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Aviation History
1949
1949 - 0257.PDF
February 10th, 1949 age from the assembly and protect the clutch plates from the grease. The whole power unit will be lifted with the aid of four eyes on the clutch driving plate. Clutches of this type are new components in aircraft and do not lend themselves very well to aircraft constructional practice. A clutch required to transmit over 500 h.p. needs plenty of body in the back-plate and weight in the cast-iron pressure plate to give rigidity, and absorb the heat generated. However, a nine-cylinder radial engine needs a small fly- wheel to run at low idling speed, and the clutch back- plate, to which are attached directly the 24 cooling-fan blades, fulfils this function. Above the clutch there is an interesting flexible shear coupling comprising the scalloped flange of the upper coupling sleeve already referred to, a series of light alloy bolts with which it engages, and eight steel cups bonded into rubber bushes through which pass bolts for attachment of the upper drive-shaft flange. This coupling is illus- trated. Good starting from cold, and steady idling speed, are engine features which assume special significance in a heli- copter. Protection of the,jnain rotors has always to be in able air loads and be aerodynamically efficient. Quick- release toggle fasteners secure the four detachable sections, and the whole is supported on its square-section tubular ring by light tubular stays from the main crankcase bolts and the front cover plate studs. The cowling is located on the flanged edges of the baffles at the lower end and on small fibre blocks, attached to the rocker-box covers, which also support a Graviner extinguisher ring, at the upper edge. The cooling-fan shroud ring is separate from the cowling. The timing-case breather passes through the small annular space between cowling and shroud ring. Air supply to the separate oil cooler is by-passed from the top of the engine, and it escapes through a separate port. The carburettor air intake is also fed from inside the cowling, close in to the centre line (between Nos. 5 and 6 cylinders). This results in a degree of centrifugal cleaning. On the 23.HM the new accessory and pump arrangements are easily recognized. The accessories section carries the magnetos, fuel injection unit and boost control unit. The sump, or bottom cover, mounts the pressure and scavenge pumps, the r.p.m. generator and, almost horizontally, the starter motor. There is a triple oil union for entry, dilution For the Westland Sikorsky helicopter the Jjeonides VV5.51 .L is fitted with a reverse and reduction gear, and a separate sump below the accessory casing. The graceful Bristol 171 helicopter for which the Leonides 23.HM. has been developed.. mind. Intermittent slow running or fast idling for warm- ing up may both be unacceptable for this reason and be- cause of the presence of the automatic centrifugal clutch with its close limits on engagement speed. The Leonides helicopter units have a modified Hobson fuel unit which includes a special cold-starting system. The aim is to achieve smooth running when cold at 600 r.p.m. with throttle closed and with no speed increase as the engine heats up. Incidentally, the throttles are oil heated. A good deal of research has been required to produce satisfactory cooling-fan blades. Airflow characteristics through the cooling system were examined, and it seems • that some pulsation was detected. For this reason the simple pressed (twisted) steel blades were abandoned in favour of the present forged alloy type with forked roots which bolt to the clutch back plate. For cooling purposes the engine is closely cowled. Air drawn in from above, ahead of the rotor gear box, is circu- lated over the .cylinder fins and out over the accessories below and to the rear. The circular cowling is of much lighter construction (22 gauge) with simple, quite loosely fitting sections by comparison with the precision fit of a Standard Leonides cowling, which must withstand consider- and thermometer, and Graviner flame detector switches are also to be found on the rear cover. Oil is only drawn from the valve gear of the lower cylinders on the standard Leonides; in the horizontal engines, "Y" drain pipes have been fitted to all the rocker-boxes, one scavenge pump has been eliminated, and there is a large external scavenge pipe from the position of the original between-cylinders sump. Furthermore, oil drainage occurs through the supercharger gears, extra holes being provided in the guide vanes (see illustration). _,-... Mounting Features Two. Graviner extinguisher rings, one above and one below the cylinder bank, are provided, and some ingenious rearrangement of ignition harness has been necessary due to space restriction. Mountings are different on each of the helicopter engines. On the 23.HM there are three main attachment bolts to the crankcase. Bristol designers claim certain virtues for the triangulated mounting which is illustrated. The mount- ing ring is supported by six tubular struts with ball-end attachments. The flexible portion, known as the Dynaflex
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