629,639. Fluid-pressure servomotor-control systems. VIALE, S. M. May 12, 1947, No. 12819. [Class 135] [Also in Group XXVI] In a gas turbine fuel system having a variablestroke fuel pump, the stroke of the fuel pump is regulated by a piston and cylinder or equivalent pressure responsive device having one element resiliently loaded which is influenced by the pressure before or across an orifice through which liquid is delivered by an enginedriven positive-displacement hydraulic pump. A positive-displacement hydraulic pump 10, Fig. 2, driven at a speed proportional to the engine speed delivers liquid through a fixed orifice 11 in the wall of a cylinder 12 within which operates a piston 13 loaded by a spring 14. The movement of the piston 13 is transmitted to the pump stroke controlling means 18 by links 15 and 17 and rocking lever 16. The length of the lever arm of the rocking lever 16 between the pivots 19 and 20 is controlled by the pilots' speed selecting lever 24 through a link 21 pivoted at 22 to the link 15 so that the velocity ratio between the piston 13 and the pump stroke controlling means is dependent upon the selected speed. To maintain the engine speed constant over the altitude range, the velocity ratio is also controlled by a barometric capsule 26 connected to the link 17. The stop 25 limits the maximum stroke of the fuel pump. In a modification, Fig. 4, the movements of the piston 13 are controlled by the pressure drop across a variable area orifice 33 through which the liquid from the engine-driven positive displacement hydraulic pump 10 flows. The area of the orifice 33 is controlled by a valve element 34 which is operated by a servo-system which includes a beam 45 pivoted at 46 to the stem of the valve element 34 and at 47 to the connecting-rod of a piston 48. This piston is urged into the central position by springs 49 and 50 and is subject to the pressure drop between the chambers 36 and 38. The beam 45 is also pivoted at 51 to a valve member 52 the position of which is set by a speed selecting lever 53. The member 52 controls the flow of liquid through a setting orifice to the suction side of the pump 10. A spring-loaded capsule 55 subject to the pressure drop between the chambers 38 and 54 controls a valve 56 controlling the flow between chambers 36 and 38. The chamber 36 is supplied with the liquid through a fixed orifice 35. The action is as follows. The valve 52 is set to a desired rotational speed, any increase in this speed will cause an increase in delivery pressure of the pump 10 which, in turn, will cause the piston 13 to move and decrease the pump stroke. Further, this increased pressure will increase the pressure in the chamber 38 which will cause the capsule 55 to be compressed and the valve 56 to close. Closing of the valve 56 will increase the pressure drop between the chambers 36 and 38 and cause the piston 48 to move against the spring 49 and close the valve 34. This closure intensifies the pressure rise on the piston 13 causing a more rapid return to the selected speed. To change the speed, the valve 52 is opened or closed by the speed selecting control 53, which results in a change in the pressure in the chamber 38. The capsule 55 opens or closes the valve 56 and increases or decreases the pressure drop between the chambers 36 and 38, thus permitting the piston 48 to rise or fall and open and close the valve 34. This results in a rapid increase or reduction of the pressure on the.piston 13 which moves to increase or reduce the pump stroke. Arranged in series with the orifice 34 is a pair of orifices 40 and 41, the function of which is to ensure a minimum pressure drop exists across the piston 13 thus determining the maximum stroke of the fuel pump. The orifice 40 is fixed and the orifice 41 is controlled by a valve 43 which is loaded by a barometric capsule 42 subject to atmospheric pressure such that the loading is increased with increase of altitude, and also by means of a spring 44 so that the spring load is increased with altitude. The minimum permissible pressure drop across the piston 13 therefore increases with altitude and the maximum possible stroke decreased. Specification 607,189 is referred to.