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Old 09-27-2015, 06:51 PM   #1
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Interior skin necessary?

In my 79 I've placed the bed at the rear so the head of the bed is against the rear window, thus the interior skin below the window is out of sight. Does this interior skin below the rear window serve any structural function? If not, I'd like to remove it make inspection of water leaks in this area easy. Any problem with doing this?
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Old 09-27-2015, 07:40 PM   #2
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There is no problem removing any interior panel in an Airstream to inspect for leaks, do rewiring, etc... BUT, the interior panels ARE part of the structure of the trailer so make sure to put them back before towing.
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Old 09-27-2015, 08:31 PM   #3
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Thanks. I was wondering if leaving that bottom panel off permanently would be an issue, which it sounds like it would.
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Old 10-02-2015, 11:36 AM   #4
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can the inside walls be paneling instead of the original skin? I have removed all for new insulation and wiring and the skin is in rough shape what alternatives are there instead of replacing alum skin?


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Old 10-02-2015, 12:13 PM   #5
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In my humble opinion the sheet under the front window will add very little to structure, I think you could leave it off if you desire.

Why not put it in with screws instead of rivets so that inspection can be quick and easy?




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Old 10-02-2015, 12:19 PM   #6
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Structure

All of the interior sheet metal, is part of the "monocoque" construction.

The interior and exterior metal work together, which all adds up to a "load bearing shell".

The front area gets the most beating of any part of the shell. To weaken it in any way, would in time prove to be a disaster.

Andy
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Old 10-02-2015, 02:03 PM   #7
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I'll second j Morgan that piece under the window doesn't add to much support but does add some torsional support so I'd try to keep as much as possible. Can you cut off the bottom couple inches so you can see in there easily.
You don't want to replace the alumimum with wood paneling alone. You can veneer over the aluminum.
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Old 10-02-2015, 03:18 PM   #8
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Look "monocoque" construction" on Goggle to understand what Andy is saying.

It is like 'sandwich' construction' - take away one side of the sandwich and you have no strength left.
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Old 10-02-2015, 03:47 PM   #9
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If you remove the bottom couple of inches of the panel, then you have removed the connection between the inner skin and the c-channel at the bottom, greatly reducing the torsional (twisting or racking) resistance of that section of the trailer. Think of it this way - take a cardboard box all taped closed to form a cube. That's pretty strong. Now, cut 2 inches off the bottom of one side, and see how much more it will flex missing that connection to the bottom. A better plan would be to cut one or two small inspection holes rather than remove the entire bottom part of the panel. Keep the inspection holes as small as possible - maybe big enough to stick a finger in to see if water has collected inside the wall.

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Old 10-02-2015, 04:06 PM   #10
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I put screws in my front panels below the windows and I don't see a problem with it. The inner skin might provide some support to the shell but it is really not much. Any significant loads on the inner skins would shear off rivets which sometimes happens. I would say the ceiling panels are the most important to structure. It won't hurt to put panels over the inner skins. The weaknesses in the structure are the poor connections between the shell and frame. Airplanes fly all the time with no inner skins.

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Old 10-03-2015, 09:32 PM   #11
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One small panel missing will not cause any structural issues. Your door is a missing panel. A window is a missing panel. Water heater, furnace, etc...
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Old 10-03-2015, 09:47 PM   #12
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So the Airstream expert in So Cal knows nothing according to the arm chair "experts".

WRONG ! Andy knows of which he speaks .

The inner panels are part of the structure of the shell and need to be intact.

BTW the 'holes' in the shell are filled with structural frames that both the inner and outer panels are riveted to.
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Old 10-03-2015, 11:58 PM   #13
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Quote:
Originally Posted by Glenritas View Post
So the Airstream expert in So Cal knows nothing according to the arm chair "experts".

WRONG ! Andy knows of which he speaks .

The inner panels are part of the structure of the shell and need to be intact.

BTW the 'holes' in the shell are filled with structural frames that both the inner and outer panels are riveted to.
"Monocoque (/ˈmɒnɵkɒk/ or /ˈmɒnɵkoʊk/) is a structural approach whereby loads are supported through an object's external skin, similar to an egg shell. The technique may also be called structural skin. The word monocoque is a French term for "single shell" or (of boats) "single hull"."

We're not saying that the inner shell provides no support, but some missing is not going to hurt the trailer.
Not saying Andy doesn't have a lot of knowledge either and that his advice often isn't spot on.
As for being a armchair expert, I'm a PE mechanical engineer, I haven't crunched any numbers or put anything into a 3D solid modeling program to run any model simulations, but my understanding of material strength and mechanical design easily tell me that the interior skin is fairly insignificant to the overall strength of the airstream semi-monocoque.
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Old 10-04-2015, 01:25 PM   #14
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OK here's is how the inner panels add structural strength .

Both the inner and outer panel are attached to the ribs and the C channel.

The shell is bolted to the Chassis extensions through the C channel between the ribs.

Remove the inner skin and the C channel is only held on one side and thus is free to move on one side . In other words the structure that holds the shell to the chassis is compromised .

Remember when in motion the whole structure is in an earthquake and needs all the structure in tact .
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Old 10-05-2015, 09:17 AM   #15
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Like HiJoe I think that the importance of the inner skin is overrated. It does add strength to a degree, but my common sense analysis of the construction tells me that the bulkheads add more to roof strength than the inner skins.

I also contend that the front of the trailer under the window is one of the strongest and most rigid sections of the trailer even without its inner skin.

There is a lot of bracing here, and then we have lots and lots of structure imparted by the slow 90 degree bend in both sides supporting the structure torsionally and otherwise.

Just my opinion of course.


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Old 10-05-2015, 09:24 AM   #16
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Quote:
Originally Posted by Inland RV Center, In View Post
All of the interior sheet metal, is part of the "monocoque" construction.



The interior and exterior metal work together, which all adds up to a "load bearing shell".



The front area gets the most beating of any part of the shell. To weaken it in any way, would in time prove to be a disaster.



Andy

Agreed ...
Read the door straightening threads for further evidence ...
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Old 10-05-2015, 11:25 AM   #17
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Quote:
Originally Posted by HiJoeSilver View Post

We're not saying that the inner shell provides no support, but some missing is not going to hurt the trailer.
Not saying Andy doesn't have a lot of knowledge either and that his advice often isn't spot on.
As for being a armchair expert, I'm a PE mechanical engineer, I haven't crunched any numbers or put anything into a 3D solid modeling program to run any model simulations, but my understanding of material strength and mechanical design easily tell me that the interior skin is fairly insignificant to the overall strength of the airstream semi-monocoque.
I'm a Civil Engineer and agree with Joe. A little inside sheeting missing will not make the whole trailer collapse in a wreck of aluminum.

Consider this:
If the interior sheet metal were considered VERY important, they would have used more rivets.
My Streamline trailer has large sections of inside aluminum missing and in place of those came with wooden paneling, from the factory. Those wood panels provide very little support.

Just my $0.02
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Old 10-05-2015, 03:09 PM   #18
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I have seen this topic argued on many threads, and it always boils down to an apparent matter of opinion, supported by resumes, anecdotes, and conventional wisdom on either side. So just for a thought experiment, I decided to get out my slide-rule and put some numbers to this problem (bear with me, this gets exciting at the end).

The strength of both the inner and the outer skins themselves is greater than the rivets that hold them together. That is to say, if I have two sheets of aluminum riveted together at a seam, and I pull them apart axially, the rivets will shear before the aluminum sheets will tear. The rivets themselves, being interior or exterior are the weak links in this system. Therefore, one way to approximate the relative contribution to the strength of the shell “system” is to compare the strength of the rivets on the exterior shell to the interior skins.

If I examine a linear foot of seam in my exterior shell, I observe that there is a rivet approximately every ¾”, so for 12” there would be 16 bucked rivets, the original diameter of which was 1/8”. I perused around the internet and found a table of shear strengths of aluminum solid rivets, and not knowing the actual alloy of these “soft” rivets, I chose the weakest/softest solid rivets on the table (just for a “worst case scenario), which were made of 5056-H32, and have a shear strength of 363 lbs. So that 12” section of shell seam has a total shear strength of 363*16=5808 lbs.

Now, if I look at a seam joining the interior skins together, I could generously observe that there is approximately one blind rivet (with aluminum mandrel), also with a 1/8” diameter, every 4 inches (three rivets per linear ft.). A similar table of shear strength values puts the strength of these rivets at 120 lbs each, so my 12” length of interior skin seam has a total shear strength of 360 lbs.

So, the total shear strength of the one linear foot of the “system” (inner skin + outer skin) is 5808 + 360 = 6168. Interior contribution amounts to 360/6168, or just 5.8%. My conclusion: both sides are technically correct, inner and outer skins contribute to the system, but the truth is their contribution is profoundly lop-sided toward the outer shell.

Is 6% enough to care about? Well, I did a back-of-the-napkin “wind load” calculation. I assumed that my trailer has 49 square ft of surface exposed to the on-coming wind, and that I am driving at a leisurely 75 mile per hour in a super low profile Lamborghini that has been rigged up to tow by Can-Am. I assume the car’s super sleek profile does not block the wind on the face of the trailer substantially. For simplicity, I’ll treat the trailer as a square box, with a drag coefficient of 2. Under these circumstances, the wind is putting 1411 lbs of force against the front of my trailer, trying to rip the shell off of the frame. Since the trailer is about 7ft wide, that means I have 7 * 5808 = 40,656 lbs of shear strength between my exterior shell and the U-channel in front. I believe I have two rows of about two feet long that form the front hold-down plate (but these rivets are spaced out), so let’s add another 2 * 5808 = 11,616 lbs. I end up with a total 52,272 lbs of shear strength holding the front of my exterior shell to the trailer frame (assuming the bolts that hold the U-channel down don’t just pull right through the channel). So the wind force on the trailer on an average Sunday drive is just 1411/52,272 =2.7% of the available strength of the riveted structure.

Take-away: Make sure your rivets that hold the exterior shell to the front hold-down plate are all intact, and ensure that your subfloor is not rotten, your outriggers are intact, and that the bolts going from C-channel to frame/outriggers are solid.

I can also offer my own actual experience, in which I towed my trailer with absolutely no interior skins for hundreds of miles and observed no ill effects (no guarantees, individual results may vary….). The feeling of towing interior skin free, is quite liberating, something akin to going commando.
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Old 10-05-2015, 03:45 PM   #19
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Originally Posted by Belegedhel View Post
75 mile per hour in a super low profile Lamborghini that has been rigged up to tow by Can-Am.
Oh no, now it's a towing thread!

Thank you for taking the time to put some numbers on it, Belegedhel.
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Old 10-05-2015, 05:01 PM   #20
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I agree with Belegedhel, and I am an aerospace engineer with courses in aircraft structures and a lot of practical experience. The inner skins are NOT A BIG DEAL. If you were cutting a hole in both skins without a frame to support the area around a hole, then you have a problem. This is the exact same thing that most folks do when they put in a fantastic fan. They cut out the structural frame around the vent and put in flimsy plastic. You will notice that doors and windows have a frame around them that carry the shear loads that would normally be carried by the shell. Yes it weakens the structure to some extent but at least there is a frame there to compensate. The inner skins really help keep the shape of the bows in the ceiling. If the bow bends the outside and inside skins try to move relative to each other. The rivets help prevent the bend but by how much is hard to tell without doing some math.

Perry

Quote:
Originally Posted by Belegedhel View Post
I have seen this topic argued on many threads, and it always boils down to an apparent matter of opinion, supported by resumes, anecdotes, and conventional wisdom on either side. So just for a thought experiment, I decided to get out my slide-rule and put some numbers to this problem (bear with me, this gets exciting at the end).

The strength of both the inner and the outer skins themselves is greater than the rivets that hold them together. That is to say, if I have two sheets of aluminum riveted together at a seam, and I pull them apart axially, the rivets will shear before the aluminum sheets will tear. The rivets themselves, being interior or exterior are the weak links in this system. Therefore, one way to approximate the relative contribution to the strength of the shell “system” is to compare the strength of the rivets on the exterior shell to the interior skins.

If I examine a linear foot of seam in my exterior shell, I observe that there is a rivet approximately every ¾”, so for 12” there would be 16 bucked rivets, the original diameter of which was 1/8”. I perused around the internet and found a table of shear strengths of aluminum solid rivets, and not knowing the actual alloy of these “soft” rivets, I chose the weakest/softest solid rivets on the table (just for a “worst case scenario), which were made of 5056-H32, and have a shear strength of 363 lbs. So that 12” section of shell seam has a total shear strength of 363*16=5808 lbs.

Now, if I look at a seam joining the interior skins together, I could generously observe that there is approximately one blind rivet (with aluminum mandrel), also with a 1/8” diameter, every 4 inches (three rivets per linear ft.). A similar table of shear strength values puts the strength of these rivets at 120 lbs each, so my 12” length of interior skin seam has a total shear strength of 360 lbs.

So, the total shear strength of the one linear foot of the “system” (inner skin + outer skin) is 5808 + 360 = 6168. Interior contribution amounts to 360/6168, or just 5.8%. My conclusion: both sides are technically correct, inner and outer skins contribute to the system, but the truth is their contribution is profoundly lop-sided toward the outer shell.

Is 6% enough to care about? Well, I did a back-of-the-napkin “wind load” calculation. I assumed that my trailer has 49 square ft of surface exposed to the on-coming wind, and that I am driving at a leisurely 75 mile per hour in a super low profile Lamborghini that has been rigged up to tow by Can-Am. I assume the car’s super sleek profile does not block the wind on the face of the trailer substantially. For simplicity, I’ll treat the trailer as a square box, with a drag coefficient of 2. Under these circumstances, the wind is putting 1411 lbs of force against the front of my trailer, trying to rip the shell off of the frame. Since the trailer is about 7ft wide, that means I have 7 * 5808 = 40,656 lbs of shear strength between my exterior shell and the U-channel in front. I believe I have two rows of about two feet long that form the front hold-down plate (but these rivets are spaced out), so let’s add another 2 * 5808 = 11,616 lbs. I end up with a total 52,272 lbs of shear strength holding the front of my exterior shell to the trailer frame (assuming the bolts that hold the U-channel down don’t just pull right through the channel). So the wind force on the trailer on an average Sunday drive is just 1411/52,272 =2.7% of the available strength of the riveted structure.

Take-away: Make sure your rivets that hold the exterior shell to the front hold-down plate are all intact, and ensure that your subfloor is not rotten, your outriggers are intact, and that the bolts going from C-channel to frame/outriggers are solid.

I can also offer my own actual experience, in which I towed my trailer with absolutely no interior skins for hundreds of miles and observed no ill effects (no guarantees, individual results may vary….). The feeling of towing interior skin free, is quite liberating, something akin to going commando.
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