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Aviation History
1957
1957 - 1362.PDF
452 FLIGHT, 13 September 1957 ON THE GROUND ... POWERED FLfING CONTROLS PROGRESS, rather than brand-newdesign, was the keynote of the powered flying controls and systems on display atthis year's show. A firm which can boast new production orders for well-proved fly-ing controls is Automotive Products, whose Lockheed Servodynes are to be used onthe D.H. Comet and Sud Caravelle. Many manufacturers are looking aheadto the time when the linkage between the control column and the pilot is replaced byelectrical signalling and a great deal of development work is being done on thereliability and refinement of such systems. An important step that has been takenin this direction is to replace D.C. systems, with their attendant potentiometer wiperproblems—delicately controlled loading, arcing at altitudes and the "staircase" effectas the wiper moves from wire to wire—with smaller and neater A.C. pick-off s and torquemotors with very few moving parts. Where there is still a divergence of opinion (or per-haps a determined open-mindedness) is in the degree of standby and emergencymeasures that should be provided. The opinion generally held twelve months agowas that the first electrically signalled sys- tems should have a manual linkage (notmanual control-surface operation) standby system, with clutches operating automatic-ally in the event of failure of the primary electrical signalling. Now, manufacturersare not quite sure, although Boulton Paul Aircraft very wisely used an early designof powered control that would give emer- gency manual reversion from the stick tothe surface when they flew the first elec- trically signalled system in the Tay-Viscount. Although this was a D.C. system, theydemonstrated an A.C. torque motor system on their stand. A pair of torque motorsmounted on a differential bridge control the servo valve master piston. A pick-off This Boulton Paul flying control is the first electrically sig- nalled system to be flown. is mounted on the torque link (which pre-vents rotation of the screwjack output) to provide a position feed back, and a ratefeed back pick-off on the slave valve pre- vents input signal overshoot. The company were also showing theirsplit control surface aileron jack, which is now coming into production. These areused in multiples—one to each section— and operate independently. Both radial andswash-plate pumping elements are used. The Fairey Aviation Company alsoshowed electrically signalled A.C. systems (in conjunction with Newmark, Ferrantiand Maclaren), although to demonstrate the sensitivity and accuracy that could beobtained the dead-beat qualities of the con- trol had been somewhat sacrificed. Faireyare among the companies who prefer to wait for control thoughts to crystallize furtherbefore they finalize their signalling system. At present, a single circuit is used; duplica-tion and triplication are both possible, they say, and can readily be provided ifneeded. R.A.E. assessment of Fairey's work in this field is to be made in the nearfuture. The company has now accumulatedmuch valuable experience with the flat plate, balanced-delivery type of slide valve,which has been specifically developed for the electrically signalled control application.It has the advantage that the operating loads under threshold conditions are virtuallyzero, but Bernoulli effects require that flow Lockheed servodynes for the Comet (top) and Caravelle have long overhaul periods. rates higher than 3 g.p.m. should behandled by two-stage valves. Controls with both single and double stage weredemonstrated on Fairey's stand, and a miniature control column proved that theoperating loads were very light. The company were also demonstratinga feel simulator in which stick rate could be varied with altitude and Mach number.It consists of a circular leaf spring loading a cam which is restrained between a slide.A roller on the stick follows the profile. The cam is shaped to give V2 feel up toMach 1.0 where the spring force—and consequently stick rate—is modified by anelectro-hydraulic actuator operating on a closed loop in much the same way as anelectrically signalled control system. Other types of simple and Mach-cor-rected V2 feel simulators—in production for a number of aircraft—were shown onthe H. M. Hobson stand. Hobson's are also developing a simulator following a"V" law, which makes use of the variable- area property of a dished diaphragm asit is deflected. The company has also con- tinued with electrically signalled poweredflying control development and—following the general trend—makes use of magneticamplifiers and A.C. pick-offs in conjunc- tion with the Type 309 piston-jack-poweredflying control. It can now be said that Hobson have beencollaborating with the A. V. Roe Weapons Division in this work for about ten months;and to prove its reliability a Hobson-Avro non-active system will shortly be flown for1,000 hr in a Dakota. Hobson, who have long been in the forefront of flying controldesign, consider that triplication of the electrical stick/control connection, leavinga "majority-rule" signal if a failure occurs, is probably the optimum that can beachieved. Quadruplication and signal- monitoring systems have been studied, butthese give progressively less reliability return as complication is increased. Onlypractical experience can prove the best course to take, but by putting forward posi-tive arguments, Hobson seem likely to set the pace.A new flying control to this country appears on Turner Manufacturing Com-pany's stand. Designed by M. Rene Leduc, these controls are of the fixed cylinder typeand feature automatic mechanical locking for manual operation following totalhydraulic failure and depletion of the main and emergency accumulators. The aircraftcontrol linkage is very simple as the inter- nal ball-type valve is operated through anaxial piston rod. Rubber-teflon sealing rings are used on the main piston and thecontrol force required can be adjusted between 1.8 and 3.5 oz; but the fluid resist-ance to be overcome in manual operation
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