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).
Recent developments in aircraft ignition systems and radio-shielding principles.
| Identifier | ExFiles\Box 37\1\ scan 162 | |
| Date | 1st July 1927 | |
| Vol. XXI July, 1927 No. 1 RECENT AIRCRAFT-IGNITION DEVELOPMENTS 31 FIG. 1—DIAGRAMMATIC ILLUSTRATION OF RADIO-SHIELDING PRINCIPLE As the Potential a, a Source of Electrical Radiation, Varies, the Electrostatic and Electromagnetic Fields Surrounding It Vary Similarly. If a Closed Circuit, b, Is Placed around a, a Current Will Be Induced in b by the Varying Field around a.{Mr Adams} This Induced Current Establishes a Field Opposed to That of a, To Neutralize Which Completely in All Directions the Source of Radiation Must Be Completely Surrounded by a Conducting Surface That Offers No Resistance to the Flow of the Induced Current. Such Absolute Shielding Is Impossible, Owing to Imperfect Conductivity of the Shielding. As the Opposing Field Is Proportional to the Conductivity of the Surrounding Electrical Paths, Any Change in Resistance of These Paths, Caused by Intermittent or Sliding Contacts, Will Vary the Shielding Effect and Set Up Noises in the Radio Receiver shielding is impossible, the results being in proportion to the conductivity of the shielding. This point is of the greatest importance in designing shielding, for there is a widespread notion that ignition shielding is similar to magnetic shielding, in that any sort of metallic housing around the ignition system will suffice. Since the opposing field is proportional to the conductivity of the surrounding electrical paths, it follows that any change in the resistance of these paths, such as may be caused by intermittent or sliding contacts, will cause the shielding effect to vary and to set up noises in the receiver. In practice, it is necessary to surround the entire ignition system with well-bonded metallic housings that enclose the magneto, distributors, ignition cables, and as much of the spark-plugs as possible. The ignition-cable tubes must be grounded at each cylinder, as high-frequency static charges will build up on the tubes and arc across to neighboring conductors if the tubes are not grounded at least every 12 in. Fig. 2 shows the radio-shielding manifolds for the Liberty-12 engine and the method of shielding the spark-plug leads with copper braid over the cables that also serves to ground the manifolds. This shielding, developed by the Signal Corps and Radio Unit at McCook Field, is one of the most practical installations in use in this Country. On many engines, the use of radio shielding will necessitate a re-arrangement of the accessories to allow the installing of the necessary manifolds and the proper housing of the distributor. The question naturally arises, why not use the cowling on all-metal ships as a means of shielding. This has been tried, but the necessity for bonding wires between the various sections of the cowling makes the inspection and care of the powerplant very tedious, and the shielding is only about one-half as effective as is the manifold type. The effect of shielding on the ignition is of considerable importance also. The addition of grounded surfaces close to the secondary cables throughout their length increases their electrical capacity from 50 to 100 per cent. In some types of ignition in which the energy from the coil is limited, this increase in secondary capacity may be sufficient to prevent the coil from charging this capacity up to the required sparking-voltage, and missing will occur. When the spark energy is ample, however, the additional capacity causes the spark to be "snappier," by increasing the initial capacity-component of the spark. In one of the first shielded installations on the Liberty-12 engine, the secondary cables were each covered with a tightly woven copper-braid, and the entire bundle of wires was grounded to the engine at frequent intervals. This increased the secondary capacity to such an extent that missing occurred in service. By removing the copper braid from the cables and using metal covers over the distributors that were connected to the manifolds in the V of the engine, the capacity was greatly reduced and no further trouble was encountered. The following interesting examples illustrate some of the obscure sources of radio interference. An engine was equipped with what was considered to be a satisfactory shielding manifold, but there was still considerable interference. Upon removing the equipment and substituting a set that had worked successfully on another engine, the spark-plugs were also accidentally changed to another type, and the interference disappeared. When the original manifolds were replaced, they worked satisfactorily, much to the surprise of those making the test. It was found that the original spark-plugs were of the auxiliary-gap type with a short spark-gap incorporated in the terminal nut. This gap acted as a small broadcasting-station that was not shielded by the copper braid on the cables. It has also been found that the primary-switch lead and the booster-magneto lead on a magneto will broadcast, unless properly shielded by a copper-braid covering. Incidentally, the entire wiring-system of the airplane, including the storage-battery and FIG. 2—RADIO-SHIELDING MANIFOLD ON LIBERTY-12 ENGINE This Shielding for Housing the Distributors and Cables, and the Method of Shielding the Spark-Plug Leads with Copper Braid Were Developed by the Signal Corps and Radio Unit at McCook Field and Is One of the Most Practical Installations in Use in This Country. The Addition of Grounded Conducting Surfaces Close to the Secondary Cables throughout Their Length Increases Their Electrical Capacity from 50 to 100 Per Cent | ||