Patent GB-628354-A (GB)
Title: Improvements in or relating to a system for controlling the air pressure in an aircraft cabin
IPC: B64D13/04
Filed: 1947-07-30
Abstract: 628,354. Automatic control systems for aircraft cabins. DOUGLAS AIRCRAFT CO., Inc. July 30, 1947, Nos. 20612 and 20613. Convention dates, Feb. 7, 1942, and June 6, 1942. [Class 38 (iv)] In an aircraft cabin air pressure control system, a pressure-sensitive device is adapted to control the flow of pressurised air through the cabin, subject to the influence of an anticipating control means sensitive to differential transient changes of state at the intake and outlet passages. In one embodiment, Fig. 7, compressed air is passed through the cabin 120 from the ducts 126 ,by blowers 121, associated with the ram duct 124 in the nose of the fuselage, and the outlet valves 122. The latter are normally controlled by the series system 132, responsive to the cabin pressure regulator 134, Fig. 8, which comprises first, the coacting sealed bellows 138, which react to the pressure in the case 136, and secondly, the bellows 140, the interior of which is connected to the external atmosphere through the tube 164. The interior of the case 136, which is connected to the pitot tube 185, located at the output, i.e. the cabin side, of the blowers 121, and also to the cabin side of the output valves 122 through a venturi tube in duct 181, is normally at cabin pressure, and changes in the cabin air flow condition are reflected immediately in changes in the conditions pertaining in case 136 which therefore anticipate the alterations in the pressure state of the cabin atmosphere. The arm 142, operated by the movement of the bellows 138, 140, is positioned in accordance with the ratio of between the external and cabin air pressures, between the contacts 150, 152, adjustable by the knob 156, to vary this ratio. The relay switch 174 is thereby selectively energized, through the relay 175, to drive the reversible motor 176 in a direction to correctively operate the valves 122. At the two extreme positions of the latter, circuits are completed which effect the rotation of motors 178 to open or close valve gates in the air inlet from pipes 124 to the blower 121. In a second embodiment, not shown, the two bellows incorporated in the cabin pressure regulator are each separately adapted to operate an associated contact arm which effect, electrically, the operation of the outlet valve to maintain the cabin pressure constant up to a predetermined altitude and thereafter to follow a selected pressure schedule. In a third embodiment, not shown, pitot tubes and venturis are employed to detect the air pressure at the points of input and exhaust respectively; a rate of climb meter is also incorporated, which comprises a pressuresensitive capsule, the interior of which is connected with the exhaust venturi, enclosed in a shell having an orifice and situated in the cabin; contacts associated with the capsule over-ride the main regulator, if the cabin pressure change, caused by rapid altitude variations, exceeds a predetermined rate; an orifice may be provided in the duct leading from the venturis or pitot tubes to allow for mechanical differences in construction of the ducts. In further embodiments, not shown, the effects produced at venturis at the cabin air inlet and outlet are transferred to the regulator by electrical and mechanical methods. The former method comprises a potentiometer at both the inlet and outlet, both of which are positioned by a diaphragm according to the state of the air flow and which control the speed of separate air pumping means to develop a pressure detected by the cabin air regulator which positions the outlet valve accordingly. The latter methods comprise, in one case, two diaphragms, positioned in accordance with the condition of inlet and outlet venturis respectively, and which each control a valve in an air pressure system connected to the cabin regulator; in the second case, the electrical contacts controlling the outlet valve setting motor are positioned by a piston and lever arrangement activated directly by the outlet and inlet venturis.