NMF - what is the history of this?

[RLWP]

NMF - Natural Metal Finish seems to be quite popular on models, I'm wondering how common it was in reality

 

One of my earliest engineering lessons was - paint is for protection. It is there to stop metal corroding, fabric from going saggy and wood from rotting.

 

It just so happens, paint is available in lots of different colours, so you can decorate aeroplanes, put words and insignia onto them and camouflage them. This is all a bonus on top of stopping the thing rotting away. Paint is also handy for hiding stopper, sealant and filler and making everything more presentable

 

So, how many full sized aeroplanes were actually finished either in bare metal, or in a laquered only finish? It suggests having to put a lot more effort into finishing the structure in the first place, and in maintaining the surface

 

Richard

[RLWP]

First piece of evidence from here:

 

http://www.kagero.eu/index.php?option=com_content&view=article&id=230:10-p-38-lightning-at-war&catid=101&Itemid=705&limitstart=1

 

Quote

On the 2nd of January, 1944, so during the production of P-38 J, the instruction T.O.07-1-1 was enforced and ordered to leave aircraft in their natural color which was officially called the Natural Metal Finish. In the case of P-38 it meant the use of putty and painting some parts of the aircraft silver. The other surfaces were left unpainted, yet they were polished and plated very precisely and at a very high esthetic level. The whole surface of the aircraft was additionally coated with one or more layers of nitrocellulose varnish. 

 

Richard

[Lothar]

During a museum tour ( I think it was Cosford ) I asked my friend from Ipswich why so may British aircraft were painted speed silver. Short answer:

"Because of the weather, otherwise they would grow moss within three minutes after they've'been rolled out of the factory"

[RLWP]

I have found a couple of articles on T.O.07-1-1 on the internet, none refer to NMF

 

Richard

[fastterry]

The finish on metal aircraft is obviously up to the user. The RAF always painted their aircraft for corrosion protection because of the climate, except of course for the early Lightnings. The Americans in WW2 stopped painting their planes to reduce production time and cost however when they started producing natural metal jets they almost always ended up being painted in silver lacquer. ie F-100 & F101. Un-painted metal aircraft do need a lot of maintenance, I remember my first job (1967) with MMA, a local airline in W. Australia who had a large fleet of DC-3's. Due to a government subsidy these planes were maintained to within an inch of their lives and I often used to see guys with floor polishers going over the wings and tailplanes. Basically money was no object in keeping them nice and shiny. I wonder if American Airlines have a clear coat over their aircraft?

TRF

[RLWP]

I can almost imagine those DC-3 wing skins getting thinner with each polish 

 

TBH, I'd be amazed if American Airlines were not lacquered. Churning 'planes out for combat is one thing, airliners are expensive pieces of equipment that have to be carefully maintained

 

Richard

[DonH]

35 minutes ago, fastterry said:

The RAF always painted their aircraft for corrosion protection because of the climate, except of course for the early Lightnings. 

TRF

 

The Lightnings went so fast that nothing would stick to the metal surface regardless of climate.

[RLWP]

16 minutes ago, DonH said:

 

The Lightnings went so fast that nothing would stick to the metal surface regardless of climate.

 

Ho ho...

 

 

Richard

[Dana Bell]

Hi Richard,

 

It's all simply a question of which metal is covering the aircraft.  Aluminum (Alumium/Aluminium) alloy can be strong and light, but is subject to intergranular corrosion, hence the need for some sort of surface finish coat.  Pure aluminum needs no surface coat, but has no strength when rolled into sheet form.  In the late 1920s the Aluminum Corporation of America (Alcoa) created a method to coat aluminum alloy sheet with pure aluminum creating a product they called Alclad.  Free oxygen would bond with the outer surface, but wouldn't migrate into the alloy core to damage the sheet's integrity.

 

If you think of a solid block of iron in the rain, the outer surfaces will rust, eventually rotting through the entire block.  On a block of pure aluminum, the outer surface will also corrode with a fine coating of aluminum oxide (alum), but that layer will NOT allow the corrosion to migrate to the center of the block.

 

Finally, since pure aluminum adds no strength to a sheet of alloy, a sheet of Alclad is weaker than a simple alloy sheet of the same thickness.  For most of the airframe, the difference is of no import, but on the skin between the wing spars the difference can be critical.  For this reason, you'll notice that many natural metal aircraft have painted spanwise sections of the wing.

 

There's a lot more to the story, but this might be a good starting point...

 

Cheers,

 

 

Dana

[RLWP]

Alclad: https://en.wikipedia.org/wiki/Alclad

 

And: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081004.pdf

 

To add to the story, I'm familiar with the process of welding aluminium (I'm a Brit) and how the surface oxide layer has to be broken through

 

Richard

 

MORE And something from the FAA: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43-4B.pdf

 

Edited May 22, 2020 by RLWP

[Dana Bell]

41 minutes ago, RLWP said:

Alclad: https://en.wikipedia.org/wiki/Alclad

 

And: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081004.pdf

 

To add to the story, I'm familiar with the process of welding aluminium (I'm a Brit) and how the surface oxide layer has to be broken through

 

Richard

 

MORE And something from the FAA: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43-4B.pdf

 

 

Excellent!  So the answer to your original question - the "natural metal finish" was very common in the US before the war, less common during the war, and more common again postwar - though there's no way to put a number on just how many.

 

For example, pre-war B-17s, P-36s, A-17s, and C-33s were unpainted Alclad for all the metal skin panels.  (Fabric, of course, was doped and coated with aluminized dope or enamel.)  When Hap Arnold tried to convince the Materiel Division to apply permanent camouflage finishes to his combat planes, Wright Field protested, going as far as to claim that primers and paints wouldn't adhere to Alclad.  (Arnold wrote "Nonsense!" in the margin of the original memo.)

 

In 1943 Arnold began pushing to return to aluminum finishes on all aircraft.  Again, Wright Field resisted, but so did the Navy, some AAF field commands, and Allies who expected added corrosion protection and camouflage on their aircraft.  Arnold won in most cases, but made many exceptions.  By October 1943 P-38 Lightnings and B-29 Superfortresses were being delivered in aluminum finish - they were the only two AAF aircraft that no other force was using.  (The Tech Orders came after the fact in 1944.)

 

Arnold was soon notified of a new problem - many manufacturers had stopped using Alclad to save money.  They reasoned that the aircraft were protected by zinc chromate primers and lacquer finish coats, so Alclad wasn't necessary.  (I've seen no records showing that the savings were passed on to the government.)  As such, several aircraft had to be painted silver for corrosion control.

 

Aircraft with laminar-flow wings (such as the P-51, P-63, and P-80) were puttied along leading edges - ugly as sin in unpainted form.  They too needed a surface finish, though not for corrosion control.

 

In both cases, the need for aluminum or gray surface paint defeated the original goal of saving production time and costs.

 

Post war metallurgy introduced new alloys, which I suspect you're much more familiar with than I am.  The aircraft I grew up with in the 1950s did, indeed, fly with unpainted skins (again, excepting areas between the wing spars and non-skid wing walkways.  Transports began getting white tops to reduce internal temperatures.  Strategic bombers got white bellies.  F-110s (F-4Cs) were accepted in Navy colors because their composite skin panels made them too ugly.  And then the cold war began heating up and most tactical and strategic aircraft were camouflaged.

 

Anyhow, I hope this helps a bit with your question.  

 

Cheers,

 

 

 

Dana

 

 

[RLWP]

It helps a lot, thank you

 

How was the problem of corrosion between the plates, and between the platework and the underlying structure handled?

 

Richard

[Dana Bell]

Hi Richard,

 

The galvanic dissimilar metal corrosion was handled by a coat or two of zinc chromate primer.  The primer could be sprayed on the interior surfaces of the skin, rolled onto the faying surfaces of the structure, or both.  Some companies dipped the structure or skin in primer for a more complete coating.

 

Some companies got into trouble for skipping the coating.  Some surfaces were insulated with a thin strip of leather.  At one time Curtiss assembled the structure, then sprayed primer over the completed assembly, somehow missing the whole purpose.

 

Boring and North American (California) were exempted from interior primer requirements on B-17s, AT-6s, B-25s, and P-51s - but both companies were still required to prime the faying surfaces.  Somewhat famously, North American primed the entire P-51 wing spar prior to wing assembly - a pain for all modelers trying to duplicate the effect.  (Zinc chromate also reduced static electricity buildup - an advantage with the main fuel tanks just aft of the spar.)

 

Cheers,

 

 

 

Dana

[RLWP]

I wonder what the availability of Alclad was over time?

 

I can imagine it being more available in the US during WW2 than in the UK. Was it used only on US designs or on any aircraft built on that side of the Atlantic (Lancasters, for instance)

 

This is partly about curiosity, and partly about the correct use of NMF on models - not all silver aeroplanes were bare metal. The Dewoitine thread made me start thinking about it this time although various Mustang builds had me wondering too

 

Richard

 

 

[jenshb]

In the early 70s Scandinavian Airlines experimented with removing white and grey paint from the fuselages of a few DC-9s to save weight and hence fuel.  They found that the maintenance effort required outweighted the savings from the lower fuel burn, so the remaining fleet stayed with the white crown and grey or silver belly.