Patent GB-1055300-A (GB)
Title: A device for sensing impending stall of an airfoil in a moving air stream
IPC: F15B21/00
Filed: 1964-06-03
Abstract: 1,055,300. Fluid-pressure servomotor systems; controlling fluid flow. BENDIX CORPORATION. June 3, 1964 [June 25, 1963], No.23012/64. Headings G3H and G3P. [Also in Division F1] In a device for sensing impending stall of an aerofoil in a moving air stream, such as a compressor aerofoil, a pressure signal supplied transversely into the air stream adjacent the convex surface of the aerofoil causes separation of the air from the surface when stall is imminent and the resulting increase in pressure is sensed downstream of the separation pressure signal. In an arrangement for controlling fuel flow to a gas turbine engine 360, Fig. 15, a variable frequency tuning fork oscillator 400 receiving a pneumatic pressure through a line 82, produces a pressure pulse signal having a frequency determined by the position of a tuning slug 402 adjustable by throttle lever 404. The tines of the tuning fork are energized by the pneumatic pressure. The signal passes through line 406, a pulse shaper 408 which restores the pulse shape and strength, a pneumatic gate 410 and a line 138 to a controller 24. The controller 24 compares this signal with a pressure pulse signal representing the actual speed of the engine 360, which speed signal is generated by rotating a toothed wheel driven by the engine shaft 374, adjacent a nozzle 62 receiving a pneumatic pressure from a line 58, and is conveyed to the controller 24 through a pulse shaper 418 and line 420. The error between the two signals sensed in the controller 24 causes adjustment of a pivoted lever 202 of a fuel control unit 28 and consequential variation in the size of orifice opening 194 receiving fuel at a pressure P 1  from a gear pump 152. This causes adjustment of a piston 178 carrying a valve 176 which meters fuel flow through line 150 to the engine 360. When stall in the compressor 364 is imminent a pressure pulse of fixed frequency generated by a tuning fork oscillator 424 conveyed through line 426 to a compressor aerofoil 210, causes the air streams to periodically separate from the aerofoil surface in synchronism with the pulse and produce a pressure pulse in line 428. The latter pulse passes through a pulse shaper 430 and line 432 to a pneumatic gate 410 wherein it modifies the throttle signal and prevents any further increase in engine speed. More than one stall sensing system may be associated with the compressor. The pneumatic gate 410 is in the form of a pure fluid amplifier supplied with high pneumatic pressure through a line 224 to form a main jet deflected between a vent 232 and the line 138 by the throttle and stall signals in lines 406 and 432. In an alternative form of pneumatic gate, Fig. 13, there are three deflecting jet passages 238, 242, 246. Each pulse shaper 408, 418, 430 is in the form of a pure fluid amplifier provided with a depression 268, Fig. 14, and a variable volume 274 controlled by a movable piston 276, bellows or diaphragm. A low pressure created in the depression 268 by the main jet when deflected into outlet 264 by a pulse from nozzle 262, causes the main jet to continue flowing into outlet 264 when the pulse has ceased until the volume 274 is filled and a jet emerges through a nozzle 282 to re-direct the main jet into vent 258. Thus the duration of each pulse is controlled by this means. In the controller 24, Figs. 8 and 15, the signals representing the throttle setting and the actual engine speed deflect in opposite directions, a pneumatic jet emerging through a nozzle 92 to cause the jet to periodically supply lines 116 and 114 in accordance with the frequency of each signal. Thus, depending on the difference between the two frequencies a pressure differential will occur between bellows 118 and 120, resulting in movement of a rod 122 connected through levers 124 to the lever 202 of the fuel control unit 28. The bellows may be replaced by diaphragms and used with springs. A neutral line 102 for the main jet is vented to atmosphere through chamber 106 and passage 108, and a chamber 96 is vented through outlets 98, 100. Feedback lines 126, 128 provide pulses in opposition to their originating signals and thereby provide a degree of gain dependent on the size of bleeds 130 and 132. By providing the feedback lines 126, 128 with appreciable volume a delay in the pressurization of feedback nozzles 134, 136 results, thereby providing a frequency variant gain control. In the fuel control unit 28, Figs. 9 and 15, the space 182 below the piston 178 is connected with the upstream side of pump 152 through a bleed 200 and lines 198, 164. The space 180 above the piston 178 receives fuel at a pressure P<SP>1</SP>1 slightly lower than the pump output pressure P 1,  through a rate bleed 188 which controls the rate of movement of piston 178. A rate feedback bellows 204 provides a force opposed to movement of the lever 202 and proportional to the velocity of piston 178, thereby forming an integrating servomotor system. A pair of diaphragm-operated valves 158 control the metering head (P 1  - P 2 ) in accordance with the setting of an adjustable spring 174, by controlling the amount of fuel returned through passage 164 to the low pressure side of pump 152.