Patent GB-621669-A (GB)
Title: Improvements in and relating to aircraft pressure cabin controls
IPC: B64D13/02
Filed: 1942-07-21
Abstract: 621,669. Fluid-pressure servomotor-control systems. BOEING AIRCRAFT CO. July 21, 1942, No. 10202. Convention date, Oct. 18, 1941. [Class 135] A pressure cabin control for aircraft cabins and the like is arranged to regulate only the out-flow from the cabin and is subject to an absolute-pressure sensitive control to initiate supercharging at a selected altitude and at a rate such as will maintain a predetermined relationship between cabin pressure and atmospheric pressure and to a differential-pressuresensitive control arranged to prevent the cabin differential pressure exceeding a predetermined value. The cabin pressure is slightly above atmospheric pressure up to a critical altitude, say 8,000 ft., when supercharging commences. From this point the cabin pressure may be maintained constant or may rise or fall slightly until the differential pressure limit is reached at, say, 30,000 ft. The cabin pressure may be controlled so that the difference between cabin pressure and some datum pressure, say atmospheric pressure at sea-level, bears a constant relationship to the difference between cabin pressure and that of the surrounding atmosphere. The cabin pressure may be arranged to vary at a certain predetermined rate without actual change of altitude. Air may be supplied to the cabin in excess of requirements by a blower driven by a propelling engine through a variable speed gear. Said gear being varied by a power piston under the control of an auxiliary valve actuated by a diaphragm mounted in a chamber connected on the one hand to a duct leading to or from the blower and on the other to a venturi diffuser within said duct. The diaphragm may also be made sensitive to cabin pressure. One form of air outflow valve is shown in Fig. 1, the valve being fully open to prevent any substantial supercharging. The valve member 1 co-operates with a seat 2 formed in a venturi passage and slidably engages a cylindrical wall 21 to form a chamber 17. This chamber is connected to the cabin through a passage adjustably throttled by a needle 15 and to a region of low pressure by apertures 14 or an external pipe connection. By these means a low pressure is produced above the 'valve to offset the low pressure due to the venturi action of the air passage and the pressure differential between cabin and atmospheric pressures, tending to close the valve. This action may be assisted by a spring. The valve is actuated by a diaphragm 13 secured to a spindle 10 guided at 11. The chamber below the diaphragm is subject constantly to cabin pressure admitted through a port 23. The chamber 24 above the diaphragm is supplied with air under cabin pressure through a passage adjustably throttled by a needle 16. In the position shown, air from this chamber escapes by way of passage 25, pipe 26, valve-controlled passage 34 and pipe 27 to a region of low pressure in the outlet conduit, to atmosphere or to the vacuum system of the aeroplane. The valve 33 controlling the passage 34 is carried by the movable wall of a spring-loaded evacuated bellows 3 adjustably mounted in a chamber 39, subject to cabin pressure by a screwed spindle 50 engaged by a rotatable cap 5. As the aircraft approaches the critical altitude at which supercharging is to commence, the expansion of the bellows moves the valve 33 to throttle the passage 34 so causing pressure to build up in the chamber 24 and move the valve 1 towards its seat. This movement acting through a plunger 40 and lever 35 moves member 32 formed with the passage 34 to give a follow-up action. The leverage of the lever 35 is of the order of 12 to 1, so that the critical cabin absolute pressure is not appreciably altered by any responsive movement of the valve. When the cabin differential pressure attains the chosen limiting value, a spring-loaded piston 4 subjecton its lower face to cabin pressure admitted through a passage 41 and on its upper face to atmospheric pressure admitted through a pipe 42 moves into contact with a shoulder 44 on the plunger lifting the end of the plunger away from the spindle 11. A byepass controlled by a throttle is fitted to vary slightly the control point. This vents the chamber 24 to atmosphere by way of passage 45, ports 45<SP>1</SP> and pipe 42, so permitting the diaphragm 13 and valve 1 to lift and lower the cabin pressure. The opening movement of the valve 1 recloses the aperture in the end of the plunger. In the event of atmospheric pressure exceeding cabin pressure, air flows into the cabin by way of pipe 27, passage 34, pipe 26, passage 25, chamber 24 and non-return valve 19. If the bellows 3 becomes ineffective it may be cut out by closing a valve 27<SP>1</SP> in the pipe 27. Provision may be made for manually moving and holding the valve 1 to its seat. This may comprise a plunger slidably mounted in the cover opposite the plunger 40 and carrying a swivelling screwed cap adapted to be screwed on to a boss on the cover. The cap 5 may be rotated by a cable from a remote point or it may be rotated by a reversible electric motor so that the cabin pressure is slowly increased or decreased irrespective of change in altitude. The control circuit of the motor is fitted with a reversing switch, a rheostat for varying the speed and switching means adapted to stop the motor after it has made a predetermined number of revolutions. In a modification, provision is made so that the elevation of cabin pressure above atmospheric pressure is always a fixed percentage of the difference between atmospheric pressure at sea-level or some other arbitrarily selected datum and the actual atmospheric pressure. To this end, the chamber housing the bellows is divided into two parts by a diaphragm secured to the casing and the movable wall of the bellows. The chamber above the diaphragm is subject to cabin pressure while the chamber below the diaphragm is connected to the cabin by a restricted orifice and to the outside air by a variably restricted orifice. Specifications 521,623, 577,349 and 621,670 are referred to.