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At the outbreak of World War I in August 1914, only eleven years had passed since the Wright brothers had been the first to fly in a heavier-than-air machine. Consequently it is not surprising that such machines were still in the early stages of development as load carrying vehicles. The military authorities considered that, in time of war, the only suitable role for these rather flimsy aeroplanes and seaplanes would be for reconnaissance and intelligence collecting rather than as offensive or defensive weapons. Consequently the pilots or observers were armed only with revolvers, rifles or hand grenades.
However, it was soon realized, perhaps more by engineers than others, that enormous tactical advantages would accrue if aircraft could be turned into aerial gun carriers capable of firing straight ahead at a target. It was also evident that such machines should, ideally, be faster and more manoeuvrable than their opponents and have very good climbing rate to rise to the attack quickly or escape from ground defences. "Pusher'' type planes, (propeller behind the wings) with the pilot's cockpit in front of the wings offered a solution to the mounting of guns with an unrestricted arc of fire, but these aircraft lacked the manoeuvrability, speed, and climb rate obtainable with ''tractor" type planes (propeller in front of the wings) with the pilot's cockpit behind the engine. Thus the development of efficient fighter aircraft had to be based on the tractor type machine, with more and more powerful engines, but some means would have to be found of enabling a machine gun to be fired forward without hitting the propeller blades.
It is evident that this problem had been addressed by both the Germans and the British before the outbreak of the war. Some research work had been done at the Royal Aircraft Factory at Farnborough, with the idea of interrupting the fire of a forward-pointing machine gun when a propeller blade was in the line of fire. This approach had to be abandoned as being too unreliable and dangerous. It was technically difficult to interrupt the firing of the gun during the split second that the narrow propeller blade was in the line of fire with sufficient accuracy and only slight maladjustment resulted in stray shots damaging the propeller. The alternative and more reliable system would be to synchronize the firing of the gun with the revolutions of the engine in such a way that it would fire through the open space between the revolving propeller blades.
It so happened that a mechanism of this kind was first patented by the Swiss aeronautical engineer Franz Schneider on 15th July 1913 (Patent No. 276,396). Schneider, who was chief designer at Luft-Verkers-Gesellschaft of Berlin, had already demonstrated his system to the German Army in 1912, but they rejected it on the grounds that the aeroplane would never become a fighting machine and would only be used in war for reconnaissance. Strangely enough, Schneider's system was published in the German aviation publication Flugsport after the war had started, in their issue of 30th September 1914, and was available for all to see.
Presumably however the French engineer Eugene Gilbert who developed the first device used in combat was unaware of Schneider's invention, because he used an entirely different system. He fitted steel deflectors to the propeller blades which diverted the bullets from the continuously firing machine gun. Gilbert's system was available by December 1914 but official interest was slight. In February 1915 the famous French pilot Roland Garros used the idea on his Morane Saulnier Type H monoplane and is reputed to have shot down five German planes between 1-18th April 1915. The system had several weaknesses, such as severe shocks imposed on the wooden propeller blades, and the danger of ricocheting bullets damaging the airframe or even the pilot.
Nevertheless, the Germans were duly impressed, because when Garros was forced down by ground fire and captured, they examined his aircraft with interest with a view to copying it. But by this time Anthony Fokker, the Dutch aircraft designer at the disposal of the German High Command, had taken note of Sneider's patent for a synchronizing gear and the improved version of it put together by the three German engineers Heber, Leimberger and Luebbe [6]. Fokker considered this to be far superior to the Garros system and had it in operation on his E-series monoplanes by June 1915. It was a simple mechanical device with a cam operated by the engine which actuated a lever and rod system connected to the gun trigger mechanism. It soon proved to be a formidable weapon in the hands of the German pilots and gave them initial supremacy in the air. A redesigned and improved mechanical gear was introduced early in 1917, in which the gun was operated from the cam shaft through a clutch operated by the pilot, and a flexible shaft connected to the gun. The ''Fokker scourge" as it became known, galvanized the Allies into urgent action to find an antidote.
Like the Germans, the British military authorities had done little to encourage engineers and inventors with ideas for arming aircraft before the war, as they too regarded the aeroplane as useful only for reconnaissance. The first British patent for a synchronizing gear to fire a machine gun between the blades of a revolving propeller was taken out by the Edward Brothers in 1914 (No. 23,790). A working model was sent to the War Office for consideration, but apparently it went astray, and no more was heard of it.
In November 1915, Captain (later Colonel) Blacker of the Royal Flying Corps and the Department of Military Aeronautics was marked ''light duty'' and was ordered to report to General Brancker, to assist him in the urgent search for a suitable system to enable pilots to fire forward through the airscrew disc1 without hitting the blades. Thus Captain Blacker became closely involved with promoting the development of such devices. He approached the Vickers firm with the problem and shortly afterwards, the first British synchronizing gear to be patented and put into production was that invented by George Challenger, an engineer of the Vickers establishment, in December 1915. It was manufactured by Vickers at Weybridge and used with the Vickers machine gun.
Preliminary work had been done by Vickers to match the Challenger gear with the Lewis gun but it was unsuccessful because of the long travel of the firing mechanism and the weight of the bullets. The Challenger gear was a mechanical device which operated the gun trigger with a lever controlled by a cam and reduction gear on the engine crankshaft and worked in much the same way as the Fokker gear. It was reasonably successful and a Bristol Scout is credited with being the first machine to go on active service fitted with this first of British gun synchronizers early in 1916.
About the same time an officer of the Imperial Naval Air Service, Lt. Dibovsky, on a mission to England, was working on a similar idea in conjunction with Warrant Officer Scarff, who later produced the Scarff gun mounting. Their idea interested the British Royal Naval Air Service, who fitted several of the gears to their Sopwith machines.
Another mechanical synchronizing gear similar in conception to the Vickers gear, but different in construction, was produced by the Sopwith firm for use with the Vickers gun. This was based on the design of Kauper and became known as the Sopwith-Kauper gear. Again it relied on a cam driven by the engine actuating levers and rods to operate the gun trigger.
By July 1916, several mechanical gears such as these were in production and worked reasonably well but they all suffered from the disadvantages of mechanical link motions - wear and backlash, the effects of changing temperatures at different altitudes causing contraction or expansion of the rods and linkages and deformation of the airframes - with consequent danger of shooting off the propellers. The disadvantages were mitigated when the breech of the gun was close to the engine and the connecting links or rods were not too long, but short links were not possible with some aircraft, such as the B.E.12 and R.E.8.
The problem then, was to devise a system for synchronizing the firing of the gun which would not be affected by varying distances between gun breech and engine, and not be affected by changes in temperature and flexing of the airframes. In addition, it would have to be markedly superior to the Fokker gear if the war in the air was to be won.
Captain Blacker, who had been monitoring the progress of all these devices, proposed to Major Hopkinson of the Department of Military Aeronautics that a hydraulic system should be considered. About mid-February 1916 Major Colley of the Royal Field Artillery, who had been wounded in France and was also on ''light duty" as a foundry inspector at the Royal Arsenal, Woolwich, eventually ''discovered" the man who was to produce the final answer to the problem, namely George Contantinesco [7].
This arose through a curious set of circumstances, the first of which was a chance meeting in Whitehall between Major Colley and Admiral Lord Fisher. Fisher was looking for a soldier with gunnery experience to join the new Invention Board, to liaise with the Navy and keep an eye on inventions that might be suitable. Major Colley was just the man he wanted, and he had him appointed as Chief Experimental Officer and Artillery Adviser of the War Office Munitions Invention Department.
Soon after this appointment Major Colley attended a committee meeting of the Invention Board where a suggestion to heat trenches by means of ''Wave Transmission" was dismissed because the inventor, ''a mad Romanian'', had stated that water was compressible. The Wave Transmission file, placed on top of the high pile of dossiers overbalanced and slipped on to the pad in front of Major Colley. Being disinterested with the rest of the proceedings he opened the file and began to read the theory of Wave Transmission submitted by George Constantinesco. He was so interested in the inventor's claims that he took charge of the dossier and on the following day visited George at his laboratory at Alperton. He was so impressed with what he saw that at the next committee meeting he urged that the case for Wave Transmission should be reconsidered. The result was still negative and he was informed that he was not to encourage the inventor and that no funds could be made available for further investigation or research in connection with the invention.
Major Colley was so incensed at the attitude of his department that this was the cause, eventually, of his leaving, and having asked for a written order to the effect that he was to have nothing to do with the invention, one was supplied. Subsequently he received orders from the War Office to join a battery.
In the meantime Major Colley had many interviews with George Constantinesco in an endeavour to see in what manner his invention could be put to practical use in war. George suggested many, one of which was the silent and flashless guns for firing projectiles, but which had already been turned down because their operation relied on the compressibility of liquids.
It was during one of these many interviews that Major Colley suggested that George attempt to so synchronize the firing of a machine gun so that the bullets would not strike the propeller. He did not know what gave him this "brain wave'' because he had nothing to do with aircraft at the time, but as soon as he was assured by George that he could construct such a device, he called (unofficially, since he was ordered to have nothing to do with Mr Constantinesco) on the Flying Corps and ascertained that such a device would be invaluable. That department was in a state of transition as the Royal Air Force was on the point of formation, and had no funds available for research and no spare equipment. He decided, therefore, to go to the Arsenal and see his old friends where he managed on his own responsibility to obtain the loan of a Vickers machine gun and 1,000 rounds of ammunition. He took these to the Flying Corps and placed them in touch with George Constantinesco.
The officers that Major Colley placed in touch with George Constantinesco included Colonel Blacker, lieutenant Colonel Caddell and Major Hopkinson, all from the Department of Military Aeronautics. The Vickers machine gun and 1,000 rounds of ammunition were duly delivered to George at his laboratory at Alperton. He immediately started work on developing a gear to operate the gun trigger synchronized to the speed of the engine using his wave transmission system, which he had already applied to his rock drill. Within 10 days, in May 1916, he had prepared the first drawing and an experimental model of what became known as the Constantinesco Fire Control Gear or the "C.C. Gear''. George submitted his first provisional patent application for the Gear on 14th July 1916 (No. 512). The Patent for the complete specification was granted on 17th July 1919.
In the first test of the model an electric motor drove a disc, at 1,500 revolutions per minute, with the position of the propeller blades marked in chalk. When the gun was activated with a lever 120 rounds of ammunition were fired, most of which passed through the same hole about midway between the chalk blades. Occasionally a round would go slightly astray by up to an inch, which did not satisfy George. He found this was due to defective rounds. The model was then used to test the quality of ammunition. With good quality rounds, all the bullets passed through the same hole in the disc at all engine speeds.
The next problem was how to complete development of the "C.C. Gear'' to the point where it could be used in fighter aircraft, because George did not have the funds to do so and officially he was not to be encouraged. After witnessing the trials Major Hopkinson was so impressed that he defied his superiors and gave George an order, on his own responsibility, for 12 sets of the gear at £50 each. With this assistance from Major Hopkinson George was able to perfect his device, but not without further delays and frustrations in obtaining materials from official sources. In this respect Colonel Blacker did much to ensure that he obtained the materials he needed.
The first aircraft to be fitted with a fully developed C.C. Gear was a B.E.2c, in August 1916. After successful trials, officialdom was fully convinced of the perfect reliability and superiority of the gear over all the existing mechanical systems in use and its adaptability to any type of engine and airframe. It was clear that all mechanical systems had become obsolete and orders were given to manufacture the C.C. Gear in quantity. Over 6,000 gears were fitted to machines of the Royal Flying Corps and the Royal Naval Air Service (later amalgamated into the Royal Air Force) between March and December 1917 and 20,000 more were fitted between January and October 1918. A total of 50,000 gears were manufactured eventually, having become standard equipment for both the British and American Air Forces.
No. 55 squadron of DH4s, the first aircraft to go into active service fitted with the C.C. Gear, arrived in France on 6th March 1917. Two days later No. 48 squadron of Bristol Fighters and No. 56 squadron of S.E.5s arrived so equipped. From then on a whole range of aircraft were fitted up with the C.C. Gear and arrived in France at regular intervals. The C.C. Gear was designed at first to operate only one Vickers machine gun, but later George produced a modified version which could fire simultaneously two Vickers guns mounted in parallel. The first aircraft to be fitted with the twin gun system was the Sopwith Camel, fig. 7. Figure 8 shows the trigger motors in a typical twin gun installation.
The C.C. Gear was capable of firing at a rate only limited by the maximum speed of the engine and the maximum firing speed of the Vickers gun. A modification of the Vickers gun by Lt. Comdr. George Hazelton of the Royal Navy boosted the gun output up to 1,000 rounds per minute but the maximum rate of fire was officially set at 850 rounds per minute to ensure reliability and safety [8]. With two guns in parallel, the C.C. Gear could produce a safe and reliable fire power of up to 2,400 rounds per minute through the arc of one propeller without hitting the blades2.
Fig. 7 Sopwith Camel with twin-gun C.C. Gear
It appears that the Germans never succeeded in copying the C.C. Gear and according to George Constantinesco this was because they did not understand it, believing it to be a conventional hydraulic system, which is just as well! When it was disclosed to the French they also rejected it for the same reason. The British and the Americans also thought at first that the gear was just an ingenious hydraulic system which worked better than anything else available and that was good enough for them. However, when the time came for manufacturers and the Air Services to prepare installation, operation and maintenance manuals for the use of pilots and mechanics, the operation of the gear by the generation of pulses, or wave transmission, was explained. The gear was in fact an application of George Constantinesco's Theory of Sonics, which remained secret until after the war. Thus, in accordance with that theory, the gear was operated by a hydrosonic transmission.
Fig. 8 Typical twin-gun installation showing trigger motors
In principle, the energy producing device for the gear was a single phase wave transmitter'', or sonic generator of the kind described in the previous chapter, which produced percussive waves or pulses of very short duration at twice the frequency of the engine revolutions. These pulses were transmitted through a main pipeline, containing a non freezing liquid mixture, 90% of which was paraffin oil, to a "motor'' attached to the gun, which actuated the gun trigger.
A secondary small-bore pipe branched off from the main pipe to a cylinder, being a reservoir containing a small amount of the paraffin mixture under a constant pressure of about 5 atmospheres (about 73 psi), maintained by a spring loaded piston.
The trigger motor consisted of a miniature hammer device containing a "sonic transformer'', being a piston of larger diameter on the input side compared to the output side, which increased the pressure on the output side, and this actuated the gun trigger against a powerful spring. In addition, there was a reflected wave absorber in the trigger motor assembly.
Every impulse produced by the generator moved at the speed of sound applicable to the liquid3 and actuated the trigger of the machine gun with absolute precision. To move the hammer in the trigger motor, the minimum pressure required was 70 atmospheres (1,029 psi) but the main and secondary pipelines were in communication under a constant pressure throughout of only 5 atmospheres (73 psi). When the engine operated the generator at a low speed the liquid would reciprocate slightly in and out of the reservoir through the small bore pipe. As soon as the engine revolutions increased to a predetermined level, the higher frequency waves produced by the generator preferred to travel down the larger bore main line and operated the trigger motor against its powerful spring. As engine speed increased, upwards of hundreds of atmospheres were created to move the mass of the hammer in the very short time available.
The important and fundamental difference between this sonic system and a conventional hydraulic transmission system was that owing to the compressibility of the liquid in the main and feeder lines, the liquid columns in them did not move as rigid connecting links, but merely acted as the vehicles for the transmission of the successive energy waves produced by the generator, just as sound waves would travel along a tube containing air. The energy thus transmitted had to be completely absorbed by the trigger motor, otherwise reflected waves travelling to and from the generator to the motor would operate the trigger at the wrong time, and this is why a reflected wave absorber or damper was included in the motor assembly.
In the production models fitted to aircraft means were provided for the pilot to control the firing of the gun(s) as required from the cockpit. There was a thumb-press lever on the joystick attached to a Bowden cable, connected at the other end to a needle valve at the bottom of the reservoir cylinder. This cylinder was divided into two parts, an outer chamber which served as a reservoir of oil for the system at atmospheric pressure, and an inner chamber containing a spring loaded piston rod attached to a handle at the top. When this piston rod was pulled up against its spring as far as possible by the handle, oil entered the inner chamber from the outer reservoir to fill the partial vacuum and the piston rod spring kept this oil under high pressure. The oil could not escape when a ball valve at the bottom of this inner cylinder was closed. There was another ball valve in the bottom of the outer reservoir cylinder. Both these valves were in communication with the secondary pipe line.
When the pilot pressed the Bowden cable lever, the ball valve in the ''low pressure" reservoir was pushed shut by the needle valve and the one in the high pressure cylinder was pushed open by the needle valve. This sequence allowed the oil under pressure to enter the system through the secondary small bore pipe line. The increase in pressure caused the piston in the generator to move up and contact the cam ring driven by the engine. Every time a ''hump" on the cam came into contact with a roller on the top end of the piston, the piston was depressed and this caused a pressure wave to travel down the main pipeline and fire the gun.
When the Bowden control was released by the pilot, the high pressure ball valve closed and the valve to the ''low pressure'' outer reservoir opened. The next strike on the generator piston returned a small quantity of oil to the outer reservoir through the secondary small bore pipe line. The generator piston then returned to its original position out of contact with the cam ring and the gun stopped firing.
Before starting a flight, the pilot would ensure there was no air in the system by opening an air release valve while the Bowden control was pressed. The high pressure piston rod would then sink. When all the air was expelled the air release valve would be closed and the high pressure piston rod would be pulled up again as far as it would go. Sometimes this process had to be repeated until all the air was expelled. Then the gear was ready for firing.
In November 1917 George had prepared a provisional specification for an entirely new system of operating the trigger on the Vickers gun, although a Patent was not granted until July 1919 (No. 15,941). It was still based on his sonic theory, but instead of using a liquid as the medium for transmitting the energy, he used a steel wire encased in a steel tube, attached at one end to the generator and at the other end to the gun trigger motor, fig. 9. In the non-firing position, the steel wire was held in tension by means of strong springs at each end. The gun was fired by suddenly releasing the tension in the steel wire at the transmitting end. This caused a wave of energy to travel along the wire of sufficient force to operate the gun trigger.
Fig. 9 Model of sono-mechanical C.C. Gear operating gun trigger by impulses through steel wire
The generator was operated in the usual way by a cam driven by the engine, fig. 9. There was a control device in a space between the top of the generator piston and the bottom of an attachment to the spring. When this was withdrawn by a Bowden cable operated by the pilot, the generator piston imparted an impulse to the member attached to the spring, which caused the gun to fire.
Fig. 10 Section of sono-mechanical generator and trigger motor
It was claimed that the system could be designed to produce impulses in the trigger motor of a duration of one-thousandth of a second, and to produce a fire power of four thousand shots per minute.
By the time the concept of this device was sufficiently appreciated to be considered for production the war was over, and no further funds for development were sanctioned. Notwithstanding the Government's control of George's patent rights during the war, he was free to sell rights to allied Governments on patents which they allowed to be communicated. Thus the British Government had no objection to George offering the invention the U.S. Government. The U.S. Scientific Attaché to London inspected the new gear and was most impressed. He suggested to the Research Information Committee in Washington that the U.S. Government would get a bargain if they accepted George's outright price of £150,000 for the patent, of which he would agree to pay £50,000 to the Red Cross. However the U.S. Government declined the offer, being quite happy with the existing C.C. Gear.
The original hydrosonic C.C. Gear or minor modifications of it, continued to be standard equipment on British and American aircraft throughout the post-war period and was still in use at the beginning of World War II. One of the last single-engine fighters to carry a C.C. Gear installation was the Gloster Gladiator, fig. 11, which came into general service with the Royal Air Force in 1937. Three of these took part in the defence of Malta at the beginning of the war. The latest versions carried four Browning guns, two in the wings fired outside the arc of the propeller. The two in the fuselage were operated by the C.C. Gear. It was finally superseded by the Hurricane, with all the guns in the wings.
Fig. 11 Gloster Gladiator - last aircraft fitted with C.C. Gear
By the end of World War I it was recognized by all concerned that the C.C. Gear had enabled the Allies to regain supremacy in the war in the air from 1917 onwards. This was proclaimed publicly by senior Air Force officers, one of which was Air Vice Marshal Sir John Maitland, as reported in "The Times'' of 29th March 1920.
However it was not until 1922 that the Royal Commission on Awards to inventors sat to consider what recompense should be paid to George Constantinesco for his work and where experienced fighter pilots gave supporting evidence on the excellence of the C.C. Gear under operational conditions. Bearing in mind that the Government had requisitioned all George Constantinesco's inventions and employed him as a servant of the Crown under the Defence of the Realm Act in April 1917, the Royal Commission awarded George Constantinesco ex-gratia payments totalling £90,000, which he shared with his partner Walter Haddon, in lieu of royalties after April 1917. Much to his astonishment George was charged income tax on this award, and suffered considerable financial hardship after fighting and losing, his court case against the Inland Revenue for withdrawal of the charges.