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Aviation History
1964
1964 - 1159.PDF
RIM TANKS <GINE FEEO 652 FLIGHT International, 23 April /964 Fig I Simplified plan of the Concord fuel system. Tankage has been extended during the past year, but precise capacities remain restricted. The excess fuel required for trimming purposes is less than the cruising penalty which would be exacted were such trimming not employed DESIGN FOR MACH 2.2... 2: CONCORD SYSTEMS AND INSTRUMENTATION By H. HILL, British Aircraft Corporation (Filton) WHAT I intend to do is to describe the kind of systems whichwill be required in any supersonic civil transport, and examine the instrumentation requirements. The loop will then be closed by suggesting ways in which the instrumentation can benefit from the techniques being developed in aircraft systems. Hydraulics This uses engine-driven pumps to drive fluid at pressures up to 4,0001b/sq in to operate flying controls, under- carriage and engine-intake controls. The system employs multiplex and multiplicatedj arrangements of pumps and actuators to provide a very low probability of system failure. Two main systems, each with a standby, provide supplies for the flying controls. Separate supply systems feed the intake controls for each engine. One of the main systems, together with its standby, provides for landing gear and hinged-nose operation and wheel brakes. Fuel Over half the take-off weight is fuel. This fuel not only feeds the engines, but on its way provides a sink for the surplus heat rejected from other parts of the aircraft. Ram air is no longer Of any use, because Stagnation temperature at Mach 2.2 is about 150°C. The fuel is put into the aircraft at as low a temperature as is convenient to provide, and has its delivery temperature to the engines limited primarily by the need to minimize problems of fuel pumping and metering. The difference between these tem- peratures may be as large as 80 or 90°C. Inevitably the arrangement of heat exchangers required for this energy transfer will complicate the fuel system. A further com- plication arises from our decision to use the fuel to control the position of the centre of gravity. In the transition from subsonic to supersonic flight the centre of pressure on an aircraft moves aft. If we have the e.g. ahead of the subsonic c.p. so as to provide a positive static margin, this margin will increase considerably when the aircraft goes supersonic. This high stability of the aircraft will require large control-surface deflections for trimming to the flight conditions desired. These large deflections increase aircraft drag and therefore fuel consumption. We can reduce this drag by either of two systems. We can shift the aerodynamic centre by employing variable-geometry wings. Alternatively, we can transfer fuel and move the e.g. aft to follow the c.p. For [many reasons we have chosen the latter, so a further task imposed upon the fuel system is to adjust the position of the aircraft e.g. In doing this it must be capable of transferring fuel forward with high reliability, in order to cover supersonic/subsonic transition. Air-conditioning Associated very closely with the fuel system is the air-conditioning and de-icing system. This has to provide cooling air for passengers and delicate equipment. It uses hot air bled from the engine compressors, and reduces its temperature by several hundred degrees Centigrade. It must provide a large enough mass flow to maintain comfortable interior conditions. In an aircraft which cruises at around 60,000ft with a pressure differential of 11 or 121b/sq in, the importance of this system cannot be over-emphasized. Associated with it is de-icing, which must prevent dangerous accumulation of ice in any flight condition; and amongst these flight conditions, protracted stand-off is probably the most significant. Flying Controls Flying controls will be essentially completely power-operated, deriving their energy from the hydraulic system and employing tandem jacks. The precision required for actuation of conventional trailing-edge surfaces at high flight speeds creates very difficult demands on the system which transmits the movements of the pilot's hands and feet to the hydraulic actuators. Because of this, considerable attention is being paid to electrical signalling systems. These can be designed to have higher precision than mechanical ones, since they are less vulnerable to the effects of structural distortion and vibration. An obvious problem is that they will have to have a reliability which is many orders higher than exists in any other electrical system to date. A mechanical reversionary system employs tandem jacks, which also provide autopilot actuation. The hydraulic systems also power artificial-feel systems, which are controlled by signals from air-data computers and the fuel e.g. control system. Electrical System Electrical generation and distribution, although appearing conventional when viewed as a schematic diagram, becomes unconventional as a result of two factors. First is the high temperature at which the alternators must operate and, secondly, the extremely low failure-probability which the system must have because of the safety implications which arise from its extensive use. Flight at Mach 2.2 results in ambient temperatures around generators of 170 to 180°C, whilst the reliability require- ment means that complete system failure-probability must be around 1 x 10'9 or lower. Completely duplicated systems with careful separation will be employed, with redundancy present in each sub-system. Generation will be constant-frequency a.c, with rectification providing d.c. supplies. Static inverters will provide for loads which require high stability a.c. Communications The communications systems will largely be quite conventional, but there will be more complex peripheral equipment to provide a greater degree of automaticity. The first step in this direction is being taken in the introduction of more complex reply codes from secondary radar transponders. We will shortly be introducing an altitude-reporting code, and this will be followed by additional modes which will provide information of value to Air Traffic Control. Extension of this approach to routine messages can result in a considerable reduction of work-load on the crew which will be extremely significant. On certain routes today's aircraft saddle their pilots with a work-load which is about 50 per cent communi- cations duties. The higher speed will make it sensible to expend more structure weight on the suppression of aerials within the envelope of the aircraft. The criterion employed is that the take-off weight of the aircraft shall not be increased over the minimum weight with a protruding antenna. Instruments Rising aircraft performance requires that instrument accuracies be increased over a wider range of the parameters which they display. In addition, the extent of equipment increases the number of instruments. The instruments which then have to be monitored by each crew member are further increased by the desire of operators to have smaller numbers of flight crew. All these factors require us to examine new approaches to the instrument problem. Cockpit displays might eliminate any information which merely indicates satisfactory functioning. Work is proceeding on the use of computers which will compare parameter values with computed tolerances which can vary throughout the flight. The system will then display those values which matter, so that action can be taken by a crew member. The integrity required of monitors which control information displays is harrowing to contemplate. Whilst the time-scale of this approach may be a little long, there is con- siderable scope in the immediate future for monitoring systeiv.* which will provide automatic checks of aircraft behaviour and provide warnings, or even disconnections, in the event of ma1-
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