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Aviation History
1948
1948 - 0120.PDF
104 FLIGHT JANUARY 2?.ND, 1948 Turbine Accessory Systems delivery may have to be recirculated, difficulties associated withgasification and aeration were encountered. Several methods of attempting to control on temperatureshave been tried, including differential thermal expansion, the rise of pressure in gas containers, the flow of the hot gasesthrough systems of orifice and electrical means. The last named is now the most favoured and can be said to havealmost arrived. Promising results were reported early from America on the use of a differential expansion system, usinga quartz rod and metal sheath as the members. The immediate cause of acceleration in a jet engine is a risein gas temperature. It follows that a temperature control with immediate response would control acceleration. The avoidanceof rich extinction on suddenly opening the throttle at altitude requires even more rapid response or anticipation. Tempera-tures are a direct function of fuel/air ratios and if these can be controlled the temperature can be limited with the- required"anticipation." Starting Conditions An early method, used to-day on one British engine and 011American engines, was to fit a separate electrically driven stort- ing pump. A pressure valve limited the maximum fuel flowto the engine until speed was high enough for the main fuel pump to deliver the flow, when the starting pump was cut. Formerly, starting was accompanied by a burst of flamepassing right through the turbine and jet pipe. This was thought to be damaging to the blades and the accumulatorsystem was designed to eliminate flames. In its essentials it comprises a "dump" valve, and a hydraulic accumulator."Vhe " dump" valve remained closed until fuel pressure rose to a given value, during which time the accumulator wascharged from the delivery of the main fuel pump. The capacity of the accumulator was sufficient to fill the manifold with alittle over. This surplus, added to the flow from the fuel pump, resulted in a burst of fine spray which was readilyiguitable. The correct degree of excess was found by trial and error; too little causing difficulty iu accelerating from the startand too much an excessive amount of flame. The dump valve was also employed to drain or '' dump '' the fuel in the mani-fold on shut-down of the engine. Referring to bursts of flame on starting, it was eventuallyappreciated how seriously this flame beyond the turbine hin- dered the acceleration of "the engine by setting up a back pres-sure in the jet pipe. In particular cases, flame vibration and resonance occurred and starting could then be effected only byaccelerating the engine through the resonating speed band by brute force. An accumulator must first be charged from the fuel pumpdelivery and this means an inherent delay before fuel issues from the jet. This led to the development of the "flowlxxjstw," for quick starts. It can be likened to a single-stroke piston-type starter pump. Two forms of this unit have beenmade—one for moderate starting times of 5 or 6 seconds and one for "immediate" starting.On certain of the newer, smaller engines a simple two-stage starting system is adopted. It is arranged that initially allfuel shall be supplied through a proportion of the total number of burners only. Due to the relatively large quantity passingthrough each operative burner, good atomization is obtained. Roy£ M The starting of larger engines is now accomplished withoutaccumulators. This is made possible by burners giving a finer atomization under starting conditions (Duplex), by the useof the torch igniter and by the far greater fuel pump capacities now provided.The ignition system adopted on most engines to date has been to employ a trembler type booster coil and sparkingplugs, basically of standard design. To get a consistent start the plug points must be immersed in the fuel spray, but theymust not get too hot and be damaged when the engine is operating at maximum power. Lubbock and Duplex burnershave the pleasant characteristic of large cone angle under start- ing and low flow conditions, so that the electrodes can be,immersed in the spray on ignition, the cone angle shutting down to a smaller one at the higher flows. Electric starting was universal at first, but as engine sizesincreased, the weight of the necessary electrical equipment rose. An alternative system, cartridge starting, is now in an earlystage of development. It is particularly applicable to military aircraft, for on civil airlines, the noise might disturb the pas-sengers' peace of mind. The reasons for the heavy weight of electrical equipment c;'-<Lbe stated. The turbine engine, unlike the piston engine, has to be run up to, say, 1/10 maximum r.p.m. or higher beforeit will maintain itself. Ignition takes place at perhaps 1/20 maximum speed and starter assistance is required to the highervalue. The starter motor can be of similar size to a piston engine unit, but the prolonged drain of current necessitates agreat increase in lottery capacity. Modern engines tend to increase the problem as" axial bloweis are coming to the foreand these appear to have a higher moment of inertia than centrifugal blowers. Slow-burning cartridges appear to be the easiest methodof providing power and the greatest economy in cartridge powder will be effected by accelerating the engine up to speedas quickly as possible, the majority of the starter power will then be absorbed in overcoming the inertia of the engine andthe minimum by friction. The power of the cartridge gases can be extracted in a pistonunit, as in the Coffman starter, by a vane-type motor or by means of a turbine, which offers most promise. The design ofthese starter turbines is rather different from other turbines, for they do not run at a constant speed. Impulse turbineswork most efficiently when the wheel tip speed is equal to about one-half the jet velocity. Maximum efficiency, i.e.,minimum cartridge charge, will result from running the turbine wheel at as high a speed as "possible, but the choice of pro-pellant used must not be dictated by considerations of turbine efficiency. A propellant with low energy content per poundwill produce lower gas velocities, therefore higher turbine effi- ciencies are possible, but the power of the turbine starterobtained from a given weight of propellant is materially reduced. An experimental starter, designed to explore the differentproblems by practical test, has developed a peak h.p. of 425 h.p. From the tests, it has been learned that torque remaiv.sapproximately constant, independent of speed, and thatjfl'J starting time of about two seconds should be used. The presaPunit could speed an engine of moment of inertia 300 1b ft up to just over 1,000 r.p.m. with a cartridge charge of i£ 1b ofcordite. Keeping still within the range over which experi- mental evidence has been obtained, although with slightchanges to the unit, it is anticipated that the same results could be obtained from f 1b to 1 lb of charge. ETEOK IV WINTERIZATION TRIALS THE Royal Canadian Air Force is now trying out one of thelatest short-span Meteor IVs. " RA421," the aircraft con-cerned, is fitted with its four-cannon armament and full cabin pressnrization equipment, and is being put through the full'' winteruation '' programme. After arriving in Canada by sea the Meteor was dispatchedin sections by railway to the R.C.A.F. Winter Experimental Establishment at Edmonton, Alberta, where it duly arrived onOctober 14th last year. Some slight delay in assembly was caused by damage to the front fuselage section sustained duringtransit. Moreover a quantity of coal, grit and soot was found to have penetrated into the airframe during the rail journey,necessitating a very thorough cleansing of the interior. During assembly the special radio range compass and its remote controlbox, G " meter," radio compass dial, O.A.T. gauge and Ameri- can-type aerial were installed. The first test flight was made on November 10th from anicv runway, the two Rolls-Royce Derwent V turbo jets lighted instantaneously in an outside air temperature of -8 deg. C.Between November 10th and 20th, 5 hr 40 min flying was logged, and seven landings made. Note was taken of theMeteor's behaviour in landing on the ice-covered runway, par- ticular attention being paid to skidding and the condition ofbrake segments. Additional tests were subsequently made 850 miles farthernorth, at Watson Lake in the Yukon. Before the flight to Watson Lake the outer mainplanes and tailplane were sprayedyellow in accordance with the latest regulations governing Arctic flying. For the greater part the flight was made at30,000 feet with an O.A.T. of —50 deg C. Though Watson Lake is situated in a valley 2,245ft above, sea-level and is sur-rounded by the thickly wooded Rocky Mountains, the Meteor landed perfectly, with ample runway to spare. During December the average daily temperature at WatsonLake was —20 deg C. The guns were fired on the ground and in the air and no stoppages were reported. The Derwent Vswere run up during heavy snowfalls and in every instance light- ing-up was instantaneous. It was found that snow depositedon the nacelles was melted by the heat of the turbo-jet and later formed into ice. The tests continue.
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