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Author Topic: 20 mm Cannon Bulges  (Read 9229 times)
Big_guy
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« on: September 19, 2009, 01:53:26 AM »

I'm interested in the production history and retro-fitting that took place regarding 20 mm cannon bulges.  We all know that the earliest Mk.IXs had the double-width bulges to accommodate two guns,  What I'd like to know is:

  • would it be safe to say that all Mk.Vcs left the production line with double-width bulges?
  • how did Mk.VIIs come fitted off the line?
  • would it be safe to say that all Mk.VIIIs left the production line with single-width bulges?
  • when did the single bulge came into production at the factory?
  • when did the single bulge start getting retro-fitted on aircraft in the field?
  • when did did double-width bulges disappear from the operational scene?
  • is there more than one style of double-width bulge reliably documented by photos?  A lot of drawings don't agree on the pattern, and certainly neither do the model kit designers!

I know how dangerous the phrase "would it be safe to say" is when talking about the Spitfire, but this topic is one of those puzzlers that everybody knows a bit about, but few of us have the whole picture. Can anybody weigh in on this one?

Cheers,

Steve Sauvé
Ottawa, Canada
« Last Edit: September 19, 2009, 01:55:44 AM by Big_guy » Logged

Cheers,

Steve Sauvé
Ottawa, Canada
sb.sauve@gmail.com

Main Spitfire Interest - RCAF Units in WW II
Edgar Brooks
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« Reply #1 on: September 19, 2009, 08:50:00 AM »

The VIII had "single blister doors" from 15-12-43; the V & IX from 7-1-44.  Supermarine had proposed the change more than a year before, but there's no hint as to the reason for the long delay before approval.  The VII gets no mention, which implies that it had the narrow blister from the start, but I wouldn't like to place a bet on it.  Any difference in the perceived shape could be caused by different production methods/machinery, if, for instance, one company stamped the bulges, while another used wheels (in the Morgan/Shacklady book, there's a report, by Alex Henshaw, on adverse handling on one IX airframe, due to the extra-large cowling bulge); I see no reason why the same couldn't be true of the cannon bulges.
Edgar
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Antoni
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« Reply #2 on: September 19, 2009, 05:39:16 PM »

From Five to Nine by Wojtek Matusiak Air Enthusiat No 95.

A new canon bay cover was duly developed, with a single narrow blister over the inboard ammunition feed drum, the only one left. The Modification 782 to introduce single blister gun door seems to have been issued in November 1942, but the new covers were probably not fitted to production aircraft until those delivered in late]anuary/early February 1943. Photographs from early April 1943 prove that EN459 featured broad blisters in operational service, while EN464 of the same batch, photographed factory fresh at the beginning of February 1943, was already fitted with the narrow ones.
The Castle Bromwich Aircraft Factory started turning out Mk.IXs at just about that time. Photographs show that the earliest F.IXs originating from that plant (built as Mk.VCs, and converted at RollsRoyce Hucknall) featured the large, double-blister covers. The narrow-blister covers had been introduced in that factory by May 1943, probably before the MA serial range. Mk.VC Tropicals (Trops) made in the same batch during May, also featured the new singleblister covers - so depending on the time of production, this must have been another modification rather than another version.
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Edgar Brooks
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« Reply #3 on: September 19, 2009, 10:38:19 PM »

Mod 782 was not issued to the shop floor in December, 1942; that was the date that Air Ministry approval was sought, and the mod was not cleared for production until 7-1-44.  782 was conditional on mod 683 "To standardise the armament on the Mk. VC & IX 2 cannon & 4 Browning," being cleared for production, and that didn't happen until 23-12-43, two weeks before 782's clearance.  These dates are open for inspection in the Vickers Spitfire mod ledger, held in the RAF Museum's library, under reference B3606.
Edgar
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Antoni
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« Reply #4 on: September 20, 2009, 12:53:49 PM »

MA304 RF*H 303 Squadron June 1943. CBAF-built narrow cannon blisters.
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Antoni
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« Reply #5 on: September 20, 2009, 12:58:58 PM »

152 Squadron Italy 1943. MA454 UM*V narrow cannon blisters.
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gingerbob
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« Reply #6 on: September 27, 2009, 01:16:16 PM »

  • how did Mk.VIIs come fitted off the line?
  • would it be safe to say that all Mk.VIIIs left the production line with single-width bulges?

AFAIK, VIIs had the single-cannon door from the beginning (of production)- they were not expected to carry 4 20s in order to keep the weight down.

Early VIIIs definitely (Oh, I'm in trouble now!) were delivered with the big bulge (and pointed wingtips- ick!)

I think there was discussion starting around June/July '42 about eliminating the big bulge.  Initially it was retained on the VIII because they still thought it might want to carry 4 20 (the Admiralty also hoped for 4 20 on the Seafire!).  It was agreed that the IX was probably going to be limited to 2 20 due to weight/stressing.

In early '43, if not sooner, there was a program of cleaning up the V (this is about when the LF.V started happening) and one of the points was replacing the big bulge with the single cannon type.  It is implied that spare doors were made available for this purpose.  Another suggestion was deleting the unused cannon stub, which involved a fair amount of hacking and filing, I believe.  I think this was thought to be worth about 2 mph- which may explain why not many erks went to the trouble!

bob

p.s. This is all off the top of my head, so probably enough imprecision to generate a flurry of rebuttals!
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Antoni
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« Reply #7 on: September 27, 2009, 03:04:23 PM »


AFAIK, VIIs had the single-cannon door from the beginning (of production)- they were not expected to carry 4 20s in order to keep the weight down.

It was agreed that the IX was probably going to be limited to 2 20 due to weight/stressing.


With the Mk IX the four-cannon option was abandoned for reasons of flight safety. The Certificate of Design for the type 361 stated; "to offset the increased weight of the powerplant the armament is limited to (a) 2 cannon and 4 Browning guns (b) 8 Browning guns." This was followed by Modification 683, dated 11th August 1942 (also aplicable to other marks, including the Mk V), to standardise armament as 4 Browning .303 in. and 2 Hispano 20 mm. guns."
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Edgar Brooks
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« Reply #8 on: September 27, 2009, 10:17:05 PM »

Mod 683 was not dated 11-8-42; that was the date of minute 817 of the L.T.C., when permission was sought from the Air Ministry; the mod (which specifically mentions only the VC & IX, and no other Marks) was not "Cleared" until 23-12-43 (and, yes, I do have a copy of the relevant page from the Vickers ledger.)  The Seafire armament was covered by mods SEA 77 & 134; as yet I don't have copies of the Seafire pages, but 134 is dated 28-12-43.
Edgar
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Editor
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« Reply #9 on: September 28, 2009, 12:35:54 PM »

AFAIK, VIIs had the single-cannon door from the beginning (of production)- they were not expected to carry 4 20s in order to keep the weight down.

That is true. EN474, which was so "early" Mk. VII as to feature a Mk. VI cockpit canopy, did have them. It was of March 1943 production.
http://www.spitfiresite.com/photos/historic/2008/04/spitfire-mk-vii-in-united-states.html

Could it be so that even though the large blisters may have remained a MK. IX production standard for some time afterwards, the sinlge-blister covers were made available as conversion parts to squadrons which wanted to standardise on 2+4 armament? Just a thought.
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Edgar Brooks
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« Reply #10 on: September 28, 2009, 02:14:31 PM »

The narrow-bulge mods were made retrospective, so, if the airframe survived, it was entirely possible to see a Spitfire, built before the mods came into force, carrying the later covers.
Edgar
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« Reply #11 on: September 28, 2009, 09:44:18 PM »

The narrow-bulge mods were made retrospective, so, if the airframe survived, it was entirely possible to see a Spitfire, built before the mods came into force, carrying the later covers.
Edgar

Understood. But, for the note, EN474 was shipped directly off the prod line to the United Sates and is shown on all US photos with narrow blisters. That, I think, would indicate that it had them already from the beginning.
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Edgar Brooks
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« Reply #12 on: September 29, 2009, 01:52:46 PM »

Neither 769, or 782, mention the VII, at all, which leads me to agree that it was never envisaged with wide bulges.  The wording of 769, for the VIII, is somewhat intriguing "To introduce a cannon gun door having two small individual blisters in place of the existing single large blister."  From that, one could deduce that there were still plans to fit 4 cannon to the VIII, since the V/IX mod, 782, calls it a "single blister door."  Many years ago, when I looked at a XVI door, I noted a join line down the centre, so replacing the second blister, with a plain plate, would have been simple.
 It's possible, too, that another mod might have lead to the deletion of that second bulge, for example 839 "To introduce new design of blanking off cap for outboard cannon position," which was for the VII (and the VIII) and dated 30-9-43.  While permanently blanking off the mounting, the planned bulge could have gone with it.
Edgar
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Edgar Brooks
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« Reply #13 on: October 04, 2009, 09:39:13 PM »

I've just found, in a file in Kew, confirmation that, as late as April, 1944, consideration was still being given to fitting 4 x 20mm cannon (so two bulges on each cover do make sense,) or possibly "E" armament, to the VIII, but to wait for the latter until it was seen how it worked in the IX & XIV.  No sign of a follow-up, but these papers pop up in all sorts of files, and there are hundreds of them; maybe it was just left to founder as the war's end approached.
Edgar
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