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Aviation History
1955
1955 - 0737.PDF
FLIGHT, 27 May 1955 735 JET-AIRLINER SYSTEMS... yellow and red are stand-bys. The supplies serve components asfollows: green, half of all the flying controls, and undercarriage, flaps, air-brakes, wheel brakes, nosewheel steering and passengersteps; blue, powered controls and feel-simulation. Yellow is a stand-by for powered controls and feel-simulation and red is thestand-by for undercarriage lowering, flaps, wheel brakes, nose- wheel steering; and it is normally used to open the freight door. The green reservoir is on the port side aft of the rear cabinbulkhead and holds 39 litres in all, while the blue reservoir is in the corresponding position to starboard and accommodates 16litres. A section in the bottom of each of these reservoirs holds a supply for the yellow system. The red reservoir, in the forwardfuselage and holding 15 litres, is divided into two compartments, one to supply the red motor pump and the other for a hand-pump.The blue and green systems are fed by four engine-driven hydraulic pumps, one blue and one green being mounted on each engine.The blue pumps are Lockheed Mk 7 with a capacity of 14 litres/min and the green pumps are Lockheed Mk 8,28 litres/min.The yellow system has a Lockheed Mk 7 (14 litres/min) electric- ally driven pump which, in case of double engine failure, can berun on the batteries alone for 30 min. Another Lockheed Mk 7 (14 litres/min) pump is electrically driven for the red system.Pressure in all circuits is 2,500 lb/sq in. To sum up, blue is flying control primary; green is flying controlsecondary and services primary. Controls normally run on primary and secondary together. Yellow can draw fluid from either theblue or green reservoirs and supply pressure to either the blue or green sections of flying controls, whichever should require it.In the event of both blue and green supply to the controls failing, electrically generated yellow power can be fed into either side ofthe Servodynes. Should all generators fail the yellow pump can be driven by the batteries for 30 min; after that permanentlycharged accumulators can maintain flight control for a little more than ten complete deflections of all control surfaces. With carethis can be made to last a considerable time. The control system is described in full below; suffice it to say here that it is fullypowered and irreversible in all axes without provision for manual reversion. There are seven main permanently charged accumulators. Onein the blue system provides the ten full deflections of all controls; another is in the yellow system. There are several in the greensystem, one to maintain control pressure for 12 sec, which is the time during which undercarriage lowering or retraction requiresthe majority of the running effort; two more provide for one under- carriage operation in case of engine failure on take-off and anotherprovides for passenger-staircase actuation while the engines are stopped. Two accumulators in the red circuit give stand-by powerfor all services and pressure for emergency operation of wheel brakes and parking brake. The undercarriage can be lowered bygravity and a special valve is provided so that fluid can run freely into the jack to eliminate drag from this source. A few strokesof the hand-pump in the red circuit are sufficient to engage the down-locks. Flaps and droop-snoot leading edge are operated bytwo Hobson type 197, integral 10 h.p. hydraulic motors through shafts and screw-jacks. One side of each motor is in the greencircuit and the other in the red. Either supply can be selected from the cockpit as required. Full deflections of the droop-snootand flaps require 300 turns at 600 shaft r.p.m., and after 300 turns the motor will stop within plus or minus a quarter of a turn ofthe pre-selected position. Rapid movement of the control lever, however, would result in hammering in the lines, and the cockpitcontrol is therefore arranged with a spring follow-up so that flap position is selected with the lever, and a pointer moving in thelever slot shows the actual position of the flap linkage as it follows the spring pressure from the selector. The air-brakes are normally worked from the green circuit bytwo piston jacks in each wing. It is not considered necessary to link these to any of the stand-by circuits. The undercarriage canbe lowered, but not raised, on its red stand-by supply. All the fairing doors are mechanically linked to the undercarriage legsso that dropping the wheels by gravity will also operate the doors. There is, however, a secondary jack which assists in the movementof the large mainwheel door; the services of this jack can be dispensed with in emergencies. Nosewheel centreing and shimmy-damping are performed by the two hydraulic jacks which are used for nosewheel steering. The control in the cockpit is in theform of a small steering wheel mounted on the elbow of the port control column—an arrangement identical with that of theComet. Each Hispano Suiza main undercarriage leg carries a bogie offour Dunlop wheels fitted with Maxaret-controlled disc brakes and dimpled tyres. Brake power is drawn from the green under-carriage-lowering circuit and can be applied differentially to each main bogie by pedals on the rudder bar. Brake pressures, bothapplied and available from green and red systems, are indicated on a panel of three instruments in the roof of the cockpit. Under-carriage position indication is by micro switches and electric warn- BLUE ESERVO1RGREENRESERVOIR CABIN PRESSUREYELLOW _SUMP PUMP ON 'PORT AND ST'B'DENGINE PUMP ONPORT,, AND STBENGINE YELLOWBATTERY- DRIVENPUMP REDBATTERY DRIVENPUMP Schematic diagram of the hydraulic supply. Flying controls normally operate on primary and secondary supplies simultaneously, the stand-by taking over whichever of them might happen to fail. The freight hatch, passenger stairway and air brakes need only one supply. ing lights in the cockpit. The position of the wheels themselvesare shown by the standard red and green light patterns but, in addition, there are warning blinkers indicating nosewheel unloaded(this for the prototype only) and undercarriage doors unlocked. A warning lights up, and a horn blows, if throttle is reduced belowa certain point with undercarriage retracted. The horn can be switched off if desired. Windscreen wipers are hydraulic, but are operated from a com-pletely independent hydraulic circuit with two small electric pumps. If necessary, they can be worked manually, much ascan those of a car. The entrance ladder under the tail is actuated from the greencircuit by two piston jacks and has hydraulically operated mechani- cal locks. A hand-pump is available as well as an accumulatorfor operation when the engines are not running. It was foreseen that, in the event of a wheels-up landing, the latter steps wouldlie at an angle to the horizontal, making it difficult and possibly dangerous for passengers to run down them in a hurry. A link-age was therefore arranged by which, regardless of the ladder's angle, the steps themselves would always be horizontal. The freight-loading door to port in the forward fuselage ishinged along its upper edge and operated by two jacks in the red circuit only. The door can therefore be opened by starting thered circuit electric pump, or by using the red general accumulator or through the handpump in the red circuit. Powered Control.—The Caravelle's controls are fully powered,irreversible and hydraulically operated. In general terms, this means that the control cables from the pilot's cockpit operate thevalves on the control jacks only and that all the effort required to move the control surfaces is supplied by hydraulic power. Noneof the forces acting on the surfaces is transmitted back to the control column. But the pilot must have some indication of thestrain he is imposing on the airframe and a system is therefore incorporated to provide a force which resists stick movement.This artificial feel is made proportionate to the airspeed. When the pilot is not applying a force to the stick this feel load, likethe aerodynamic load on plain controls, returns it to a neutral position determined by the setting of the stick neutral-positionadjustment, which is the trim control. To displace the valve on the jack at the control surface requiresvery little force and a very small movement. Any incidental distortions in the control-cable circuit, if they affect the positionof the valve, will cause control-surface movements which, firstly, have not been selected by the pilot and, secondly, will be difficultfor the pilot to counteract. The controllability of the aircraft— that is, its responsiveness and its stability—will be affected byany imperfection in the design and mounting of the mechanical section of the control system. If anything so vital as the control surfaces of an aircraft arehydraulically operated, some form of stand-by must obviously be provided against the possibility of failure in the air. This canbe done either by reverting to direct manual control (manual reversion) or by taking power from another source. Manualreversion, however, introduces a great number of problems, since the finely balanced mechanical section of the powered-controlcircuit will have to be stressed to take the full air-loads produced by control surfaces that have not necessarily been balanced andadjusted for manual control. In any case the loads in such a system on a fast, large aircraft would probably be beyond thecapabilities of the average pilot. The most effective way of pro- viding the necessary precaution is to use a stand-by source of
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