Warbird Skin Thickness Question...

[Crew29]

So as we're rebuilding the tails on the 61, I've been dealing with a lot of what I think is thin skin... Most of the verticals are .025”, with two pieces of .032” and two pieces of .040”. The horizontal skins are pretty much the same. Most of the airplane is flush riveted, so the skins and much of the sub-structure are dimpled. The booms are .032” and flush riveted or spot-welded.

Rich (51fixer) mentioned in his horizontal rebuild thread the skins there were .040”. How does that compare to the rest of the airplane? I've been wondering how the skin thickness varies from airplane to airplane, manufacturer to manufacturer, etc....

So I figured I'd ask WIX.... What are typical skin thicknesses for various types of A/C? Numbers on specific fighters, transports, and bombers would be great

Is the fighter genre typically heavier skinned, or does it depend on manufacturer/design team/sub-structure? I know this is probably a huge question, with facets in all sorts of directions and no simple answers. Thanks for the help...T.J.

[51fixer]

So I figured I'd ask WIX.... What are typical skin thicknesses for various types of A/C? Numbers on specific fighters, transports, and bombers would be great

Is the fighter genre typically heavier skinned, or does it depend on manufacturer/design team/sub-structure? I know this is probably a huge question, with facets in all sorts of directions and no simple answers. Thanks for the help...T.J.

It boils down to engineers and company philosophy.
NAA, Douglas, Lockheed, Ect all had different ways of doing things.
Most struggled to reduce weight, well except Republic.
You also have a minimum thickness you can countersink or cut a hole to match the head of a countersunk rivet. This varies by the size of the rivet.
It happens that .040 is the thinnest you can use to countersink a 1/8" rivet into. Any larger countersunk rivet and you must dimple as well as any thinner metal you must dimple for the same rivet.
On the P-51 horizontal the flush rivets used are 1/8" and 3/32" therefore the .040 for the skin.
Now other factors are edge distance and rivet spacing which also determine construction methods.
Rich

[bdk]

I certainly agree with Rich's analysis. Basically, you can use thick skins like the P-51 with widely spaced stiffeners, or a thin skins like on a P-38 or a P-47 with more closely spaced stiffeners. If you've ever seen the corrugated skins used on the inside of a P-38 or Lodestar wing you'll know exacly what I mean.

It is all designer preference.

[snj-5]

Tailcone skins on a T-6 are .025 or .032 (.032 is used in the very
rear area where the tailwheel is, and around where the baggage
compartment is). Fuselage Side panels are .025 up front, with a short
section towards the rear that are .032. Accessory panels are .032.
Empennage has .040 leading edges & .025 skins.

Haven't tore into the wings yet, but I'm guessing it's the same story
(.025 most places, .032 where heavier loads are concentrated, and
.040 or more on the leading edges.

Bela P. Havasreti

[DryMartini]

The B-17 has .020/.025 in the nose,
mostly .032 & .040 everywhere else.
There is some .051, like around the
waist gun windows.

[mustanglover]

Concerning countersunk skins and material thicknesses.

You really can't use a .040 skin with a standard countersunk AN426 rivet. You will almost always get a knife edge condition when countersinking the skin for the full head. You can countersink less than the the full AN426 head and then microshave the head of the rivet to some degree.

The minimum type head for .040 skin would be a NAS1097 type head rivet. And that would be the absolute minimum.

Typically .063 ( and maybe .050 if you are careful) is the thinnest skin than can be countersunk for a standard AN426 rivet.

Just a lil structural lesson.

[bdk]

Better tell North American Aviation!

[51fixer]

Concerning countersunk skins and material thicknesses.

You really can't use a .040 skin with a standard countersunk AN426 rivet. You will almost always get a knife edge condition when countersinking the skin for the full head. You can countersink less than the the full AN426 head and then microshave the head of the rivet to some degree.

The minimum type head for .040 skin would be a NAS1097 type head rivet. And that would be the absolute minimum.

Typically .063 ( and maybe .050 if you are careful) is the thinnest skin than can be countersunk for a standard AN426 rivet.

Just a lil structural lesson.

The FAA says I need to follow the Structural Repair Manual and Tech Orders to maintain the P-51.
NAA built it with a large percentage of .040. The horizontal is all .040 skin with 1 exception. The tailcone is all .040. The rear fuselage sides and top are .040. Most of the wing outboard of the fuel tanks are .040 as well.
All are mostly c-sunk with a small amount of dimpling.
The P-51H used mostly dimpling and developed the hot-coin dimpling machines to dimple 2014 and 7075 skins which would crack until this technology was developed.
Rich

[mustanglover]

Better tell North American Aviation!

They really didn't know any better and Cessna really didn't know any better up through the early 80s.

You see all kinds of thin skins with countersunk AN426 rivets in them and they are just waiting for a shear of the rivet head.

I always recommend going to a NAS1097AD rivet when using a "thinner" skin.

[mustanglover]

The FAA says I need to follow the Structural Repair Manual and Tech Orders to maintain the P-51.
NAA built it with a large percentage of .040. The horizontal is all .040 skin with 1 exception. The tailcone is all .040. The rear fuselage sides and top are .040. Most of the wing outboard of the fuel tanks are .040 as well.
All are mostly c-sunk with a small amount of dimpling.
The P-51H used mostly dimpling and developed the hot-coin dimpling machines to dimple 2014 and 7075 skins which would crack until this technology was developed.
Rich

Very true as to the FAA and SRM data.

But, I would always watch my countersinks and make sure I don't have a knife edge at the bottom of the countersink.

We never use the manufacturers called out rivets when we do any structural repair on the early Cessna jets. If we did, we never would get structural approval for the damage tolerance analysis because of cracking in the skins and cutting of the rivets.

[mustanglover]

Concerning countersunk skins and material thicknesses.

Typically .063 ( and maybe .050 if you are careful) is the thinnest skin than can be countersunk for a standard AN426 rivet.

Of course I am talking about No. 4 (1/8") rivets here, you can go to a .040 skin with a countersunk rivet if you use a No. 3 rivet (3/32"). But you still need to be careful of knife edging.

[Rossco]

Starting at the firewall..Upper bulkhead 2 skins of .022" with a asbestos fireboard of .125" in between.Bottom bulkheads are .080".
Frames 6 and 7 .035" bottom webs with c channel caps for fuel tanks and hatsesetions for side structure.Frame 8 is a double web of .035" and frame 9 is the same construction as 6 and 7.Frame 10 sits under the pilots seat and is .050" due to it having the rear wing spar attach points.Frame 11 is another double web and is .035" and the rest of the fuse rearwards are single frames of .035" as well.Until you get to the tail separation where it is .080" .Stringers (intercostals) are zed section of .035" and longerons are .063".
Skins are .035" IIRC and depending on which model either dimpled or domed.They overlap upwards due to Supermarines experience with Flying boats.Wing leading edge skins are .080" and are pressed and the rest .035" IIRC.Tail plane is .035" with internal skin stiffeners.

[hang the expense]

P-40s start from firewal back to frame 7 as .040 with 050 and 063 doublers at the wing attatch area.Frame 7 to 13 is mostly 025 top to bottom with one skin 032 right below waterline.13 to 16 040.All dimpled except for the 050 and 063 which is countersunk.78 degree rivets make for a very sharp countersink.Hard to re-counter sink to 100 degree because the hole will get larger.Wings can double as bridges just like republic.

[hang the expense]

P-40s start from firewal back to frame 7 as .040 with 050 and 063 doublers at the wing attatch area.Frame 7 to 13 is mostly 025 top to bottom with one skin 032 right below waterline.13 to 16 040.All dimpled except for the 050 and 063 which is countersunk.78 degree rivets make for a very sharp countersink.Hard to re-counter sink to 100 degree because the hole will get larger.Wings can double as bridges just like republic.

[Crew29]

Hey thanks for the response guys... so it looks like it just depends on the group of engineers and the thought processes for that project. I kinda thought that was the case....

BDK, got any pics of the corrugated skins used in P-38 & loadstar wings? I'm not quite sure what they look like...

Rossco, what is meant by domed? Got any pics?

Expense, is the entire P-40 riveted with 78-degree rivets? Are those rivets easily available? How about dimple dies and what not... I've seen countersinks, just never thought about what uses them...

Also I heard rumors 1097's are getting harder to find... Has anyone had problems ordering them?

Allright guys, thanks for the replies... T.J.