Spitfire machinegun "smoketrails" ?

[Olmec Head]

I have just had a flick through the Wingleader book on the Spitfire Mk 1  Pretty much most of the underwing photos show  various levels of stained / darker underwing panels - both prewar (where they are not brand new or on show) and wartime - particularly the BoB era photographs.  Some also show staining from the gunports back to the panels (AZ-H captured by the Germans and LZ-N being re-armed as examples) - where the guns have been fired as part of the sortie.  Its less easy to see upper wing staining as it does not show up as well on dark colours of course - if it is there.  I think Ray_W's model shows a good example of what a BoB Spitfire would have looked like.  It depends in my mind on the pressure on the ground crew on re-arming and servicing as to how shiny or not the aircraft would be at anyone time.  

 

The Wingleader book is very good for the Spitfire Mk1 and I would recommend those with an interest to buy it, it does show examples of Spitfires from pristine to less pristine and some are downright scruffy.  I don't want to reshow their photos as mentioned above as it would be unfair on the authors and publishers.

 

and to Work in Progress - I always seemed to get covered in cordite 'stuff' after a day on the range and that was only small arms on single shot only, my point was it does get every where and unless cleaned off seems to stick, weapons not cleaned properly were always a bugger to subsequently clean for inspection.

 

 

[Graham Boak]

The key point here is the BoB, where there was a high rate of operations.  The last time this subject appeared I had on my desk the Stratus book on Polish use of the Mk.IX, with lots of high quality photos.  Only a few showed any residual staining, and only light when they did.  At this period of the war the pace of operations was much lower, even in the most active areas.  Few of the photos showed the underside, naturally.  However the regularity and evenness of the darker shade on the panels makes me suspect a different paint job rather than just being gunsmoke staining, though I can't come up with a convincing reason why.

[Retired Bob]

1 hour ago, Graham Boak said:

However the regularity and evenness of the darker shade on the panels makes me suspect a different paint job rather than just being staining, though I can't come up with a convincing reason why.

I read somewhere that before refitting the lower panels, the grubby/oily finger/hand prints were wiped off with a rag dampened with some waste petrol.  This of course did not clean the panel too effectively but spreads the grime making the panel appear darker than the rest of the wing.

[Rolls-Royce]

22 hours ago, JackG said:

The A wing gun layout.  Inner and outer placement look to be near equal distance to gun port opening:

 

 

regards,

Jack

The overhead installation diagram in Morgan & Shacklady page 56, if it is to scale, shows the outboard gun muzzles closer to the leading edge than the inboard pair, mainly due to its curvature. The outboard guns have flash hiders proud of, and at, the leading edge of the wing. This can be seen in photos of early Mk Is and before the muzzle cover patches were adopted.

Edited December 6, 2020 by Rolls-Royce

[JackG]

The outer wing is thinner,  and the gun opening results in a more open oval shaped hole, thus exposing the gun flash suppressor much more than the inner most one.  I'm certain if viewed from underneath, these two lengths are near equal.  Unfortunately I've only a top view to illustrate this:

 

 

 

regards,

Jack

[Graham Boak]

These were removed early on.

[JackG]

Yes, gun muzzles were removed quite early, but the reason for their discussion here is the suggestion that there was a difference in the degree of staining between the two inboard and two outboard gun positions.   Both the diagram and photos show innermost and outermost are the same length away from the gun port opening.  If there were to be any discernible difference of stain density,  it would be the middle two gun positions, as they have most staggered placement.

 

Anyhow, here is a bottom view, and to me gun muzzles 1 and 4 appear to protrude the same distance - or was there a variety of suppressor lengths utilized?

 

 

 

regards,

Jack

 

 

[Work In Progress]

8 hours ago, Graham Boak said:

the regularity and evenness of the darker shade on the panels makes me suspect a different paint job rather than just being gunsmoke staining, though I can't come up with a convincing reason why.

Me too, or me neither, or whatever.

 

The only idea that springs to mind is that they may have come off a different airframe, at a time when there were a lot of variations in underside colours. But that is not a theory I'm in love with. They _ought_ to have been interchangeable, but lots of Spitfire parts were hand-fitted.

[Graham Boak]

I'm willing to be convinced by Retired Bob's explanation - notice that the prototype K5054 does not show any difference in the panels.

 

The position of the gun inside the wing may be relevant (though given, as said, the lack of differential staining from the inboard guns I'm not convinced either) but the blast muzzles are a complete irrelevance, as they were not present for any significant gun use.

[Ray_W]

I've seen Bob's explanation before. This image seems to support it showing the greasy finger marks, streaking and variability between the panels. Why the panel in the roundel is almost always cleaner though I cannot determine unless the roundel colours were an easier to clean or less likely to get dirty, gloss finish compared to the Sky underside.

 

 

And the often used image, note the staining back from the panel and again why is the panel cleaner in the roundel and yet not through to the aileron?

 

 

And here is an early version. What was the wing underside finish on K9795? 

 

 

There is something going on with these panels. They seem to like to pick up oil and soot. If someone has the armourer's operating procedure saying to wipe down the panel before refitting then the question would be answered. Then it would keep the demarcation to the abutting panel. 

 

Ray

 

 

[Graham Boak]

If you can see the serial, it is Aluminium.

[Olmec Head]

The picture of K9795 shows that even brand new and well kept pre- war Spitfires got grubby underneath when operational, even taking into consideration the graininess of the reproduced picture.  I wonder if anyone knows of a clearer copy of this photo as I have only seen it in 'average' quality.  

 

I would also go for the panels being stained by the accumulation of cordite dust and being incompletely cleaned down causing the panels to be discoloured until they got a proper going over.  I would guess that the amount of cordite produced by several hundred rounds of .303in would be greater than that produced by the 20mm cannons firing at a slower rate with less rounds.

 

 

 

[Work In Progress]

14 hours ago, Olmec Head said:

I would guess that the amount of cordite produced by several hundred rounds of .303in would be greater than that produced by the 20mm cannons firing at a slower rate with less rounds.

 

Realising I didn't have the charge weight figure for 20x110mm Hispano at my fingertips I've just looked it up.  A Hispano 20mm round is pushed downrange by 29 grams of nitrocellulose propellant. A .303 Mark VII round is powered by 2.4 grams, so the Hispano burns 12x the propellant per round.

An airborne configuration .303 has a cyclic rate of around 1150 rounds per minute, a Hispano II around 600 rpm. So the Hispano burns roughly 6.3x the amount of propellant per second of fire than the .303. 

A fully-loaded Hispano in a B wing has six seconds of fire, in a C wing with more ammunition it has 12 seconds of fire. Each .303 has about 16 seconds of fire. Not enough to catch up.

[FinnAndersen]

1 hour ago, Work In Progress said:

 

Realising I didn't have the charge weight figure for 20x110mm Hispano at my fingertips I've just looked it up.  A Hispano 20mm round is pushed downrange by 29 grams of nitrocellulose propellant. A .303 Mark VII round is powered by 2.4 grams, so the Hispano burns 12x the propellant per round.

An airborne configuration .303 has a cyclic rate of around 1150 rounds per minute, a Hispano II around 600 rpm. So the Hispano burns roughly 6.3x the amount of propellant per second of fire than the .303. 

A fully-loaded Hispano in a B wing has six seconds of fire, in a C wing with more ammunition it has 12 seconds of fire. Each .303 has about 16 seconds of fire. Not enough to catch up.

In his autobiography, Ginger Lacy mentions "DeWilde" rounds for his Spitfire (?), which was not popular with his armourer, as, AFAIR, they fouled the guns more than the standard rounds. Did they have a different propellant?

 

/Finn 

[Work In Progress]

4 minutes ago, FinnAndersen said:

Did they have a different propellant?

No. The projectile is different, being an incendiary.

Incidentally, while I have no doubt his armourer's comment was made in good faith, or possibly misremembered, it makes no sense. Incendiaries don't do anything different from ball until they hit the target. 

Even tracer doesn't ignite until it leaves the barrel.

Edited December 7, 2020 by Work In Progress

[Work In Progress]

3 hours ago, Work In Progress said:

No. The projectile is different, being an incendiary.

Incidentally, while I have no doubt his armourer's comment was made in good faith, or possibly misremembered, it makes no sense. Incendiaries don't do anything different from ball until they hit the target. 

Even tracer doesn't ignite until it leaves the barrel.

 

In fairness to Ginger Lacey's armourer, and having looked more into tracer, I'm going to walk this back a bit. There is a significant amount of testimony about tracer ammunition giving accelerated fouling.  So I now have no difficulty buying into this for tracer. But I haven't yet seen anything about incendiary rounds going off in the bore, which would seem like a whole bigger problem.

Edited December 7, 2020 by Work In Progress

[Graham Boak]

Tracer may have fouled the barrel because they burned in the base of the projectile, in order to be seen.  This wasn't true for De Wilde, AFAIK.  Whether this had a different cartridge propellant I don't know.

[RMP2]

If it is any help - there are various types of powder used for various calibres. They have different burn rates, temperatures and indeed amounts of smoke produced - as pointed out earlier smokeless is just that, not smokefree. Point being - I wouldnt be at all surprised if the .303 powder was different to the 20mm, in fact I expect it was.

Re tracer rounds - they only "properly" ignite after leaving the muzzle, or at least are only visible a way afterwards, not immediately. I would assume that is down to accessible oxygen, the initial firing of the round producing only the heat required to get them started.

 

Now I read that back its not really so much help, more an observation and maybe raises more questions. But still, having hand reloaded and fired ammunition of many different calibres with occasional tracer use, I can assure you I have some idea of what I am talking about.

[Work In Progress]

31 minutes ago, Graham Boak said:

Whether this had a different cartridge propellant I don't know.

It doesn't

[RMP2]

5 minutes ago, Work In Progress said:

It doesn't

 

Out of interest, what were they using?

[Work In Progress]

46 minutes ago, RMP2 said:

 

Out of interest, what were they using?

 

A couple of variations, but the propellant varied independently of the projectile.

.303 began its service life in 1888 as a black powder cartridge and went through eight major marks and dozens of sub-types, and was made in many factories around the world. 

 

During WW2 the dominant family was Mark VII, and there were many, many types of projectile - different combinations of projectile composition, projectile aerodynamic form and so on - but all using the same two propellants. Which were:

1. Probably most famously, cordite, properly Cordite MDT 5-2. You will be aware that cordite comes in a tubular form, shaped to control the burning surface and thereby the form of the detonation. It looks pretty much like short lengths of fine spaghetti, or perhaps vermicelli.  MDT is Modified Tubular Cordite. Cordite is a double-base propellant. Originally it was 58% nitro-glycerine, 37% gun cotton and 5% mineral jelly but to reduce bore erosion they tweaked it to 30%, 65%, 5% . The 5/2 indicates the external diameter of the filament (five hundredths of an inch) and internal diameter (two hundredths).

2.  A simpler single-base graphite glazed nitro-cellulose propellant.

 

There are headstamps indicating production of standard ball, incendiary, and tracer, all with the two different types of propellant. Rounds designed to the same pattern with the two propellants were interchangeable. 

 

20x110mm Hispano, incidentally, used a single-base nitrocellulose, not double-base cordite.

 

Edited December 7, 2020 by Work In Progress

[Olmec Head]

5 hours ago, Work In Progress said:

 

Realising I didn't have the charge weight figure for 20x110mm Hispano at my fingertips I've just looked it up.  A Hispano 20mm round is pushed downrange by 29 grams of nitrocellulose propellant. A .303 Mark VII round is powered by 2.4 grams, so the Hispano burns 12x the propellant per round.

An airborne configuration .303 has a cyclic rate of around 1150 rounds per minute, a Hispano II around 600 rpm. So the Hispano burns roughly 6.3x the amount of propellant per second of fire than the .303. 

A fully-loaded Hispano in a B wing has six seconds of fire, in a C wing with more ammunition it has 12 seconds of fire. Each .303 has about 16 seconds of fire. Not enough to catch up.

A good point, one cannot argue with science and maths.  

[RMP2]

6 minutes ago, Work In Progress said:

 

A couple of variations, but the propellant varied independently of the projectile.

.303 began its service life in 1888 as a black powder cartridge and went through eight major marks and dozens of sub-types, and was made in many factories around the world. 

 

During WW2 the dominant family was Mark VII, and there were many, many types of projectile - different combinations of projectile composition, projectile aerodynamic form and so on - but all using the same two propellants. Which were:

1. Probably most famously, cordite, properly Cordite MDT 5-2. You will be aware that cordite comes in a tubular form, shaped to control the burning surface and thereby the form of the detonation. It looks pretty much like short lengths of fine spaghetti, or perhaps vermicelli.  MDT is Modified Tubular Cordite. Cordite is a double-base propellant. Originally it was 58% nitro-glycerine, 37% gun cotton and 5% mineral jelly but to reduce bore erosion they tweaked it to 30%, 65%, 5% . The 5/2 indicates the external diameter of the filament (five hundredths of an inch) and internal diameter (two hundredths).

2.  A simpler single-base graphite glazed nitro-cellulose propellant.

 

There are headstamps indicating production of standard ball, incendiary, and tracer, all with the two different types of propellant. Rounds designed to the same pattern with the two propellants were interchangeable. 

 

 

Thanks for that. I wanst sure if generic cordite was still on the go then or if things had moved on to more specific brandings of powder. Kind of nulls my thoughts on powder types back then!
 

[Ray_W]

Back in the following thread from 2012, Edgar attached a couple of documents hosted by Photobucket and now no longer accessible. Did anyone make a copy? I would be interested to see them and what was stated in "Paragraph 9". 

 

Ray