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Are tube spars an insult to the engineering community?

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stanislavz

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I did read about chemical etching of aluminium tube for some ultralights. Maybe this could be a solution ?
 

wsimpso1

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The discussion needs to focus on a particular design. Ultralight fabric skin? Reno racer? Composite canard?
D tube designs are for fabric skin only. Faster airplanes, the skin of the torsion box between main spar and drag spar is also contributing to torsional stiffness, usually bigger than an integral D-tube.
 

BBerson

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That's my point. Pick a particular specific spar design instead of broad general comments about tube spars. Every design is different. For some a tube spar is ideal. Others don't need it.
 

Jay Kempf

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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.
 

wanttobuild

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Jay K.

I read all of your posts. #1smartcookie.

We have discussed this before, but this time you caused a little more thought.

A CFRP round tube, tapered from tip to root, or incremental steps increasing in diameter from tip to root could be optimized even further with the addition of a vertical "web" inside the tube? ie two "D" shapes back to back, then wrapped +45_45? I guess for that matter a cross inside the tube?

In this manner the stiffening of the tube could be done internally and still leave the ease of rib placement.

obtw, I am having problems coming up with a "reusable" mold for a truss style cfrp rib. Short of a silicone mold it has been one tuff nut to crack.
 

Norman

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I don't know how this became an argument about the merits of the BD4 or Jim Bede's ethics. The question that started this was simply about the merits of a round tube vs other shapes. Several questions actually and ultralights were also mentioned. Since the OP is in the United States that word "ultralight" implies a severe weight restriction. When you're building to a weight restriction efficient use of materials should be a priority. A pound or two in the spar can mean the difference between having a faring around the pilot and sitting out in the wind. A round tube could look like a good choice for the spar because it is 21.5% lighter than a box of the same height and wall thickness but that box is 27% stronger and 30% stiffer. Not a huge difference but real and there's a lot of room for improvement. The vertical walls of the square are thicker than they need to be. So let's either use 1/2 thickness side walls or just throw one away and move the remaining wall to the middle to make an I-beam. This wide flange I-beam is 3/4 the weight of the constant thickness box beam but the same strength and it's still not quite optimum because the flanges are kind of thin and can buckle before narrower flanges so let's squeeze that material to make it a proper narrow flange I-beam. Now we've got something that's about the same weight as the round tube but 30% stronger and stiffer and unlike a round tube a built up I-beam can be tapered to fit in something other than a hershey bar and if we want to save a few more ounces we can taper the cross sectional dimensions to match the load along the span. Building up this shape from wood is easy (as I tried to point out yesterday) but the strength to weight ratio of aircraft grade spruce is not all that impressive when one compares it to either S-glass or carbon fiber. Building with modern composites is a bit more work and time consuming but still not all that hard (even a dolt like me can learn it in a few days).
Spar_Square-vs-Round.jpg
 
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TFF

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Building a tapered composite tube spar is fully left field. It has the complexity of designing any other composite structure but leaving the usefulness on the drawing board. It’s not simple anymore. Super complex with out any of the easy of pre fabbed off the shelf stuff a tube spar is supposed to take advantage of.
 

wanttobuild

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a stepped cfrp spar is easy to manufacture. tapered is a little harder but manageable.

Norman, I was just trying to stick to a round spar, easier for me to manage, and rib attachment is a snap. I don't want to disagree with Norman but a cfrp round tapered or stepped spar is easy to build
 

Norman

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a stepped cfrp spar is easy to manufacture. tapered is a little harder but manageable.

Norman, I was just trying to stick to a round spar, easier for me to manage, and rib attachment is a snap. I don't want to disagree with Norman but a cfrp round tapered or stepped spar is easy to build
If a round tube meets your strength and stiffness requirements and you're not down to the point where every ounce count's do whatever works.
 

wanttobuild

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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
 

TFF

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I don’t think anyone disagrees about the best engineered structure would be the I beam. It also requires someone to execute it during the build. That does cut out a number of people. Lazy or not comfortable building it. The round spar brings them back in to a possible builder.

If you are penning the perfect design, no. If you are building for a race, efficiency or world record, no. Best all around airplane, no. An airplane that anyone could build with minimum pieces and construction simplicity, maybe.

Engineering something that takes some hard out of the build and keeps cost down, is a win. I think it could be done up pretty good. The best, no, but still useful.
 

Norman

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a stepped cfrp spar is easy to manufacture. tapered is a little harder but manageable.

Norman, I was just trying to stick to a round spar, easier for me to manage, and rib attachment is a snap. I don't want to disagree with Norman but a cfrp round tapered or stepped spar is easy to build
Steps in a beam are not good. Every abrupt change in diameter will be a stress riser. What Bede did to address the stiffness problem on the BD5 was to stuff an I-beam into the tube spar and call it the SuperSpar suplimintary wing spar system. As I understand it it is 80% of the span and only adds 10 pounds. The other solution to the oil-canning problem that builders talked about was using thicker skins. More ribs would also do it.
 

wanttobuild

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The steps would be a short taper, no abrupt step. A stepped tapered spar was considered to provide a constant diameter for the rib spar connection.
Thanks for the feedback Norman, hopefully Jay will weigh in
 

BJC

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IIRC, Dick Schreder designed and built an experimental wind turbine blade that used two aluminum tubes for the spar, with PVC foam ribs and aluminum skin, all bonded. They were joined by bonding a tapered joint. BoKu, please correct and or elaborate.


BJC
 

Gregory Perkins

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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.
 

stanislavz

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I don’t think anyone disagrees about the best engineered structure would be the I beam. It also requires someone to execute it during the build. That does cut out a number of people. Lazy or not comfortable building it. The round spar brings them back in to a possible builder.
Big +++ for tube - you could just buy it. And then make all small rib/panels section from one small mold. in a month or so.. A number of small steps.

But - one could go to Borabee direction too with I spar build up from two tubes will be more efficient, but will need more of metal work-skill. Same could be seen in I beam made from carbon fibre pultrusion. + is no big/tricky mold, only chordwise connections.
 
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