Patent GB-811564-A (GB)
Title: Control systems for power plants having at least two power units
Inventor: BUCKINGHAM DAVID EDWIN JAMES
IPC: B64C27/14
Filed: 1956-05-14
Abstract: 811,564. Gas turbine plant. NAPIER & SON Ltd., D. May 3, 1957 [May 14, 1956], No. 15014/56. Class 110 (3). A control system for a power plant having at least two turbines, comprises torque-sensing devices associated with the individual drives from each turbine, a power control mechanism for each turbine arranged to maintain the output of the turbine at a value determined by the setting of a control member, an override device for each control mechanism adapted when operated to cause the mechanism to provide emergency power irrespective of the setting of the control member and cross-connections between the torque-sensing device of each turbine and the override device of the (or each) other turbine so that when the torque from a turbine falls below a predetermined value, the override device of the (or each) other turbine is automatically operated. The invention is described with reference to a helicopter power plant comprising two gas turbine power units driving two independent helicopter rotors. In the drive shafts of each of the units torque-sensing devices are provided which determine the hydraulic pressure difference in a pair of pipes as a function of the torque being transmitted. The pair of pipes are connected to a conversion unit 18A, Fig. 2, one of which is provided for each unit. This conversion unit is sensitive to the pressure difference in the pipes 17A due to the torque of its turbine. If this pressure difference falls below a predetermined value the unit 18A brings the override device or devices of the other turbine or turbines into operation and simultaneously actuates a fuel cut-off and dump valve 20A of its own turbine. The pipes 17A communicate with two chambers 231, 23<SP>11</SP> between which are arranged two telescopic pistons 24, 25 acted upon by adjustable springs 26, 27. The spring 27 is stronger than the spring 26. The pistons 24, 25 are connected by push rods 28, 29 to opposite ends of a lever 30 pivoted on an hydraulic valve 31 the upper end of which acts on the actuating rod 32 of an electric switch 33. The switch 33 when closed operates the override device 13A of the other turbine or turbines. The valve 31 controls the supply of servo-fluid to and from a servomotor 36 controlling the dump valve 20A. Stops 44, 45 controlled by cams 46, 47 actuated by the control lever 11A, bear on the ends of the lever 30 to hold it immovable. The cams 46, 47 operate on spring- loaded valves which control the flow of servofluid to and from pistons 53 which actuates the the push rods 56. In normal flight, the stops 44, 45 are in their inoperative positions, and the torque transmitted by the turbine is such that the pressure difference between the chambers 23<SP>1</SP>, 23<SP>11</SP> is sufficient to keep both of the pistons 24, 25 depressed against the action of the springs 26, 27. The valve 31 is then in its lowest position, the switch 33 open so that the override devices of the other turbine or turbines are inoperative and the fuel cut-off and dump valve 20A in its normal position. If the power unit or its drive shaft fails, the torque sensitive device lowers the pressure difference between the chambers 231, 2311. The springs 26, 27 will extend, the spring 27 extending faster than the spring 26. The pistons 24, 25 will rise and raise the valve 31. At a predetermined value of the transmitted torque which is a little below the torque normally transmitted in normal flight the valve 31 will have risen sufficiently to close the switch 33 and operated the override device or devices of the other turbine or turbines. It also actuates the valve 20A to cut off the fuel supply to the turbine and dump the fuel still being supplied by the control mechanism 10A. When the piston 25 is locked out of operation, the piston 24 will operate the control system but only when the pressure difference between the chambers 23<SP>1</SP>, 23<SP>11</SP> and thus the torque has dropped to a considerably lower value. The cam 47 is actuated by the control member 11A so that at all settings of the control member below that corresponding to the predetermined value of the torque at which the control system would be operated by the piston 25, the piston 25 is locked out of operation by means of the stop 45. This prevents operation of the control system when the torque falls below this predetermined value due to deliberate movement of the control lever. The cam 46 is arranged to lock out of operation the piston 24 at settings of the control lever 11A corresponding to the idling range. The failure of one of the other turbines will cause its switch 33 to be closed and the solenoid 60 to be energized so as to actuate the valve 61. The valve 61 controls the flow of servo-fluid to and from the servomotor 65 which adjusts the position of the fulcrum 14A in the linkage 12A between the control member 11A and the power control mechanism 10A so that for a given position of the control member 11A the flow of fuel through the mechanism 10A is substantially in excess of the normal amount. A device 70 bleeds fuel off the pipe 39 or may be used to vary the stroke of a variable delivery pump when the member 71 in the turbine inlet reaches a predetermined temperature. When the override device 13A is operated the valve 61 connects the pipe 73 to the pipe 66 so that a pressure is applied to a piston which acts on the bleed valve 70 to hold the valve closed. If the torque exceeds a predetermined value, the piston 75 is depressed against the action of the spring 76 and opens a bleed valve 78 which reduces the fuel supply to the engine. Alternatively, the valve 78 may be used to control the servomotor of a variable-stroke fuel pump. To permit starting and idling on the ground at very low torques, means may be provided to render the override devices inoperable. This can be done by disabling the override devices themselves, by interrupting the intercon. nections between the torque-sensing devices and the override devices or by putting the torque-sensing devices out of action. Provision may also be made to inject a coolant into a turbine when the temperature-limiting device 70 is put out of action. Specification 811,563 is referred to.