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Aviation History
1946
1946 - 0661.PDF
APRIL 4TH, 1946 FLIGHT 339 ARMSTRONG SIDDELEY TURBINES through, the compressor. The valves also give some aid to acceleration after starting by preventing the compressor blades from stalling. They are controlled by a lever in the cockpit. On the Python these separate blow-off valves have been abandoned in favour of a single common valve at the front of the compressor casing. Between the diffuser elbows and the combustion cham- bers there is a flexible expansion joint, at present in the form of copper bellows; this is likely to be discarded, prob- ably in favour of a piston-ring joint. Thin-gauge stainless steel is the constructional material of the eleven combustion chambers, and the-flame tubes inside are of Nimonic 75. Of Armstrong Siddeley design, they differ considerably from others based on designs developed by the L'ucas company, and employ the prin- ciple of vaporizing of fuel rather than its atomization by means of high-pressure spray jets. The soundness of the principle has been proved so far by trouble-free running up to the highest altitudes yet flown on the A.S.X. unit. There is a mixing chamber fed with air from a duct, and positioned roughly in the middle of the flame tube. The primary air for combustion—approximately one-fifth of the total air supply—enters the flame tube through this mixing chamber, and fuel is sprayed into it by a jet in the entry duct. Primary combustion takes place in the flame-tube nose piece where the ratio of air to fuel is about 15-to-i, and the burning gases flow back over the mixing chamber, heating and vaporizing the new fuel fed into it. A portion of the secondary air for combustion enters through a hole in the nose of the flame tube, which has a deflector plate on the inside. The cold-air stream is directed over the interior surface of the dome by the plate and prevents carbon formation in the rich-mixture combustion area. A.S.X. AND PYTHON DATA A.S.X. DIMENSIONS Maximum diameter Overall length (lO propelling nozzle) WEIGHTS Net dry weight PERFORMANCE Maximum speed of rotor Static sea-level thrust (take-off and combat)Fuel consumption at take-off r.p.m Fuel consumption at cruising r.p.m. A.S.P.I (PYTHON) DIMENSIONS 42in 14ft 1,9001b Max. dia. over cowling Overall length over cowling Length mounting flange to C/L rear airscrew Annular air-intake duct area Propelling nozzle area WEIGHTS Estimated net dry weight (including starter and Accessory Box Drive Shaft) Estimated installed weight (with airscrew).. Oil consumption Oil circulation PERFORMANCE 8,000 r.p.m. 2,600 1b 1.03/lb/hr/tb thrust 1.0 Ib/hr/lb thrust S.P.I-I S.P. 1-2 54.5 in 48 in lift 4in S.P. l-l S.P. 1-2 7ft I in 7ft9.5in 2.5 sq. ft 2.6sqft ..: S.P. l-l S.P. 1-2 3,0101b 2,9801b 4,1001b 3,9501b 0.5 gal/hr 1,200 gal/hr Rating Max. take-off, max. combat Max. climb Max. cont. cruising ... Engine Sp eed r.p.m. 8,000 7,800 7,600 Aircraft Speed m.p.h. 0 200 300 400 500 0 200 300 400 500 0 200 300 400 500 Airscrew shaft h.p. 3,670 3,950 4,290 4,860 5,520 3,150 3,450 3,800 4,200 4,950 2,720 2,960 3,2603.680 4,250 Net Jet Thrust ib. 1,150 760 590 420 280 1,060 660 490 320 170 950 570 390 220 60 Fuel gal/hr 359 372 385 401 425 323 334 346 361 384 290 300 308325 341 Further air enters the flame tube through flutes around the base of the domed cap, and through holes in the tail section, downstream of the mixing chamber. Final mixing of combustion gases and secondary air is brought about by four shovel-like deflectors attached to the outer casing just aft of the flame tube exit. Starting and the Fuel Supply Igniter plugs provided in two combustion chambers are fitted with a small auxiliary starting fuel jet with electric- ally operated valve. Starting jets may later be fitted to all the combustion chambers, although during develop- ment the present system has proved satisfactory for re- starting in flight at altitudes up to 30,000 ft. From the combustion chambers the gases are led to the turbine manifold by separate manifold pipes. These, clamped at each end, pass through the main air-intake body be- tween the eleven intake throats. in keeping with the description of the engine assemblies, the components of the latest fuel system as used for the Python are listed in order as follows: — Fuel tank. Pulsometer pump. Low-pressure filter. Main fuel pump. Main isolator valve. • , Throttle and altitude control unit containing metering orifice. Fuel distributor with starting pressure valve. Combustion-chamber sprays (eleven). The names of the components are for the most part self-explanatory. The Lucas variable-stroke fuel pump in- corporates a maximum-speed governor, and a maximum- pressure relief valve. The isolator valve is a manually- operated, taper-seat, high-pressure shut-off valve. The Hobson impeller-type throttle and altitude control, linked mechanically with the airscrew speed control unit and the pilot's throttle lever, meters the supply of fuel to the engine. The fuel distributor divides the metered fuel supply evenly between the eleven combustion chambers and does not function until the fuel pressure is sufficient to lift the starting valves. A maximum temperature over-ride device is fitted in the exhaust cone to bring about a restriction in fuel supply- in the event of the maximum temperature being exceeded. The Python has a single-lever cockpit control for the whole of the power range from slow running to full power, and each position of the pilot's lever both selects the appropriate airscrew pitch and assures that the correct quantity of fuel is fed to the engine. Corrections for alti tude, forward speed and air temperature are made auto matically. Torque required when starting a large gas turbine engine is less than that for a piston engine of comparative power, but it is required for a relatively long period, and this is one of the main problems of starting. The weight of a suitable electric starter together with its cables is exces- sive, and the drain on batteries would be prohibitive ; for this reason a specially designed gas-starter-motor is em- ployed on the Python. The automatic starting sequence is initiated by means of a press-button in the pilot's cock pit, or, should it become necessary to re-start in flight, advantage may be taken of the power provided by the wind-milling airscrews. On the A.S-X. an electric starter motor, geared to the front extension shaft, may be used, and the special gas-starter is an alternative. A feature of gas-turbine engines used in conjunction with airscrews is that the power output falls off very rapidly as engine speed decreases, and, to quote the case of the Python, under 100 h.p. is produced at half engine r.p.m. This corresponds with the slow-running speed of the engine and it will be realized that the airscrew will also be running at half speed. In order to absorb as little power as pos- sible, the airscrew must slow-run with blades in practically
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