Patent GB-605971-A (GB)
Title: Improvements relating to speed governors for rotary wing aircraft
IPC: B64C27/54
Filed: 1945-12-20
Abstract: 605,971. Fluid-pressure servomotor-control systems. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 20, 1945, No. 34584. Convention date, Dec. 30, 1944. [Class 135] [Also in Groups XXXIII and XXXV] A governing system for a rotary wing aircraft having a lifting-rotor driven by power means and a throttle for varying the output of the power means, comprises a servomotor for changing the pitch of the rotor, and means influenced by both the position of the throttle and the speed of the rotor for controlling the servomotor, whereby the pitch of the rotor is adjusted in response to movements of the throttle in anticipation of the consequent change in power. Fig. 1 shows a helicopter having a lifting-rotor 3 driven, through a transmission 6 incorporating a free-wheel, by an internal-combustion engine 5 whose power output is controlled by a pilot's throttle lever 15. The pitch of the rotor is adjusted by a vertical rod 11 connected, through a bellcrank lever 13, to the servomotor 17 and to a manual control lever 12, movement of which in a clockwise direction increases the pitch of the rotor. The servomotor comprises a piston 19, Fig. 3, actuated by fluid from a pump 29 driven from the rotor and delivering through a relief valve 33, filter 34, manually-operated " on-off valve 35, an electrically-operated valve 36, having a valve member 48 biassed by a spring 49 to the position shown in which pressure fluid is delivered to a conduit 24, and an electrically-operated control valve 23, which controls the operation of the servomotor. A. three-phase alternator, driven from the rotor and having stator windings 59b, supplies current to a tachometer indicator 60 and is also connected, through a three-phase rectifier 61, to a solenoid 54 comprising a phmger 55 supported by spider-springs 56 and engaging the valve member 25 of the control valve 23, whereby the plunger and valve member, which are biassed upwardly by a spring 58, are urged downwardly by the solenoid. Thus, a change in the speed of the rotor alters the current flowing in the solenoid 54 and causes the valve member 25 to be moved in a direction to supply fluid to one end of the servomotor cylinder whilst exhausting the other end, such that the servomotor changes the rotor pitch in, a  a direction to restore the original rotor speed. A coil 62, supplied with alternating current from the alternator causes a slight oscillation of the plunger 55 to minimise the static friction of the valve member 25. A rheostat 63, in series with the solenoid 54, is adjustable by a knob 64 to vary the control point of the system. Also connected in series with the solenoid 54 is another rheostat 65 whose resistanceadjusting arm is mechanically connected to the throttle lever 15 so that anti-clockwise movement of the lever 15, e.g. to increase the power of the engine, reduces the resistance in. series with the solenoid 54, causing the valve member 25 to move downwardly, thereby admitting pressure fluid to the left-hand end of the servomotor to increase the pitch of the rotor simultaneously with the movement of the throttle lever. Similarly, movement of the throttle lever in the other direction reduces the pitch of the rotor. To prevent a change in the governed speed, which would occur by changing the resistance in series with the solenoid 54 consequent upon altering the throttle setting, a rheostat 66 is provided also in series with the solenoid 54, and whose resistance is adjusted by movement of the servomotor so that when the throttle lever is adjusted to reduce the resistance of the rheostat, the consequent movement of the servomotor increases the resistance of the rheostat 66, and vice versa. For controlling the rotor pitch.manually, a switch arm 68 is moved from the position shown in Fig. 3, to an alternative contact 70, to de-energize the solenoid 54 and energize a solenoid 50 for moving downwardly the valve member 48 of the valve 36 to cut-off fluid from the conduit 24 which is then vented through a conduit leading to a fluid-supply tank 37, whilst pressure fluid is supplied to a conduit 43. A spring-loaded piston valve 39, which is normally retained in the position shown by fluid pressure in a conduit 42 connected to the conduit 24, is then moved downwardly to interconnect the two ends of the servomotor cylinder to permit manual operation of the rotor pitch-changing means by the lever 12, Fig. 1. If, during manual operation of the pitch-changing means, the speed of the rotor should be reduced below a safe minimum value, the current flowing through the solenoid 50 is too low to hold the valve member 48 against its spring 49, and the valve member then takes up the position in which pressure fluid is supplied to the conduit 24, and the valve 39 is moved to cut off communication between the ends of the servomotor cylinder. Since the solenoid 54 will, in this case, be deenergized, pressure fluid will be delivered to the right-hand end of the servomotor cylinder, i.e. to move the latter in the direction to reduce the rotor pitch, until the speed of the rotor has risen above the minimum safe value, when the valve member 48 will be returned by the solenoid 50 to its lower position. Spring-loaded ball valves 46, 47 in conduits interconnecting the ends of the servomotor cylinder permit, in an emergency, of manual operation of the pitch-changing means when the valve 39 is closed. Should the pressure fluid supply fail during automatic operation of the governing system, the valve 39 is moved downwardly by its spring, thus permitting manual operation of the pitch-control means, whilst should the alternator fail, the valve member 25 is moved to the position corresponding to minimum pitch, thereby preventing the speed of the rotor from falling below its safe minimum value. Specification 470,492 is referred to.