Abstract: A cylinder (3) is positioned to rotate on a fixed axle (1) in such a way that several leakproof chambers (3B, 3D) are formed: a second, longer cylinder (4) is positioned to slide along the first (3) and to turn on the fixed axle (1) in such a way that several leakproof chambers are formed (4A, 4B). Communicating pressurized fluids into the various chambers makes the cylinders (3, 4) turn a distance proportionate to a value x, and can make the second cylinder move axially along the first cylinder a distance proportionate to another value y; these values x and y being derived from several variables and sensors (10a to 10d) "read" the surface contours of the second cylinder (4), representing, for example, a function f (x,y).

Abstract: A fuel flow control assembly comprises a metering piston which can slide within a casing and defines with that casing an upstream chamber at a pressures P.sub.1 and downstream pressure P.sub.2. Relative movement of the piston and the casing varies the effective opening of a metering orifice and providing that the pressure drop P.sub.1 -P.sub.2 is kept constant the orifice has linear characteristics.The assembly also includes a pressure drop monitor and a slide control valve which uses as its casing an internal bore of the flow control piston. With a view to rendering the assembly more compact the pressure drop monitor is also disposed within the bore of the flow control piston.