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August, 1931  U.S. AIR SERVICES  33
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damage on the main bearing, or upon ball bearings. Since this discharge lasted for a fraction of a microsecond only, the tests do not prove that damage from this source can be entirely ignored. In general, however, it would seem that a discharge striking a metal propeller would flash from the shaft to the face of the engine case without causing any damage. However, should the discharge take place in the bearing, it is possible that trouble would result. Owing to the resistance of the bearing, it is comparatively easy to provide a shunt path between the propeller shaft and housing so that the oil film will not be punctured by a discharge. 
While the energy stored in the test condenser is of the order of 10,000 wattseconds compared to 10,000,000 wattseconds in the discharge of lightning, it must be remembered that the energy it dissipated almost entirely in heating the air so that the energy dissipated in a bearing may not be any more with a lightning stoke than that under laboratory conditions, unless the discharge consists of a number in rapid succession liberating a considerable amount of energy.

8. SUDDEN CHANGE IN PRESSURE ON ADJACENT SURFACES

THE sudden change in air pressure following a heavy stroke will probably not exceed that frequently occurring under normal flight. Should a discharge be close to and parallel a surface, it is possible that a heavy effective pressure may be set up tending to cause the collapse of same. It is this sudden 
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change in pressure which probably accounts for the tearing of fabric. 
It is evident that the larger the spread of the conducting surfaces, the greater will be the danger of a stroke including the airplane in its path to ground, or from cloud to cloud. While the use of aerial extending some distance below the plane will tend to increase the danger of a stroke, it must be remembered that this will at least keep one of the points of contact of the discharge at some distance from the plane. This advantage may more than offset the increased probability of a strike. The use of a loop set does not change the hazard in any way over that where no radio set is used. The use of a strut or mast extending above the plane for an aerial would seem to be an added protection as it would tend to keep the point of contact at a distance.
The insertion of a resistance or impedance between the antennae and instrument with a shunt path to the frame of the aircraft will provide ample protection, the protection probably being much more effective than that provided for the ordinary house radio set using an outside aerial.
While hot gas is a good conductor, the question of the engine exhaust forming a conducting path which would tend to induce a stroke to the plane does not seem to be an appreciable hazard. In all of the tests the effect of the exhaust gases could not be noticed. In many tests made there was no indication that a low resistance or conducting path was created by the hot gases. In fact, it would appear that the dielectric strength
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of the air was apparently increased by the wind or pressure produced by the propeller. While the gases are conducting for a very short distance from the exhaust, this hot gas is soon cooled by the mixing of the strong air current produced by the propeller, so that no effect upon a discharge can be expected. 
IN conclusion it may be said that while information is lacking as to the effect upon the person, much can be accomplished to remove the danger of this effect by giving attention to the various factors involved. While the hazard is exceedingly small, it is possible that still further improvements may be effected by taking advantages as opportunities in design and construction present themselves. 
Further information upon the effect of shock will doubtless show that the hazard in not very great, although it may appear to be necessary to protect the pilot from sound of other conditions during a storm. Where the pilot has a fear of lightning, it is possible that tests or checks might be devised which would remove this fear.
Anything which will permit the safe landing of the plane or provide automatic control while the pilot is stunned would do much to eliminate the hazard that now exists. Owing to the increased reliability of aircraft, less attention will be paid to storms. While this will tend to increase the lightning hazard, it would seem that the present hazards can be more than offset by careful attention to the various factors tending to produce reliability. 
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Air Corps Photograph of an Observation Squadron in the recent maneuvers over New York City
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