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Aviation History
1945
1945 - 1503.PDF
I2O FLIGHT AUGUST 2ND, 1945 sive temperatures, vulnerability to combat damage, and ability to withstand abuse and improper operation. Evaluated on this basis, constant-speed drives and syn- chronising controls appear likely to require somewhat greater maintenance and a somewhat higher standard of technical ability to operate them than do any components of the existing 24-volt systems. Motors for use with A.C. systems, on the other hand, will require considerably less mainten- ance than D.C. motors now require. So far, electric systems have been discussed rather A.C. ELECTRI generally as to the effect that the system may have on the aircraft as it carries out its primary function of carrying payload. For proper overall perspective, it is now necessary to consider the fact that all large aircraft carry some equipment which must be operated from direct cur- rent and some equipment which must be operated from alternating current. Solenoids and some forms of control apparatus function best when designed for direct current, whereas many instruments and all high-power radar equip- ment operate from alternating current. Regardless of the type of power provided for the main system, conversion equipment will be required to suit many of the devices to be operated. Where possible, of course, devices will be designed to operate from the basic power supply without the use of conversion equipment. Although an A.C. motor will weigh about 60 per cent, as much as an equivalent D.C. motor, it will not pay to install an inverter to permit the use of A.C. motors on D.C. equipped machines unless compelling reasons other than weight exist. On the other hand, radar tubes require power at a D.C. potential of many thousand volts. The lightest and most reliable method of obtaining this power is by transforming high- frequency A.C. power to a suitable voltage and then recti- fying it .to provide the required D.C. potential. The weight required on D.C. aircraft for inverters or for special alternators provided solely for radar operation is consider- able, but it is negligible compared with that required for D.C. generation at the necessary voltage. Constant-speed Device Needed With so much progress in deyeloping equipment for A.C. systems already recorded, it is necessary to report that one tremendous obstacle has not yet been overcome. In Machine Design for June, 1941, Dr. E. E. Minor writes on the subject of "high-frequency power for aircraft" and points out the need for a constant-speed device to couple the alternator to the main engine. Not much more can be said in 1945, at least at this writing. The variable- stroke, friction-drive device to which Dr. Minor alluded was abandoned some time ago, but there are at least three hydraulic devices which bear some promise of success, although at much higher weights than were originally con- templated. Recent developments in the gas-turbine field also suggest'that an auxiliary gas turbine ma ' be a suitable prime mover in spite of high fuel consumption, and that development of such a turbine may make it possible to abandon the long quest for a constant-speed drive. It is thought that A.C. systems will be justified for high (CONTINOED) values of connected load, and for aircraft a large size with power-transmission distances exceeding 100 or possibly 200 ^ feet. The dividing line between preference for D;C. systems and preference for A.C. systems is not too clearly estab- lished, and will shift somewhat according to the success in developing an alternator drive, and, as pointed out pre- viously, according to the particular make up of the utili- sation devices required on the machine. One question of para- mount importance cannot be answered at this time, CAL SYSTEMS namely: When will the new A.C. system be in actual operation? Some of the urgency has disap- peared since 1942. Then again, much oi the early impetus came from the high- altitude brush problem, a major obstacle to D.C. apparatus in 1942, now also a forgotten problem thanks to the work of Dr. Howard Elsey, of the Westinghouse Research Labora- tories, and his associates. Even so, with so much oi the equipment already completed, it does not appear that -*d flight tests will be long delayed, although it is admitted that the first installations cannot be as complete as pro- ponents of the A.C. system would like to see them. Preparing for Change If this prediction is correct, it behoves the designers of aircraft accessories to be ready with A.C. versions of air- craft electric equipment. In general, conversion of exist- ing D.C. motor-driven devices will be relatively simple, and in most cases, the A.C. powered device will be simpler, lighter and easier to maintain than its D.C. counterpart. Often, an AC. motor will merely be rubstituted for the D.C. motor, with negligible modifications of the accessory. Many designers, however, will want to take advantage of the possibilities inherent in A.C. motor application by redesigning the accessory for a higher motor speed. The most popular speed for D.C. motors is probably 7,500 r.p.m.; 11,500 r.p.m. is expected to be the most popular speed for 400-cycle motors, although conservative designers may hesitate to exceed 7,500 r.p.m. except for intermittent ratings or for continuous ratings below one horse-power. Very large motors are likely to warrant special lubrication systems which will permit opejation at 11,500 or 23,000 r.p.m., speeds which are already exceeded on gas-turbine compressors and on superchargers. Use of built-in A C. motor parts is considerably simpler than is the case with D.C. motor parts, since no brush rigging is used. As a compromise between motor parts and complete motors, the U.S.A.A.F. are sponsoring the development of "three-quarter" motors, the bearing housing for the drive-end being a part of the accessory instead of being part of the motor. This method of construe- L- tion saves some weight compared with complete motors, <""^ and facilitates motor replacement compared with integral motor parts, but is being resisted by many manufacturers because it complicates problems of ventilation, factory testing, and responsibility for satisfactory operation. Standardisation of three-quarter motors is considerably more difficult than standardisation of complete motors, since many internal features of motor design are affected. Book Review Air Power for Peace. By Eugene E. Wilson. McGraw-HillBook Co., Inc., New York and London. $2 net. NO, Mr. Wilson, you are not correct on page 21 of your bookwhere you speak of Radar as " American-invented." It was a British invention Likewise it is not accurate that in1918 the French had " the largest and perhaps the best air force in the world " (page 22). The Royal Air Force was thenthe largest, and few with knowledge will doubt that it was the best There are other points in this book to which we might takeexception, such as the omission (on page 119) to mention that Bomber Command of the K.A.F. was active, as well as theFortresses and Liberators, in helping the invasion of Normandy, and the remark on page 133 that " the failure of the guns inthis action (i.e., against the Bismarck) is significant." The guns did not fail; it was the guns of H.M.S. King George V andRenown which silenced the Bismarck's turrets and left her helpless Then it was obviously more economical to use marinetorpedoes for the coup de grace. The main contention of this book, thot air power consists ofthe three elements of air forces, air transport and aircraft pro- duction is interesting and sound. The description of howAmerican aircraft industry was built up is an instructive section, and the' outline of the war has value for referencepurposes. Based on history, it is a book which looks ahead. It is not easy reading, but it is certainly a book of merit.
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