FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1947
1947 - 0387.PDF
MARCH 2OTH. 1947 FLIGHT Airborne Power Station Self-contained Auxiliary Genera- ting Plant for Large Aircraft : * First 3-phase A.C. System on the Shetland SUCH is the convenience and comfort provided by alarge modern flying boat like the Short Shetland,that it can be regarded not so much as an aircraft which operates from the water, but rather as a small ^^Juxury liner that flies. This conception is further sup- ported by the fact that the flying boat is liable to be away from its base, sometimes in remote parts of the world, for longer periods than is a land-based aircraft of com- parable size ; which stresses the need for its equipment to be self-contained, easy to maintain, and, above all, as reliable as man's skill and ingenuity can make it. These primary considerations formed the basis of an article in our associated periodical, British Engineering Export Journal, by Mr. R. G. Rayden, B.Sc, of B.T.H.'s industrial engineering department, on the sub- ject of electrical installations in flying boats. Saying that the importance of the electrical installation in large aircraft was not always appreciated, Mr. Rayden pointed out that power had to be constantly available for a variety of duties ranging from starting the main engines to cooking meals. Once upon a time a windmill-driven dynamo was enough to supply, via «, small accumulator, the few lights and indicating devices found on aircraft. But now there are a host of items demanding electric power for their operation—flaps, pumps of various kinds, air-compressors, de-icing equipment, flying instruments, wireless and radar services, automatic pilot and compasses, besides heating, lighting, cooking and refrigeration and a dozen-and-one other services on which the safety and comfort of crew and passengers depend. Modern Arrangements Up to the present, engine-driven generators have sup- plied the power for these various items of equipment, but the modern move in flying boats is to provide self-con- tained auxiliary generating plants. These may either provide tha whole of the power needed, or they may be of sufficient capacity to meet all the electrical loads when on the water, and be augmented by generators driven by the main engines to cope with the additional power re- quirements in flight. As has already been pointed out, the electrical needs of a modern flying boat are liable to be greater than those of a land-based airliner in current use, but at the same time the greater roominess of a large hull facilitates the accommodation of an auxiliary generating plant to meet these added electrical loads. It is pretty certain, however, that with the still increasing size of land- based airliners, they, too, will require, and be able to A&ccommodate, similar plants. Indeed the Bristol 167 is pile first land-based airliner in which provision is made for such equipment. The airborne '' power station '' presents its designer and manufacturer with a number of special problems. Weight and size must be kept at a minimum, but the plant must be capable of functioning with complete reliability in tem- perature* ranging from arctic cold to tropical heat (Mr. Rayden mentioned —50 degrees C to +50 degrees C in his article). Since repairs and replacements may sometimes have to be carried out in primitive circumstances, the design must make these tasks as simple and easy as is consistent with operational efficiency, and special facilities must be provided to identify the various circuits easily, locate faults q .ickly, and check the installation accurately. Generally speaking, the electrical supply for aircraft has,U P to now, been 24-29 volts D.C. from the engine-driven The auxiliary generating plant and main distribution boxeswhich are located in a separate compartment on the Short Shetland. generators, and most accessories have been designed for this voltage. But in certain instances (notably on flying boats of the Sunderland type) a small petrol-driven auxiliary unit with a D.C. generator has been installed foi stand-by duty .when the craft was at her moorings. Now, however, a substantial increase in voltage becomes necessary to keep down the weight of cable and accessories, while a further saving in weight with, it was felt, increased reliability, suggested changing Jrom D.C. to 3-phase A.C. in order to take advantage of the greater simplicity and ruggedness of "squirrel-cage" motors on which mainten- ance is reduced 'to the occasional changing of bearings. Shetland Installation The first 3-phase A.C. installation in this country was, in fact, carried out on the Short Shetland which has a span of 150ft 4m, a hull 110ft long, and an a.u.w. of 120,000 lb, and it marked a definite stage in the development of air- craft ancillary equipment. After full consideration of all the various factors, it was decided that no volts 250 cycles represented optimum value, but as a certain amount of D.C. at 24-29 volts was needed for standard components, the system had to be divided into two parts. The 3-phase. 250 cycles no volts A.C. supply fed the main engine starters, flap motor, feathering pumps, vacuum and hydraulic pumps, bilge and refuelling pumps, cowl gill motors, cabin heating and cooking. This side of the sys- tem also supplied the interior lighting, heated clothing, navigation lights, etc., but through transformers which re- duced it to 29 volts A.C. single-phase. The 24-29 volt D.C. supply looked after accumulator charging, the windscreen wipers, de-icing equipment, immersed fuel pumps, gauges, instruments, indicators,
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
