Patent GB-709816-A (GB)
Title: Pressure control mechanism applicable to pressurized aircraft cabins
IPC: B64D13/02
Filed: 1951-06-15
Abstract: 709,816. Fluid-pressure servomotor-control systems. GARRETT CORPORATION. June 15, 1951 [June 28, 1950], No. 14190/51. Class 135 A pressurized aircraft cabin is so controlled that up to a first predetermined altitude, e.g. 10,000 ft., the cabin pressure is substantially the same as atmospheric, between that altitude and a second altitude, say 27,000 ft., is maintained constant, thence to a third altitude of e.g. 32,000 ft., is maintained at a constant differential of e.g. 5 p.s.i. with respect to atmospheric pressure, above which and up to a fourth altitude, say 50,000 ft., cabin pressure is kept at a constant ratio to atmospheric pressure, and beyond the fourth altitude is maintained at a constant low differential of just over 1 p.s.i. above atmospheric pressure. Means is provided for rendering ineffective the mechanism for ratio and low differential control. As shown in Fig. 3, a cabin outflow valve 41, 47 coacting with a seat 45 is subiect to cabin pressure which is admitted, through openings 52, to a chamber 51 bounded by the part 47 of the valve and by a diaphragm 49 supported by a fixed member 50, whilst the upper side of the part 47 is subject to pressure in a control chamber 28. Thus cabin pressure is maintained at a value substantially equal to control chamber pressure plus a small addition necessary to balance a spring 60 loading the valve. The control chamber 28 has a restricted opening (not shown) to cabin pressure and two valve-controlled outlets to atmosphere. One valve, 83, is actuated by an evacuated bellows 88 subject to control chamber pressure and arranged to vent the control chamber freely until the first altitude is reached, whereafter, and up to the second altitude, the valve 83 throttles the outlet to atmosphere to maintain a constant cabin pressure. At the second altitude, the other valve, which is housed in a casing 101 shown in section in Fig. 5, takes over the control. This valve is constituted by a standard tyre valve 185 carried in a tubular body 177 mounted on a diaphragm 141. The lower side of the diaphragm is exposed to control chamber pressure and the upper side to atmospheric pressure, whilst a second diaphragm 190 carrying an abutment 210 for coacting with the stem 186 of the valve 185 is subject on its lower face to atmospheric pressure and on its upper face to control chamber pressure. A spring 213 urges the diaphragms apart. Between the second and third predetermined altitudes, when the cabin pressure is kept at a constant differential with respect to atmospheric, the diaphragm 190 is urged, by the preponderance of control chamber pressure over atmospheric pressure, to its lowermost position in which a plate 195 engages a stop 203, and control of the pressure in the chamber 28 is effected by adjustment of the opening of the valve 185 in response to movement of the diaphragm 141. For the ratio control between the third and fourth altitudes an evacuated bellows 170 is provided, an end-plate 168 of which is arranged to engage the body 177 at the third altitude. However, the bellows only becomes effective when an electromagnet 135 is energized to raise an armature 150 upon which it is mounted. At other times the armature is urged by a spring 161 to a lower position, as shown, in which the cabin pressure is controlled by the diaphragm 141 alone throughout the range above the second altitude. When, during control by the diaphragm 141 in combination with the bellows 170, the fourth altitude is reached, at which a change from ratio to low differential control is required, the bellows will have expanded to an extent to bring a cupped member 169 carried thereby into engagement with a stop 175, and at the same time, the difference between cabin and atmospheric pressures will have fallen to an amount insufficient to keep the diaphragm 190 depressed against the spring 213. The valve 185 will then be controlled by the diaphragm 190 to maintain a fixed, low, differential between cabin and atmospheric pressures. The restricted opening between the cabin and the control chamber 28 is so chosen that if, during control in the ratio or low differential ranges, the electromagnet 135 is deenergized, the rate of increase of cabin pressure to the value determined by the diaphragm 141 alone is restricted to a low value of, e.g. 2<SP>11</SP> Hg. per minute. The two valves 83, 185 together with their associated bellows and diaphragms &c. may be housed in a chamber separate from that above the cabin outflow valve and connected therewith by piping. In this case, the valves 83, 185 may control more than one cabin outflow valve. In a modification, a valve corresponding to the valve 83 is actuated by the bellows through a variably spring-loaded lever, and the mechanism corresponding to that in the casing 101 is modified so that the ratio control bellows is rendered ineffective when the electromagnet is energized. Also, the connection to atmosphere of the two valves may be controlled by a three-way valve. Specification 655,601 is referred to.