Spitfire machinegun "smoketrails" ?

[P.o Prune]

I know this topic will probably fit plane with machineguns installed in leading edge of the wings. So forgive me for just using the Spitfire as an example.

 

When looking at the Spitfire (in this case) models, I have noticed that people often paint smoketrails from the muzzle going aft. Is that a real thing?

Considering that the Spitfire was flying around 300 mph, I would imagine that the muzzleflames (smoketrails) would be all over the place and not neatly going back over the wing.

 

Any thoughts?

[wellsprop]

The whole leading edge gun staining is questioned in other threads, there certainly doesn't seem to be much evidence of it existing as it is often represented (long lines over the wing) . There would be staining no the lower surface from the ejector ports however.

 

Despite this, yes, there would be neat staining, in the direction of the airflow (to a point). 

 

The airflow over the leading edge of a wing will be laminar (the flow lines are all parallel), therefore you would see straight lines from the leading edge, in the direction of the airflow.

 

The airflow will usually detach/separate from the wings surface where the pressure gradient switches from positive to negative (which is approximately the location of maximum thickness ie 30-40% of the chord). Following this, the airflow will become transitional then turbulent, which would disperse the staining. 

 

Aerospace engineers utilise this when testing aircraft. A wing section can be placed in a wind tunnel and have smoke blown over it, or a drop of oil placed on the leading edge. The smoke/oil will stick to the wing and follow the airflow. 

 

So yes, it would happen IF the guns did produce huge amounts of smoke/spot from the barrel - that topic has been well discussed on other threads. 

[Ray_W]

1 hour ago, P.o Prune said:

I have noticed that people often paint smoketrails from the muzzle going aft. Is that a real thing?

 

The short answer is yes, heading back in the airflow.

 

However, quite often modellers overdo the smoke staining, but that is their weathering choice. Just check the photo evidence for what you're modelling if you wish to portray it accurately. There are so many variables at play based on the type of aircraft and airframe age, gun useage, ammunition type, traffic areas, whether it has been wiped down purposely like for example with the addition of muzzle leading edge gun patches or accidently rubbed off on armourers' clothing. Also need to consider where most staining occurs (e.g case/link ejection ports).

 

For example I note with Battle of Britain Spitfires you will see very subtle staining on the top of the wing if at all. More visible (still subtle) on the underside lighter colour, and usually most pronounced staining from ejection ports (although my thoughts on this is that this is a mixture of gun oil and smoke). 

 

Here's a Spitfire photo. In this case, some underside smoke staining in the direction of airflow. 

 

 

No hard and fast rules. Some aircraft exhibit more pronounced staining and some will probably post photos to suit.  And yet other photos can be produced of the same aircraft with very little staining. And again, yes all moving back in the airstream.

 

Ray

Edited December 5, 2020 by Ray_W
Added Photo

[Werdna]

Potentially, the other considerations are gun calibre and propellant mix.  The bigger the round, the bigger the amount of powder detonated and some powders create more smoke than others.  Much like the exhaust staining on Luftwaffe aircraft - the residue generally became more oily and sooty later in the war, as fuel quality deteriorated.

[alt-92]

Great photo, @Ray_W. Lots of little details visible. 

@Werdna : true. AFAIK the Browning .303 Mark II would be firing 'smokeless' cordite propellant rounds, where smokeless really should be taken with a grain of salt. It is smoke-less, not smoke-free.

Besides the blast residue, the panels can be seen on the ground face down. There was a discussion recently about darker coloured panels underwing - it might very well be caused by having been laid down on a wet surface and wiped down.

Ailerons slightly deflected, elevators down, and it looks like the flaps are slightly drooped as well.

 

[Work In Progress]

It does get radically overdone by some people. This is not the pre-WW1 black powder era and anything with a nominally smokeless cartridge like these guns is relatively polite when it comes to residues, though the cyclic rates are high so even a small amount does build up a little.

Start this from about 0:50 for a fair impression

 

 

Edited December 5, 2020 by Work In Progress

[mackem01]

Perhaps something else to bear in mind - greases and lubricants from gun maintenance may heat up and find their way out when the gun is fired. Depending on how much is used may produce a dirtier stain.

[Ray_W]

2 minutes ago, alt-92 said:

Besides the blast residue, the panels can be seen on the ground face down. There was a discussion recently about darker coloured panels underwing - it might very well be caused by having been laid down on a wet surface and wiped down.

 

I tried to replicate these effects in my recent BoB GB build.  I am still uncertain what causes the underwing panel discolouration.

 

 

On the upper wing I did some smoke staining, it just looked too naked without, so in this case modeller's prerogative. 

 

 

I think the reality should be very little upper wing smoke stains at it was deposited on the armourers' backside. Why do I believe this? Another great video on YouTube. Take a look at 6:38.

 

 

Ray

[JohnT]

2 hours ago, Ray_W said:

 

The short answer is yes, heading back in the airflow.

 

Here's a Spitfire photo. In this case, some underside smoke staining in the direction of airflow. 

 

Ray, interesting photo and something I hadn’t noticed before. Is it my eyesight/ imagination but are the smoke stains more pronounced on the inboard guns and less the further outboard you go?  And is specific to this airframe or generally applies to Spitfires due to the effects of airflow etc?  I think I am right in that the gun installation is the same for all eight guns so the variable is location on the wing?

 

 

[Work In Progress]

2 minutes ago, mackem01 said:

Perhaps something else to bear in mind - greases and lubricants from gun maintenance may heat up and find their way out when the gun is fired. Depending on how much is used may produce a dirtier stain.

Generally for high altitude work MGs had to be stripped of lubricants and assembled dry in order to function at the -40c which is fairly typical at 30,000 feet

[alt-92]

...which was such a big problem early on, and why the gun heating ducts were implemented.

[Work In Progress]

2 minutes ago, JohnT said:

Ray, interesting photo and something I hadn’t noticed before. Is it my eyesight/ imagination but are the smoke stains more pronounced on the inboard guns and less the further outboard you go?  And is specific to this airframe or generally applies to Spitfires due to the effects of airflow etc?  I think I am right in that the gun installation is the same for all eight guns so the variable is location on the wing?

 

 

Don't treat this as typical or highly causal, it is just as likely to have been the result of someone giving the wing a partial wipe off before getting interrupted. This stuff does come off easily

[Ray_W]

24 minutes ago, Work In Progress said:

Don't treat this as typical or highly causal, it is just as likely to have been the result of someone giving the wing a partial wipe off before getting interrupted. This stuff does come off easily

 

My thoughts as well. A lot of variables at play. Why I like to find some period photos and make a judgement.

 

 

[Rolls-Royce]

The differences in powder staining at various locations on the Spitfire wing shown could be a result of the distances the guns are buried back from the leading edges of the wings and the length of the resulting blast tubes. The bullets (and gases) begin to decelerate as soon as they leave the muzzle. The outboard guns are closer to the wing leading edges, meaning their gases and residue are traveling somewhat faster than those of the inboard guns when they exit the blast tubes, giving a slightly better chance of dispersal in the airflow and less concentrated staining than the inboard guns. I may be way off base here, but that's my story, and I'm sticking to it! 😉

[Selwyn]

Another thing to consider is that you would not get a great build up of gun staining. The residue from gun propellant is very corrosive and it has always been mandetory to clean off these deposits as soon as possible after gun firing has taken place, This took place at least on a daily basis.   Doing this clean was, (and still is) the norm.

 

Selwyn

[noelh]

All good advice here. As ever try and use photos for reference and avoid copying other modellers work. 

 

Having said I would definitely copy Ray's Spitfire above. Very nice it is too. 

[Rolls-Royce]

37 minutes ago, Selwyn said:

Another thing to consider is that you would not get a great build up of gun staining. The residue from gun propellant is very corrosive and it has always been mandetory to clean off these deposits as soon as possible after gun firing has taken place, This took place at least on a daily basis.   Doing this clean was, (and still is) the norm.

 

Selwyn

Back then, the most corrosive element was the priming compound used in the rounds. This contained traces of mercury and formed powerful hygroscopic salts that needed to be removed very soon after firing.

[JackG]

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

 

 

regards,

Jack

[11bravo]

Nice video posted by Ray W.   In addition to showing the gun soot (or lack thereof), it does a nice job of showing oil leakage on the lower cowling and general weathering of BOB Spitfires.   One thing I found interesting was the nearly complete lack of paint chipping on the leading edges of wings, tailplanes, etc.   

 

Regards,

 

John

[Work In Progress]

14 minutes ago, 11bravo said:

One thing I found interesting was the nearly complete lack of paint chipping on the leading edges of wings, tailplanes, etc.   

 

Regards,

 

John

 

Aircraft flown exclusively off grass and only extremely rarely in precipitation. There was nothing to wear the paint off the leading edges

Edited December 5, 2020 by Work In Progress

[Ray_W]

If you want more, different Spitfire (no blown gun patches) but great example to study for "A" Wing Spitfire underwing smoke streaks plus a pile of other useful info. A very useful short film.

 

 

 And you no doubt have seen it before. This film is a delight. "Put those cigarettes out chaps!" Sorry my narration. Always smile when I see this. You will see what I mean. Also make sure you wear those mud splattered rubber boots so you do not damage the wing surface. Interesting comments regarding gun lubrication at 5:10. 

 

 

Ray 

[Ray_W]

9 minutes ago, Work In Progress said:

There was nothing to wear the paint off the leading edges

 

Just sliding boots at the wing root.

[Olmec Head]

11 hours ago, Work In Progress said:

Don't treat this as typical or highly causal, it is just as likely to have been the result of someone giving the wing a partial wipe off before getting interrupted. This stuff does come off easily

I have always found that cordite residue to be hard to remove, particularly when a few hundred rounds have gone through.  

[Work In Progress]

20 minutes ago, Olmec Head said:

I have always found that cordite residue to be hard to remove, particularly when a few hundred rounds have gone through.  

What, off cold painted metal a couple of feet away from the detonation? It's nothing like what you get in the throat or the bore where it gets mashed in with heat and 50,000 PSI of pressure. It's completely indirect.

Bear in mind also that this is not particles of residue that have impacted the surface of the aircraft with any of the energy of the muzzle efflux. It's a cloud that has emerged from the muzzle at speed, met the oncoming airflow, stopped completely, and been blown cold back onto the leading edge. It's basically just soot at that point.

Edited December 5, 2020 by Work In Progress

[Work In Progress]

4 hours ago, Ray_W said:

Interesting comments regarding gun lubrication at 5:10. 

Yes, being a Vb it's well into the era where they had got the gun heating sorted out