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Aviation History
1956
1956 - 0056.PDF
FLIGHT, 13 January 1956 HOT AND COLD . . . Fig. 3. The circuit which controls the cabin-air temperature in the Bristol Britannia. Broken lines enclose each portion of the circuit housed in a separate box; the main Wheatstone-bridge cabin-control amplifier is at the top and the port and starboard duct controllers can be seen to left and right. when flight conditions are relatively stable.For larger types of high-performance aircraft, a more precise form of control may be necessary.For example, the FLM/A/6 originally developed to control de-icing equipment, is a magnetic-amplifier system of exceptional sensitivity, capable of controlling to within ±i deg C. This unit, which weighs 7 lb, employs twostages of amplification and runs on 400 c.p.s. three-phase current. A principal feature of thisamplifier is the form of stabilizer incorporated. An ad hoc resistor is inserted in each of two adjacentarms of the bridge, and each resistance has its own heating coil. When the relay is energized, currentis passed through the heaters, so that, as the temperature rises, the value of the resistors alters and causes the relay to be de-ener-gized. The heaters then cool and the bridge is returned to its original setting, the on/off ratio of the stabilizer being matchedto the characteristics of the aircraft system. As a result, hunting is eliminated, the output from the amplifier being in the form ofa progressively shortening series of signals. As a convenient means of rounding off this section, it is instruc-tive to examine the cabin temperature control equipment of the Britannia. As one might expect, this is typical of the most up-to-date and sophisticated practice. Both thermionic valve and mag- netic amplifier circuits are employed, the former being used forprecise control of temperature in the cabin, and the latter being well-suited to the conditions in the supply ducting (Fig. 3). Air for the Britannia cabin is tapped from the delivery end ofthe compressors of all four Proteus powerplants. From the fire- wall, the air is ducted through the wing at a gauge pressure of65 Ib/sq in and then makes four passes through an air/air heat exchanger in which the temperature is reduced and excess moisturecondensed. The temperature is then further reduced in a cold air unit before the flow is passed to the fuselage.Suppose the cabin temperature falls: the consequent signal from the cabin-sensing element is amplified and then operates themotorized selector shown in the centre of Fig. 3. This comprises three rotary potentiometers ganged together and driven by asmall split-field motor similar to those employed in Teddington time switches. The potentiometers form part of the bridge cir-cuits of the cabin controller and the duct controllers, one port and one starboard, in order to draw the required quantity of heatfrom each side of the aircraft, while at the same time limiting the maximum duct temperature. A signal of falling cabin temperature causes the split-fieldmotorized selector to turn in the appropriate direction and move to a position of higher duct temperature. In general, an equalincrease will be selected on port and starboard sides, although the port supply can be biased to a constant temperature higher orlower than the starboard, if necessary. At the same time, the gang shaft of the motorized selector drives a follow-up (rotary poten-tiometer) resistance which cancels the cabin signal, the movement of this resistance being a function of the off-temperature differencesensed by the cabin element. Having selected the higher duct temperature, the system drives port and starboard control valvesand their associated follow-up resistances, the former being moved to the amount re-selected by the follow-up circuit. A ductstat is fitted in each supply duct, and shrouded andunshrouded ductstats are fitted in the common main duct to the cabin, the latter being in the bridge circuit of the main controlbox. The design of the duct elements is such that, when the motorized selector calls for "full hot," the duct temperature doesnot exceed 248 deg F, and in the "full cold" condition does not fall below 35.6 deg F. A stabilizer is inserted to limit the speedof selection; when the motorized selector is energized it runs only until the call is cancelled by the stabilizer, this occurring before TURBINE TAPPING VALVE 75O°C CABIN .ELEMENT i TEMP. CONTROL VALVE the signal is cancelled by the movement of the selector itself. Asa standby, in the event of failure of either port or starboard systems, the remaining circuit can still be automatically controlled, andmanual over-rides are also provided. On die Britannia, all tail surfaces are de-iced by electric resist-ance mats. The wing, however, is heated by warm air obtained in the manner shown by Fig. 4. Hot gas is extracted betweenthe second and third stages of turbine blading and ducted through a large gate valve. This valve, also a Teddington product, oper-ates at a throughput temperature believed to exceed that normally employed in any other airborne gate valve known to be in pro-duction, namely 750 deg C. It handles gas at this temperature in quantities up to 600 cu ft/min at a pressure of up to 30 lb/sq ingauge. The sliding gate is manufactured in Ni-resist nickel cast iron operated by a remote electric actuator. From the turbine-tapping valve the gas passes to a stainless-steelcross-flow heat exchanger and is then discharged overboard. Fresh air, rammed in at the main engine intake, is heated in the heatexchanger and then ducted along the wing leading-edge. The latter comprises an outer skin to which is Reduxed an inner skinwith chord-wise corrugations. The hot air escapes through these corrugations and finally is discharged through flush vents in thewing under-surface and near the tips. As Fig. 4 indicates, the actual leading-edge temperature issensed by thermistors spaced across the span. Each thermistor is mounted on the inner side of one of the corrugations adjacent toa Reduxed bond; the elements are protected by Bakelite-type mouldings and are in contact with highly conductive shunt barswhich ensure that the temperature recorded is the mean of that of the inner and outer skins. In operation, the thermistors passa signal to a sensitive relay which governs the position of the turbine-tapping valve. Modulation of the primary air throughthe heat exchanger then maintains the temperature of the leading- edge at 80 +5 deg C, which is a suitable temperature for theremoval of ice without weakening the Redux bonding. The remainder of this account is concerned with equipment inwhich a change in temperature is recorded by the differential expan- sion of a bi-metallic sensing element, which is then made toactuate the control circuit directly. Such a device is not only inherently rugged and simple but it can be designed to operatesatisfactorily over a relatively wide range of ambient tempera- tures. Using virtually no materials other than stainless steel andhot air, Teddington Aircraft Controls are at present engaged in the development of a range of control systems, some of veryadvanced conception, which should be uniquely suitable for the requirements of some of the fastest aircraft at present on the draw-ing-boards. In highly supersonic applications it is likely to be difficult, if not impossible, to maintain ambient temperatures atany level less than 150 deg C, and it is therefore becoming increas- ingly difficult to employ electric or electronic control systems. As a simple introduction to this new family of completelypneumatic control equipment, Figs. 5a and 5b depict the exterior and a longitudinal cross-section of the FOW temperature-contro! BAKELITE SHUNT BARS SENSITIVERELAY CONTROLLER Fig. 4. Thermistors are used to sense actual leading-edge tem- perature on the Britannia durinp flight in icing conditions. These units, one of which is shown a' a schematic detail, are distri- buted along the span and, by wav of an amplifier, control the sup- ply of de-icing air.
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