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Aviation History
1955
1955 - 0816.PDF
814 FLIGHT, 10 June 1955 The J. and H. G282 alternator (below) is an advanced air-cooled unit capable of accepting 120-deg C intake air. The curves show altitude influence upon blast-cooling airflow and (far right) flight-speed effect on inlet air temperature. 100 90 80 60 50 30 20 10 0 V v V\ \ \ > \ Ah V V\ c 8 LA! CONST/ oc s \ s s T COOLED WITH kNT PRESSURE DRO \ s s s s, P 10 20 30 40 50 60 70 ALTITUDE (ft X 1000) a UJ £ S°O 200 5 Q 160 2 s •«l!0 S => IU K si 80 t ^ 40 5 0 |-4n -80 w L ir y A / /k / / A/ J // f / A/ J A / / >f /f i/ i 'ft/ yf SE 20,000 FEET 30 40 ,000 FEET 000-100 000 2 4 6 1 FLIGHT S«£D(n Oil-cooled Generators ADVANCED ELECTRICAL MACHINERYFOR FAST, HIGH-FLYING AIRCRAFT THIS article is based on a report prepared by Jack and Heintz, Inc.,an American firm specializing in aircraft electrical machinery. The principal authors of the report are O. E. Buxton, project engineer onoil-cooled A.C. generators, and Charles R. Dunfee, district manager, N.Y. office. The company state that they would welcome specific re-quests for further information from other engineers working in this field. FOR many years, air has been the accepted medium forcooling aircraft generators and other rotating electricaccessories. Such an arrangement is essentially simple, air being rammed into a duct leading to the accessory andthen dumped overboard slightly hotter than it was before. Many present aircraft will continue to employ air cooling anddevelopment is attempting to extend the limitations of such equip- ment. Modern air-cooled generators are being redesigned withhigh temperature components, new insulations, "free flow" air passages and other innovations. The American firm of Jack andHeintz, Inc., of Cleveland, Ohio, recently announced an air-cooled generator—it is illustrated on this page—capable of operating withinlet-air temperatures as high as 120 deg C (248 deg F). Never- theless, this company—and others—have appreciated that air-cooling can no longer be relied upon either in supersonic flight or at sustained altitudes greater than 40,000ft. In the first caseram temperature rise is excessive and in the second the weight of air that can be handled by normal ducting is considerablyreduced. These factors do not of themselves preclude the use of air; the governing factor is the necessity for making funda-mental design changes to increase the cooling effectiveness. Studies have indicated that the practical limits of air-cooledmachines are determined by inlet-air temperature, the maximum permissible values being about 120 deg C at sea-level, 40 deg Cat 50,000ft and minus 12 deg C at 65,000ft. Air cooling might be taken beyond these limits, but Jack and Heintz feel that theeffort expended would not be worth while in view of the new liquid-cooled machines now available. It was in 1949 that the American firm initiated developmentof the liquid-cooled G37, a 30V, 300-amp D.C. generator weigh- ing 66 lb. This generator was available before aircraft had en-countered flight conditions requiring it, but it served to alert the industry to the feasibility and exceptional potential of oil-cooled units. During the past six years, therefore, Jack and Heintz engineers have steadily explored the various factors associatedwith oil-cooling, such as installation, sealing, bearings and cool- ants. The company is now 6729 oil-cooled D.C. generator. in pilot production of severaloil-cooled units and Stan- dards for such equipmentare being evolved. Four such units are illus-trated. The internal cooling system of one of the newmachines is also shown schematically. Oil enters atthe mounting pad, where it returns to the engine afterbeing circulated through the stator and rotor. A major difference, compared with a blast-cooled machine, is that the mainrotor mounting need have only one bearing (at the anti-drive end), the bearing at the other end being supplied by the drive. Theliquid used would typically be lubricating or hydraulic oil tapped from, and returned to, an existing circuit.Oil-cooled machines are smaller in diameter, and possibly shorter, than comparable air-cooled machines, since air ductingis replaced by tubing. For example, oil pipes of half-inch overall diameter can replace three-to-four-inch ducting and still largershrouds. Fundamentally, the cause of the slimmer configura- tion is thermal. Temperature gradients exist in air-cooledmachines both axially and radially and air generally loses its effectiveness as a coolant before it penetrates fully into a genera-tor. Therefore, provision has to be made for a considerable air blast against the face of the generator and this accounts for recentincreases in diameter. In oU-cooled machines, axial temperature gradients are substantially lower and cooling can be effected byemploying axial oil ducting. The latest air-cooled machines require a fan and water-separa-tor, which again increase the overall bulk of such equipment. It appears that the minimum diameter of an oil-cooled generatoris determined by the diameter /length ratio compatible with good electrical design. The reduction in bulk and die elimination ofducting has already been found substantially to alleviate conges- tion around turbojets, even when allowing for slight increase inthe capacity of the oil heat-exchanger. Air cooling has always involved some drag. This is eliminatedor considerably reduced with oil-cooled accessories, and under certain conditions—such as high speed at low altitude—the fuelconsumption is sufficient for fuel/oil heat exchangers to perform all the cooling of both the engine and the accessories. At highaltitudes, air/oil heat exchangers may also be required, but the air requirements will be low owing to the high cooling efficienciespossible. It has been found possible, in fact, to employ a recircula- ting system which can be switched from one heat-sink to anotheraccording to the environment or operating condition. In some cases, bleed-air ducts from the engine can be eliminated, so im-proving the engine performance. The altitude limitations of air-cooling necessitate the deratingof generators at high altitudes. Liquid-cooled machines, on the other hand, have a performance independent of flight speeds andambient conditions so long as proper oil inlet temperatures and flow rates are maintained. For example, the J. and H. G190 willdeliver 40 kVA at 0.75 power factor as long as five gal per min of oil is available at a maximum of 300 deg F. In the designof air-cooled generators, the operating level is increased until some part of the machine reaches its critical limit. Successive improve-ments then shift the design problem from one component _ to another until, in theory, the perfect machine is reached in whicheach component reaches its critical limit at the same temperature. Nevertheless, even with careful design, it is practically impossibleto eliminate high spots in blast-cooled machines. In liquid-cooled machines, gradients are mainly radial anddepend principally on the load (since the coolant flow is independ- ent of altitude). Furthermore, the temperature rise of the oilis low enough to ensure that all components receive cooling liquid at roughly the same temperature. It is thus possible
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