One device we haven't covered is the Davis recoiless gun - a weapon that never realised its potential. I enclose a write up (note this includes the bit on the Supermarine Night Hawk I've posted elsewhere).
Attempts had been made started to fit large calibre weapons to aircraft even before the First World War had started. A Short S81 Seaplane was fitted with a one and a half pounder semi automatic cannon for trials in 1913. When the gun was fired in flight the recoil effect was so fierce that the aircraft was stopped dead and fell several hundred feet before regaining flying speed – not a desirable feature for a combat, or indeed any, aircraft. It was obvious that some sort of recoilless weapon was required if large calibre weapons were to be practical in the aircraft of the day. An American naval officer one Commander Davis had been experimenting with recoilless weapons specifically for deployment in aircraft. His first attempt was a single shot weapon where the shell emerged frontward and the rest of the gun was fired backwards to cancel the recoil. This was not very practical but by the outbreak of the First World War he had refined the weapon into a gun with two barrels, one facing forward and one rearward with a common breach in the middle. When fired the shell was shot towards the target whilst a counter weight of tallow containing lead balls was blasted backwards. The gun was manufactured in Connecticut and came in a number of sizes the smallest being a 1 1/2 pounder with a calibre of about 35mm, this was regarded as a suitable anti Zeppelin weapon whilst the larger guns were regarded as having a potential to be used for ground attack or anti shipping (particularly against U boats).
Ingenious as it was the Davis gun posed a number of problems the major of which was the need to ensure clear space behind it so that the counter weight did not damage the aircraft – it would discourage pilots if firing the weapon blew off their own aircraft’s tail. Coupled with this was the design of the breach that required manual loading and reloading so that the gun could not be mounted over wing and away from the reach of the pilot or a gunner. Another issue was the fact that the breach was not always completely gas tight so that flames and smoke often spurted from it on firing. The British Admiralty (The Royal Navy was initially responsible for defending the UK against airships) decided that specialised aircraft would be needed to carry the Davis gun into action.
The first two designs were the ADC Sparrow Scout of 1915 and the Blackburn Triplane of 1916 respectively. Both were designed by the same person and very similar in concept being single seaters with pusher engines and propellers mounted behind the pilot. The tail assemblies were mounted on a spindly looking framework attached to the upper and bottom wings. The nacelle for the pilot and engine was, unlike most pusher aircraft, mounted high up on the top wing, presumably to maximise the chances of the pilot being killed if the aircraft should nose over on landing! This was encouraged in the ADC design by a tall main undercarriage with the wheels extremely close together. Maintaining the engine on both aircraft would have been awkward for the ground staff and the airman who swung the propeller to start the engine must have had an interesting time as he would have needed to do this from a stepladder (and probably been blown off it by the prop wash). Both nacelles had long deep noses to house the Davis gun, these must have greatly impeded the pilot’s forward view. It seems probable that both aircraft were designed without their designer being aware of the full characteristics of the Davis gun.
If fitted in the Sparrow the breach would have been between the pilot’s legs, giving full scope for the effects of the escape of fire and smoke, whilst in both aircraft the only direction in which the counterweight could be fired would be through the propeller with a high probability that this would be smashed. In fact neither aircraft was fitted with its intended armament and both never proceeded past the prototype stage.
The Robey Peters Gun Carrier completed in spring 1917 adopted a different approach. This was a large ponderous single engined tractor biplane with two gunners nacelles fitted to the top wing, one port and one starboard. Both gunners would have a clear field of fire and there was no part of the aircraft that would be hit by the counterweight. It would still have been an interesting experience sharing the gunners elevated cockpit with a flame spitting breach of a Davis gun. The pilot however sat so far back in the main fuselage behind the wings that his cockpit was almost in the tail. His view in any useful direction would have been negligible and he had no means of communicating with the gunners. This would have made locating and intercepting any airship problematic let alone taking off and landing on airstrips at night. As it was the prototype Robey Peters Gun Carrier damaged its undercarriage on the first take off attempt and did not become airborne. After repairs were made a second flight attempt was successful in that the aircraft took off and flew round the airfield – before crashing – appropriately enough on the local lunatic asylum. No more Robey Peters Gun Carriers were built.
The last, and most outré, attempt to produce a Davis gun armed anti airship fighter was made by the newly formed Supermarine company (formerly Pemberton Billings and Co). This was the Supermarine P.B.31.E Nighthawk, a very large twin engined quadruplane. The fuselage, which was mounted between the middle wings, was surmounted by a enclosed cockpit, somewhat reminiscent of a small conservatory, on top of which was built a gunner’s position for the Davis gun and a rearward firing Lewis gun position with a Scarff gun ring. This was level with the top wing. The Davis gun would have a clear field of fire all around the horizon (but the counterweight would decapitate the rear gunner if he was in his cockpit when the main weapon was fired forwards). A second Lewis gun position was stationed in the nose of the main fuselage together with a small searchlight and a 5hp petrol engine to drive a generator for the light and provide heating for the main cockpit that contained sleeping facilities for spare crew members. The pilot was positioned at the rear of the enclosed cockpit doubtless to reduce the possibility that he might actually be able to see an enemy airship whilst at the same time adding extra interest to the process of taking off and landing on ill lit night time air strips.
The whole contraption was powered by two 100hp rotary engines. It is worth noting that the total power available to the Nighthawk was less than that used by the majority of light two seater aircraft today and with this it was expected to haul a crew of between three and five, two machine guns and a Davis gun (all with ammunition) and up to 18 hours worth of fuel around the night sky. In fact the Nighthawk could take off and climb very very slowly to its cruising altitude, it could then amble slowly (almost gliding) on its patrol. Its top speed of 60 mph was not that much greater than that of the later Zeppelins and its best chance of intercepting one of these was by collision if one accidentally flew into its path. By the time the Nighthawk was undergoing flight trials in mid 1917 conventional aircraft were shooting down Zeppelins using ordinary machine guns loaded with an ordinary round/tracer round combination of ammunition and there was no need for other approaches. The Nighthawk did make one useful contribution to British defence – one of the junior members of the design team was a Reginald J Mitchell and this was his first experience of aircraft design. He later went on to design the Supermarine Spitfire.
No more aircraft were specifically designed for the Davis Gun although a considerable amount of Allied effort and resource was wasted on trying to devise suitable mountings for existing aircraft to use the weapon for ground and anti shipping use. Only one Davis gun is known to have been used in combat – the crew of an RFC RE8 serving in the Middle East extemporised a simple mounting on the side of their aircraft for a Davis gun at a fixed angle of 45 degrees downward (so that the counterweight exited upwards and rearwards). With this they carried out successful ground attack missions against the Turkish army. It is interesting to consider that a similar (but reversed) mounting on the side of any of the conventional aircraft available for the defence of the UK, allowing the Davis gun to fire forward and upwards at 45 degrees, could have provide a most effective anti Zeppelin weapon.