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Aviation History
1964
1964 - 2169.PDF
186 FLIGHT International, 30 July 1964 "Flight International" photograph One of the P.53I-0/N development aircraft—lighter, much less powerful and differing in almost every detail from today's Scout and Wasp SCOUT AND WASP... rear cross-tube is anchored centrally, but the front cross-tube is fixed to both the main girder webs. To overcome the elasticity of the body, diagonal struts connect the ends of the rear cross-tube to the girder main webs, thereby stiffening the whole airframe vertically and laterally. The finished chassis was resonance tested, with artificial cyclic-control disturbances, to check that the margin between the actual and allowable blade angles would meet the severest field conditions. The Wasp undercarriage, by Automotive Products, seems to have been easier to perfect from the ground-resonance aspect, the involved needs of deck landing having amply met this case. The allowable blade swing angle is well over 10° at all r.p.m., and the largest that could be induced artificially was only 0.6°. The four units of the Wasp chassis are virtually identical, being parallel- action linkages carrying wheel units and diagonal shock-absorber/ damper oleos. The track has been kept as wide, and the wheel base as long, as possible which has necessitated the addition of stub winglets to the tapered rear fuselage. The lower V-links are pinned to the main girder webs, while the front and rear upper V-links are fixed to the reinforced floor edge and the winglets, respectively. The castoring posts are designed so that each wheel is free to move through 360°, can be locked fore-and-aft or locked at 45° port and starboard. The deck crew insert the locking pins, the angled positions being used in crosswinds or to turn on the deck. There is a tie-down shackle and winching bracket on each castoring post, and a jacking pad on each lower V-link. Each wheel has a brake which is really a lock; it is either on or off. Applied by a push-down lever, it consists of a planet gearwheel and a small segment spring-locked in to prevent sliding on the deck. The brakes are unlocked hydraulically, and so are fail-safe. Powerptont The derated (potentially 1,050 s.h.p.) Bristol Siddeley Nimbus is completely uncowled; accessibility approaches perfection, and the engine deck forms an ideal working platform for tools and parts. Only when the upper engine items or rotor are involved is there any need to mount the platform. From the aerodynamic viewpoint the absence of cowling makes little difference, since the local airflow is rather complex, and in forward flight the engine is shielded by the main-rotor gearbox and cabin roof. The Nimbus is sufficiently well known not to warrant a descrip- tion here, but a word about the ratings is needed to clarify its application in the Scout/Wasp airframes. The main rotor (two- stage axial/centrifugal compressor and two-stage turbine) revolves at a constant 35,000 r.p.m.; the single-stage power turbine turns at 30,000 r.p.m. and drives a 13.33 : 1 reduction gear to the output Main-rotor pylon, drive and control linkage: I, collective-pitch control beam; 2, cyclic-pitch servo unit (port and starboard); 3, cyclic rods; 4, cyclic bellcranks; 5, collective servo; 6, collective rod; 7, spider spindle; 8, spider piston; 9, spider arms; 10, pitching-moment compensator rods; II, main-rotor shaft; 12, compensator flyweights; 13, bearing oil feed; 14, main-rotor gearbox; 15, spiral bevel; 16, oil pump; II, Layrub coupling; 18, input pinion; 19, hydraulic pump; 20, crown wheel shaft at 2,150 r.p.m. Power is governed by varying the fuel flow which, apart from the effect of the throttle described later, is metred to limit the gearbox torque. In the Scout the maximum power requirement is 685 s.h.p., while in the Wasp it is 710 s.h.p. The requirements can best be represented tabularly:— Scout 5 minutes 685 s.h.p. up to ISA-^ 30° at 7,000ft and to specific humid- ity 0.04. i hour 685 s.h.p. up to ISA+ 25° at sea level or ISA at 13,000ft. Max continuous 600 s.h.p. up to ISA + 27° or ISA at 16,000ft. Wasp 710 s.h.p. up to ISA + 25" sea level. 685 s.h.p. up to ISA + 25' at sea level. 600 s.h.p. up to ISA + 270 at sea level. In other words, the naval engine is adjusted to give an extra 25 s.h.p. to cover the additional 2001b normal take-off weight increase, and this is available to the same temperature limit as on the Army aircraft. The naval helicopter's normal duties are at sea level, for anti-submarine work. The Nimbus is carried on a four-point mounting. The forward fitting is a steel-tube stirrup with a Metalastik bush at the top of each arm supporting fittings on the air-intake casing. Lugs on the stirrup are pinned to two of the pylon struts on the engine deck. This arrangement takes up the expansion of the engine as it heats. The rear attachment consists of two rubber-covered trunnions on the reduction-gear casing held in machined fittings riveted to the engine deck. The engine spur reduction gear lies between and below the splayed exhaust pipes and is coupled through a freewheel to the main-rotor shaft, running forward under the engine, and to the tail-rotor shaft. On the port side of the engine deck is an oil- cooler duct fed by an Airscrew Company fan driven from the engine gearbox. The Nimbus is supplied as an ECU (engine change unit) complete with accessories. Control of the Nimbus is combined with rotor constant-speed governing. Gas-generator speed and power are controlled by a twist-grip throttle on the collective-pitch lever, with an adjustable friction damper to stop creep. The free turbine is governed within relatively close limits to pre-selected r.p.m., which maintains constant rotor speed irrespective of loading due to changes in blade angle. The desired speed is selected by a faster/off/slower switch on the collective-pitch lever. Originally, an automatic torque-limiting system was superimposed to prevent the engine from exceeding the transmission limit. Now all Scouts and Wasps have an audible warning by Shipton Auto-
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