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Aviation History
1946
1946 - 0819.PDF
APRIL 25TH, 1946 FLIGHT 423 ME'TRO-VICK CAS TURBINE sions and is gathered in a circumferen-tial duct externally surrounding the compressor outlet, whence it is ventedto atmosphere. External pipes also supply air to a similar scent-spraynozzle at the front bearing for cooling and lubrication—principally the latter—and the drain is by a short pipe carried inside the bottom spoke of theintake shroud and vented to atmo- sphere. Cooling air is supplied to the frontface of the turbine disc through a channel arranged between the innerand outer cone extension, the air sup- ply being abstracted from the finalcompressor stage. At the wheel face a deflector plate ensures that the airis impinged on the disc near the centre, and it flows radially outward to escapeinto the jet stream past the roots of the turbine blades, which it also cools.The crowns of the turbine blades, and the roots and crowns of the statorblades, are cooled by tappings from the secondary air supply in the combustion chamber. It isalso worthy of note that air from the same source is supplied to the burner shroud tubes in order to cool the burners,escaping through vent holes at the rear of the shrouds into the flame chamber. Vent points at the sixth compressor stage supply airunder pressure for piping to the apices of the " star " frame supporting the fan turbines, the air being conducted downinside the tubular members of the frame to the interior of the hollow turbine shaft. Part of this air supply is divertedto the bearings where a feed of oil in one of the bracing frame tubes makes an oil/air mist for lubrication; the restof the air is devoted to cooling both th/<rear face of the IGNITER BURNER A section through the combustion chamber illustrating rows of air inlet "wedges "and outlet throat to turbine. The flame chamber is supported by peg locations projecting internally from the combustion chamber, and is thus fixed radiallyalthough free to move axially for thermal expansion and contraction. Rear view of unit with augmenter showing outlet orifices and tubular bracingmembers. The flange forward of these is a bell-mouth intake to the fans for test bed purposes only. after which it is supplied to a manifold feeding the twentyburners. The barostat is simply an aneroid device for regulating the fuel supply according to the atmosphericconditions obtaining; in short, it compensates for drop in density with altitude. The governor is incorporated as asafety measure to limit the speed to maximum r.p.m. by cutting down the fuel supply and thus prevent overspeeding.A tapping from the fuel pump delivery line taken off just before the throttle valve supplies a solenoid valve and.subsequently, two igniters at roughly 11 o'clock and 1 o'clock on, or in, the combustion chamber. To start the unit, the pilot switches on and the startermotor, solenoid, valve and igniter sparking plugs are energised. The opening of the solenoidvalve permits fuel to flow to the igniters, and the spark initiates a localplume of flame. At the same time the starter motor is accelerating the tur-bine and compressor, and, as soon as a rise in fuel pressure is registered onthe gauge, the high-pressure fuel cock is opened and fuel is at once suppliedto all 20 burners. The flame plumes at the igniters immediately promotecombustion throughout the chamber and thus the unit starts and picks upspeed ; when' the load on the starter motor is relieved, a throw-out switch breaks the starting circuit. compressor-turbine disc and the front face of the foremostfan-turbine disc. The mist from the fan-turbine bearing^ is allowed to escape into the interior of the bullet tail-cone,and is exhausted into the jet at the outlet orifice. It must surely be agreed that these measures of cooling and lubrica-tion show a degree of design quality more than deserving of parity with other aspects of this highly interesting unitThe fuel and control system is really very simple. Fuel is supplied from the aircraft tanks to a low pressure niter,whence it flows to the suction side of the engine-driven fuel pump, being delivered from there to the throttle valve,with a tapping taken from the main line to feed a barostat. From the throttle valve the fuel is piped to a centrifugalgovernor and thence to the main high-pressure cock control. Normal flight operation is by .1single throttle lever actuating the throttle valve—it is the only enginecontrol the pilot has to concern him There can be little doubt that, Metro-Vick having pointedthe way, ducted-fan thrust augmenters have come to stay, and although the overall diameter of the unit is thus in-creased, the gain in thrust obtainable by this means, and the amazing drop in specific fuel consumption, render theenlarged nacelle a very worth-while payment. In any case, working on a specific basis of pounds olthrust per square foot of cross-sectional area, the pure jet part of the unit has a ratio of 321.5 lb/sq ft when usedalone, whilst when incorporated with the augmenter, although the diameter has increased, the jump in thrustgives a ratio of 318 lb/sq It. On this basis, then, which is regarded as a criterion, the -."rearer size of the augmentedunit is shown to occasion a negligible difference.
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