrived on the flight line (as it would do very shortly) it wouldliterally have a flying start, with the early experimental stage of the turbine left well in the background. The Westminster. Westland entered into a licence agreementwith Sikorsky for the S-56. This, the largest helicopter so far in the Sikorsky range, was initiated as a marine assault type, buthad also been used by the U.S. Army. A considerable number of these large helicopters were now in squadron service with theU.S. Forces. The S-56 was powered by two Pratt and Whitney R-2800engines mounted in pods on a stub wing set in the high-wing position. The helicopter was designed for loading through frontclamshell doors and loading ramps. In the United Kingdom the only suitable engine that wasavailable was the Napier Eland turbine. Fortunately this power- plant was already under development with a rear drive and itappeared that the best proposition would be to dispense with the stub wing and the engine nacelles and to mount the turbines onthe cabin roof, forward of the main gearbox. The Westminster project was developed as a combined private venture effort betweenWestland and Napier. "The Napier Eland," said the lecturer, "is a fixed-shaft turbinewith which we had had no actual experience, and again we were making the problem more difficult by using a pair of fixed turbinesdriving on to a common rotor shaft. Fortunately Napier had ' under development a clutch which was also a fluid drive and weseized on this as a method of avoiding direct mechanical connection between the high-energy rotating parts of the two turbines, andin a similar way avoiding direct mechanical connection between the rotor head and the turbines."The Westminster project, although based on an S-56 licence agreement, in actual fact made use of only part of the S-56; butthe parts used nevertheless comprised some of the most difficult components, requiring the greatest amount of development work.Thus, the main rotor hub, the main rotor blades with the rotor shaft, tail rotor assembly, duplicated power controls and a numberof other items were actual S-56 parts. They had behind them a large amount of accumulated flightexperience and, in fact, as a result of this experience numerous design changes had already been incorporated. "At the start of the project," Mr. Hollis Williams continued,"there was no firm operational requirement, and we realized that we could spend a vast amount of engineering time and moneyproducing a helicopter which, when complete, was not exactly what was required. We therefore decided to prove the turbinesin association with the new parts of the transmission, by building in the first instance a flying test-rig, and our aim was to get ourresults in the cheapest and simplest possible manner. However, there is no such thing as cheapness or simplicity with a flyingprototype of any description. Everything is relative, and we eventually decided that our flying test-rig had most of the elementsfor a successful crane, and should be so developed as a forerunner to the Westminster transport. This, therefore, is our immediateline of development. We intend by means of this rig, and by a second similar one which is now under construction, to buildup flying hours and so accumulate the experience and background which will lead up to certification and good serviceability on theactual Westminster transport." Turbine Control Problem. The Napier Eland turbine, said thelecturer, had a full-authority fuel governor and hence the main engine control was a speed selection lever. In the Westminster,each of the turbines was coupled into a single gearbox via a hydraulic clutch, a reduction gearbox and a freewheel. Theimportance of accurate speed selection of the two engines in order to prevent large asymmetry of power distribution had beenindicated by a simple analysis and subsequently confirmed by a more elaborate dynamic investigation carried out on a digitalcomputer. The slipping clutch (up to 2 per cent slip at maximum torque) was an important feature in the control of this coupledinstallation. The engine control in the cockpit comprised two speed-selectionlevers with appropriate r.p.m. "gates"; at the top of each lever was a trimming knob so that fine adjustments can be made to theengine speed selector. Engine control has therefore been removed from the collective-pitch lever; the pilot had only to select con-stant-speeding r.p.m. In the event of failure of one turbine, the other turbine automatically developed full power without consciousaction on the part of the pilot. Installation. The engine installation was in general orthodox,but the rear drive turbines have fairly long high-speed drive-shafts and the mounting of turbines and gearbox on a flexible airframeintroduced problems associated with shaft mal-alignment. The original toothed couplings on these shafts were replaced byspecially designed ball couplings, and these were capable of standing a greater degree of mal-alignment; they were based ona similar coupling that was operating successfully in the Gazelle installation. The high-speed shaft on each turbine was connected to theclutch, which was also the fluid drive, and then through the primary reduction gear, which incorporated the torquemeter, intothe lower part of the main gearbox. The fluid drive introduced a heat-dissipation problem, and reacted on consumption; but theadvantage of a non-mechanical link in the transmission would, they were confident, outweigh the disadvantages.The lower part of the main gearbox, of Westland design, was connected to the upper part, which was Sikorsky designed andmanufactured. This upper gearbox contained the final double epicyclic gear reduction, and the total gear reduction was 12,500turbine r.p.m. down to 197 rotor r.p.m. Slimming up, the lecturer said: "Based on experience to datewith one trial installation of a single turbine, I would say without hesitation that, in comparison with a piston engine installation ofcomparable power, development time and effort can be saved. "The piston engine has been the mainstay of helicopterdevelopment up to this date, has given good service, and where first cost is an important consideration will compete with theturbine for some years to come, particularly in the smaller sizes where engine development continues. The turbine is the moresensitive to altitude combined with high ambient temperature. Where such conditions have to be met, as in a world-wide opera-tional requirement, an oversize turbine must be installed, probably run in a gated condition at sea level I.S.A. This tends to increasefirst cost and the gating reacts unfavourably on consumption. "However, taking the all-round view, I am sure that the manyadvantages of turbine power outweigh the few disadvantages, and that better helicopters will result from its use." R.O.C. GOES UNDERGROUND SO that the Royal Observer Corps can fulfil the twin role oftracking enemy aircraft and plotting nuclear fall-out, its posts are being equipped with measuring instruments and undergroundreporting stations. A post which already has this equipment— "November 1," two miles north of Farnham in Surrey and over-looking Farnborough airfield—was shown to the Press recently and A. Cdre. J. H. T. Simpson, Commandant of the R.O.C., andMr. R. H. F. Firth of the Home Office were there to answer questions.The instruments are a ground zero indicator, which gives a bearing and azimuth, so that by the intersection of these from twoor more posts a position can be plotted by triangulation at the control centre; a bomb power indicator, which measures vibra-tions and thus indicates the height of a bomb-burst; and a gamma radiation detector, readings from which can be taken undergroundin a reporting post 15ft below the surface. There is room in the latter for up to four observers, who have sleeping accommodationand an 8-10 hr supply of air. The R.O.C. is now holding its biggest annual summer campsince 1948—at R.A.F. West Mailing, with which the Corps was closely associated during the war. Between June 23 and July 26some 3,000 men and women volunteer observers will have attended. The R.O.C. is now some 15,000 strong, but with itsnew responsibilities still more members are needed.
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