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Aviation History
1945
1945 - 2114.PDF
OCTOBER 25TH, 1945 FLIGHT 449 t , integral and bent hot after machining. The impeller at % its maximum speed of 16,600 r.p.m. has a tip velocity of approximately 1,500 ft. /sec. (This is considerably in excess of sonic speed, which is approximately 1,100ft. /sec. at sea level and less at altitude.) From the impeller the air enters a difiuser, and is thence delivered to the ten combustion chambers. To drive the compressor approximately 100 h.p. is re- qujred for each lb. of air delivered per second, or a total of about 4,000 h.p. This power is delivered by the single- stage axial flow turbine. • Some idea of the vital import- ance of the turbine blades will be gained if it is realised that each one has to transmit about 75 h.p., whilst at a temperature of about 850 deg. C. and subjected to the stress of rotation at over 16,000 r.p.m. The blades alone constitute a considerable engineering achievement They are fully machined from a special Mond Nickel alloy and in production every one receives a labora- tory inspection for dimensions, form and finish. It is of interest to record that, although dimensions have been in- creased, aerodynamically they are replicas of Air Commo- dore Whittle s first design. The " fir tree " type roots of the blades are fitted in slots broached in the periphery of the Jessop G.18B steel turbine disc and peened over on each side of the extremity to retain them axially. Initially, however, they are not rigidly secured in the radial direction but permitted a slight but limited rock at the tip of the blade. In operation, as the metals accommodate the temperatures and loads, the assembly tightens up. Nozzle Guide Vanes Before reaching the turbine blade the combustion gases are directed to the appropriate angle of attack by the tur- bine nozzle guide vanes. The 48 vanes are individual precision castings of a heat-resisting steel and, as cast, are :••.." correct to form within 0.0005m., and require no machin- " ing but merely a surface polishing. They are cast on the " lost wax" method, reputed to have been invented in ;r the 16th century by the Italian artist Benvenuto Cellini for casting works of art and widely employed in modern times in dentistry. The blades are produced by Rolls- Royce on a modernised system which to a considerable degree mechanises the "lost wax" process. Rotating components must be most carefully balanced, 1. both statically and dynamically. It is noteworthy that the compressor impeller and the turbine rotor are each dynamically balanced when assembled with their respective shafts. It has been found that balance is likely to be upset if rotors are unbolted and reassembled, and this consti- tutes an important reason for the division of the drive from J«L turbine to compressor. ^^ The combustion system and the fuel supply and control components were developed in collaboration with Joseph Lucas, Ltd., of Birmingham, which throughout gave the most intimate co-operation and is now largely responsible for the manufacture of these items for Derwent engines. Early engines, including the Welland, had reverse-flow combustion chambers in which air from the compressor entered the forward end of the outer shell of the chamber, and at the rear end made ;.• 180 deg. turn to enter the rear end of the flame tube in which the burner was A typical Whittle-type double-sided impeller and, at right, the turbine rotor. mounted. Combustion products were ducted from the tor- ward end of the flame tube through the outer shell to the turbine nozzle. The Derwent engines have straight flow combustion chambers, with the burner located at the tor- ward end, and exhaust to the rear. On the reverse How system the combustion air was preheated but there was a pressure drop due to flow losses and, on balance, the straight flow system yields a somewhat improved combus- tion efficiency. The reason for the change, however, was primarily due to the altered disposition of the main -com- ponents. The reverse flow chamber was designed to deliver the air and combustion products to a turbine located as close as was practical to the compressor in order to use the shortest possible shaft connecting the rotating elements. With the use of the divided shaft on the Derwent this con- sideration no longer applied and the straight flow chamber was adopted. Practical advantages were also gained in the fabrication of the chambers and in reducing the problem of accommodating expansion. Combustion Chambers Ten chambers are employed on Derwent I engines, but as few as -seven and as many as eleven have been fitted to different types. The choice is not made arbitrarily but as a result of close study of the requirements of a specific design. The aim is to secure the largest total cross-sectional area to obtain a relatively low velocity and the least pressure drop. These factors would be best secured by a small number of large diameter chambers. Practical difficulties, however, prevent this policy being pursued to extremes. A large diameter chamber is more difficult to fabricate, is struc- turally weaker and presents greater problems in the atomi- sation and distribution of the fuel. Smooth combustion is likely to be jeopardised and tends to yield a less eco- nomical rate of fuel consumption. Another factor affecting the number of chambers assumes an importance if a double- sided compressor is employed. To "reach the rear intake, air must flow between the necks leading from the diffuser to the combustion chamber and an adequate area must be maintained or the efficiency of the compressor will be impaired. As there are no working surfaces in sliding contact and only three main shaft bearings, lubrication presents no insuperable problems even at the high rotational speeds employed. Functioning on the dry sump principle, the total quantity of oil used in the system is only 2| gal. carried in a rectangular tank mounted on the wheelcase of the engine. The rate of circulation is about 200 gal./hr. at the cruising rating and 215 gal. /hr. as maximum. Built into the tank is a pendulum-actuated device to ensure the maintenance of the supply under negative " G" conditions liable to be encountered during aerobatics. A triple gear-type pump unit driven at one-fortieth engine speed is mounted on the wheelcase. The lowest unit is the pressure pump delivering approximately 30, 50 and 70 gal. /hr. to front, centre and rear bearings respec- tively and also a supply to the auxiliary gearing in the wheelcase. Pressure at cruising r.p.m. is 35-45 lb./sq. in. Immediately above the pressure pump is the scavenge pump for the centre and rear bearings, and above that the pump scavenging the front bearing and the wheelcase. The combined delivery of the two scavenge pumps' is passed through a cylindrical Serck cooler, mounted vertically on the starboard side of the compressor casing, and dis- charged to the tank. The rate of con- sumption under all conditions of operation does not exceed one pint per hour. x The arrangement of the cooler is in- genious ; the air passages being ducted from the upper end to the exterior of the engine nacelle. When the aircraft is stationary on the ground there is a depression inside the nacelle and air flows in from outside the nacelle and passes down through the cooler. In flight, however, ram effect raises the
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