Adding to the story... I now have a copy of the three chapters of AP1659C regarding BP Turrets that apply to the Defiant's A MkIID turret....a copy of AP 1562C the Pilot's Notes for the Defiant MkII.....and today a copy of theAP1592A the Pilot's Notes for the Defiant MkI. There are many amendments in all three booklets; ALL of them marked out in the text, and with an amendment note in the top coner of the page concerned, AND a date-stamped entry in the document control page at the start of the AP.....and NONE of these amendments done in the correct process for correcting faults in the publications or taking on board changes are anywhere near the instructions on the pilot's gun control, the gunner's master switch and how tio swap control of the guns between the two... In other words, wherever the hell Clarke got that strange idea that the pilot's part of the firing circuit was simply disconnected from the master switch, he didn't get it from ANY of the RAF primary material on the Defiant's firing circuit! The three chapters of AP1659C I've got from Hendon on the A MkIID are VERY enlightening about how the turret worked....in particular the interrupter mechanism. Until now, what I'd gelaned from all over the place was "there was a ring of electrical contacts on the turret, and plastic non-conducting parts in the ring. When the guns traversed and something on the turret went over the non-conducting bits....it broke the circuitand the guns stopped firing"..... The truth is both weirder, and more elegant at the sme time! First of all - the A MkIID is an entirely self-contained mechanism, apart from all its electrical connections to the rest of the planes, and of course where its bolted into the bloody thing! Under the gunner's "table" in front of him, just under it but above his left knee - was the electrically-driven hydraulic pump that drove the turret traverse, it was bolted to the side of the inside of the turret, and was the size of a medioum saucepan, and the same flattish cylindrical shape... Its drive shaft stuck out the bottom of the pump, and the was a toothed sprocket on it. A chain went round the inside of the left side of the turret body to the top of another cylinder down BELOW the drivers left thigh...brought round the curve of the turret by a nifty idler gear. This second cylinder was about the size of a 1970's stainless coffee percolator. A geared sprocket on top of IT meant that what was inside IT was driven round at the same speed as the turret traversed!!! Now, INSIDE this cylinder was a SECOND cylinder - and while the OUTSIDE one was connected to ONE side of the firing circuit, the inner one was wired to the other side of the circuit. The outer one was static, the inner one was the turret-speed-driven one Anyone who's familiar with an old magneto or dynamo will see right away where THIS is going! The inner cylinder had a copper cover....and in TWO places on it a plastic insert. This insert equated to the shape of the various parts of the Defiant that the guns WEREN'T supposed to hit! Between the two were TWO springloaded pickups like magneto or dynamo bruishes - as the inner copper-covered cylinder turned, and the pickups were on copper...there was a through current and the guns fired! When one pick up hit the shaped plastic "cut-out"....it's bank of guns, right or left, STOPPED firing. Sounds great, eh? :p VERY simple..... On no. 1/ the inserts could burn because of the voltge...meaning that the "edge" of the cut-out could....un...."vary"... 2/ there was a world of possible ways to badly adjust the chain drive and the geared sprockets! The chain could wear, the idler gear could be badly adjusted, the sprocketes themselves were attached to the hydraulic pump/interrupter body on verniers, which could be one ickle tooth out after being dismantled for servicing or replacement! Then there was a one VERY horrible complication!
As anyone who knows about the Defiant knows, there was a whole separate electrically-driven hydraulic circuit in the body of the aircraft. Behind the cockpit....and behind the turret....were two aerodynamic fairings, put in to improve the performance of the aircraft. As the guns traversed....these moved up and down OUT of the field of fire of the guns!:p Weird or what??? This was all well and good - there was an "interlock" system tying this hydraulic system into the one traversing the turret....but there was one teensy detail issue.... The fairing immediately behind the pilot's head....couldn't go down ENOUGH to clear the guns! So there was what Boulton Paul called a "stirrup" system... Basically there was an arm attached to each of the hydraulic rams in the gun elevating mechanism...and as the turret traversed round to where the fairing was behind the pilot's head....HUP!...and the extra "stirrup" rod pushed the guns up and out of the way THEN brought them back down to the elevation they had been at.....but on the OTHER side of the cockpit fairing! Wonderful...but how does this affect the gun interrupter??? Simple. To make sure the guns couldn't fire under 19 degrees when facing forward...and shred the prop!...there was a THIRD element to the interlock/stirrup system! A motorcycle-like Bowden Cable was attached to the stirrup rod at ONE end.....it wound down under the gunner's table....to the Interrupter mechanism! It entered the top of that....and was clipped to the top of the holder holding the two aforementioned spring-loaded pickups! To be "sort of" be foolproof - the two plastic non-conducting inserts in the inner cylinder face simply stopped the guns firing ahead AT ALL. But what the cable to the pickups from the stirup mechanism on the guns did was....simply PULL the pickup holder UP as the stirrup forced the guns UP...so that when the guns were elevated over 19 degrees by the stirrup, the guns could fire straight ahead because the cable pulled the pickups up over the top of the plastic inserts, maintaining contact with the copper inner cylinder - thus maintaining a circuit! So what stopped the guns specifically chopping up the prop was...if the guns were forward but UNDER 19 degress...the stirrup "cable" didn't pull the pickups out of the way of the insert, they went OVER it and hey presto - no connection! And no bangbang... Sounds great! And if you've ever owned a motorcyle, you'll know just HOW prone to wear and breakage and bad adjustment a Bowden control cable is!!!
Oh, and nearly forgot ANOTHER possible issue with this complicated mechanism... Periodically, if the interrupter wasn't simply removed and dismantled for a total service with all the problems of then realigning it with the hydarluic pump, ensuring all the elements of the gear drive were correctly located/adjusted.... A fitter had to pull a rubber bung out of the outside body of the interrupter...stick in a cloth on the end of a wooden stick or screwdriver....and clean the copper inner cylinder and the plastic inserts! Obviously, no matter now "heavy duty" all the components were (or not!) an intrusive procedure like this was just begging for damage to be done!:p
Actually - I should qualify "retired"... Yes, the two Fighter Command Defiant squadrons were withdrawn from dayfighter operations over the Channel and Kent - but through the end of 1940 as noted previously in this thread, a number of squadrons formed later in the year provided daylight patrolling over the Irish Sea as they began to work up as nightfighter units later, with training in GCI (ground controlled interception). Looking at 307 Sqn's online history - it seems that the Defiant was actually quite useful in THIS daylight role...for of course any lone German aircraft at THAT sort of range from the Continent could only be bombers sans escorts The kind of foe the Defiant was actually designed to engage! Arguably the effect of nos. 264 and 141 Defiant Squadrons being pulled during the BoB was negligible - given that Dowding ALSO and at the same time was keeping his seven Blenheim F squadrons firmly on the apron! By August 1st 1940 141 Sqn was at Prestwick in Scotland - in daylight able to patrol areas that might see Luftwaffe bombers but never fighters
Strangely - the Defiant was suprisingly advanced in several areas of its construction. Some of the above posts might make the reader think that the whole turet thing and all the interconnected systems was some sort of a Heath Robinson affair; on the contrary, given the limits of late 1930s technology - it's a fascinating study in the application of all that technology to one end. When you see some of the system diagrams and illustrations in AP1659C you understand how they applied 1930s technology to problems we'd be taxed to deal with now in such a limited space. Another example are the Defiant's wings; by the time it was being designed and the prototype constructed, the advantages of elliptical wings were well-recognised...but Supermarine were beginning to discover how difficult and time consuming the construction of such a wing really was for the Spitfire When Boulton Paul wanted to fit an elliptical wing to the Defiant they came up with a similarly elegant answer to these production difficulties... Each wing consisted of three seperate sections...that when bolted together resulted in a very close to fully elliptical wing! And yet each separate non-elliptical section could be constructed using the more traditional skills available at Boulton Paul!
Well I still believe it filled a niche and is well documented as "much maligned" All credit to the brave chaps that flew it at a very difficult time. [YOUTUBE][/YOUTUBE] Aeroflight ยป Boulton Paul Defiant RAF - Boulton Paul Defiant Aircraft of the Indian Air Force - Boulton-Paul Defiant TT I & TT III [www.bharat-rakshak.com] also Corgi are releasing the Defiant this August 1/72nd scale
