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Aviation History
1955
1955 - 0738.PDF
^RUDDER 736 FLIGHT, 27 May 1955 HYDRAULICFEEL JACK Trim, stick centring, feel and variable response gear under the cockpit floor. Input from the sticks is divided to serve the trim gear and cable pulleys individu- ally. Movement of the trim linkage offsets the stick neutral position dictated by torque rods and feel jacks. The oval pulleys will later be moved out to the Servodynes leaving at this point only the autopilot input pulleys which are not shown here, but are concentric with the oval pulleys. JET-AIRLINER SYSTEMS.., * power, and diis is done in die case of the Caravelle. The main power units in die Caravelle's control system areLockheed tandem Servodynes, one for each aileron, one for eleva- tors and one for rudder. They are respectively, two AIR 42756for ailerons, AIR 42762 for rudder and AIR 42556 for elevator. The units are made up of two jacks mounted back to back, wididieir pistons anchored to die structure and dieir bodies operating die control surfaces direct dirough push-rods. The power supplyenters at die piston mounting, passes dirough die piston and is exhausted from the body. All supply lines are rigid pipes, andonly die return lines are flexible. The maximum effort of each jack is 3,100 lb at 2,500 lb/sq in,making 6,200 lb for each Servodyne unit. Not all die power avail- able is required at each surface, but all the Servodynes are similarin construction and die effort supplied by each is fixed by using a different stroke obtained by varying die lengdi of the hingeradius arm. The aileron units give 3,037 lb-ft widi 4.72in stroke; die elevator unit 3,978 lb-ft with 5.9in stroke; and the rudder unit3,580 lb-ft. To provide die maximum rigidity in die surface/ Servodynelinkage, the Servodynes for die elevators and rudder are mounted at one end on die control surface hinge-bracket itself. The hinge-bracket structure is dien carried direcdy forward and die odier end of die Servodyne mounted on it. In the case of die ailerons,the power unit lies in between two sections of aileron between two reinforced trailing-edge ribs abutting direcdy on die rearspar. The aileron hinges pass through tiiese ribs and swinging links mounted on diem are actuated by the push-rods from theServodyne body. To achieve maximum stability and minimum friction, die con-trol cables are kept as much as possible in long straight runs and tensioned at 110 lb. In die fuselage tiiese are situated immedi-ately beneadi die cabin floor and straight down die centre-line from die cockpit to the rear pressure bulkhead. Each controlcircuit passes only once through die pressure cabin wall and at this point die two cables are converted into a single torsion linkage.The seals at die bulkheads are glands around a rotating shaft which give at once die best pressure seal and die minimum friction. The elevator Servodyne mounted on top of the tailplane torsion box. One end is anchored to the elevator hinge axis itself. The two halves of the unit normally work simultaneously in the blue and green circuits. HYDRAULICPOWER INPUT The aileron control cables pass through the pressurebulkhead immediately behind the rear spar and are thence carried in a straight line along the rear spardirect to the Servodynes. Those for the elevator and rudder pass through the rear bulkhead on either sideof the sill of the passenger door and, in further short cable runs, to the Servodynes. The elevator Servo-dyne is mounted on top of the central torsion box structure of the tailplane within the fin, while thatfor the rudder is mounted on its side on a box beam running from the lower rudder hinge bracket upwardsand to the left to a fuselage frame. This frame is here steadied by a triangulated structure from the reinforcedfuselage frame immediately ahead of it which re- inforces the passenger-access tunnel. The cables cantake the greatest force applied by two pilots—1,100 lb. Valve operating force is 1-2 kg (2.2 — 4.4 lb) andsensitivity less than one-tenth of a degree. Beneath the cockpit floor lies die mechanism con-cerned widi stick-centring, feel-simulation, trim and control-surface response variation. The controlcolumns are identical with those of the Comet and the linkage between them and the beginning of die cable- -' „ • runs is by push-pull rods. These rods lead to two axes mounted centrally under the floor where each-.:.• circuit is duplicated, one side leading direct to the cable - pulleys and die odier to the trim linkage and stick-centring mechanism. From die accompanying illustration it can be seen diat die pulleys are oval. By this means, the rateof control surface response to stick movement is rendered non- linear, so diat towards die stick-neutral position the surfaceresponse is reduced. This gready assists control at high speeds, when a powered control system tends to become over-sensitive.On the production aircraft die oval pulleys will be moved out to the Servodynes in order to maintain the full range of cablemovement right out to die control surface, thus avoiding over- sensitivity to any play which may have remained in die circuits. Stick force is determined by die centring action of bronze-beryllium torsion bars and by a further force proportional to die aircraft's airspeed. This is done through a Hobson feel-simulatorcontrol, type 159, which regulates hydraulic pressure in diree Hobson centring jacks, sometimes called "q-pots," according todynamic pressure sensed at the starboard pitot head. The trim wheels, mounted in the natural sense on die controlpedestal, each operate a worm wheel and a tootiied quadrant which, through a push-rod, operates the differential linkage thatoffsets die neutral-position of each control as required. The differential linkages are connected by cable to the centringmechanisms. Since feel and trim in such a control system are entirely artifi-cially produced, it is possible to gain some idea of the handling qualities of die aircraft by operating die ground rig. Break-outforce on die stick appears to have been eliminated and stick- centring is positive and pleasant. Stick forces have been adjustedin relation to die operational use of the aircraft and are therefore fairly large. Loads are varied according to E.A.S., thus producingq-feel. There is no compensation for compressibility effects. Air Conditioning and Pressurization.—In conformity widimodern practice die air conditioning and pressurization systems are linked with each odier and rely for their charge air supply onengine compressor bleeds. Air thus drawn at comparatively high temperature and pressure from the engines is treated for tempera-ture and for moisture content and fed into die cabin. Pressuriza- tion is obtained by dirottling the outlets, and all air is dumpedoverboard widiout being recirculated in the cabin. Principal design responsibility has been shared between S.E.and AiResearch in America, practically all the Com- ponents being supplied by the latter company. Thede-icing system uses a proportion of die air bled from die engines. Cabin air is bled from die 15di—and last—stage ofeach Avon compressor at a temperature equivalent to ambient plus 200-250 deg C, and at a rate of 75 lb/minper engine at sea-level and 45 lb/min at height. This is equivalent to 1.2 lb/person for 75 occupants at40j000ft. Air from each bleed is passed, as required, dirough a heat-exchanger, brake-turbine-type expan-sion unit and water extractor mounted close to each engine. Each of diese units can be cut out accordingto die temperature and moisture required, but above 19,200ft die moisture content of die air is low and diewater extractors are automatically cut out by electric valves controlled by a barometric capsule on die rearbulkhead. Air reaching the cabin is automatically regulated by tiiermostatic cut-off so diat it does notexceed 100 deg C at the delivery nozzles; diis is in order to avoid any risk of melting the vinyl-sheetcovering of the glass-wool insulation layer in die walls of die pressure cabin.
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