629,086. Fluid-pressure servomotor-control systems. MILES AIRCRAFT, Ltd., and TREU, J. F. Sept. 11, 1944, No. 17328. [Class 135] A double-acting fluidpressure servomotor apparatus for connecting an output member to an operating member has in addition to the normal control valve movable to either side of a neutral position in which the servomotor is locked by the pressure of incompressible fluid against displacement by the output member, and arranged to supply fluid to one side of the servomotor and exhaust from the other, an additional valve which when the pressure supply fails or drops unduly permits fluid to pass from one side of the servomotor to the other when the output member is shifted directly by the operating member as shown, the servomotor for operating the control surface of an aircraft comprises a piston 3 carried by a hollow piston-rod 4 and moving in a cylinder 2. Sealing washers 7, 8, 9 are provided for the ends of the cylinder and the piston. Within the piston-rod 4 is a pilotcontrolled hollow operating member 10 provided with packings 23, 27, 26, 28. Pressure fluid is supplied through a connection 12 past a non-return valve 29 to the interior of the operating member at the end 11 connected to the pilots' control. An exhaust connection 14 is provided at the end of the piston-rod adapted to be connected to the control surface. The operating member 10 is prevented from rotating in the piston-rod by keys 25, Fig. 2. The member 10 has two lands 17a, 17b with diametral pairs of passages 19a, 19b connected with the central pressure passage 20. In the neutral position of the member 10, the passages 19a, 19b are both cut off from corresponding passages 16a, 16b in and leading to the two sides of the piston 3. The exhaust of pressure fluid when the operating member is moved away from its neutral position is by way of a recess formed by a waisted portion 18 and connected with the remote sides of the lands 17a, 17b by longitudinal channels 21 through the lands. Exhaust fluid passes through ports 32 and past a check valve 30 in an extension 10<SP>1</SP> of the operating member to the outlet connection 14. A further check valve 31 between the supply and exhaust parts of the system opens when the pressure supply fails so as to allow transfer of liquid from one side of the piston to the other. The valve 31 is lightly loaded compared with the valve 30. The operating member 10 is biassed to a central position by a spring 15 arranged between slidable collars 34, 35 having their inward movement limited by a distance sleeve 36. Outward movement of the collars is arrested by stops 37, 38 on the operating member 10 and by stops 39, 40 carried by the piston-rod. A member 41 on which the stop 40 is formed can be adjusted by screwing it more or less into the piston-rod 4. The stop 39 is formed as a cap nut and can be adjusted independently on the member 41. The valve passages which are of elongated or oval section may be arranged so that the passages 16a, 16b are nearer together than the passages 19a, 19b, or vice versa. In the one case, the pressure fluid from the cylinder ends exhausts between the lands 17a, 17b whilst in the other case it exhausts to the outer sides of the lands. The lands 17a, 17b may have their sides cut away at 21<SP>1</SP>, Fig. 8, to form substantially a figure 8 shape, these cut away parts taking the place of the channels 21. The servomotor may comprise a vane-like member movable to and fro in a segmental cylinder. Specification 569,924 is referred to. According to the Provisional Specification the control valve may be as described in Specification 579,535 or it may comprise a valve member floatingly mounted on the control rod and urged into contact with a plane surface on the rod by synthetic rubber or other blocks interposed between the valve member and the power piston. The relative movement of the piston-rod and the control rod may be limited by stops in conjunction with synthetic rubber or other resilient members.