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Are tube spars an insult to the engineering community?

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Jay Kempf

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Double D can maybe be construed as something else. But maybe not on an airplane techno geek forum....

A shear web sub assembly inside a tube is something I have looked at inside a tail boom. A shear web in the direction of the largest aero load is a really good design option.
 

b7gwap

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The single biggest advantage to tube spars is that they come pre manufactured as an extrusion.
Part of the pioneer, Knievel-seque spirit of ultralight design is the TLAR nature of a tube ladder wing with external bracing. I doubt very many of those designs ever underwent a FEA/FEM, or even a test to destruction. They knew it worked on quicksilvers and hang gliders, so just make it look right. The top speed (drag loads) and wing loading requirements probably kept the philosophy safer than it could be. Yeah tubes make good columns to load in compression, but not when you have transverse loads applied at the same time. (Think standing on a soda can and pushing against the side.).

I like that pioneer spirit. I also see the structural elegance of an optimized structure, like the ULF-1, or the Lazair. I think the experimenter gets just a little gun shy when they sit before the task of designing an I beam spar wing. It’s a lot harder to TLAR web thicknesses, stiffener and rib placements, cap reinforcement, etc than it is to buy the next size higher OD or thicker wall tubes.
 

Sockmonkey

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Double D can maybe be construed as something else. But maybe not on an airplane techno geek forum....

A shear web sub assembly inside a tube is something I have looked at inside a tail boom. A shear web in the direction of the largest aero load is a really good design option.
For a tail boom stiff in horizontal as well as vertical, use four quarter-circle extrusions so the internal bracing forms an X. Before you glue them together, drill some lightening holes along the flat portions.
 

stanislavz

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Heck the world is open for someone to make an I beam composite spar with slip on tapered buckets. A wing kit panel that comes as a spar, 5 buckets, and a completion kit.
If you look at the bending moment curve of just about any wing, and three times were enough at the root, one would likely be enough for the outer half.
It was just guesstimate + other tubes will be smaller in diameter..
 

stanislavz

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It all depends on goal. YMMV - If you are a kit builder, buying tubes is a good bussines case (cheap, straight, consistent). Same as molding 10 or 100 airfoil shaped buckets to mount it on that tube.

Build up I beam is not as safe for kit builder as tube - you can't test it before delivery. + you need kind of jig to build it untwised.

Shipping build-ed up D tube without rear ribs - is ok, you can test it and buyer will know what he is buying. + it is straight, not twisted etc.

For one off plane - this is my third cnc foam cutter, build from drawer slides 700x700mm working area. build int two evenings. Have to be scrapped 4 years ago, but is still used occasionally. Used to cut foam for lost a foam casting of aluminum mostly.

1601286840891.png

And - you can mount two of them on both sides of your long table, glue eps blocks to table, cut your female molds of D cell and I beam, Cover with any foil, and have nice and straight parts. Or you could cut front D cell with upper skin in one part. Or whole airfoil. And build it from inside. It will need kind of vacuum to hold it in mold, but I do not see why it can not be build..

But - here you have expendable, cheap mold, and as straight as possible. For one off. Not for production.
 

opcod

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Just to point also.. Tube spar are nothing new and quite old. Many plane did flew without issues and all design by true engineer at the time of the war. No inovation except re-using the same old tech.
 

TFF

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I don’t think this thread is about safety. It’s about is buildable and useable vs best at any cost. One end you get LEGOs and the other you get NASA. It’s always a sliding scale in between. No matter how good you think your idea is, there is a better way, in both directions.

There are so many threads that say, I wish my plane would just assemble by its self. Just as many threads are impossible goals with a NASA program. This is homebuilt airplanes. Home, like in your house. Some people have great tools and some don’t. Some have great budgets and some don’t. I wonder how many are only interested in the unobtainable? Makes a great safety valve of not really doing it.

It’s seem people are attached to their ideas more than actually owning and operating an airplane. It’s very much in line with the Wainfan presentation of being in love with your ideas and having to make crazy concessions down the road. If the concessions are not that crazy, it’s probably not that bad of an idea. If the baby is thrown out with the bath water, you need to have a moment. Most don’t want to be wrong and never do that.
 

pictsidhe

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Light, strong, easy to assemble, cheap.
Pick any three.

If I was designing a strutted fabric covered aeroplane, I would be taking a long, hard look at tube spars. I'm not, so I'm looking at other options.
I am attempting to design a kit 103 that is fairly easy to assemble, rugged and not too expensive. It is difficult to nail all three. I have to be able to make all parts at a reasonable price to meet target weight, strength and ease of assembly. I'd like to use as much off the stuff as possible, that really helps with cost. A widget made by the million is 10 times cheaper than anything I can do.
 

BJC

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Aesquire

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Paul MacCready built those tube spars by starting with aluminum tubes, then doing tailored laminations of graphite fiber and Nomex honeycomb with vacuum bagging, then dissolved the aluminum from the spar with hydrochloric acid. Lots of acid.
Yep. Elegant use of a fairly cheap aluminum tube as fixturing/moldless core in an otherwise impossible to remove place...

And structurally, the Gossamer Albatross was the same as the Gossamer Condor, a single spar wing, a keel tube fore & aft to hang the wings, canard, and pilot "pod" on, and a vertical tube going up to act as a king post and one going down to hold the pilot. In other words, a giant "jack" from a kid's game stretched out & bent.

Also the structure of a Bowsprit hang glider. Here's a current example.
The extra drag of the wires is somewhat compensated for by the reduction in weight by eliminating the cross spars. As seen in the "normal" configuration that is the dominant flex wing design today.

In the Gossamer Condor/Albatross the wing spar is not right at the leading edge, like the hang gliders it was inspired by, as it uses built up ribs and no need to fold to fit on a car roof. MacReady was well aware that the tubular beam was suboptimal, and that the wire bracing was a parasitic drag nightmare, but the speed regime of a human powered aircraft, a slow running pace, made the wires a good compromise where weight was a critical factor.

The use of tubes on the Condor was both a MASSIVE time, labor & material cost saver. When ( not if ) they crashed during testing, it was a simple matter to just plug in a new, readily available piece of tubing cut to length and drilled for bolts. If it had been a lovely "I" beam built up structure, testing would have resumed in months or years, not a few hours. Since they Did crash repeatedly, ( with no injuries because... 10 foot altitude and slower than a bicycle ) it was a brilliant choice.

And with the Albatross, they were able to tailor the loads much better, plus had a now experienced pilot/engine so it wasn't as bad on needing frequent repairs as the Condor...Or the Wright brothers who would fly until they broke the glider, then rebuild it in the shed overnight at Kitty Hawk. ( which was my pattern with my first hang glider, a Batso Rogallo. Run, hop, crash, repeat. Both Wilber & I had the disadvantage that there were no schools to learn to fly then )

As to why Dr. MacReady used round instead of square tubing as the disposable cores on the Gossamer Albatross, which would have been a logical improvement, I suspect it was a combination of "if it ain't broke, don't fix it" and that the loads on the wing are not the "easy" strut braced or cantilever type we are used to. Cable bracing loads the spar in compression, torsion, and weird ways....

That's why hang gliders, where weight is a major factor, still use tubular members. The Cross Spar is loaded mostly in compression. The Leading edges are loaded in a highly variable direction from root to tip ( near BSLD but not quite... ) and also need to take really nasty loads in a tumble or tail slide.

Also, there's the cost factor. The SWIFT and other rigid wings use a Composite D tube, and the advantages of reducing drag with the cantilever structure come at a not insignificant price.
 

Jay Kempf

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My guess why Dr. MacReady used round cores is they weren't vacuum bagging. Harder to control fabric over a square in a wet hand layup using grad student labor. For round you can wrap tight over wax paper or release film. This was in the stone ages of composites. Lots of good history on those projects. 20 kids getting dispensed a few grams of epoxy each in a tiny Dixie cup and told to cover 2 feet of spar. So in other words they made the tube, they didn't buy it. QC of the build was a huge issue. They didn't buy tubes, they made each one and each one had a laminate schedule with ply drops. There are really good videos of building it or there used to be. Don't remember it being transported on car tops. They rigged in a huge hangar and waited for dead calm cause the thing was so fragile it couldn't take a gust.
 

BrianW

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I did read about chemical etching of aluminium tube for some ultralights. Maybe this could be a solution ?
Contrary to popular opinion, the Gossamer Condor used an aluminum main spar, chemically etched in a pipe to thin the outboard sections. By contrast, the Channel crossing Albatros used carbon fiber wrapped on an aluminum tube, which then had a groove etched in it in order to pull it out, because the composite shrank a little.
 

Aesquire

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The Gossamer Condor was brought to the Smithsonian in a van, and had to be assembled in place. The funny part was 2 little "errors" on the Museum staff's part, the first being prepared to lift in the plane with a crane capable of moving a medium bomber. ( the Condor could be lifted by one person ) and the second that the staff didn't quite know how big it really was, and had to change plans to fit it in. "The Wingspan is WHAT?" ( btw, they will deny that today... )
btw, the tubes about 2/3 of the way out on the wings, going up/down/forward are called "deflexors" and hold the cables bracing & shaping the wing from tip to root on each side.
The Albatross is considerably cleaner in drag overall.

Good point on the difficulty of wrapping a square tube in a hand applied wet layup without vacuum bagging.
 
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