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Aviation History
1962
1962 - 0741.PDF
FLIGHT International, 10 May 1962 739 VICKERS VC10 . . . The spoilers also act in unison as airbrakes, in response to opera tion of the pilot's airbrake lever. Airbrakes and lateral control can be applied simultaneously, and it is the function of the mixing box to make this possible. Thus if control-wheel movements are applied when airbrakes are in operation, the spoiler on the up-going aileron side will open still more, while the spoiler on the same side as the down-going aileron will remain extended to the position selected on the airbrake lever. Trimming Lateral-control trimming is effected on the ailerons only, by means of two electric actuators. Rudder trimming is incorpor ated in the artificial-feel unit, by adjusting the datum setting by handwheel. Longitudinal trimming, both normal and autopilot, is carried out by hydraulically varying the incidence of the tailplane. The controls are duplicated, one hydraulic motor on system B being controlled by the first pilot, while the other motor on system A is operated by the second pilot; but the first pilot can operate the latter by means of subsidiary levers. To operate the tailplane trim two spring-loaded levers have to be moved simultaneously: one to arm the actuating system and one to select the rate of move ment, infinitely variable up to a maximum of 2/3° per sec. This is to safeguard against runaway; release of the levers will immedi ately shut down the arming valve in the hydraulic circuit. Autopilot and autoflare A detailed analysis of the philosophy and engineering of the automatic flight control system and flight director instruments of the VC10, particularly in relation to auto matic flare-out, has already been published.' Here it is proposed to pick out only the broad main points. In order to retain full automatic control after any one failure in the system, a duplicated autopilot system has been adopted. The system, supplied by Elliott Brothers (London) Ltd, is based largely on components of the well-proved Bcndix PB-20 autopilot, made under licence by Elliott, and interchangeable with American- built components as installed in Boeing 707s. However, the system as a whole—i.e., the dual autopilot concept—is novel, and designed entirely by Elliott. Only one autopilot is used to fly the aircraft, and the two systems, including power supplies, are completely independent. Each autopilot has a comparison monitor which detects faults and, in flight, will disconnect the system if these faults are likely to lead to dangerous conditions. For autoflare the system provides for automatic changeover to the second monitored autopilot system, in the event of fault in the first. Under these conditions the second autopilot is primed and ready to take over. If for any reason the monitoring system fails to prevent an autopilot runaway, the control movement is limited to a safe amount by the yielding of a torque-limiting spring. In addition to the usual modern autopilot facilities, the VC10 autopilot provides Mach-number and IAS locks, and adjustable datum for the height, Mach and IAS locks; automatic throttle control of airspeed on the approach; automatic flare-out control of the pitch channel; and yaw stabilization. The automatic throttle control is, of course, for use only at low speeds, and it can be used under manual control as well as under the autopilot. A friction coupling in the autothrottle drive to each powerplant control linkage enables the pilot to override the system manually without electrically disengaging. Yaw dampers are fitted to each section of the rudder, and it is possible to operate them without engaging either of the autopilots. Under autopilot control, the yaw damper on the bottom section of rudder is controlled by one autopilot, and that on the middle section by the other autopilot, the other two sections in either case being slaved to the master. The third yaw damper is not assoc iated with an autopilot; it is provided mainly as a standby, and for use in the event of Elrat being brought into operation. Associated with the autoflare system are the necessary radio aids: duplicated self-monitoring glide/slope receivers, Marconi AD.260; and dupli cated self-monitoring radio altimeters STR.4; specially developed for automatic landing by Standard Telephones and Cables Ltd.* Electrical System Since the primary flying controls are electrically powered, a flight-proved generating system is essential. Accordingly Vickers-Armstrongs have selected constant-frequency Westing- house 40kVA, 200V, 400c/s brushless alternators (a.c. generators) as used on the Boeing 707, and manufactured under licence by Plessey. One generator is driven by each engine through a Sund- strand constant-speed drive, made under licence by English Electric, and rated at 57 h.p. with an input speed range of 3,600-7,000 r.p.m. and an output speed of 6,000 r.p.m. Very comprehensive protection is provided against under-voltage, under-excitation, over-voltage, over-excitation, differential current protection, stability protection, negative sequence voltage protection and ground supply phase sequence protection. As noted, there are two completely separate systems, each supplied by one port and one starboard engine-driven alternator. Thus alternators 1 and 3 supply system A and 2 and 4 supply system B. For the critical take-off and landing cases, the two systems always keep their identity as separate systems; but in cruising flight, should one alternator fail, it is desirable to share the load between the remaining three, and therefore the two systems can be paralleled. Similarly there are two separate 28V d.c. systems, each supplied by a silicon transformer rectifier unit (TRU). A third standby TRU can be connected to either of the d.c. busbars. Each system also has a 17A, 24V battery to cater for various emergency loads. In order to limit TRU loading and save weight, certain lighting and heating loads are supplied with 28V a.c. from 115/28V transformers. Pitot-head-heater loads are shared between the low voltage a.c. and d.c. systems. Elrat, the emergency ram-air turbo-alternator, incorporates a very rugged Rotax solid-rotor alternator. Switching from the main to the emergency busbar, and switching-out of non-essential services, is brought about automatically on mechanically lowering Elrat into the airstream by a lever within reach of all crew-members. Elrat provides for the following services: five p.c.u.s, one artificial- feel unit, one emergency yaw damper, one TRU, one VHF or HF transmitter/receiver, one ADF, one Polar Path compass, one Flight Director system, one turn-and-slip indicator and one emergency artificial horizon. Very great care has been taken in the installation of power supply and distribution to ensure that the two discrete systems cannot be short-circuited together. In the electrical/electronics bay, forward of the forward freight compartment, electronic components of duplicated systems are rack-mounted on opposite sides of the aircraft. Likewise, the electrical control and protection equipment and associated busbars for each system are on opposite sides of the aircraft. The two channels forming one system are separated by a sandwich panel on which they are mounted. The main cables from the two systems are run in different ducts spaced well apart. Hydraulic System The split hydraulics on the VC10 have the advantage of eliminating the conventional, rarely-used, emergency hydraulic system and its associated drills. Again chosen for proved reliability, power for the hydraulic systems is provided by four engine-driven Vickers Inc variable-displacement pumps, made under licence by Sperry. The pumps on Nos 1 and 2 engines supply system A and those on Nos 3 and 4 engines system B. Approximately 80 hydraulic h.p. is provided. The fluid used is Skydrol 500A, and maximum working pressure is 3,0001b/sq in. Each system has its own fluid reservoir, from which the Vickers pumps are fed by hydraulic motor-driven booster pumps. In cruising flight, when the demand is low, one pump in each system can be off loaded. Accumulators are installed in each system to improve response and meet peak demands. Certain services operated hydraulically call for both systems: these are the undercarriage (port units operated by A, starboard by B), flaps, slats and spoilers. The tailplane incidence can be varied by either system. Nosewheel steering is normally by A, with B as an alternative; main wheel brakes are actuated by B, whilst A powers the standby brake-system not providing antiskid facilities. Windscreen-wiper parking is on A only. In each system, a pressure-maintaining valve opens only when the pump delivery pressure exceeds 2,0001b/sq in. The lines to the tailplane motor and spoiler jacks are taken off the main line upstream of this valve, thus ensuring that these services are not deprived of power when simultaneous demands are made on the general services. Although the A and B systems normally work separately, it may occasionally be necessary to ferry an aircraft on three engines. To ensure that the landing gear retracts com pletely in this case, the two systems can be temporarily connected through a commoning valve for take-ofF. In the event of complete engine failure, power is still available through system B for operating tailplane incidence, spoilers, flaps and slats by lowering the Hyrat, the emergency ram-air driven turbopump. This draws fluid from the return circuit of system B
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