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Aviation History
1949
1949 - 1660.PDF
424 FLIGHT, 29 September 1949 THE STORY Of THE BRABAZON . . . gallons, of which 13,500 gallons is usable. These tanks are disposed between the spars and ribs. At the front and rear of each are branch pipes which feed into two main trunk lines supplying electrical fuel pumps in a collector box faired in beneath each wing. In emergency, or if the collector boxes require servicing, electrically operated cut- off cocks can be used to isolate the boxes from the fuel bags. These cocks are shut electrically, in the event of a crash, by the action of inertia switches. From the collec- tor boxes fuel is pumped through non-return valves and a control cock in the main supply line, from which each engine is fed. Pressure-refuelling is effected via a valve at the rear of each fuel collector box on the underside of each wing. Intercomm. sockets at these points allow verbal communi- cation with the cabin. Fuel for the anti-icing heaters is delivered by a pump in each of the starboard under-wing gravity-fed collector boxes, from which the two main fuel pumps distribute to the engines. On the port side the system is extended to supply the three cabin heaters. Each anti-icing heater consumes 5 gal/hr. The fuel system for the tail anti-icing heaters is entirely self-contained. Power-operated Controls '' Although there have been exponents of aerodynamic balancing (of controls) for the largest aeroplanes envisaged, I believe that the uncertainty of over-balance or under- balance on the first flight alone demands the use of power operation." These are the words of Mr. A. E. Russell, in his revealing lecture. Some Engineering Problems of Large Aircraft, delivered before the Royal Aeronautical Society in 1946 ; and it is now of interest to observe that, although the flying controls of the Brabazon I are power-operated, these will be abandoned on the Mk. II in favour of spring- tab operation. In the early days of Brabazon development an electro- hydraulic system of control was chosen but was rejected in 1947 in favour of a purely hydraulic scheme of much simpler layout. As alternatives, an electrical system and a second type of hydraulic system were developed simul- taneously. To simulate the loads on each of the control surfaces and undercarriage, ground rigs were constructed, and for over two years development was in progress with these trial assemblies to produce the most efficient system of hydraulically powered actuators. During August, 1948, after many hours of durability tests on the ground, air-testing was sanctioned and a Brabazon-type hydraulic elevator-operating unit was installed in a Lancaster. Before flight-clearance was given for the complete Brabazon system the elevator unit underwent fifty hours' air-testing, and during this period each of the hydraulic units for the other controls was being subjected to a 500-hour actuation and durability test. The hydraulic system for the Brabazon's power-operated controls is separate from other hydraulic services, and com- prises a power supply (from twenty engine-driven Lockheed pumps, five on each auxiliary gear box) and four power units, for each aileron, the nkkfear and the elevator, Each operating system jp divided into two half-systems as below : (1) Ailerons: Four half-systems (2,000 lb/sq in) with 12pumps (three to each half-system). (2) Elevator: Two half-systems (1,900 Ib/sq in) with fourpumps. (3) Rudder: Two half-systems (1,900 lb/sq in) with fourpumps. Except for details all four main units are similar: the movements of the pilot's controls are conveyed by rods to "follow-up" mechanisms in the appropriate hydraulic units, and thence to rotary control valves, which bring jacks into action to move the control surfaces- As the sur- faces move into position the follow-up mechanisms return the rotary control valves to their central position ; thus the pilot's controls and the control surfaces are synchronized. Should a rotary control valve jam, movement of the follow- up mechanism turns a transfer valve, so reducing pressure in the supply line to a by-pass valve and, by allowing this valve to open, shortening the supplies from the rotat- ing control valve to the jack. The jack is rendered useless by this automatic operation and the other half-unit is free to operate. A spring device is incorporated in the control system to simulate aerodynamic "feel." .. * Test Instrumentation Every hour the Brabazon I is in the air will yield its measure of technical experience, and with such a com- plexity of systems and services involved it is not surprising to discover that more than half the fuselage is occupied by approximately a thousand instrument dials (including oscillographs), many made at the Royal Aircraft Establish- ment, Farnborough. All readings will be recorded photo- graphically or plotted automatically for analysis in Bristol's flight-research laboratory; thus valuable time will be saved in the checking of performance (jointly with the Aeroplane and Armament Experimental Establishment, Boscombe Down), in completing and refining design, and in accelerat- ing production of the Mk II aircraft. The test instruments used are mainly of synchronous electrical type, each with a transmitter containing an element sensitive to the quantity to be measured and with means of electrically repeating the indications of these ele- ments to dials in the cabin. In order that each group of dials shall give a complete picture of some function or aspect of performance without the need for cross-reference, the dials are grouped in twelve panels, relating to the following: (1) aircraft performance (i.e., speed, height, rate of climb, etc.); (2) performance of essential services (i.e., hydraulic systems—excluding the power-operated flying controls—and the fundamentals of the electrical system ; (3) operation of powered flying controls; (4) engine cooling; (5) engine temperatures; (6) engine oil temperatures; (7) airscrews; (8) oil temperatures in the hydraulic systems; (9) output, etc., of the electric generating system; (10) pres- surization; (11) anti-icing (including external skin tempera- tures) ; (12) aerodynamic pressures and flows. Two types of camera are used. For panels where read- ings remain approximately constant with a given flight con- dition, the periodicity of photographic record varies from one picture every ten minutes to one every other second and a negative size of 5in x 5m (sufficient to cover 170 dials) has been adopted. Where the rate of indication change is greater (e.g., in the aircraft's response to control movements) the photographic periodicity may be as high as four per second. Specially built 35mm cinematograph cameras meet this requirement. The twelve cameras are controlled from a master station on the flight deck, where the chief test engineer is stationed, but observers can immediately override the master control should they notice any trend or condition which, in their opinion, should be recorded. Normally, however, all cameras operate simultaneously to obtain a full record of performance under selected conditions. Two mirror galvanometer oscillographs, each capable of showing 15 quantities simultaneously, record low-frequency vibrations of up to 60 eye/sec (such as those caused by aerodynamic forces) and steady strains in the structure. High-frequency vibrations, up to 1,000 eye /sec (e.g., those caused by engine forces) are recorded by cathode-ray oscillographs reading from four stations at one time. A continuous watch on the operation of various emer- gency devices is maintained by three '' lamp recorders,'' each containing 120 lamps. Should any lamp light up, it will produce a line on a slow-moving film, thus recording the timing and duration of the associated operation. Special test-requirements are inevitable during the Brabazon trials, and to meet them, various portable and self-recording instruments are provided. In conjunction with two double-beam cathode-ray oscillographs they en- able any flight phenomena to be observed. I
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