The guns will jamb on land when in normal use. It was obvious that they would do so in a greater degree in the air.
The Squadron was using pairs of Vickers light machine guns, lying parallel on the fuselage just behind the propeller, the firing mechanism being the
Sopwith-Kauper Interrupter Gear. The bullets from each gun had to pass between the blades of the propeller, which would be revolving at say 1400 revolutions a minute. Consequently a propeller blade passed the mouth of each gun barrel about 2800 times per minute.
To fire the guns by means of an interrupter gear was in itself an unnatural way of working them. Furthermore, the speed of the propeller put a tremendous additional strain on the firing mechanism of the gun, because the interrupter gear was taken directly off the propeller shaft. It is clear, therefore, that the slightest fraction of a “hang fire," the smallest hold-up or slackness in the Sopwith-Kauper gear, or a minute delay in the firing operation caused by frozen lubricant, would certainly result in a holed propeller. Many cases are known of propellers which had their blades sheared off entirely. In any case a propeller with one or more holes pierced through its blades was in danger of breaking up, or of becoming so unbalanced as to affect its even circular movement, and so cause great vibration and also upset the running of the engine.
If a pilot had reason to suppose that his propeller had been holed, he would have the additional anxiety of knowing that at any moment one or both of his blades might carry away, and either smash up part of the wings or fuselage, or put him at the mercy of enemy aircraft or anti-aircraft gunfire. This question of holed propellers was almost as important as that of gun failure, and raised many new problems. Towards the end of my time in the Squadron, a method was found of doubling the rate of fire of the guns. The wear and tear was consequently terrific, and raised a fresh series of troubles which had to be overcome. Pilots told me that the effect of the first scraps they had with this gun was to chase the Huns right out of the sky for several days. The effect produced by the speeded-up gun was terrific, and the great disadvantage, if any, of this new development was that the ammunition became exhausted in a few moments.
When timing the gear on the ground by turning the propeller over by hand, it was found best to allow the gun to fire when about two to three inches of propeller blade had still to pass the barrel. With the engine running,
