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Aviation History
1951
1951 - 1187.PDF
738 CARRIER FLIGHT CO-AXIALS . . . flange of the collet which, in turn, bears against the forward rim face of a flange on the oil transfer sleeve. The eyebolt/operating-pin assembly for the rear airscrew is similar to that used in the front airscrew, the eyebolts being connected direct to the pitch-change piston. Although seemingly far more complicated than the method used for the front airscrew, the transfer of oil to the rear airscrew for pitch-change and lock actuation is, in fact, not difficult to understand. In the annular cavity between the outside diameter of the inner airscrew shaft and the bore of the outer airscrew shaft is accommodated a cavity-wall sleeve. The annular cavity in this sleeve is axially divided into three separate volumes, each of which accommodates oil for one of the three actuation functions of the airscrew. From each individual oil cavity, tappings give into land-divided volumes in the oil transfer sleeve, whence the oil is conducted via drillings to the appropriate places—that is to say, to the front of the main piston for effecting a coarsening of blade pitch, to the rear of the piston for fine pitch, and to the rear of the lock piston for actuating the flight-safety stop lock. These dispositions of oil apply similarly to the front airscrew assembly. In the opening paragraphs of this article, the high rotational inertia characteristics of the gas turbine were touched upon, and by the same token that this inertia introduces control problems in the transitional phase of power change, so does it also introduce problems in engine starting. What it amounts to is that the compressor/turbine assembly has to be spun up to a rotational speed sufficiently high for the compressor delivery to be enough to permit combustion. This takes a little time, and the demand made upon the starter is not small. In turbo-prop power units where the airscrew is directly coupled (through a reduction gear) to the compressor, it is obvious that the starter has to turn the airscrew as well as the com- pressor/turbine assembly and, even with the airscrew blades at their normal fine pitch setting of about 20 deg, the additional load imposed on the starter as a result of the airscrew blades thrashing the air is very considerable. Third Oil Line To reduce to a minimum the airscrew load in engine starting, it has for some time been the practice to fine-off blade pitch to a low angle, and this has been effected by the incorporation of a withdrawable fine-pitch stop in the airscrew cylinder/piston assembly. In the past, this over-riding of the normal fine-pitch stop was obtained as a result of a temporary increase in fine-pitch oil pressure, but there were drawbacks to this system which made it desirable that the stop-lock should be actuated as an entity quite separate from the normal pitch-change actuation of the airscrew. Thus the third oil line was introduced. During his landing approach, the pilot presses a switch to energize the lock circuit and, the lock having been collapsed, the blades are able to fine-off to (and constant-speed at) an angle below that of the flight-safety stop in order to match the high-revs/low-power condition of the constant-speed engine. It should be noted here that "flight-safety stop" is merely a new term for what used to be called the "fine-pitch stop." The new name has something to commend it, however, in that although the stop acts as the normal fine-pitch setting of the blades, this setting is also the angle to which the blades will revert and at which flight can safely be sustained in the event of engine/airscrew control failure. When it is required to shut down the engine, the h.p. fuel-cock lever is pulled back beyond the normal closed position to the feathering position. The flywheel effect of the engine once more becomes manifest here, for it takes an appreciable time for the engine to run down to a stop, and this characteristic is put to use for, whilst the engine is running down, the c.s.u. is delivering oil. Movement of the h.p.f.c. lever back to the feathering position lifts a valve in the top of the c.s.u. which causes the residual oil delivery to be passed into the "coarse-pitch" line to feather the blades. By this means, not only does the high drag of the blades thrashing the air cause the engine to come to rest fairly quickly but, the blades being feathered, there is little tendency for the airscrew to "windmill" the engine in the comparatively high wind speeds which usually prevail over an aircraft carrier's deck. When the engine is once more to be started, the h.p.f.c. lever is moved forward out of the feathering position so dropping the valve in the c.s.u. to cut off delivery to the coarse-pitch line. The electrically-driven feathering pump is then energized and delivers oil to the c.s.u. whence it is directed to the fine-pitch line to unfeather the blades. A micro-switch on the oleo leg of the aircraft undercarriage acts as a safety unit in the lock circuit in flight, but when the aircraft is on the ground a solenoid is automatically energized to actuate a servo valve which admits high-pressure oil from the feathering pump to the lock cylinder. This withdraws the support ring of the lock collet, so that the main pitch-change piston can move fully forward This impressive drawing of the co-axial c screws for use with Double Mamba /x>v installations shows the internal mechoni of this complex assembly in unusua! c intricate detail. .
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