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Aviation History
1960
1960 - 2239.PDF
FLIGHT, 7 October 1960 577 V' One main vertical gyro and its comparison gyro and a main and comparison air data sensers. Note the greatly simplified comparison units parison computers, Only two of a considerable number of pick-offs are shown in the diagram. One pair of synchros, again upstream of the clutches, provides the i.a.s. signals requiredrespectively for the automatic throttle controller and its com- parison system; and a set of pick-offs governs the steady reductionin autopilot sensitivity during the approach. As the ILS beams become narrower near the transmitters, the resultant angularsignals become coarser in relation to lateral displacement. A height gain signal from the air data senser is therefore nulled atthe beginning of the glide-path and autopilot response steadily reduced as height is lost and distance to touch-down decreases. The integrity of gear systems is adequate because all the gain-adjustment signals are taken upstream of the electro-magnetic clutches. Gearing gain values, which may be critical during auto-matic landing, are derived quite separately from the outputs used for manometric locks during en route flying. Common shaftdrives for the pairs of synchros avoid problems of matching which might occur with electrical pairing of completely separatesensers. The clutched synchros are also always engaged in the zero position with resultant improvement in signal noise. It isstill important that the pairs of pitot heads be sufficiently matched to prevent angle of attack, turn and gust errors from varying andcausing unnecessary difference signals. The main computer of each autopilot contains all the addersand shapers required to produce the control demands related to the inputs being used. The circuitry is built up in plug-in cardmodules on the Bendix model. About 25 different electronic and electro-mechanical modules are involved and a special automatictest installation has been designed to isolate faulty cards in boxes. Each main computer is housed in two 1 ATR cases. The com-parison computers are stripped down containing only the minimum number of modules essential for effective monitoringand therefore occupy only one short | ATR case each. Each main computer is divided into longitudinal and lateralamplifier and computer channels. All longitudinal aerodynamic parameters, throttle control, pitch autostabilization, height, i.a.s.,Mach number and glide-slope functions are in the longitudinal channel and the lateral channel accounts for yaw damper, azimuth,aileron and rudder, aileron damper and the dynamic vertical senser functions. Errors revealed by differences in signals between main andmonitor units are passed to the error disconnect system, described in a preceding article, which then disengages the autopilot byreleasing a lock integrated with the powered control unit. From the computer, amplified commands are passed to thetorque motor actuator mounted directly on the powered control unit at the appropriate control surface. The first summary ofthe automatic flare-out system of the VC10, published in Flight for February 19, described the mechanical control linkage andarrangement of split surfaces and individual actuators used in the VC10. In the elevator channel, the one most directly involved inautomatic flare-out, there are four surfaces and four powered control units, and the output of No 1 autopilot is passed to theinner port unit, the output of No 2 autopilot going to the inner starboard unit. Mechanical interconnection ensures that auto-pilot demands are passed to all four surfaces from any one auto- pilot and spring links ensure that jamming of one unit cannotimmobilize the remainder. Under manual control, mechanical feed-back positions the powered control valve in the normal wayand the pilot's input controls are reacted against the artificial feel loads. When an autopilot is engaged, part of the powered controlfeed-back linkage is locked and autopilot demands are fed directly into the main valve operating linkage by the torque motoractuator and associated servo valve. The connection is thus direct and the control cable and feej system loads are taken by thepowered control and not the autopilot actuator. Control is there- fore extremely close and the autopilot can position the surface to±0.3° with a band-width in excess of lOc/s. This offers con- siderable improvements over present-day stabilization systems. Inthe engaged condition, surface movement is reacted jointly against the feel jacks and against preloaded springs at the locking point,so that, if the powered control output demanded by the autopilot exceeds the normal feel loads, the preloaded springs yield andprevent any further displacement. Such a torque limiting method is advantageous in that it ensures that limits are not exceeded, butdoes not actually remove the control demand. The maximum permissible effort remains applied until autopilot demands oncemore subside. Nuisance cut-out in turbulent weather can there- fore be largely avoided. With the autopilot engaged, the normal mechanical feed-backof surface movement is replaced by an electrical synchro posi- tion feed-back directly to the autopilot servo; and the servoamplifiers are thus included in the autopilot powered control loop. Because the second autopilot is electro-mechanically slaved to thefirst during an automatic landing, only a failure in one of the powered control units themselves could cause one surface to movein opposition to the others. The flight director instrument system is considered as an alter-native to the autopilot for all modes except automatic flare-out and for this reason is integrated with the automatics to only alimited extent. It derives its attitude and compass signals res- pectively from the main vertical gyros and Polar Path compassesand takes radio information from either group of radio receivers. These signals are electrically separated from the autopilot as faras is practicable. The system is of necessity duplicated in that each pilot has his own course deviation indicator and horizondirector indicator but only a single flight steering computer is Diagrammatic view of the pilot's datum adjuster, main air data senser and comparison senser for one of the autopilot channels. The adjuster has two more synchros MONITOR serving the second autopilot DISCONNECT TRANSDUCERS COMPARISON SENSER ' COMPARISON COMPUTER
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