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Old 09-08-2006, 12:25 AM   #1
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Smile Help me design an aluminum frame please.

Hello, I tried to post before but I don't think I did it right so this is my first real post. I rescued a 21' early 60's? Airstream (21 feet inside dimension) headed to the scrapyard, free to me.

Cabinets, appliances, floor, all junk. Had it completely gutted, leaving only the frame, wheel wells, the front floor section, torsion axle and wheels.
It was too far gone to restore. It will surely be painted, not polished, too much corrosion/scratches, but no major dents.

I did salvage the fiberglass end caps which are in good shape.

Anyway, the frame from just aft of the wheel wells is pretty much gone. The rest of the frame has surface rust but is probably salvageable.

I have a source for delivered aluminum at commercial "by the pound" rates.

I would like to simply make a nice aluminum frame from scratch.

I can work in a friend's commercial shop on the weekends with facilities to keep it straight. I would include the frame "wings" to support the shell supports.

My goal would be to save some weight, perhaps some redesign (longer tongue, rear bumper longer for storage), size for tanks, and avoid rust, etc.

As mentioned in the other posts, there is no need to be stronger than the factory.

Can someone with a little experience or background give me some ideas on I beam size, grade of aluminum, distance apart, etc. or tell me where to look?

I would like to have the middle I beam start at the hitch and as one piece go all the way back to the bumper, with the rest of the front angling to meet the center, sort of like a wooded boat in 2 dimension. I would consider more smaller I beams, since I am also considering a welded aluminum floor, and not having a belly pan at least for awhile.

I know this will be a project but it will give me a chance to learn aluminum welding. I especially like the idea of aluminum welded floor (to have some covering to soften it up...cork?).

Or is there a fundamental problem with using aluminum?

Next question-2 or 4 wheels?

Most of my friends this is a crazy idea, just not as crazy as building my own airplane.

Thank you. Skip Simpson Daytona Beach Florida
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Old 09-08-2006, 12:38 AM   #2
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WOW! Wouldn't finding a fixer-upper, not a re-builder-upper be better? But I admire your vision. Isn't this how Wally began? Go for it and keep us interested with pictures.....lot's of them.
Neil.
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Old 09-08-2006, 07:16 AM   #3
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Aluminum has 1/3 the flex modulus of steel so you will need to increase the size of the beams considerable amount to retain the same stiffness. You would do this by increasing the moment of inertia of the beams by three. If you do not know what I am saying, it is time for you to hire a mechanical engineer with some trailer or aircraft experience. An aluminum frame is not something that anybody has commercialized, even Wally.
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Old 09-08-2006, 07:50 AM   #4
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Quote:
Originally Posted by SkipS
...I would like to simply make a nice aluminum frame from scratch....
Sounds like a nice project. I would keep the belly pan in mind during the planning phase. It's important for many reasons, not the least of which is aerodynamics! But it will protect your black and (new)grey tanks as well.

One thought on the floor: The aluminum needed to support walking weight would be very thick. You might consider an aluminum honeycomb floor. Some Argosys used it way back when (with questionable success) and it's the standard in airliner floors and ambulance cabinets among other things.

I found a fair amount of info after doing a Google search for "aluminum trailer frame". Of course manufacturers support both sides.

As your engineering plan advances I'd be interested to hear what you calculate the weight savings will be, and also what the extra cost will be. I'm not sure it's worth it. With the advances in rust proofing paints the longevity argument doesn't really fly.

Best of luck,

Steve
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Old 09-08-2006, 08:03 PM   #5
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hello SkipS,

check with Jim golden here on the forums ,check the threads for his avatar.
he has been planning an aluminum frame ,although i think he is going steel now do to costs ,but he might be the guy you need to talk to as he sounds like hes got alot thought out on this idea .

Scott
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Old 09-09-2006, 09:44 AM   #6
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An aluminum frame with the strength and rigidity of a steel frame will weigh about the same as a steel frame.

To obtain the same level of rigidity, the tubes will have to be substantially larger (how much? It would take an experienced engineer to know) and the resulting tubes will be far less resistent to crushing.

The cost will be several times higher.

The resulting frame will have a shorter fatigue life by an order of magnitude. Any cracks which develop will tend to continue to catestrophic failure in a very short time.

Mark
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Old 09-09-2006, 10:44 AM   #7
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Skip--What is the worse thing that would happen.--you would spend a lot of money and energy on something that didn,t work and at the same time learned a lot and had a lot of fun.
Or---if it does, you would have spent alot of money and energy on something that gave you a lot of satisfaction, learned a lot and had a lot of fun. pieman
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Old 09-09-2006, 05:12 PM   #8
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look at the post at weight reduction remodeling. I remember hearing that welding is bad.

s.com/forums/f46/weight-reduction-remodeling-methods-22907.html
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Old 09-09-2006, 08:49 PM   #9
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Hi Skip,

I think your idea has a lot of merit. I would do an aluminum frame if I could get the materials cheap.

Yes, aluminum has a lower modulus of elasticity than steel, which means it will spring more. But that's easily solved.

If you use 2000 series aluminum, it will be stronger than the steel that the original frame was made of. 6000 series aluminium will be about the same. I'd stay away from 3000 series. Also, I'd avoid 7000 series due to its fatigue properties. Probably your best bet would be 6000 series. It's about the same strength as normal chassis steel and it's got very good corrosion resistance.

As for a size, If you used a 6" deep standard channel section, you'd probably be fine. You could also use an I-beam if you want. Email me the sizes of the materials you have access to and I'd be glad to check the numbers for you. I'll need the overall height of the beam, the overall width, the web thickness (that's the vertical portion of the beam/channel),and the flange thickness (that's the horizontal part of the beam/channel). Most of the standard shapes have flanges that taper, being thicker at root and thinner at the tip. Give me both measurements.

Something to remember is that aluminum gets its strength from heat treating. So if you weld it, it ruins the heat treat and makes it revert back to its lowest strength, which is about 1/3 that of normal chassis steel. Not good. So, you should probably look at riveting the structure together, not welding it.

Anyway, I'd be happy to help you however I can.

And, I don't think you're nuts at all. I've designed an airplane I'm going to build too

I used to live in Daytona. Nice town. I attended Embry Riddle Aeronatical U. in the late 80's. I like that town!

Take care,
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Old 09-09-2006, 10:06 PM   #10
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Alloy Steel when properly heat treated have a specific flex fatique strength. If you design to keep below this limit, the product will last forever. All aluminums (even the properly heat treated ones) only have a endurance limit. That means that, no matter what stress you design a structure for, it will eventually fail. That is why they put new wings and replace high stress members on planes after so many hours or take offs and landings. The design process of picking just the right alloy and heat treatment as well as the assembly methods is not child's play. The very highest strength aluminums are also very notch sensitive and must be designed carefully. That is why there are more civil (structual) engineers hired by plane companies than they have aeronatical engineers on staff. The Award company (now defunct) had some great engineering designs for frames. If you can find early one around some where you might get some good ideas. I still do not think this a project for beginers, unless they have unlimited consulting budgets and sources of materials. I am a graduate mechanical engineer with a materials engineering experience and worked in areospace and high performance sports equipment.
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Old 09-10-2006, 01:49 PM   #11
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Well since we're talking about letters after out names

I'm a licensed Professional Engineer who does stress analysis for a living. My degree was in Aerospace Engineering, and I used to work for the Boeing Company where I did structural analysis and loads and dynamics. I'm also a pilot and an avid EAA member. I'm first and foremost an airplane guy. I am very familiar with aluminums.

There are more civil guys hired by aircraft companies simply because there are 75 times as many guys out there with civil degrees. I do civil work now and sit in an office full of civil guys. None of them know much about any materials other than iron or concrete. They've no idea about aluminums or other high stress steels. Nor do they know anything about dynamics. This is not a slam on them; they were never trained in it. Nor did they ever need to be for what they do. It's a different industry.

I did work with fatigue at Boeing. They have an inhouse publication that's about 800 pages long devoted just to the topic. The bottom line is that it's not that hard, especially with a more basic, ground-bound application like this one. It's all about the stress reversals, combined with the severity of the stress. Keep the stress levels low, and parts will last pretty much forever. I.E. make the endurance level so high that you'll never reach it in this lifetime or the next and you're fine. Certainly one can bury himself in the theory and testing, but what it boils down to for practical use is that you keep the stress low enough and the number of permissible reversals becomes bascially infinite. When you can do that, your airframe lasts forever. When you can't do that, you have to design for change outs. Landing gear is very highly stressed. It's designed to be replaced. The fuselage keel beams are designed to stay forever.

Yes, for certain, aluminum is crack prone. But at 1/3 the weight of steel, we can have double the section and still only 2/3 the weight. If we double the height of the section, we'll get about 7 times the bending stiffness, so even at 1/3 the elastic modulus, we'll have more than twice the flexural stiffness of the original frame and be 1/3 lighter. There are a lot of 707's still flying with the original airframe.

If Skip used a standard size 6" channel section in 6061, he'll have about the same strength level as the mild steel original, but he'll have about four times the section as an original 4" frame. So he's got 4/3 the flexing stiffness and four times the bending strength, and reduced weight. What's wrong with that? It's a simple ladder frame with some outriggers. It doesn't get much easier.

They build aluminum framed 53' box trailers every day. I see them all the time. And the Airstream's shell is aluminum.

I do agree with Dwight in that a deal of thought needs to be put into this. But, it's not an impossible task. Actually, I think it's very doable.

I didn't elaborate much in my earlier posting so as to spare the readers the details. But if we'd like to get into detail, that's cool. I wouldn't recommend a 4" frame of the same section. I'd use a deeper section with a greater moment of inertia. That knocks the bending stresses down, and the stiffness goes up exponentially. There was a lot of detail hidden in my recommending him use a 6" deep 6000 series channel section.

On my own 31 footer, I plan to do an 8" deep frame. I plan to use steel, as I don't have access to cheap aluminum. If I could get aluminum at the same price as steel, I'd look hard at it.

I thought Award had some neat ideas too. Problem was I could never actually see one. They sold them factory direct, at least when I wanted to look at them. And not being in Canada, it was tough to look at one. I did contact them once, and they were delivering one to somebody down south off I-95. They said they'd call me when they came by my area and I could take a look at it at a rest stop or something. I told them "Cool, just call me the day before you're going to be there." They never called.

I think nobody's done an aluminum frame commercially because of the expense. Award went under just trying to punch holes (we call them "embossments" in industry) in their frame.

Anyway, my vote is Do It! If it'll last 45 years on a 707, it'll last forever on your camper.

Beefy section + low stress levels = longevity
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Old 09-10-2006, 09:17 PM   #12
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Numbers

OK, my inner dweeb is showing through here, but this has been bugging me, so I ran some numbers. Here we go.

Dwight is 100% correct on the issue of having to pump up the moment of inertia to balance out the lower elastic modulus. It is indeed a direct relationship. While not necessary to match for strength, it's necessary to match for deflections. I'm not sure how much the shell can deflect, but let's assume that we're going for about the same as factory.

E = Elastic Modulus (=29E6 for steel, = 10E6 for aluminum)
I = Moment of Inertia (=5.3in^4 for a 4" by 2" by .25" wall square tube)

I assumed a load of 400lbs at 10 feet out from the back axles. This is a simplification for mathematical purposes, but probably isn't too far off. This gives us a moment of 4000 ft-lbs.

OK, so we do the deflection equation:

Y=-PL^3/3EI

The term EI is the elastic modulus times the moment of inertia, and its commonly referred to as "flexural stiffness". It's not directly related to overal strength, but how much the beam will spring back and forth under load.

OK, so we do the math and we find that the "stock" frame I've listed above will deflect 1.50" under the imposed load (actually 1.499 but we'll round it.)

OK, let's look at the 6" channel I recommended. Do the math and get I=14.18 in^4. Not quite triple the box beam, but close to it. Do the deflection calc and for the same load, we get a deflection of 1.61". So we've deflected the frame about 1/8" more with this frame. I think that should be fine.

How about strength? Stress = M*Y/I = 4000(12)*2/5.3 = 18,113psi for steel, = 10,155psi for the aluminum frame. So the aluminum frame, if made of a comparable stress grade (about 36ksi) to the steel, is actually stronger. It will just deflect a hair more because aluminum is more springy.

How about the weight? Stocker weighs about 560lbs for 60' of it, new one weighs about 221lbs for the same length. Weight savings = 339lbs, or about 60%, for roughly equal stiffness.

You could go to a 7" deep section and be both stiffer and way stronger, as well as probably 50% lighter.

Anyway, sorry to bore. I'm obsessive/compulsive like that

And that, ladies and gents, concludes tonight's show.
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Old 09-10-2006, 10:11 PM   #13
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That about says it all

Quote:
Originally Posted by JimGolden

On my own 31 footer, I plan to do an 8" deep frame. I plan to use steel, as I don't have access to cheap aluminum. If I could get aluminum at the same price as steel, I'd look hard at it.

Out 25' is in the same boat. There are some people around that can weld aluminum but the welding requires higher skill and a purging gas. I know there are stick rods that do aluminum but realistically tungsten inert gas (TIG) is required.

Not only is aluminum more expensive but requires higher skilled welders and more sophisticated equipment. Mistakes become more expensive.

We would like aluminum but will settle for steel when the time comes.

R
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Old 09-11-2006, 11:17 PM   #14
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aluminum frame

Thanks Jim and all for the analysis and encouragement. I will check my source's price for 6 and 7 inch c channel 600 grade aluminum. 300 pound savings sounds significent; I am hoping to do a lightweight, ultra-modern final product, cozy for 2.

Never had a camper trailer before. Bear with me if my ideas aren't in the norm.

Took a look at an aluminum boat trailer today, it was bolted together. I assume for an Airstream simple high grade aluminum angle brackets would be used. Bolts of dissimilar steel metal vs. aluminum rivets?

Compared to building an airplane, I think a trailer frame would be easy.

Skip Simpson
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