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Old 10-05-2015, 10:17 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, 10:24 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, 12:25 PM   #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, 04:09 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, 04:45 PM   #19
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Quote:
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, 06:01 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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Old 10-05-2015, 07:53 PM   #21
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I learned long ago when to quit trying to explain. 1989 trying to explain to a co worker that there was more to NY( where I'm from) than New York City, seriously could not convince him that NY was not not one big giant NYC. Where was the internet when I needed it.

When you think about the AS though it's easy to forget that the only things really holding the shell down are the rivets around the bottom into the chanel, and there aren't nearly as many there as other places like where exterior panel seams.
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Old 10-05-2015, 07:57 PM   #22
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So some holes drilled along the bottom with a hole saw and covered with solid outlet covers make good inspection ports and keep the panel essentially intact. Everybody happy?
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Old 10-05-2015, 08:12 PM   #23
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Originally Posted by perryg114 View Post
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

It seemed obvious to me to leave the roof vent frames in place when upgrading to a modern sized vent style. I capped the top with sheet metal to accommodate the larger sized hardware.

I might have should been an engineer, I can "see" things like that without having to think a lot.


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Old 10-05-2015, 11:36 PM   #24
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J Morgan, seeing the work you've done on your trailer I think you'd qualify! It can be a curse at times as things get over analyzed to Sunday. Or as we used to say in the navy, "nuked".

Belegedel, like your going commando notion. It would feel free of all the other clutter that ends up filling the trailer.
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Old 10-05-2015, 11:57 PM   #25
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As this idea digested, images of the insides of C130s, 141s, 17s, etc. came to mind - none of which have complete interior skin or have fallen from the sky because of a lack of that interior skin; although I will admit to more takeoffs than landings in any of them.
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Old 10-06-2015, 02:34 PM   #26
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I have been following this thread but had decided not to comment earlier. But here it goes, Belegedhel you are right on the money with your calculation for the "B" rivets. I do however have the shear number for the "A" rivets which have an alloy number of 1100-F. The shear number for a 1/8 inch "A" rivet is 130 Psu, if it's important to you all, you can do the math.

I have basically removed most of the "A" rivets on my trailer and replaced them all with 5/32 "AD" rivets (2117-T4) which have a Psu number of 596.

I put my Fantastic fans in surrounded by aluminum framework if anyone cares and I have never towed it yet with interior skins installed and have towed it a lot. The C-130 that I am the crew chief and Flight Engineer on does not have interior skins, except on the doors and ramp, and my take off and landing cycles match.
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Old 01-08-2016, 05:24 PM   #27
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To the engineers both physical and civil. Much attention in your calculations has been given to the force at which rivets pull apart. Please apply your knowledge and experience to the following:

Let's imagine a downward force on the roof with only an outer skin. When that force exceeds the structural strength of the shell, that shell will buckle and collapse in on itself. Sections twist. Panels fold. We don't have a situation with whole sections of rivets shearing off. It crumples. By having an inner skin when the load exceeds that of the outer skin a section will try to twist but is prevented from twisting by the inner skin. The same thing can apply to a torsional force applied to the shell. The panels will buckle and fold before the rivets shear.

Think of the door to the airstream. With just ribs and an outer skin the door twists and flexes. It is wobbly and unstable. Put the inner skin on it with even just three or four well placed rivets and the whole door becomes rigid.

As to all the people talking about C-130's not having inner skins... Well we aren't camping in C-130's now are we. They obviously are designed to only need one skin. Just because it works for a C-130 doesn't mean it was a design factor when AS designed our trailers.

I'm not an engineer. Luckily we have some here. Maybe they can address the concerns I expressed regarding the compressive forces and torsional forces with and without inner skins.
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Old 01-09-2016, 10:00 PM   #28
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My back-of-the-napkin math above was an effort to simplify the question (which was originally a variation on the question"do I dare to pull my trailer down the road without interior skins/missing a single section of skin") right down to the "does it matter or not" level without the use of a super-computer.

In order to provide a complete analysis of all the possibilities (What if I have 2 ft. of wet snow on my roof? What if Andre the Giant stands on my AC unit? What if an asteroid falls on on my endcap?), I would need a 3D model, a Finite Element Analysis program, and would have to know how to model things like the strength of the ribs, which have a varying and wonky cross section, and are typically not even continuous from the crown of the structure to the floor and so will contribute to the structure in variable ways. I would then need several hours/days of Engineering time, which doesn't come for free.

In the case that you get a downward force on the roof of your trailer substantial enough to overcome the rigidity of your ribs and to crumple the exterior skins, I can promise you, without any further analysis, that even if your inner skins keep that fallen tree from hitting your subfloor, you are still in a bad way.

I can't speak for the design of any but the 70's era trailers, but in these trailers the center section of the interior ceiling, is a free-floating panel that is slid into two parallel channels. If you push down on the roof enough to deform it, that center section is going to pop right out of that channel and provide zero support as the ceiling becomes convex to the inside. There aren't even any longitudinal rivets at the edge of the panel to shear.

At the end of the day, I am not trying to convince anyone to live without interior skins. Furthermore, I encourage all trailer owners, whether their interior is complete or they are going 'commando," to avoid sources of downward force capable of deforming their outer skins and ribs. Nothing good can come of that exercise. As they say, "don't bet against the house (trailer)."

Interior skins are actually pretty handy even if they aren't particularly structural:
-They keep the insulation from falling into your breakfast cereal.
-If you shine them up, everywhere you look its like a fun-house mirror (mine make me look thin and young, but sort of stooped).
-You can put screws/rivets into them to hang your self-portraits without worrying about rain water pouring in from the outside.
-They keep your electrical outlets in a consistent place so that you don't have to go looking for them every time you want to plug in.
-They provide a "barrier" between you and the bugs/rodents that have take up residence in your trailer walls.

Now I'm inspired...may I present--a limerick:

An Ode to the Interior Skin

All hail the sheet we call "inside skin!"
'tis aluminum, not steel, not tin!
Holds the insulation in its place,
Reflects, and improves my charming face,
but its structural support's a bit thin!

Hmm... That rhyme was a bit thin. But not bad for an engineer.....
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