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The Great War (1914-1918) Forum

Remembered Today:

Interrupter Gear


PhilB

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When firing a Bren gun, I was always surprised by the delay between pressing the trigger and the first bang. I imagine Lewis guns would have been similar. It`s why I find it difficult to think that an interrupter gear could so accurately time a shot as to enable it to pass through the propeller arc in the extremely short time available between the blades passing. I roughly calculate that, at 2000 rpm, there would be a blade passing every 0.015 (about 1/70)seconds. Did they have to finely tune the system by adjustments on test? Did they have special propellers available to test on? How exactly were they set up?

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It was practically impossible to sychronise the Lewis gun (as it was with most gas operated mgs). A gear was eventually created but never used in service as it was too complicated, expensive and still relatively unreliable Almost all the sychronised guns used in WW1 were on the tree that starts with the Maxim. The standard KuK weapon the Shwarzlose with a non locking breech block was also near impossible to sychronise which is why the early KuK fighters have the gun in a housing on the top wing. Forward firing Lewises were generally mounted on the top wing, in pusher aircraft or fired through a propeller bullet deflector

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For the RFC/RNAS it was the Vickers that was fitted with the interrupter gear: Lewis guns were mounted to fire outside the arc of the propeller.

There was a piece in Aeroplane Monthly about this very subject (which I will try to find.....).

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The delay is the lock time when firing from an open bolt and is not related to whether the gun is gas or recoil operated. The bolt is in the open position and when the trigger is pressed the bolt has to move forward, collect a round from magazine or belt, chamber it and fire it. If this delay is known, then it is a relatively simple matter to time the interrupter gear to take account of the delay.

With the Constantinescu (Sp?) gear used with the Vickers, the gun effectively fires a series a single shots rather than a prolonged burst. each shot being fired when the trigger is tripped by a pulse from the hydraulic actuator of the gear.

I have some contemporary data on the interruptor gear somewhere and will look it out when I get back from a work trip at the weekend.

Regards

TonyE

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Not as simple as that. I'll post more in the AM - but I'd like to hear of any gas operated mgs that were successfully synchronised.

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I will shortly be uploading a copy of the handbook for the Constantinesco Interrupter equipment. This will be available on the Vickers MG website, along with the RAF manual for the Vickers MG from 1919. Let me know if you would like a copy and I will let you know when it is up there.

Regards

Richard

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The delay is the lock time when firing from an open bolt and is not related to whether the gun is gas or recoil operated. The bolt is in the open position and when the trigger is pressed the bolt has to move forward, collect a round from magazine or belt, chamber it and fire it. If this delay is known, then it is a relatively simple matter to time the interrupter gear to take account of the delay.

With the Constantinescu (Sp?) gear used with the Vickers, the gun effectively fires a series a single shots rather than a prolonged burst. each shot being fired when the trigger is tripped by a pulse from the hydraulic actuator of the gear.

Gogu Constantinescu was a Roumanian (not Spaniard) who moved to Britain before the war and changed his name to George Constantinesco.

Both Britain and France had been independently attempting to syncronise the Lewis before the advent of the Fokker synchronised gun. These attempts failed because "the long travel of the firing mechanism - something like four inches - introduced a variable time factor" In other words the delay is unpredictable. This is a common feature on gas operated weapons.

The Constantinesco gear was by no means the first not the only Allied gear and the gear used was partly engine dependant. The man responsible for starting and encouraging the development of most British gears was Major R V Stewart Blacker (later to one of the aircrew in the first flight over Everest and the originator of the Blacker Bombard and the PIAT of WW2). He was asked to coordinate the application the lessons of the failed Lewis attempts to the Vickers. The very first fruits of this were the Vickers Challenger Gear patented in Jan 1916 (the first plane shot down with it was an Albatross CI by Albert Ball flying a Bristol Scout). The gear involved a very long flexible rod to drive the trigger motor actuated by a gear attached to the rotary engine pump spindle but this spindle also drove the tacometer and was too fragile for all the demands placed on it, frequently breaking. This gear was replaced by the Scarff Dybovsky Gear which was an improve met but very soon replaced by the Sopwith Kauper Gear with an improved method of connection to the gun trigger. This became the standard British Gear for rotary engines. At the same time that Blacker was in discussions with Vickers and Challenger he was also encouraging Constantinesco (who was working on pneumatic trench mortars) to apply his talents to a synchronsation gear based on the transmission of pressure waves through liquids. This resulted in a series of Constantinesco Gears usually driven by a take off from the front of the prop shaft on an inline engine. The first aircraft to appear in France with the Gear was the Airco Dh4 in March 1917. The Constantinesco gear became the standard for online engines, because it did not rely on mechanical connections it was relatively easy to apply it to all sorts of aircraft.

This is very much a summarised account. There were other British gears (and some French Russian, Italian and American ones)

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Gogu Constantinescu was a Roumanian (not Spaniard) who moved to Britain before the war and changed his name to George Constantinesco.

Oh dear Centurian..

I know he was Roumanian! When I wrote "Constantinescu (Sp?)" the "Sp" was querying the Spelling, not his nationality.

Regards

Tony

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Not as simple as that. I'll post more in the AM - but I'd like to hear of any gas operated mgs that were successfully synchronised.

Two points:

First, I said that the set up was "relatively" simple, not that it was simple. Semantics I know, but still important.

The total time T between the impulse from the propellor cam to the passage of the bullet through the plane of the propellor is made up of:

t1: the time for the impulse to travel through the liquid and depends of course on the length of the tube.

t2: the lock time of the gun, i.e. the time for the striker to fall.

t3: the time from the striker hitting the cartridge primer until the bullet leaves the muzzle

t4; the time for the bullet to travel from the muzzle to the plane of the propellor blades.

Without getting too complicated, t1 is typically about .00225 secs for a 9 foot long tube, whilst t2 is about .0037 sec for a Vickers gun. As Centurian has said, the Vickers was successful because it fires from a closed bolt and has a very short and constant lock time, whilst the Lewis and Hotchkiss fire from an open bolt and suffer from the same delay as the Bren.

t3 for Mark VII ammunition was chronographed at .0019 sec. and t4 depended on the aircraft, varying from 2 inches to 4 feet. The total time T was typically 0.0095 seconds. Knowing the number of revolutions of the propeller gives the distance the blade will travel in this time and hence the point at which the gun must be fired.

The gear could be set up by firing through a disk that replaced the propellor so that the precise position of the bullets could be seen.

Two factors were vitally important. The lock time of the gun, which had to be maintained well, and the consistency of the ammunition for both speed of ignition and muzzle velocity. This was why special batches of ammunition were selected for air service, first as "Green Label" and then specially manufactured as "Red Label".

Secondly, the US Marlin was gas operated and was very successful as a synchronised gun, especially on the Airco DH4 you mention. It was the first gas operated gun to be so. Some 38,000 of the Model 1917 were made between Setember 1917 and December 1918. The Marlin Model 1918 which had a reviced trigger mechanism arrived in France too late to see much action, but had the war continued into 1919 it would have become the standard synchronised gun for the US and probably for the French as well.

Regards

TonyE

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In fact the few Marlin guns fitted to aircraft were synchronised using the Constantinesco gear and were still subject to firing stray shots which risked hitting the prop. This was a more serious risk with four bladed props used on Dh 4s. A programme was set in motion to produce a reliable Marlin synchronsation which also involved producing a modified version of the Marlin - the Marlin 1918. However the war ended before the new system could be tested under operational conditions and there was no way the system would have been adopted widely until this had been achieved. As far as I know none of the Marlin 18 fitted aircraft saw action.

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Varying speed (within certain parameters, I have seen accounts that suggest that sychronisation sometimes failed if the engine was reving over a certain limit - this seems to have still been a problem in WW2 in aircraft such as the Bf Me 109 which still had nose mounted machine guns)

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A contemporary review I have of the Constantinesco CC gear suggests that in the heat of battle pilots ignored the rev range that the gear was set for and fired anyway, but that is hardly surprising. The report says that five bullet strikes on a propellor blade seemed to make no difference and that ten was not uncommon. beyond that of course the blade was liable to fail with predictable results.

Regards

TonyE

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A blade could sometimes fail with only one hit. I would be very surprised if five did not cause a failure let alone ten.

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Was the metal deflector plate on the propeller not wholly effective? I imagine a bullet hitting it would cause appreciable stress to the propeller if not the engine.

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QUOTE (Phil_B @ Nov 17 2008, 03:42 PM) <{POST_SNAPBACK}>
Was the metal deflector plate on the propeller not wholly effective? I imagine a bullet hitting it would cause appreciable stress to the propeller if not the engine.

Deflector plates were not fitted to props (although it may look that way in some photos) but were mounted on steel rods running up immediately behind the blades and attached to a revolving collar around the prop shaft. There was however often a brace to the prop ( to ensure that the plates turned with the prop) but this did not bear the weight of the deflector plate. This approach was taken because of the very issue of the shock of the bullet hitting the plate. Garros's trials had shown that only a few hits could loosen or distort the prop fitting, start the prop vibrating and cause severe problems. The steel rod approach removed the force of the impact from the prop. Deflector plates were not fitted to aircraft with sychro gear. There was a loss of prop efficiency when they were used.

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It looks like the original deflector plate may have been attached to the propeller:-

When Garros and Saulnier talked, early in 1915, they considered another approach – to attach steel deflector plates that would deflect any bullets that hit the propeller. These were triangular in shape, with the apex pointing to the pilot – the idea being that the bullets would be deflected off at an angle, but forwards of the aircraft, rather than bouncing back at the pilot.

Garros tried out this arrangement for the first time on 1st of April, 1915 with immediate success, shooting down a two seater reconnaissance plane on his first pass. This was followed up with victories on the 13th and the 18th.

The French press immediately picked up on this success, and he was the first pilot to be labelled an "ace". At the time the term was a generic one describing someone particularly skilled in a certain area. Shortly after this the term became formalized to mean anyone who has shot down five or more aircraft, a designation which has remained to this day.

The French equipped a small number of their planes with deflector gear, and sold some deflector equipped Morane-Saulnier’s to the RFC.

http://www.firstworldwar.com/airwar/deflectorgear.htm

I assumed that to mean they were attached to the propeller, but it doesn`t actually say that. Were Garros`s plates also on their own disc? Clearly the idea didn`t catch on!

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No Garros's plates were on rods as I described and for the reason I described. Garros tried putting the plates directly on the prop and did a ground run with disastrous results which resulted in the engine shaking loose. He then came up with the idea of the rods which he used when he shot down that first aircraft. Somewhere I have photos of the system in which the rods can be seen (they are only visible looking directly at the side of the aircraft.) I'll try and locate.

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A blade could sometimes fail with only one hit. I would be very surprised if five did not cause a failure let alone ten.

The report I quoted from is an unpublished typewritten manuscript by F.W.Jones, a scientific consultant to the Ministry of Munitions, written in 1921 as a record of his involvement in many of the advances in air to air fighting. It covers the CC interrupter gear, trials with electrically primed SAA, and all the advances in AP, tracer and explosive and incendiary ammunition in great detail. It is part of my research for my Ph.D thesis.

I rather presumed he knew what he was talking about.

Regards

TonyE

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I was just going on photos of smashed props and the numbers of holes (or half holes) visible

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