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Aviation History
1948
1948 - 1085.PDF
JULY 1948 FLIGHT 75 POWER- OPERATED CONTROLS New Electro-Hydraulic System Developed for the Saw SR 45 ^MONG the many problems which attend the design /~\ and construction of very large aircraft is that of "*• •*• flying-control operation. The speeds of modern large aircraft, and the physical size of the control surfaces of such giants as the Saunders-Roe SR 45, make some form of power operation essential. Further complications arise from the fact that the pilot must have available full control at all times, even if some part of the main system should fail. The sysem which has been developed for the SR 45, and which is, of course, applicable to other large aircraft, is the result of co-operation between Saunders-Roe and Boulton Paul, the latter firm having designed and developed the power unit, which comprises two identical electro- hydraulic torque converters. Three independent systems are used, one each for elevator, rudder and ailerons; all major components are interchangeable. Details of the new power-control system vvere disclosed bySaunders-Roe yesterday. The* principle of operation is as follows: Initial power is supplied by a constant-speed electricmotor operating off the aircraft 120V. DC supply, with battery iloat. The constant-speed variable-torque output of the electric motor is fed into a hydraulic variable-speed unit,comprising a variable-capacity hydraulic pump and a fixed- capacity hydiaulic motor. The output of the completeassembly is tnus a variable speed, with a limited maximum torque drive. The limit on torque is provided by a suitablerelief valve in the hydraulic circuit; the maximum output speed is governed by the respective hydraulic pump and motorcapacities. Control of the variable-speed unit is obtained by adjustment of the hydraulic pump's specific capacity. An infinitely-variable speed drive is thus obtained from theoutput of the hydraulic motor. This is taken direct to the appropriate control surface via torque shafts and gc_r trains,terminating as an arc control by means of a mechanical screw jack and radius-arm assembly. In practice, several screw jacksin parallel are employed. . Positional Control Control of the surface, so far outlined, would be purelyvelocity control. That is to say, the rate of operation of the surface would be proportional to the pump capacity settingIn order to obtain a positional control (as on an orthodox aircraft) a differential link has been incorporated whereby theoutput displacement is fed back as a cancelling signal when positional alignment has been reached. A differential gear box SARO POWER - OPERATED CONTROLS: This diagrammatic representation of the elevator control does not attempt to represent the actual physical design. The aileron and rudder systems are similar in general principle. DIFFERENTIAL FO«IH0FGEAR "-15O2 I'TRAVEL 1^** t 751 05°T MqijjjL^ FOLLOW-UP MAX TRAVEL?4629-65° UNIVERSALS '"* ELEVATOR NO MASS BALANCE TRAVEL :E9 753° MAX RATE 24 I52'/S£C MAX. AVAILABLE TORQUE 45 LBFT. MAX.SPEED 480 RPM MAX TRAVEL i 3545 4° TORQUE SHAFT MAX.SPEEO 861 82 RPM MAX. TRAVEL-6565-7" TO REDUCE TOTAL MAX HINGE MOMENT TO 5000 LEFT.
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