Patent GB-780759-A (GB)
Title: Improvements in or relating to aircraft control systems
IPC: B64C27/32
Filed: 1954-12-03
Abstract: 780,759. Aircraft controls. HOBSON, Ltd., H. M., and WESTBURY. R. Nov. 14, 1955 [Dec. 3, 1954], No. 35064/54. Class 4. [Also in Group XXIX] A power-operated flying-control system in an aircraft comprises a hydraulic servomotor for actuating a control surface of the aircraft and operable by an electro-hydraulic actuator under the control of an electrical signalling system and also by a mechanical linkage, the arrangement being such that in the event of runaway movement of the actuator as a result, for example, of the failure of the electrical signalling system switches are operated to cause a changeover mechanism to disable the electrical signalling system and take up lost motion between the pilot's control member and the linkage so that the hydraulic servomotor is controlled directly through the linkage. In an embodiment; the pilot's control lever 11 is connected through non-linear gearing 27 of the kind described in Specification 763,381 to an arm 25 pivoted at 26 and also through a link 38 to a lever 40 which is connected at 41 to a further link 42 having a connection with a lever 44 pivoted at 15 to the control surface 10. A lock 47 is movable vertically by pistons 48, 48a from an upper position in which the lower end 51 of the lever 40 is looked, resulting in movement of the lever 11 being transmitted to the link 42, and a lower position in which the lower end of the lever 40 is free. A piston 30 connected to an arm 28 of the lever 25 is subject to a pressure established by a control unit 36 as described in Specification 734,437 for imparting " feel " to the control lever 11 as a function of air speed, and the travel of the control lever required to produce unit displacement of the control surface is varied as a function of air speed by the gearing 27. The lever 25 actuates a potentiometer 77 which produces an electrical signal which is passed through a magnetic amplifier 78 and fed to a transducer 80 for operating a control valve 81 for a servo-piston 82. The piston 82 operates a lever 24 pivoting at 70 and connected to a pair of control valves 19, 19a for a pair of servo-pistons 13, 13a which are connected at 15 to the control surface 10. Positional feed-back is transmitted from a potentiometer 75 actuated by the control surface 10 to the amplifier 78 whilst velocity feed-back is transmitted to the amplifier from a potentiometer 76 actuated by the piston 82. While the servomotor is under electrical control, an autostabilizer 86 is effective on the amplifier 78 to damp out oscillations that tend to occur in high-speed flight at altitude. In the event of an electrical failure or a hydraulic failure occasioned by seizure of the valve 81 runaway movement of the piston 82 will open either of a pair of contacts 92, 92a. At the same time the consequent movement of the control surface 10 will be transmitted back through the linkage 44, 42, 40 resulting in operation by the lower end of the lever 40 of one of a pair of switches 95, 95a. In consequence an electrical circuit is interrupted resulting in de-energisation of solenoids 55, 56, 56a. The solenoids 56, 56a operate valves 57, 57a to vent the upper sides of the pistons 48, 48a enabling springs 49, 49a to raise the lock 47 to secure the lower end of the lever 40. The solenoid 55 operates a valve 61 to release pressure fluid from a cylinder 62 and allow a spring 64 to move a piston 63 to the left thereby operating a bell-crank lever 65 pivoted at 66 such as to depress a link 67 and the lever 24 to disengage the upper end of the latter from .its pivot 70 and to cause its lower end to engage a pivot 72. At the same time the lever 24 is disengaged from the piston 82. A relay also operates to disconnect the electrical system from the supply. The control valves 19, 19a are then operated directly from the pilots lever 11 through the linkage 38, 40, 42, 44, 45, 24. The arrangement of the switches 92, 92a and 95, 95a is such that the electrical system is not disabled merely on account of movement of the control surface 10 lagging behind that of the pilot's control. Pilot's control switches are also provided for selecting electrical or mechanical control. The system may also be controlled by an auto-pilot 99 which transmits signals to the amplifier 78 on closure of a switch 984 under the control of the pilot. The auto-pilot also transmits signals through a low-pass filter 100 to a trim relay 101 so causing a motor 102 to rock a plate 34 about a pivot 26 to straighten the linkage 28, 30. A potentiometer 103 provides a feed-back signal to the relay 101 to terminate the movement of the plate 34 when the linkage is straightened. When the servo mechanism is under the control of the autopilot the solenoids 56, 56a are de-energised so that the lower end of the lever 40 is locked and the pilot's control lever 11 follows the movement of the control surface 10. In the event of failure when the system is under auto-pilot control, the control surface 10 will displace the upper end of the lever 24 from the pivot 70 and open a switch 68 which causes a reversion to mechanical control. The pilot can operate an emergency cock 104 to vent the cylinders 59, 59a, 62 to revert to mechanical control should the electrical system fail to change over. The-solenoid 56 may be arranged for automatic de-energisation when a given Mach number is attained to allow the piston 48 to move up to reduce the backlash in the mechanical linkage. The Provisional Specification describes an arrangement in which the main servomotor comprises a moving cylinder connected to the control surface and in which the mechanical linkage is somewhat simplified.