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Aviation History
1957
1957 - 0134.PDF
COMPRESSOR 4 GLOW \ PLUGS \ TURBINE 136FLIGHT, 1 February 1957 FLAMETUBE DEFLECTORSOVER FLAME "TUBE TUNNELS Baffling problem: This simplified sketch shows the positions of the two deflector plates fitted as the final cure—after months of exhaus- tive research—of the de-icing problem. Note also the glow-plug— one in every other flame-tube. The photograph shows the deflector plates fitted over Nos. 1 and 8 (adjacent) flame tunnels only. BRITANNIA PRELUDE . . . made in 1,547 flying hours. Except for freezing of the flying-control gearboxes, referred to below, there were no significant troubles. With only minor adjustments the Proteus engines rantrouble-free; there was not a single unscheduled engine change. To pilots reared on a diet of ignition troubles, piston seizures andvibro-massage from piston engines this was really something— this, and the fact that the Britannia can fly without effort with twodead engines, engendered a happy content they had never before had in the air.In spite of our original misgivings, it must be freely admitted that the electrical system works with the regularity of a householdlighting system. The undercarriage has given no significant trouble whatsoever and G-ANBD, for example, has already com-pleted more landings during training than a Britannia will norm- ally do during the course of two years of route flying. This ispretty high praise for the brakes, the tyres and the hydraulic system. There were two major setbacks. One filthy night in January1956, when a Britannia was parked in the open at Hum, the flying-control gearboxes froze up. An east wind, driving sleet andsub-zero temperatures, had revealed a serious defect. This hap- pened when arrangements for introduction of the aircraft intoservice were fairly well advanced. What is sometimes described as "fire-brigade action" set in.Modifications were started the same day, and for some time the aircraft, which were being used for training, were flown in "zero-plus" temperatures. The action was prompt and efficient and the problem faded out. But on April 3, 1956, a bigger and tougher snag hit the Britannia.G-ANBE was flying from Livingstone to Nairobi when one engine after another started to "bump"; then worse happened—one afteranother they "flamed-out." Briefly, an engine "bump" occurs when the flame in one or moreof the flame-tubes is extinguished and then relit. It is, of course, relit from the other flame-tubes through the connector tube. Asthe relight occurs a "bump" is heard and felt. It would probably pass unnoticed by anyone who was not concentrating, and insome cases is so slight that a momentary fluctuation of the torque- meter is the only evidence of its occurrence. A "flame-out" means that the eight flame-tubes are extinguishedsimultaneously. The engine then loses all power. Fortunately, the Proteus 705 can be relit immediately without trouble, so thismost disconcerting phenomenon has never placed the aircraft in a hazardous position. In addition, the Britannia will hold thesame height for some five minutes on two engines, and will take about 14 hours to drift down to 8,000-10,000ft, where it can holdheight indefinitely on two engines. The Proteus had been given the full treatment at Ottawa, whenit went through winterization tests without fault. It had, in fact, proved to be the most successful engine ever tested there. Twohad then been installed in an Ambassador and flown with a spray- rig in front of each engine. Here again the tests had been a success.The engines had consumed colossal quantities of ice in cakes and sticks without trouble.It didn't seem to make sense that at sub-zero temperatures at about 20,000ft over Africa ice could cause bumps and flame-outs.The exact mechanics of flame-out are still being investigated, but this much is known. When dry-ice crystals enter the air intakeand accumulate on corners and "pockets" in the intake cowling, the build-up of ice will eventually break away and cover or partlycover the entry guide-vanes. Then the engine bumps or flames out, according to the magnitude of the ice fall. It seems that the explanation may be something like this Ataltitude the compressor is operating close to the "surge line," the ice fall is^ sufficient to create instability (rather analogous to "backpressure") and the combustion chamber is thus starved of air. At low altitudes the compressor is operating further away fromthe surge line and therefore no trouble is experienced. Clean up the air intakes and add heat at the trouble spots—that was the line of attack; and it was wrong. Heat was the main cause of the trouble—heat in the wrong place. It took a long timeand an immense amount of work to find the cure. Seven missions were flown from Filton to Entebbe in Uganda by Bristols. Compli-cated instrumentation was fitted, TV cameras were installed in the cowling. Modifications were made and thrown away as useless.Mods were re-modded. Many of them made the engines behave worse than ever. Then the right answer was found. Now the engines are ready for passenger service. The finalsolution sounds so simple. Glow-plugs have been fitted in four of the eight flame-tubes and metal deflectors have been fitted overtwo flame-tube tunnels. A number of angles have been faired and holes sealed, but these are minor modifications. The glow-plugs and the deflectors produced the "fix." First the glow-plugs. These are l|in-long, 1 fin-diameter hol-low platinum-alloy rods which are screwed into the body of the flame-tube like a sparking plug, so that they project into the heartof the flame. It is a characteristic of flame-outs that the airflow is chopped for only a fraction of a second. It returns while thehot rod is still incandescent, and the mixture is immediately relit. The fitting of glow-plugs is a sensible insurance against flame-outs. They have no moving parts, no electrical connections and require no maintenance. They have a "life" greatly in excess ofthe flame-tube. A glow-plug fulfils all the requirements of a perfect piece of equipment. Now, the metal deflectors. The hot gas from each flame-tube isejected rearwards through tunnels which pass through the air intake annulus (see sketch). The cold, dry, feathery ice crystalswhich pour into the intake strike the warm tunnels. This is where a snowball effect sets in. Squeeze a handful ofcold, dry snow and it solidifies into a snowball: the compression melts the crystals, which then fuse. The heat of the tunnels hadbeen having exactly the same effect. A cake of hard ice about 2jin x 5in was built up. Eventually this ice pancake broke awayand smacked on to the entry guide-vanes like a blanking plate. The flame-out followed. It was now clear why the Ottawa testswere so successful. Large lumps of ice don't stick—they clank harmlessly through the engines like a pin-table ball. But theTV cameras showed the dry ice forming on two of the tunnels. The next move was to fit deflectors over the hot tunnels todirect the crystals past them. These deflectors are alloy sheet- ing with a 15-thous. clearance from the tunnels. The deflectorshave the same temperature as ambient air and the ice crystals stream past and bounce off them like dust particles. But at no time did the Britannia stop flying. Training andtesting went on; then Nasser grabbed the Suez Canal. Troops were flown out to the Mediterranean in 34 Britannia flights. Thishelped a lot. These flights brought further technical and opera- tion refinements apart from the considerable help they wereto crew training. After a lull the trooping charters came in thick and fast when the shooting started in Egypt. A further thirty-fourwere operated. When we began flying the refugees out of Budapest we werepleased that we had an aircraft to do the job. We fitted 115 seats in each Britannia and pushed them day and night until wehad completed 29 flights, carrying 3,300 refugees to the freedom of the Western world—until there was no room in Britain tohouse them. This was not a Britannia party—every available aeroplane was commissioned—but we think there will be manyHungarians who will always remember the Britannias and their crews, for in their misery they were given help; their fear wasbanished by kindness. By mid-January 1957 our Britannias had carried 10,511 fare-paying passengers. Now and then, aeroplanes get tossed about pretty violentlywhen on their lawful occasions. Once in a while an airliner disintegrates; but once in a while is too often. If you read about
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