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Aviation History
1945
1945 - 1502.PDF
AUGUST 2ND, I945 FLIGHT Tig on January 18, 1943. System characteristics are as follows: — Volts, line-to-line 208 at generator Volts, line-to-neutral .... 200 at load centres 120 at generator Frequency 400 ± 20 cycles per second Neutral grounded Basic generating units are main engine-driven alterna- tors rated at 40 kva at 75 per cent, power factor. They are designed for parallel operation when suitable speed- controlled drives are provided. Four such units provide 120 kW (160 kva) and require distribution lines rated approximately at 440 amperes. By contrast, the familiar " 24-volt " system, operating, at 27 volts, would require copper to carry 4,400 amperes. FW long conductor spans, where regulation rather than thermal capacity determines the size of conductor required, the single 27-volt conduc- tor would require 17 times the copper that is required for all three of the 208-volt conductors. , Factors of Selection Motors are of course polyphase. They are operated grounded-neutral so that they will develop partial ratings with one line shot away, and will continue to run at light load with two lines shot away, although they will not restart under the latter condition. Availability of A.C. systems does not mean that all large aircraft will or should use them. In selecting the system best suited to a particular machine, many factors must be considered, the size and electric power requirements of the aircraft being two of the most important factors. Functionally, an aircraft is a device for carrying payload from point A to point B, and systems must be studied primarily to determine the overall effect they will have on payload capacity. . Installation weight, fuel consump- tion, and, for a military machine, the probability of its reaching its destination, must all be considered in deter- mining this. Although a military aircraft must be designed to meet certain specific tactical requirements, like any other machine, once it is designed, it will lift a certain gross load. The combined weights of aircraft, equipment,- fuel, and payload must not exceed the total lift. Fuel con- sumption for operating equipment may be of far more con- sequence than the equipment weight for operation over extreme ranges. Reliability is fully as important as weight. Payload capacity achieved at the cost of reliability is not a good investment; capital would be expended faster than the fictitious high rate of return could ever justify. Where possible, of course, the designer will provide the spares necessary to assure the return of the aircraft even though some vital device fails because of enemy action or because of its inherent weakness. The weight of this reserve 40-kva alternator showing mounting flange, support brace, terminal board and air inlet. 400-cycle gear motor rated at 200 inch pounds at 27.5 r.p.m. with breakaway torque of about 600 in. lb. equipment must be subtracted directly from payload capa- city. Remaining deficiencies in reliability must be made up by supplying additional machines and crews. Conversely, an improvement in reliability cuts down losses. '' All-electric'' aircraft, according to the U.S.A.A.F., are much less vulnerable to combat damage than are hych~aulicaHy equipped machines. This is one of the reasons for the military importance of electric systems, since the transition from hydraulic to electric operation of actuators and accessories is a major factor in the expansion of electric power requirements on large aircraft. The only other factor of comparable magnitude is the great increase ia the use of radar devices. The B-29 Superfortress, although equipped with a 24-volt D.C. system, is considered to be the first all-electric aircraft, and has the largest electric system capacity of any machine regarding which infor- mation has been released. Payload and Maintenance Neither comparative reliabilities of 24-volt and 120-volt D.C. equipment, nor the comparative reliabilities of 120- volt D.C. and 208-volt A.C. equipment have yet been estab- lished by operating experience, although some experimental and much theoretical evidence is available on these sub- jects. In general, the absence of brushes and commutators on motors will increase reliability, and it is generally admitted that circuit interruption problems will be less with alternating current than with direct current at com- parable voltages. Closely related to reliability is the factor determining the time an aircraft will be grounded for maintenance. This factor may be evaluated as the payload utilisation factor. An aircraft does not carry out its function of delivering payload while a service crew is struggling with its electric equipment, or with any other equipment for that matter. As the intricacy of the equipment increases, the time for ground maintenance is very likely to increase in proportion. Some of the equipment proposed for A.C. power systems is admittedly much more intricate than equipment now being used for 24-voit systems. Too much should not«be made of this point, as 24-volt equipment is also growing more intricate as new requirements develop. Although actual service will tell the final story, some predictions regarding maintenance have been made for actual or anticipated designs on the basis of such general considerations as the number of parts involved, the neces- sity for abnormally accurate tolerances, likelihood of exces-
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