878,657. Fluid-pressure servomotor-control systems. SOLAR AIRCRAFT CO. Jan. 27, 1958 [April 22, 1957], No. 2609/58. Class 135. [Also in Group XXVI] The shroud 34 of a variable-area effluent multi-flap nozzle 32, Fig. 1, for an after-burner gas-turbine jet-propulsion engine is positioned by two pneumatic actuators 38 supplied with compressor discharge air by a conduit 40, 44, 45 over a valve assembly 42 controlled by a mechanism 46 connected by a line 48 to the conduit 40 and lines 50, 52, 54 to a point adjacent the downstream side of the turbine 24. The mechanism 46 includes three pressuresensitive units that sense the ratio between two input pressures P1 and P2, Fig. 2, each unit basically consisting of a resilient metallic diaphragm 56 to the opposite side of which these pressures are supplied, the pressure P1 passing through two parallel orifices 58, 59 and exhausting via another orifice 60; the orifice 58 is provided with a manually adjustable needle 61 whilst orifice 59 is adjusted by a needle 62 rigid with a diaphragm, bellows or as shown, piston 63 in a cylinder 64, the chambers on each side of which piston are supplied with the pressure P1, by orifices 70, 72, the upper chamber having an exhaust orifice 74 controlled by a valve 76 attached to the diaphragm 56, and the lower chamber having an exhaust orifice 78. The piston-rod 80 is connected to an indicator 84, an actuator or a valve. The orifices may be of the convergent, convergentdivergent, sharp-edged or drilled-hole type. If, in the balanced position shown, the pressure P2 increases to reduce the differential between P1 and P2 the diaphragm 56 is downwardly deflected an amount proportional to this increase whereby the valve 76 is moved closer to the orifice 74, the resulting increased pressure above the piston 63 urging the latter downwardly to increase the area of orifice 59 until the pressures across the diaphragm are again equal whereupon the diaphragm returns to the neutral position shown carrying with it the valve 76; the piston 63 remains in its new position so that for each ratio of P2 to P1 there is a unique position of the piston-rod 80. This unit may be used whenever it is required to sense, indicate or control the ratio between two fluid-pressures. The three units 100, 102, 104, Fig. 4, are arranged in relay form in a double manifoldassembly 106, 108, open to the line 48, Fig. 1, and respectively, sense after-burner lighting and blow-out, overall engine-compression ratio and turbine pressure ratio. The valve 42, comprises a piston member 232, Fig. 4, which is moved by the piston 224 of the turbine pressureratio sensing unit 104. The spaces 256, 258, on opposite sides of the diaphragm 260 of the engine compression ratio sensing unit 102 are, unlike the two other units, both designed as flow chambers, each being supplied with compressor discharge pressure; the unit 102 may have a twin diaphragm arrangement. A manually-operated valve 326 provides an overriding control of the nozzle by either venting the space 214 beneath the piston 224 or connecting it to compressor discharge pressure; in a third position of the valve 326 the pressure in the space 214 is connected to a vent line 336, the end of which is controlled by a valve 338 attached to a piston 340 operated by the pressure drop across the after-burner metering valve 348, Fig. 1, so that when the valve 338, Fig. 1, closes the line 336 the piston 224 functions normally and automatically. During engine operation without after-burning the turbine discharge pressure is such as to lower diaphragm 148 and hence the piston 178 of unit 100 so fully opening the inlet orifice 264 in unit 102 whereby its associated piston 284 moves to reduce the area of the inlet orifice 204 of unit 104 and the space 214 is vented solely through valve 338 and the nozzle is closed. The unit 102 remains in this position. During engine operation with after-burning the pressure drop across the metering valve 348, Fig. 1, is sufficient to close valve 338, Fig. 4, and the nozzle is controlled automatically. Upon ignition of the after burner the turbine discharge pressure rises rapidly and elevates the diaphragm 148 and hence the piston 178, closing the orifice 264 and reducing an orifice 158 whereby the diaphragm 148 remains in its elevated position and the unit 102 resumes control of unit 104 via the inlet orifice 204. Any change in the turbine discharge pressure during after burning is sensed by unit 104 and rectified by adjustment of the nozzle. As the overall compression ratio changes during flight the turbine pressure ratio will be set to a new value through alteration of the area of the orifice 204. In the event of a blow-out the turbine discharge pressure drops and causes lowering of the diaphragm 148 and subsequent opening of the orifice 264 whereby the nozzle is moved to a partially-opened position until reignition occurs or the throttle is moved to the non-after burning position. In a modification, the diaphragm 260, Fig. 7, of the unit 102 is clamped to the housing 262 by a cover 368 supporting a housing 372 enclosing upper and lower cylinders 374, 376 with a flat plate 378 positioned therebetween and having a control orifice 380. Compressor discharge pressure is supplied via an orifice 382 or a capillary tube to the cylinder 376, the pressure on the diaphragm 260 being less than the compressor discharge pressure due to both orifice 382 and a further orifice 388 formed between the plate 378 and the cylinder 376. A second reference pressure is established in chamber 390 formed between the orifice 380 and the annular orifice 392 comprised by the cylinder 374 and plate 378; the housing is open to atmosphere by ports 384. For a given compressor discharge pressure and ambient pressure there is one position of the plate 378 such that the pressures above and below the plate and the ambient pressure balance the plate between two columns of air. The plate is pivoted on a lever 394 with a counterweight. Any change in ambient pressure causes movement of the diaphragm and the piston 284, Fig. 4, to adjust the orifice 266 to alter the pressure below the diaphragm to re-establish the latter in its neutral position. Specification 828,689 is referred to.