From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article on recent developments and requirements for aircraft ignition-systems.
| Identifier | ExFiles\Box 37\1\ scan 158 | |
| Date | 1st July 1927 | |
| Vol. XXI X3199 July, 1927 No 1 Recent Developments in Aircraft Ignition-Systems By F.{Mr Friese} G.{Mr Griffiths - Chief Accountant / Mr Gnapp} SHOEMAKER¹ CHICAGO SECTION PAPER Illustrated with DIAGRAMS AND PHOTOGRAPHS ABSTRACT THE fundamental electrical and mechanical requirements of ignition equipment for aircraft engines are outlined and the special requirements peculiar to this service and that apply, in general, equally to military and commercial aircraft, are described. Brief descriptions are given of various new types of both magneto and battery ignition and the developments in each are pointed out. Characteristics of an ideal ignition system are enumerated as a basis for further development. Among the general requirements reliability is given place of first importance, followed by light weight, compactness, low cost and adaptability of a single model to engines of different types. The chief design-requirements are speed, ruggedness, simple mounting, light rotating-parts, resistance to vibration, ample lubrication, protection against moisture, and fire-proof ventilation. Each of these subjects is dealt with specifically. Difficulties of meeting the exacting electrical requirements are explained and means employed to overcome them are described. Too great spark energy may cause “overlapping,” which, with battery ignition, results in burning of the breaker contacts, and, in magneto ignition, reduces the intensity of alternate sparks. For ignition of supercharged engines at high altitude where the air density is much reduced, the air insulation of the ignition system is much less effective than at sea-level, and a flash-over distance to ground of roughly 0.75 in. is required. Coil failures will result unless the length of the coil is increased to provide this gap or all the air-spaces are filled with some insulating material. The principles of shielding the ignition system to prevent interference with radio communication are explained and complete shielding of the system for a Liberty-12 engine, as developed by the Signal Corps and Radio Unit at McCook Field, is shown. As a result of tests to determine fire hazards, the Experimental Engineering Section developed a type of magneto vent for ventilation and drainage which is shown. Several types of two-spark or double magnetos for supplying sparks to two sets of spark-plugs are illustrated and described, as is also a pivotless type of high-speed breaker-mechanism developed by the Materiel Division at McCook Field. Several new battery-ignition distributors as developed for use on airplanes that carry equipment requiring a generator and battery are described, and the author lists the relative advantages of battery and magneto ignition. He then tells the requirements of an ideal airplane ignition-system, states those that have been met satisfactorily, but concludes with the statement that development of ignition equipment especially adapted to aircraft engines has only begun. DURING the World War, a feverish attempt was made to adapt existing types of ignition equipment to the requirements of aircraft engines and new models were built by several manufacturers. In most cases, however, this development stopped abruptly at the end of the war and practically all development since then has been at the instigation of the Air Corps, with the cooperation of a few of the large manufacturers. This development was somewhat slow up to the beginning of 1926, as the limited market for new types did not seem to warrant any great activity on the part of the manufacturers. But the stock of surplus war-type engines has become smaller and the number of airplanes in service has increased until a rapidly increasing market for new ignition-equipment has caused at least one company to go into the manufacture of aircraft magnetos exclusively; and increased interest is being shown by other manufacturers. Virtually all American aircraft-engines, with the exception of the Liberty-12, have been equipped with two single magnetos, most of which were supplied by the Scintilla Magneto Co., whose Type-AG magneto is regarded as the best single aircraft-magneto available in production quantities in this Country. The trend of development of all aircraft equipment is to reduce the size and weight and to increase the reliability and output; and this has been the object of the work on ignition done by the materiel division of the Air Corps. The only recent discussion of ignition has been presented by Dr. F.{Mr Friese} B. Silsbee, of the Bureau of Standards, his paper² being devoted principally to the electrical behavior of the source of the sparks and the process of ignition. The object of this paper is to outline the fundamental electrical and mechanical requirements of ignition equipment for aircraft engines and to explain the special requirements peculiar to this service. In general, these requirements apply equally well to military and to commercial aircraft. The various new types of battery and magneto ignition are also described briefly and the developments in each are pointed out. The characteristics of an ideal ignition-system are outlined as a basis for further development. GENERAL REQUIREMENTS The general requirements of aircraft ignition are reliability, low weight, compactness, low cost and adaptability. Since the flight of an airplane depends upon the continuous operation of the powerplant, reliability is undoubtedly the most important requirement of ignition equipment. The increased use of aircraft for commercial purposes is making the factor of cost more and more important, but, in aircraft, cost can never be placed ahead of reliability. This is not always the case in automotive vehicles. Light weight is also an important factor, since the weight of the ignition equipment is parasitic, that is, ¹M.S.A.E.—Associate mechanical engineer, aircraft-powerplant materiel division, Air Corps, McCook Field, Dayton, Ohio. ²See THE JOURNAL, November, 1926, p. 442. 28 | ||