1. Field of the Invention
This invention pertains generally to fluid pressurized compartment regulating systems, and is more particularly concerned with the controller utilized in cabin pressure control systems for pressurized passenger aircraft cabins.
2. Description of the Prior Art
It is well settled in the prior art of fluid pressurization of compartments, as for example the passenger compartments of aircraft, to supply the compartment from a source of fluid under pressure and to exhaust the compartment to the ambient atmosphere by way of an outflow valve. Accepted practice now utilizes supercharged air or bleed air from the compressor stages of a jet engine of the aircraft as the supply source, with cabin pressure control being obtained by controlling the outflow valve by means of a cabin pressure regulator or controller. U.S. Pat. No. 2,669,175 issued Feb. 16, 1954, entitled "Cabin Pressure Control" on an invention by Richard A. Fisher, and assigned to the assignee of the present application, is typical of such a system, wherein an enclosure or cabin 10 is supplied with pressurized air from a pump or supercharger 11. The pressure of the air in the cabin 10 is regulated by a pressure setting mechanism 35 which controls the action of an outflow valve 14 disposed over the outlet opening 12 of the cabin 10.
The mechanism 35 has a control chamber 38 separated from a second backup chamber 37 by a diaphragm 39 which senses the differential of the pressures in the chambers. A rate selector valve 94, controlled by a knob 104, is arranged to vary the rate of flow through a passageway 95 communicating between the two chambers. Cabin air is admitted to the control chamber through a restricted calibrated bleed 42 and is thereafter exhausted through a valve 43 to a region of lower pressure, such as ambient atmosphere.
The valve 43 is subject to the differential of pressures across the diaphragm 39, and to the urging of an adjustable tension spring 85 and an evacuated bellows 70 disposed in the backup chamber 37. The spring 85 is adjustable by a knob 89 to vary the tension thereof. The knobs 89 and 104 are provided for the selection of landing field altitude and rate of change of cabin altitude, respectively.
As noted, the mechanism 35 depends on the bleed of the relatively low (and usually varying) cabin air pressure for the source of control pressure in the control chamber 38.
It was conceived that the control apparatus of the prior art, such as briefly described, could be simplified from fabricating, operational, and functional standpoints if there were only one chamber for control purposes, with the chamber supplied from a readily available higher pressure source of fluid admitted to the chamber at a precisely fixed rate. It was further conceived that if the fluid admission to and exhaust from the control chamber were similarly rate limited, there would be no need for elements for setting rate of change of cabin pressure. Control apparatus as thus broadly conceived would be simplified and compact and would require only minimal additional elements to provide for manual setting of the altitude of the destination airport.
Thus, in both aircraft ascent and descent the cabin altitude change would be rate limited to fixed amounts which would relieve the pilot and crew from additional duties during flight. Their sole concern would be a simple setting by a thumbwheel, for example, which would set the control with destination altitude. As will be seen hereinbelow, the "set-and-forget" procedure can be done prior to takeoff.
Stemming from the concept of a fixed fluid admission rate into the control chamber, as aforesaid, is the unusual advantage of precisely limiting the descent rate of the cabin to an absolute maximum. This is important from the standpoint of passenger comfort and well-being since it is now generally recognized that passenger discomfort (and sometimes acute physical distress) usually accompanies a cabin altitude rate of descent greater than about 300 feet per minute. As a general rule, cabin altitude ascent rate is not of much particular concern at rates up to about 2,000 feet per minute, although it is deemed that under normal circumstances an ascent rate limit of about 550 feet per minute is quite acceptable.
It was further conceived that if the fluid flow through the control chamber were to be scheduled in accordance with the ascent or descent of the aircraft, then the cabin altitude would be reliably scheduled relative to the altitude of the aircraft, and as a result the cabin would be automatically ascended or descended at a rate (between zero and the aforesaid limit rates) proportional to the climb or dive rate of the aircraft.
To this end it was conceived that the flow through the control chamber could be modulated by a metering valve arrangement under the control of a dual-aneroid having one bellows element of a particular effective area subject to ambient air pressure and another bellows element of a different effective area subject to the control chamber pressure.
With these concepts in mind, a "breadboard" model of a control was sketched, laid out and then constructed with a housing about 23/4 inches wide by 31/4 inches high (as viewed from the front or thumbwheel side) and about 73/4 inches from front to back. The housing defined the control chamber which was coupled by a conduit to a pneumatic relay provided for actuation of an outflow valve in the fashion shown described in U.S. Pat. No. 3,974,752 issued Aug. 17, 1975, on an invention by Glenn A. Burgess et al entitled "Pressure Control System."
Contained in the housing of the breadboard version were all the necessary bellows, flow limit restrictions, metering valves, pivoted beams, capillary tubes, solenoid valves and the like which will be described later. Also contained in the housing was an integral pressure regulator of conventional design to regulate the flow of air from the high pressure source in order to provide exactly 5.5 standard cubic centimeters per minute through a precision calibrated restriction means for a purpose described hereinafter. As completed, the internal volume of the controller including the volume of the interface tubing between the outlet supply port of the chamber and the pneumatic relay, consisted of approximately 35 cubic inches. The breadboard model, in an altitude laboratory chamber test for Sabreliner and DC-9 aircraft cabin pressure control systems, performed the following automatic schedules and controls of cabin altitude from lift-off to landing without any need for attention by the pilot or flight crew:
(a) Minimum .DELTA.P for passenger loading and unloading, and also at touchdown after flight;
(b) Ground .DELTA.P for flight prepressurization of the cabin during the takeoff run;
(c) Auto scheduling of the appropriate cabin altitude during all phases of flight.