Why not all Aluminum?

[JimGolden]

I'll echo a few of the comments already made....

First, it's all about the money. To do an aluminum frame right (in my ever humble opinion anyway) you'd do it as a build-up riveted structure. I'd use 2024 or 6061. 2024 is just as strong as the steel in a car frame; 6061 is close but has better corrosion resistance. Airstream could not make money building them like that. But no reason a person with time on their hands couldn't fabricate one. It's more in the labor than the materials,though the materials aren't cheap.

Young's modulus is the elastic modulus of a material; it's "springyness." Yes, aluminum is three times as springy as steel. So you do want to pump up the moment of inertia (essentially the bending stiffness). This is easy to do. And, as we all know, the old long Airstreams are too springy anway. And since aluminum weighs one third that of steel, we can make a much stouter frame and still save weight.

As for fatigue, it is no mystery and is a known quantity. It comes down to cycles. Boeing has an entire book on it that I've read (it's really painful awful boring reading but it's good info and they have it all figured out) There is no way any of us or our descendants will ever cycle an Airstream as much as a jet airliner gets cycled (a cycle is a stress reversal, from compression to tension). Airliners manage to hold together. It's all based on empiral knowledge but basically you have to lower the overall stress levels to achieve higher cycle times before failure. In English, that means you make the frame stronger so that under maximum load it sees a lower percentage of it's total capacity with each stress cycle so it lasts longer. So let's just say that with a certain size trailer and a 4" deep frame of whatever thickness that when you hit the certain size bump, you take the aluminum to 80% of it's yield stress. It's not going to last long like that. But you want the trailer to last forever. So you make the frame 8" deep. All else being equal you just made it 8 times stronger. So now you're hitting about 10% of it's yield stress. You can cycle that a bunch now before it breaks.

So the key is that you cannot simply replace steel with aluminum and keep all the dimensions the same. You have to properly design the frame. But it's not a big deal.

I designed one for my 31' Excella a few years ago. I was going to use steel because it it weldable (if you weld heat treated aluminum, you reduce it to its base state....so if you have 2024 that yields at 40ksi, where you welded it you just reduced its strength to about 10ksi...). I was gonig to make it 8" deep, channel section, 3" flanges, and I believe it was 3/16" thick. It was going to add about 150-175lbs overall weight to the Stream, which I considered negligible and I'd make up for it elsewhere (like throwing the tambour doors over the hill...). I was also going to put a 6" wide perimeter plate all around the outriggers. The U channel would bolt to this plate. The floor would then sit on top of this plate but INSIDE the shell. No more attaching the shell to the floor.

I could bore you with further details, but it was a good design and I still have it in AutoCAD. I even offered it to a guy on here when he asked me about it but he never wrote me back. The only reason i didn't do it was because I was in the middle of building a house and didn't have the time for a full shell off. I sold the Exella and bought an Avion, which has a frame that's actually more stout than the one I'd designed.

Steel does have some advantages here. It's easily repairable, it's durable, it is less prone to fatigue (though we can work around that). It's weldable. And you can get a shop to hot dip your frame once you weld it up.

I agree with the others; I think there are bigger problems that rot them up than the frame being made of steel.

Someday I'll buy two Excella's and cut the front off one and the back off another and make my custom frame and have my Railroad Super Long special with the garage in the middle for the Hog

Until then, see ya on the road!

[Chrispate]

Originally Posted by thecatsandi

I personally find TIG easier then Gas welding.

I started with oxy acetylene for 4 years so it's my foundation. So I guess it'll always come easiest to me.

[Jess Smosh]

Even aluminum corrodes i have some on mine that "rusted" you know what i mean all depends on were there used helps to keep on pavement to prevent rust on frame

[Alumination]

Regarding the comments on wet fibreglass insulation promoting frame corrosion, water leaches salt out of fibreglass insulation. At work we lost a large 316L SS pressure vessel to Stress Corrosion Cracking. It was insulated with fibreglass and gland water from a pump mounted on top of the vessel had kept the insulation wet under the cladding for years. I couldn't figure out where the chloride came from to initiate the SCC until I came across the salt in fibreglass information. We ended up stripping the fibreglass from all of our SS pressure vessels.
Replacing the fibreglass insulation with something that doesn't retain water or leach out salt should reduce the corrosion problem.

[Kevin245]

Originally Posted by JimGolden

I'll echo a few of the comments already made....

First, it's all about the money. To do an aluminum frame right (in my ever humble opinion anyway) you'd do it as a build-up riveted structure. I'd use 2024 or 6061. 2024 is just as strong as the steel in a car frame; 6061 is close but has better corrosion resistance. Airstream could not make money building them like that. But no reason a person with time on their hands couldn't fabricate one. It's more in the labor than the materials,though the materials aren't cheap.

Young's modulus is the elastic modulus of a material; it's "springyness." Yes, aluminum is three times as springy as steel. So you do want to pump up the moment of inertia (essentially the bending stiffness). This is easy to do. And, as we all know, the old long Airstreams are too springy anway. And since aluminum weighs one third that of steel, we can make a much stouter frame and still save weight.

As for fatigue, it is no mystery and is a known quantity. It comes down to cycles. Boeing has an entire book on it that I've read (it's really painful awful boring reading but it's good info and they have it all figured out) There is no way any of us or our descendants will ever cycle an Airstream as much as a jet airliner gets cycled (a cycle is a stress reversal, from compression to tension). Airliners manage to hold together. It's all based on empiral knowledge but basically you have to lower the overall stress levels to achieve higher cycle times before failure. In English, that means you make the frame stronger so that under maximum load it sees a lower percentage of it's total capacity with each stress cycle so it lasts longer. So let's just say that with a certain size trailer and a 4" deep frame of whatever thickness that when you hit the certain size bump, you take the aluminum to 80% of it's yield stress. It's not going to last long like that. But you want the trailer to last forever. So you make the frame 8" deep. All else being equal you just made it 8 times stronger. So now you're hitting about 10% of it's yield stress. You can cycle that a bunch now before it breaks.

So the key is that you cannot simply replace steel with aluminum and keep all the dimensions the same. You have to properly design the frame. But it's not a big deal.

I designed one for my 31' Excella a few years ago. I was going to use steel because it it weldable (if you weld heat treated aluminum, you reduce it to its base state....so if you have 2024 that yields at 40ksi, where you welded it you just reduced its strength to about 10ksi...). I was gonig to make it 8" deep, channel section, 3" flanges, and I believe it was 3/16" thick. It was going to add about 150-175lbs overall weight to the Stream, which I considered negligible and I'd make up for it elsewhere (like throwing the tambour doors over the hill...). I was also going to put a 6" wide perimeter plate all around the outriggers. The U channel would bolt to this plate. The floor would then sit on top of this plate but INSIDE the shell. No more attaching the shell to the floor.

I could bore you with further details, but it was a good design and I still have it in AutoCAD. I even offered it to a guy on here when he asked me about it but he never wrote me back. The only reason i didn't do it was because I was in the middle of building a house and didn't have the time for a full shell off. I sold the Exella and bought an Avion, which has a frame that's actually more stout than the one I'd designed.

Steel does have some advantages here. It's easily repairable, it's durable, it is less prone to fatigue (though we can work around that). It's weldable. And you can get a shop to hot dip your frame once you weld it up.

I agree with the others; I think there are bigger problems that rot them up than the frame being made of steel.

Someday I'll buy two Excella's and cut the front off one and the back off another and make my custom frame and have my Railroad Super Long special with the garage in the middle for the Hog

Until then, see ya on the road!

Good information Jim. Aluminum frames are viable though expensive as you state, and joint efficiency is a significant factor should someone consider a welded aluminum frame versus a mechanically fastened one. Like you, I would proceed with a built-up riveted structure should I ever get the urge to take on something like this (not likely to happen though).

Your comment about material strength loss when welding heat treatable alloys is spot on. It should be noted though that 2024 is considered by the welding industry to be one of the few non-weldable alloys. 2024 is an aluminum, copper, magnesium alloy that is susceptible to inter-granular cracking after arc welding that can lead to catastrophic part failure.

Though the welding of 2024 isn't discussed specifically, here is a post where aluminum material treatment and welding considerations were discussed.

http://www.airforums.com/forums/f36/...ost793041.html

Thanks for sharing your knowledge and experience.

Kevin.

[195Pilot]

Well my 2 cents.
I have a 31 ft frame out from under the shell. Sadden with the poor workmanship and the quality. I am welding up a new frame with rect. tube vs c channel.Also rect tube for the crossmembers. The workmanship is lower than poor. I have found 4 rivets on the roof that were never installed. The welding is pitiful.
Now there are new ways to weld aluminum STIR FRICTION WELDING, basics, lay to pieces together and a extremely high rpm router runs down the weld line the front end of the cutter fuses the material, little heat and does not disturb the temper. As I understand they are welding 2024 t-3 with no problems.
Another option for the frame would be foam core carbon fiber. The only trick there is incorparate the hard points. Foam core, epoxy, and carbon fiber you can build a frame stronger, lighter and maybe cheaper than aluminum. While your at it make the subfloor the same way.No more rot,no more rust, no more corrosion .

[dwightdi]

Golden Jim in on the money with most all his conclusions. It depends on whether you have the time and money to build a trailer with the technology of an aircraft or built something that will attract enough buyers at that necessary price. Airstream INC is a business and is making engineering and business decisions based on average potential consumers and it ability to make a profit. Some consumers may want a trailer which will last 50 years, but most do not look past 10 years of personal use. There are a few consumers that can afford multi-million dollar RV's and yachts. The small numbers are one of the reasons Airstream decided to get out of the class A motor homes.

Steel has a critical stress limit that the material will last forever. Aluminum has no stress at which it will last forever. That is why they retire planes after a given number of flight hours. Of course, this assumes the steel will not begin to rust at some point which will dramatically reduce its resistance to fatigue.

Rot and accidents are the reasons most Airstreams are put out to pasture (as hunting shacks) or end up in the scrap yards. Weak frames and tail drag in the larger units of the 1970's are why many of those models are relegated to stationary service or worse.

[Tim A.]

Originally Posted by 195Pilot

Well my 2 cents.
I have a 31 ft frame out from under the shell. Sadden with the poor workmanship and the quality. I am welding up a new frame with rect. tube vs c channel.Also rect tube for the crossmembers. The workmanship is lower than poor. I have found 4 rivets on the roof that were never installed. The welding is pitiful .

Your Airstream appears to be a 1975 model, so is 37 years old. If the quality is so poor, it is amazing it has lasted until now! Good luck with your new frame.

Tim

[Splitrock]

Originally Posted by 195Pilot

Well my 2 cents.
I have a 31 ft frame out from under the shell.. .

Hi to Pierre , South Dakota

From, Sioux Falls, South Dakota



`

[Kevin245]

Originally Posted by 195Pilot

Well my 2 cents.
I have a 31 ft frame out from under the shell. Sadden with the poor workmanship and the quality. I am welding up a new frame with rect. tube vs c channel.Also rect tube for the crossmembers. The workmanship is lower than poor. I have found 4 rivets on the roof that were never installed. The welding is pitiful.
Now there are new ways to weld aluminum STIR FRICTION WELDING, basics, lay to pieces together and a extremely high rpm router runs down the weld line the front end of the cutter fuses the material, little heat and does not disturb the temper. As I understand they are welding 2024 t-3 with no problems.
Another option for the frame would be foam core carbon fiber. The only trick there is incorparate the hard points. Foam core, epoxy, and carbon fiber you can build a frame stronger, lighter and maybe cheaper than aluminum. While your at it make the subfloor the same way.No more rot,no more rust, no more corrosion .

Friction stir welding (FSW) is a unique process and it is newer than the traditional welding methods (early 90's). Similar to inertia welding, it overcomes many of the undesirable heat related issues; however there is still some yield stress reduction using this process.

FSW is a mechanical process rather than an electrical one and while the mechanics are relatively simple, the implementation is not. FSW success depends upon having controlled tool speeds and feed rates among other variables. FSW is not a DIY process, nor is it commonly available in general industry.

[Gene]

I don't think anyone is really thinking Airstream would adopt an aluminum frame, but I wouldn't want one. They wouldn't hire expert aluminum welders because they would have to pay them more. They'd try to get by on the cheap and a year or 5 we'd be reading about frames falling apart. Airstream does adopt new techniques sometimes, but they don't seem to test them thoroughly and sometimes disaster ensues (OSB subfloor for ex.)

But if people are designing ones for their own trailers, that is interesting and maybe a manufacturer will come out with the All Aluminum Trailer.

Gene

[Friday]

If you look at automotive uses of aluminum frames, they are either hydro-formed parts welded, or cast and machined... either method doesn't lend itself to only making a few pieces due to the tooling needed.

Just taking a bunch of tubing and welding it together would require a lot more material to get the same strength.

[Cracker]

Way back in the '70s I had the opportunity to work with what was then a new material called "Tarset." It was epoxy-based and required very specific prep of the steel - i.e. a full "white" blast of the steel followed within hours by a special undercoating. If you left the steel more than about 4 hours you had to re-blast it. The Tarset coating was then applied with an airless gun to a specific film thickness (---I think that it was around 12 mils). This is ancient history - but I'm here to tell you that in many subsequent uses of this coating it developed a phenomenal reputation. The initial application I experienced was for the coating of channel markers to be installed in the gulf of mexico. Subsequently, we used this treatment for steel sheetpiling and many other marine applications. The coating was so tough that the only way to remove it to weld attachments was by using a carborundum cup grinder. I'd love to have an Airstream chassis coated with Tarset epoxy! The coating used on pickup beds seems to be similar - without the extensive prep work.

[195Pilot]

Splitrock I have seen your post,Im about 40 miles east of Pierre. Out in the middle of nothing. Thats one of the problems with my project every thing is a week to order or a trip to Sioux Falls or Rapid for material. Not to mention the 100% up charge doing business in a small town.
Hate to harp on quality control but this trailer was a park model for the last 20+ years and the PO CURED the roof leaks with 10 gallons of TAR.
I have 40 plus years in aviation,Pilot, Airframe and PowerPlant Mechanic with an Inspection Authorization and at one time DER. It's an insult to compare the "QUAILTY" of the airstream Trailer to A Type Certified Airframe. The airframe manufacturer would have his Type Certification pulled on the first inspection or on the first test flight ending with a catastrophic component failure if they had the quality of airstream manufacturing.
And yes most pressured aircraft are life limited due to cycles. These limitations are set forth by the manufacture. But there are quite a few aircraft that are flying with close to 70 years under their wings being used every day as freight dogs. With 10's of thousands of hours on their log books.
One that comes to mind is the Douglas DC-3 First flown in the early 1930's and end of production was at the end of world war ll. NOW THAT'S QUALITY.Plus the engines have been out of production almost as long.Big old Round Motors that on start up belch smoke, shake and viberate to high heavens until all cylinders come on board.
When the last of the latest Boeing fly by wire computer controlled mega jets goes to the boneyard there will be a DC-3 there to fly them home.

[ROBERT CROSS]

"One that comes to mind is the Douglas DC-3 First flown in the early 1930's and end of production was at the end of world war ll. NOW THAT'S QUALITY.Plus the engines have been out of production almost as long.Big old Round Motors that on start up belch smoke, shake and viberate to high heavens until all cylinders come on board.
When the last of the latest Boeing fly by wire computer controlled mega jets goes to the boneyard there will be a DC-3 there to fly them home."

Now that brings back some memories....

Easter leave 1966, el Toro to Pensacola Fla...low and slow.



Marine Corps C-117D, BuNo 50835 at Marine Corps Air Station el Toro on April 29, 1989. Its Douglas construction number is 26998. It was delivered to the Army Air Corps as C-47B-20, 43-49737, then transferred to the Navy as R4D-6, 50835. It was converted to R4D-8 and later redesignated C-117D in 1962.

Bob

[Gene]

Speaking of airplanes, we were on the way to Alaska and my wife was outside. A guy comes up to her and points out all the dimples in the skin around the rivets. He says he worked for many years on aircraft assembly and those dimples are a result of the rivet gun be set too high and that should not happen if the riveter knows his job.

Gene

[purman]

Originally Posted by CrawfordGene

Speaking of airplanes, we were on the way to Alaska and my wife was outside. A guy comes up to her and points out all the dimples in the skin around the rivets. He says he worked for many years on aircraft assembly and those dimples are a result of the rivet gun be set too high and that should not happen if the riveter knows his job.

Gene

What, I though AS only hired the best of the best.
They charge like they do.

[thecatsandi]

Originally Posted by CrawfordGene

Speaking of airplanes, we were on the way to Alaska and my wife was outside. A guy comes up to her and points out all the dimples in the skin around the rivets. He says he worked for many years on aircraft assembly and those dimples are a result of the rivet gun be set too high and that should not happen if the riveter knows his job.

Gene

If the dimple was consistent all the way around I would agree. If it is only on one side, it is bad technique.

[LarryGlover]

Originally Posted by CMSmith

So is there a better insulation to use once a floor is replaced so moisture does not get trapped against the frame?

I'm doing a frame-off floor replace on my 1965 Streamline (an airstream copy), and I have decided to use a spray in foam insulation that is water pepellant and helps seal all cracks . I would imagine you could also use a bedliner material such as rhinoliner or line-x liner as this would coat the bottom of your wood floors as well as the frame and provide temperature and noise insulation as well. Batt insulation is used from the factory because it is inexpensive and quick to install with no delay in build time due to drying or curing times that are required by spray in insulations.