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Aviation History
1958
1958 - 0304.PDF
318 FLIGHT, 7 March 1958 CARBON BRUSH OXIDE FILM Fig. 7. The lubricating oxide film on a commu- tator breaks down in the dry air at altitude. COPPER COMMUTATOR ROTAX LOOKS AHEAD . . . the contact face of the crystalline-structure carbon brush. At alti-tude, where the air is dry, the film does not form, and the unlubri- cated brush bears against the copper surface of the commutator,which becomes a very effective grinding wheel. The most successful approach to overcoming high brushtemperature and high wear has been to add to the brushes cores of adjuvants, such as varnish, barium fluoride, P.T.F.E. or molyb-denum disulphide, of which the last named has perhaps been the most successful. Some of this work implies pure research, but muchis still essential development for satisfactory high-altitude function- ing of existing equipment. Rotax's brush suppliers, The MorganCrucible Co., Ltd., manufacture cored brushes and carry out limited altitude tests and friction measurements. While this workprovides useful information, the searching test of a new brush design is its performance in a machine, and current density, springload and airflow temperature rise must all be checked in the Rotax chamber (the R.A.E. have devoted, and continue to devote, muchtime to this problem). The molybdenum disulphide cored brushes have been successful in reducing brush temperature, but still posewear problems at altitude; they are not yet really suitable for the long overhaul periods of civil aircraft. About O.OOlin wear per houris a reasonable aim. There are still a number of unsolved problems with coredbrushes, and their behaviour is not yet completely understood. The sketch in Fig. 2 shows the wear characteristics at ground WEAR SEA LEVEL ALTITUDE Fig. 2. Brush wear against core track area and height. BRUSH TEMP. TIME Fig. 3. Three runs at altitude on the same brush. level and at altitude, although unexplained inconsistencies haveoccurred in the measured values at altitude. An interesting pheno- menon is that the rate of wear increases with time, but stabilizesafter about six hours. These curves also reflect the manner in which the lubricant cores become effective as the operating heightincreases; at ground level the oxidant film reduces the need for the molybdenum disulphide cores, but at altitude, the larger the coretrack (the area swept by the core) the lower the wear becomes. And although in this condition the pure carbon tracks apparentlyconduct no current at all (the surfaces of the brush show no sign of electrical erosion) the brush retains its effectiveness by conduc-tion through the carbon in the core track area; it appears that the ends of the cores (which are more resistant to wear than thebrushes) become filmed-over with carbon. The conduction path to the brush-lead is the carbon in thetrack area surrounding the lubricant core (see Fig. 4). Rotax report some success inreducing brushwear by rhodium plating of the commutator (the Dalicplating process), although in the laboratory it has been found thatunder certain conditions the plating has stripped away. It is clearly apparent that this im-portant work must involve a con- siderable amount of test time in the A small Rotax magnetic amplifier unit is used on the F.27 rather than a more cumbersome transformer-rectifier unit. Fig. 4. Generator brushes with adjuvant cores. Tracks of car- bon and adjuvant form at altitude. CORE TRACK AREA altitude chamber, and a second, smaller, altitude chamber hasbeen constructed to operate within the sea level cold chamber where soak testing is performed. R.A.E. pioneered high-altitudecold chamber electrical component testing in this country, and Rotax were the first private company to provide facilities forsimilar test work. So valuable has this proved that chamber capa- city has become hard-pressed to keep ahead of production. Thisis not an indictment of Rotax foresight, but it does highlight the ever-present need for a great deal of ideas research at the designstage. Long overhaul lives and very few modifications is the aim ofevery component manufacturer in the country, but the conflicting pulls of early delivery dates, low first cost and early large scaleproduction test work must not be allowed to get out of hand: everyone is familiar with the designer who can never release hisdrawings to the shops because he has thought of something better. Yet there is still a need for larger test facilities that can do moreessential work in less time; this is part of the price of the competi- tion for reliability. So much, then, for the background facilities to the eventualproduction flow. What of Rotax ideas for the production lines of a few years' time? The company have always maintained closerelations with the aircraft and engine manufacturers, because it is upon these contacts that the next stage of systems design is decided.The process of advancement is, in fact, not very clear cut, as engine, airframe and accessory designers all have a contributionto make. At the moment, too, electrical manufacturers are at a crossroads with many lines of development to pursue ahead ofthem. It is not easy to recall when there has been such a wide divergence of opinion about the best way to generate aircraftelectric power—notably constant frequency A.C., which continues to challenge the ingenuity of designers in achieving low weightand high efficiency over a speed range now generally stipulated as about 3/1. Assuming that there is a requirement for a supply of constantfrequency A.C., it can be met by a Sundstrand constant speed drive and alternator, by other types of hydro-mechanical drive (such asHobson's 15kVA constant speed unit), by a hydraulic drive— which may be remote from the powerplant—or by an A.P.U. oran air turbine drive. The number of manufacturers involved in developing these schemes results in a bewildering array of com-ponent permutations. English Electric, for example, offer a com- plete system of their own (Sundstrand drive) or work in conjunctionwith Rotax; Hobson with B.T.H., and Rotol with other manu- facturers again. Competition for the few British civil orders isbecoming very intense, particularly since the challenge of foreign manufacturers' work must be taken into account. On exportedequipment this is a powerful force with which to reckon. Rolls- Royce Con ways exported to the U.S. for example, will carrycertain U.S. AiResearch equipment, and U.S. Westinghouse equipment is to be used on Fairchild-built Fokker Friendships. Presumably Conways for the VC.10 will be equipped withBritish accessories (the English Electric Sundstrand seems a likely choice) but Armstrong Siddeley are building AiResearch A.P.U.sand starters under licence; Rotax are supplying generating equip- ment for the Fokker-built Friendship with magnetic amplifiercontrol chosen on technical merit over the more bulky transformer rectifier unit; Rotax generating equipment is to be used on theGrumman Gulfstream and is also used on the Vanguard. These examples show very clearly the competitive pattern thatis forming. It represents a healthy fresh wind throughout the industry, but which only the robust may long survive. To the rival schemes for constant frequency A.C. generationmay be added a new Rotax suggestion for a self-contained variable speed, constant-frequency alternator. It is one of the difficulties ofthe accessory manufacturer's life that he must sell equipment at the scheme stage if he can; a longer wait to prove a unit or anidea may allow the competition to get too far ahead. Development of this particular scheme (briefly described in Flight, January 31,1957) is now being continued "behind closed doors," as certain difficulties have been encountered in obtaining competitive effi-ciencies with 60 and 80kVA units throughout the speed range. The principle is a Swedish one, and has been satisfactorily demon-strated on a 750W unit; for the moment, no more may be said. Another competitor in the constant frequency alternator marketis the French firm Labinal, whose alternator and drive is to be used (for example) on the six Caravelles for the Scandinavian Air-
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