Question on German WWII Aircraft Skin

[Robert Porter]

I have noticed on some of the German Bombers, and transports, they apparently used a corrugated metal skin, even on lifting surfaces of the wings. My question is why? On first appearance it would appear to play havoc with generating significantly more drag> Just wondered if perhaps there really was some benefit?

 

[tyrodtom]

You'll notice they didn't do it on any of their even moderately fast aircraft. Because a lot more area is exposed for higher surface drag.

But a corrugated piece of metal is a great deal stronger than just a flat piece of metal of the same gauge.

 

[Robert Porter]

You'll notice they didn't do it on any of their even moderately fast aircraft. Because a lot more area is exposed for higher surface drag.

But a corrugated piece of metal is a great deal stronger than just a flat piece of metal of the same gauge.

Makes sense for strength I guess, but the skin of an aircraft does not generally contribute to the structural strength of an airframe, its just that, the spars and frame. So I have to think the disadvantage would outweigh any minor structural gain. But I am not an engineer. Just always puzzled me. And German engineers have a deservedly great reputation as engineers or excellence so I am left having to assume it did add strength where they wanted it to.

 

[Robert Porter]

I found an interesting thread on this, it was indeed an attempt to increase stiffening and strength. And is still in use today!

Why did the Junkers Ju-52 have corrugated external surfaces?

 

[tyrodtom]

I've seen aircraft structures that use corrugated panels, but a outside smooth panel, and sometimes inner too, covers the corrugations.

 

[Crimea_River]

Keep in mind, the Ju52 design was on the drawing boards in the late 1920's, a mere 10 years after WW1, during a time of rapid innovations in aviation. The corrugated skin was adopted from the success of earlier Junkers designs. It would be unfair to apply hindsight based on knowledge that's clear to us now but that was still being learned at the time.

 

[GregP]

The skin of an aircraft creates a large percentage of the strength in monocoque construction. The skin is a stressed member of the airframe.

 

[GrauGeist]

Many of your early metal monoplane designs used a form of corrugated material to strengthen the aircraft.

Notice that it was employed in the larger airframes of the day because aircraft design was still developing.

Not only did the Ju52 (don't forget the Ju52/1M) have corrugation, but many of Junker's early monoplanes were corrugated.

So was the Stout 2-AT and 3-AT(Ford Trimotor), Fokker Trimotor, Tupolev TB-1/TB-3 and several other types (many prototypes were corrugated like the Vickers Vireo, too). The one thing they all had in common, besides the corrugation, was that they were designs of the 1920's.

 

[Koopernic]

I have noticed on some of the German Bombers, and transports, they apparently used a corrugated metal skin, even on lifting surfaces of the wings. My question is why? On first appearance it would appear to play havoc with generating significantly more drag> Just wondered if perhaps there really was some benefit?

The use of corrugated metal was fundamentally an engineering calculation problem and as the theory didn't exist they used corrugated metal to prevent skin buckling.

The Germans invented the stressed skin all metal aircraft, Junkers flew an experimental corrugated skin canard in 1912 and made a series of fighters and ground attack aircraft in WW1. Shortly after WW1, as early as 1919, flat stressed skin aircraft by companies such as Dornier (flying boats) and Rohrbach started to appear.

The reason given in "Aeronautical Research In Germany" is that when they started trying to build stressed skin aircraft (where the skin takes load not just provides covering) there was no theory or method to calculate the stresses and loads. They had two choices:
1 Use Flat Sheet and use diagonal bracing rods to stop buckling
2 Use corrugated sheet metal.
From 1929, probably earlier they had 'wagner web theory' described in patent 547624 and going back to 1921 something called Bredt Torsion Tube theory which seems to have only been used on gliders with composite wings.


When you look at the Ju 52 the corrugations clearly avoided the need for many stringers or ribs in the aircraft. This actually kept the inside of the wink quite 'clean' and fuel tanks fitted nicely.

Notice also that many modern aircraft use a corrugated skin and merely place a sheet skin over it.

When Erhardt Milch took over the running of the Luftwaffe and its procurement he desperately needed a lot of aircraft that would be quick to produce (to train crews, get numbers up) so the Ju 52 was converted into a crude bomber. (Rather than wait for Ju 86 and He 111) It was already out of date but it could be produced relatively quickly because it didn't require a lot of tooling. This ensured truly massive production of the aircraft.

This is why the Ju 52 ended up being the main transport of the Luftwaffe.

Junkers tried to replace the Ju 52 with the Ju 252 which was initially a smooth skin pressurised trimotor about the same size as the ju 52. Lufthansa then wanted it increased in size and then the Luftwaffe wanted a trappoklappe loading ramp. These delays meant few of the excellent aircraft were produced since the resource to tool up went elsewhere.

 

[stona]

The corrugations were for strength and ease of construction. Luckily we have a first hand account from the Junkers factory in 1930 from none other than Beverley Shenstone ("the man who perfected the elliptical wing"), who would later design the Spitfire wing.

"I had enough time to pay a lot of attention to the production of the Junkers Junior J-50 light aeroplane. This was an all corrugated tandem open cockpit 2-seater with an Armstrong Siddeley Genet engine of about 60 hp.
On a simple jig the frames were set up and a few longitudinal stringers were added. The corrugated skin was applied in two halves and was pre-cut to exactly the right size with corrugations beaten flat along the edges.As in all Junkers types of this date, the originally flat skins were corrugated by a special press, one corrugation after another - a slow job. They were then cut to shape using a wooden or metal pattern. After that, some or all of the edges were flattened by bashing them so that the requisite riveting could be done. I was rather concerned by this hammering out of corrugations as it must have made the dural somewhat brittle. In the case of the Junior, two sheets, one each side, covered the entire fuselage. In spite of the fact that it was not conical, but longitudinally curved, made possible by the 'give' due to the corrugations. I have never seen a simpler, metal fuselage.
The wings consisted of two spars with tubular flanges and leading and trailing members. The Junior was the first Junkers production type that did not use the former linked triangular spars, well known as in the F-13 and W-33 and others. The Ju 52, which first flew before I left Junkers, also had spars like those of the Junior."

Cheers

Steve

 

[Robert Porter]

Keep in mind, the Ju52 design was on the drawing boards in the late 1920's, a mere 10 years after WW1, during a time of rapid innovations in aviation. The corrugated skin was adopted from the success of earlier Junkers designs. It would be unfair to apply hindsight based on knowledge that's clear to us now but that was still being learned at the time.

You are exactly correct based on what I have been reading since I posted. It turned out once I figured out how to get Google to show what I meant as opposed to what I asked, there was a lot of information on the why of it. Basically it was a strengthening measure and Junkers had originally intended a smooth layer on the outside however weight became a consideration and they removed it.

As the technology improved they no longer needed the rigidity in the skin and switched to smooth aluminum for the better aerodynamics. On smaller modern aircraft they have interestingly switched back. Some Cessna aircraft use indentions in the skin on the tail sections to increase strength without having to add internal structure and weight. Whats old is once again new!

 

[Robert Porter]

The use of corrugated metal was fundamentally an engineering calculation problem and as the theory didn't exist they used corrugated metal to prevent skin buckling.

The Germans invented the stressed skin all metal aircraft, Junkers flew an experimental corrugated skin canard in 1912 and made a series of fighters and ground attack aircraft in WW1. Shortly after WW1, as early as 1919, flat stressed skin aircraft by companies such as Dornier (flying boats) and Rohrbach started to appear.

The reason given in "Aeronautical Research In Germany" is that when they started trying to build stressed skin aircraft (where the skin takes load not just provides covering) there was no theory or method to calculate the stresses and loads. They had two choices:
1 Use Flat Sheet and use diagonal bracing rods to stop buckling
2 Use corrugated sheet metal.
From 1929, probably earlier they had 'wagner web theory' described in patent 547624 and going back to 1921 something called Bredt Torsion Tube theory which seems to have only been used on gliders with composite wings.


When you look at the Ju 52 the corrugations clearly avoided the need for many stringers or ribs in the aircraft. This actually kept the inside of the wink quite 'clean' and fuel tanks fitted nicely.

Notice also that many modern aircraft use a corrugated skin and merely place a sheet skin over it.

When Erhardt Milch took over the running of the Luftwaffe and its procurement he desperately needed a lot of aircraft that would be quick to produce (to train crews, get numbers up) so the Ju 52 was converted into a crude bomber. (Rather than wait for Ju 86 and He 111) It was already out of date but it could be produced relatively quickly because it didn't require a lot of tooling. This ensured truly massive production of the aircraft.

This is why the Ju 52 ended up being the main transport of the Luftwaffe.

Junkers tried to replace the Ju 52 with the Ju 252 which was initially a smooth skin pressurised trimotor about the same size as the ju 52. Lufthansa then wanted it increased in size and then the Luftwaffe wanted a trappoklappe loading ramp. These delays meant few of the excellent aircraft were produced since the resource to tool up went elsewhere.

View attachment 356336

Very nice and informative, it jives with what I have since found out as well. When you think in the context of the time, it was during the transition from fabric to metals and of course their were issues to overcome. Seems plain to understand in hindsight it just sort of caught me off guard when I was first looking at it. One of those things I had always wondered about but never put the effort in to figure it out until returning to modelling.

Thank you very much for your detailed reply!

 

[Inspector Eric]

The Dc3 center wing section has a corrugated skin. It's even mounted in compression. Would like to have seen a factory jig used for perfoming such a feat

 

[Robert Porter]

The Dc3 center wing section has a corrugated skin. It's even mounted in compression. Would like to have seen a factory jig used for perfoming such a feat

I also read the Ford Trimotor was also corrugated and a quick pic look confirmed it.

 

[MiTasol]

Makes sense for strength I guess, but the skin of an aircraft does not generally contribute to the structural strength of an airframe, its just that, the spars and frame.

Absolutely 100% wrong.

Monocoque (also known as Stressed Skin) construction (the most basic concept behind all metal aircraft and some wooden aircraft) is all about using the skin as a primary structural member.

For a good basic intro on the subject read TM 1-410 Airplane Structures which is readily available on the net and probably even on this forum. If you cannot find it let me know and I will post it.

Most modern aircraft go one stage further and incorporate integral fuel tanks.
In those the fuel tanks are nothing more than the skin and ribs with a small amount of sealing and the minimum necessary inspection panels.

Happy reading

Mi

 

[Robert Porter]

Absolutely 100% wrong.

Monocoque (also known as Stressed Skin) construction (the most basic concept behind all metal aircraft and some wooden aircraft) is all about using the skin as a primary structural member.

For a good basic intro on the subject read TM 1-410 Airplane Structures which is readily available on the net and probably even on this forum. If you cannot find it let me know and I will post it.

View attachment 356645

Most modern aircraft go one stage further and incorporate integral fuel tanks.
In those the fuel tanks are nothing more than the skin and ribs with a small amount of sealing and the minimum necessary inspection panels.

Happy reading

Mi

Well that let my ignorance show! Oh well live and learn. Never really understood that but I do now. Thanks for the info, also did some more reading and turns out even the Wright brothers used fabric as a portion of the stiffening of their wing. So apparently the skin of an aircraft has almost always contributed, at least a little, to the strength of an airframe.

I appreciate you pointing out the serious flaw in my reasoning.

 

[Elmas]

from

The stressed-skin monoplane

 

[Elmas]

improving the 109??

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