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).
Page from a publication discussing recent developments in aircraft engine ignition systems, including double magnetos and spark plug terminals.
| Identifier | ExFiles\Box 37\1\ scan 164 | |
| Date | 1st July 1927 | |
| Vol. XXI July, 1927 No. 1 RECENT AIRCRAFT-IGNITION DEVELOPMENTS 33 requires no special tools for its operation, even in inaccessible locations in the V of the engine. The construction and method of attaching the terminal are shown in Fig. 5. An inspection of several airplane engines run on the ground at night showed the presence of many coronas and static discharges from various metal parts around the secondary cables. These were particularly pronounced with magneto ignition. Under very favorable conditions, these static discharges will ignite gasoline. Several cases were found in which the ignition sparks themselves jumped to ground where the clearance was too small. The obvious remedy for such troubles is to enclose the secondary distributing-system completely with well-grounded metal manifolds and to provide sufficient insulation, or clearance, around the spark-plug terminals. This also meets the requirements of radio shielding, as previously outlined. In general, the problems of eliminating fire hazards due to the ignition-system are mostly mechanical and have little or no effect on the electrical design of the ignition system. The general requirements outlined above are the features that are desired in aircraft ignition-equipment, but, unfortunately, all these problems have not been solved up to the present time. Some progress has been made, however, in obtaining a reduction in weight and an increase in the satisfactory operating-speed of magneto ignition by the adoption of the double magneto and improvements in the design of the breaker mechanism. DOUBLE MAGNETO The idea of two-spark magnetos is not at all new, as magnetos with both ends of the secondary winding brought out through two distributors to two sets of sparkplugs were used many years ago. In fact, the first so-called “vertical double-magneto” submitted to the Air Corps was of this general type. In an attempt to reduce the weight of the ignition system by eliminating the use of special mounting-brackets and drive-shaft assemblies, the Splitdorf Model PA{Mr Paterson} vertical double-magneto was developed. The novel features of this type are the production of two sparks from a single unit and the flange mounting for driving the magneto in a vertical position at the upper end of the accessory drive-shaft at the rear of the engine. The magneto itself, as shown in Fig. 6, is a modification of the conventional Splitdorf SS{S. Smith} model by the use of two separate high-tension coil-windings on one magnetic circuit, two breaker-levers, a mounting-flange on one end-plate, and two integral distributors. This is not strictly a double magneto, as the two sparks are not entirely independent of each other; but it furnishes sparks to two sets of spark-plugs and thereby reduces the weight of the ignition system, which was the primary object of the development. A complete analysis of the electrical characteristics of this magneto is beyond the scope of this paper, but a brief explanation will be given to explain the troubles encountered. As pointed out above, two sparks are obtained by using two separate high-tension coils on one magnetic circuit. One end of each primary is connected to ground through a breaker mechanism, the two breakers operating simultaneously. The other ends of the primary windings are carried out to the ignition switch and are either connected together or grounded through the switch, thus requiring the primary current to flow through the switch in the running position. To run on one spark, the primary of the inactive coils is open-circuited and the active coil is connected to ground through an impedance-coil, to prevent an excessive primary-current at normal operating-speeds. Unfortunately, this also reduces the spark energy when running on one spark at low speeds. It was found in service that the short-circuiting effect of one fouled spark-plug would cause the other plug in the same cylinder to miss and foul-up. It was also found that FIG. 5—SAFETY-LOCK SPARK-PLUG TERMINAL This Terminal Fastens the High-Tension Cable Securely to the Spark-Plug with the Necessary Flexibility, but at the Same Time Will Not Come off until Released Manually. No Tools Are Required for Its Operation FIG. 6—SPLITDORF MODEL-PA{Mr Paterson} VERTICAL DOUBLE MAGNETO This Produces Two Sets of Sparks from a Single Unit and Has a Flange for Mounting the Magneto in a Vertical Position at the Rear of the Engine for Driving from the Upper End of the Accessory Drive-Shaft. It Is Not Strictly a Double Magneto, as the Two Sets of Sparks Are Not Entirely Independent, but Its Construction Reduces the Weight of the Ignition System. The Two Sparks Are Obtained by Using Two Separate High-Tension Coils on One Magnetic Circuit. | ||