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Are tube spars an insult to the engineering community?

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wsimpso1

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I did read about chemical etching of aluminium tube for some ultralights. Maybe this could be a solution ?
Solution to ... what? I can not tell what you are addressing.

Aluminum really wants to go back to bauxite. Etch aluminum with a mild acid and let the air see it for even seconds and the surface layer of aluminum atoms have grabbed oxygen, and the oxidation will then proceed further. Alodine works because it dissolves off the aluminum oxide and forms a new compound on the surface atoms simultaneously. Paint sticks well to alodined aluminum, but durable high strength bonding does not. Factory methods exist for durable aluminum bonding, but AFAIK they involve processes and materials that have not found safe ways to migrate into our home shops.

When safe durable bonding processes do migrate to home shops, it will enlarge aluminum construction.

Billski
 

stanislavz

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Solution to ... what? I can not tell what you are addressing.
Solution to have it tapered. It was etched via electrochemical etching. Not plain acid.

But is a no go for one-off aircraft. Just tubes in any better alloys are hard to obtain.
 

wsimpso1

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Billski,

The BD4 wing basic wing structure has no ribs, no riveted joints, no masses of adhesive. There are 1 foot female molded sections that are the entire skin including the aileron/flap cove plus the rib. Each wing section has a flange for the round spar and a joggle for the next section. The wing is wet. Adhesive and a large hose clamp are used for the spar joint. A large plywood airfoil shaped female clamp is used to clamp the joggle at the skin, same adhesive from a caulk gun. If you tape over the joint with mold release there is no ooze. Back then he chose a version of proseal which had mixed results but only in terms of holding fuel not structurally. No etching, no prep, just the caulk gun. 10 wing sections, one spar tube per side plus a wing tip, for somewhere near 26' of span. Many have added span by making extra sections and making removable tips.

So no aluminum skin bonding, no real aluminum critical bonding at the spar, more of a caulk and seal, no large amount of ribs to install, rivet, no big critical alignment, just a parallel string rigged to the spar tube to align the upper or lower trailing edge line.

It is these details that make the compromise. There is no drag spar so all loads, drag, lift, torsion go from skin through molded in rib directly to one tube.

The wings are not heavy. I have lifted heavier wings. They could be lighter but the BD4 builds up pretty light for its wing area. It can't be compared to building composite molds.

Bede the man, a different story. No way to optimize him. But the wing structure he designed for the BD4 is a great piece of out of the box engineering especially considering the time and materials available. It was conceived as a kit from the start and the concept was something that could be built with simple plywood disposable jigs and not many of them and basically pretty much hand tools for the rest. Focusing on one component in one sub assembly sorta misses the point. The BD4 isn't the BD5. But Bede's reputation as a snake oil salesman just sort of overshadows all his activities.

I still page through the BD4 book sometimes and find little details that I use for other purposes. Just like paging through other sets of plans and other aircraft designs. Plagiarism is, after all, the most sincere form of flattery.
What ignorance are you implying? I know of Jim Bede and how he sold people on building and flying his bucket wings because they were easy to build. I also know how folks had problems with airplanes that wanted to roll because even small assembly errors resulted in crooked or twisted wings, how people had trouble keeping the fuel inside, how people could not finish their birds because the supply of tubes and buckets dried up, and so on. How many of Jim Bede's "bucket" wings have you ever seen at a fly in? I have seen plenty of conventional aluminum winged BD4's flying and in construction, but I have only seen one bucket wing at OSH and the owner complained that it was out of rig, that the tanks developed new leaks, and that he was going to have to tear the wings down to salvage the spar from that one to build a new sheet metal wing...

To start with, the spar has to be strong enough to carry the lift, bending moment, and torsion at the root. A tube big enough at the root is over size and overweight everywhere else. Like 2-1/2 to 3 times more weight than it needs to be in a cantilever wing airplane. Then just being a round tube it is another 30-40% heavier than if it were a tailored built up aluminum spar. So you get hit twice on weight for the round tube. Then every foot, you have a fiberglass rib (many of them redundant), a fiberglass flange at the tube, and a fiberglass flange for connecting to the next bucket. In most fiberglass wings, that is six extra ribs and 1 part in 12 extra wing skin per side. Then there is the adhesive. You can call it what you want, but it is adhesive and there are pounds of it required to fill the gaps at the tube and at the perimeter. I get somewhere over 1000 linear inches per side. Then the adhesive joint at the tube spar is unlikely to be long lived. All of this is weight removed from usable load of the airplane or added to takeoff weight, increasing runway needs, reducing climb and ceiling, and generally making the bird perform more poorly. Now if saving some shop time in exchange for the other issues is a good trade for someone, I say "It is your airplane and you get to make that trade". Just accept up front that it will love the ground more and be worse in a forced landing than it could have with more "efficient" design...

The first alternative is to build a more conventional wing with aluminum ribs that fit on the round tube spar, then skin it , riveting it together. I suppose if you live and operate the airplane in a desert, you can trust a bonded structure to hold up longer than you will, and avoid all those rivets. It will still be heavier than with a tailored spar and conventional ribs just because of the round tube spar limitations.

The next alternative, and one some folks faced once the very specific tubes left the market, is conventional spars, ribs, and skins, including the carry through. Serious engineering, I know, but there are lots of airplanes out there using them safely and efficiently.

Now where do tubes make all kinds of sense? In externally braced wings, just like where straight wooden spars make sense. Much lower bending moments with the maximum around the struts. The spar is a lot smaller in these designs because the max bending moment is a lot smaller. Yeah, you have to deal with compression in the spars too. Lots of good designs out there this way.

Billski
 

wsimpso1

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Norman I reread your post.

I am just thinking outloud, but I want to keep the ease of rib installation on a round. How about making the round spar a beam?
Span wise reinforcement internally top and bottom?

obtw, we are in agreement the tube is not the lightest
How are you going to build that? I have a couple ways that are difficult and thus failure prone. I have a couple more that will require capital level tools. None that will be home workshop capable.

All of this round spar effort is to supposedly make ribs a snap? Composite wings do not have ribs every few inches like in a metal wing. Composite wings only have a few ribs - one at each end, one more to close out the fuel bay, maybe a baffle in the middle of the fuel bay and one to hang the aileron bellcrank. And these few ribs are cut from a big board of sandwich and taped in on the main spar, drag spar, and top skin. If you molded your sandwich wing skins and built a decent jig on a torsion box to assemble the wing, the alignment is already set, and you are then ready for the flange transfer technique to bond the bottom skin. Light, sturdy, and rewarding work. Not so bad...

Billski
 

wsimpso1

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Interesting to note that all ( as far as I know) of the more famous human powered planes like McReady's that won the Kremer (sp?) prize used composite tube spars. If there was a lighter way to achieve their objectives they would have used it. Obviously other factors shaped the decision.
Externally braced low wing loading is where round tube beams are useful as moments are kept small. The Gossamer Albatross is a beautiful example of a light externally braced wing.

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. Environmentally sound methods for disposal of those waste products are expensive if you can find someone to take it. And these tubes were tailored, with more plies near hardpoints and along the top and bottom of the spars. The team used round aluminum tubes because they could get round aluminum tubes and then remove them later. With acid.
 

BJC

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Bill, I’m beginning to detect that you think that aluminum tube spars are not a good idea.


BJC
 

Norman

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Bill, I’m beginning to detect that you think that aluminum tube spars are not a good idea.
Most structural engineers don't take cantilever round tube beams seriously, that's why I included a link to a drone spar calculator in post #47. I spent a few hours looking for round tube beam calculators and most of what I found was for pipe supported at both ends (like when a water pipe crosses a river). Round tubes are just so inefficient for cantilever beams that it doesn't make economic sense to use them in anything larger than a toy. One can argue that single seat airplanes are bigboy toys but in a hobby where cutting grams off of parts is, or should be, a full time activity it seems to me that this is a pretty big opportunity to save a lot of grams. I can see where a tube spar would look appealing from the standpoint of attaching ribs for easy rigging that could be achieved just by attaching a 1/2 round section to both sides of a better structural shape but that would add unnecessary permanent weight for a dubious benefit on the building table.
 

ToddK

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Nothing wrong with a tube spars, especially on fabric covered, strut braced aircraft. My first airplane had tube spars. Great machine. Lots of them have been flying around now for decades and for good reason. On a fast all metal wing, if you are already dealing with lots of aluminum and rivets, might as well go for traditional metal wing construction, I cant think of a lot of good reasons to go with a tube spar over a sheet metal spar in that situation.

Personally, I like the smell of sawdust so...
 

TFF

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I can see how one of these BD wings would be not straight. It leads itself to the simplicity that anyone can build it. Not just anyone can build to tolerance. It’s also pretty heady for a 1968 airplane. Composite wing parts in 1968.

In the eyes of 2020, of course there are flaws. All easily addressed if it had been evolved. Too many bucket panels. Probably as good as they got then. It was pushing build technology in the homebuilt world, wing and fuselage.

From a flexibility standpoint, it would be easy to change the wing platform. Yes the spar gets even more ridiculous, but you want a tapered easy to build wing at home, it’s all in the bucket molds. Today you could make the buckets, 3-4 ft span with some core, much more stable materials, better quality molding with better jigging.

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. Optional wing tip buckets for taste and performance. Push that 51% rule. I call a free wing kit from the person who does it for payment of a super idea.

I’m not even a fan; but it can make consumer sense. What makes a winner kit is people making them.
 

Vigilant1

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I’m not even a fan; but it can make consumer sense. What makes a winner kit is people making them.
The question in the thread's title asks if tube spars insult the engineering community. Proposed:
- For this purpose, there is no monolithic engineering community consensus. There would probably be a convergence of opinion approaching consensus about the applicability of tubular spars if we spelled out enough other attributes/criteria of the use case.
- Even so, any consensus of the engineering community might differ from the opinions of the folks selling and buying the plans/kits/planes. There are a lot of GA planes with swept rudders, and it ain't for aerodynamic or structural reasons.
 

jedi

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Thanks for all the comments. What I have learned follows:

1. Tube spars are not all bad.
2. Tube spars are OK for low aspect ratio Hershey bar wings on simple inexpensive airplanes.
2a. Tube spars are not OK on moderate or greater aspect ratio wings with considerable taper.
3. Tube spars can handle torsion and thrust drag loads better than I beams.
4. I beam spars are better for long tapered wings with a D leading edge or rear spar for torsion and thrust/drag loads.
5. Tube spars can handle compression loads better than an I beam. Struts cause compression loads.
6. Those that do not like tube spars are generally thinking of the cantilever Hershey bar wing. Their discontent is as much about the wing as it is about the tube spar.
7. If the tube spar is tapered, stepped, machined or otherwise custom manufactured and not just the mill run extruded pipe or tube, many of the negative issues can be mitigated.
8. Tube spars are not "insult to the engineering community". They are a challenge to the engineering community.
 
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stanislavz

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Here is a photo of grunmann wings. Good as reusing it for fuel tank too.

Its got me into idea - what if you run not one tube, but three - middle to the end, front one 2/3 of span, and rear one to 1/3 of span.
 

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wsimpso1

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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.
 

wanttobuild

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Tube spars are efficient and can be produced in a cheap foam mold, fully tailored for strut attachment and fuselage attachment.
The knee jerk reaction is the mandrel. Wrong answer. Consider a mold, all internal reinforcement, ie pultruded carbon rod can be placed and a "web" can be formed in a mold.
The mold would produce a reinforced "D"
The "D" would be placed back to back, hot glued together. Now you have a straight, internally reinforced tube mandrel.
One now starts wrapping the reinforcement, based on your educational qualifications, which could range from TLAR to who knows. It is your rear end, go for what you feel comfortable with.

Only your imagination and your tolerance for weight, which btw is not applicable, to a spar, which is now a round beam.
 

Sockmonkey

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Tube spars are efficient and can be produced in a cheap foam mold, fully tailored for strut attachment and fuselage attachment.
The knee jerk reaction is the mandrel. Wrong answer. Consider a mold, all internal reinforcement, ie pultruded carbon rod can be placed and a "web" can be formed in a mold.
The mold would produce a reinforced "D"
The "D" would be placed back to back, hot glued together. Now you have a straight, internally reinforced tube mandrel.
One now starts wrapping the reinforcement, based on your educational qualifications, which could range from TLAR to who knows. It is your rear end, go for what you feel comfortable with.

Only your imagination and your tolerance for weight, which btw is not applicable, to a spar, which is now a round beam.
Ah you ninja'd me. I was going to suggest the double D.
 

TFF

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The issue with the spar used as a fuel tank is the fuel can unport the fuel pickup. It’s why AA-1s ended up with an AD to not spin them. In a spin it had a high moment, and if the tanks are not full it all gathers in the tip, not in the root. Didn’t help that they were trying to complete with C150s for that market.

The auxiliary fuel tanks there in the picture are going to be a thorn in my side soon. I told some friends that own some AA1s where an available STC rights for those tanks were. They bought it. Once they figure out the PMA stuff, guess who gets to install it and probably make them. Not really for profit, AA1 only has 22 gallons or so. You can’t go very far. Another 10 gallons will transform it.
 
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