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Old Nov 01, 2014, 05:10 PM
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Wing Spar Design

Hello Fellow Scratchbuilders!

I'm revisiting the design of a wing spar for a removable wing. The spar is for a 3D model so it needs to be able to withstand some G's. My goal is to have the easiest construction with the strongest spar.

In the pictures below, the "tube" represents an aluminum or carbon receptacle that accepts the wing tube. The spar would be made from balsa or hardwood (basswood or yardstick).

Currently, I have been using spars similar to Option 1. I have never had a failure but sometimes alignment can be a pain. This design allows the wing tube to be as long as possible.

Lately I have been thinking that if the wing itself is strong then the wing tube could be shorter. Options 2 & 3 explore this. Options 2 & 3 are also self aligning which would make construction easier. I am a bit worried about option 2 - I'm thinking the end of the receptacle tube may be a weak point. This could be remedied by a shorter tube and capping the entire spar with carbon or hardwood (Option 3).

...I think I may have just answered my question - time to do some testing! lol... sometimes sharing helps... but still very interested in what others think regarding wing spar construction!
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Old Nov 01, 2014, 05:25 PM
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Is the spar foam?
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Old Nov 01, 2014, 05:28 PM
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The spar would be either balsa or hardwood (Thanks - I'll update the 1st post!). I thought about a foam spar, but I think the receptacle tube could compress a foam spar given enough load...
I think a foam spar capped with carbon could work really well for a permanent spar in a non removable wing option.
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Old Nov 01, 2014, 06:10 PM
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Option 3 appears to be the most robust. Those carbon caps can be very thin .5mm.

Ken
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Old Nov 01, 2014, 07:01 PM
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Thanks Ken!
I came to the same conclusion as I was typing! I did not realize carbon strips were available in 0.5mm thickness - I will definitely look into that.
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Old Nov 01, 2014, 07:14 PM
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Another option is a foam spar, capped top and bottom with either pallet strapping or fiber glass mesh tape (used in drywall applications}. If you need a tube, put a plywood (2mm) rib at each end of the tube. I have also used a foam spar with 1x3mm flat CF top and bottom.

Marty
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Old Nov 01, 2014, 09:08 PM
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I beams (spars) have compression loads on top and tension loadings on bottom. Purpose of the separtion/web is to keep the top and bottom strips from deflecting.
Foam would be a V poor choice, being easily compressible
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Old Nov 01, 2014, 10:08 PM
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Quote:
Originally Posted by Bare View Post
I beams (spars) have compression loads on top and tension loadings on bottom. Purpose of the separtion/web is to keep the top and bottom strips from deflecting.
Foam would be a V poor choice, being easily compressible
with the 1x3 CF placed in the Vertical direction, 3mm high, 1mm wide, it has proven to be a very strong wing, even on my 3D,s.

That being said, you are perfectly correct about compression/Tension, and Web.

Everything is a trade off, strength vs weight.

Marty
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Old Nov 02, 2014, 12:31 AM
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Thanks for the suggestions guys! If this plane was a floater, I think carbon capped foam would work great. This wing needs to be able to take some abuse so I think the web material needs to be a bit sturdier. I'll definitely keep that one in my back pocket for a future build!

I threw together a quick "option 3" spar and did some testing. I didn't have any carbon flats so I cut strips of .8mm light ply as a substitute. I glued everything together with white gorilla glue and got to testing.

I built two spars and joined them with a carbon rod. I supported each spar at about the 60% mark and hung a weight from the center. The full wingspan was about 32". The wing was able to hold 5lbs no problem. I was going to test 10lbs and the spar flipped on the side and snapped. . The plane for this spar would be about 20-21oz AUW with the largest battery.

Maybe I can hunt down some carbon flats in what is left of the LHS' near me!!! I think the carbon flats will be the key to success on this one..
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Old Nov 02, 2014, 03:31 AM
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Regardless of the core material the carbon flats need to be in the vertical plane, This allows the spar to flex fore and aft, but very rigid up and down. So in effect, you are laminating the sides of the spar, not the top and bottom. Of course, you could laminate all 4 sides, but that is getting quite expensive for a spar.

Marty
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Old Nov 02, 2014, 07:40 AM
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Laminating both sides would be expensive and probably overkill

But what about a box, just in the area that the tube resides. Short pieces on top, longer pieces on sides. The tube would be glued to the strips,on all four walls of the box. The strips could also be glued to each other.

Also: What will be between the two vertically oriented strips within the wing? They will be separated by the diameter of the tube. I don't think you need anything strong in that area, certainly you don't need to add any weight. Some very light foam glued to the strips would provide some stability for the strips, as well as filling the void.

Regards
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Old Nov 02, 2014, 10:09 PM
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Finally got a chance to do some testing with carbon caps. I traveled to the nearest hobby shop and was able to pick up some .5mm thick by 3mm wide carbon flats. Not ideal (I wanted at least 5mm wide), but it's all they had.

I built my spars as pictured in 'Option 3' of the opening post. I did not extend the carbon caps to the wingtips - I felt it was overkill. I ended up using 1/4" basswood as the spar material. The caps were glued on with Crazy-Glue (not foam safe); and the tube was glued in with 5min epoxy. While testing 10lbs - after a partial failure due to twisting - the caps separated from one of the spars. The spar had broken but the caps remained in tact. I fixed the spar with crazy glue and dental floss. The dental floss really helped to strengthen the tube/spar area. I will definitely incorporate the floss (or something similar) in the final version. I tested the spars to failure. All failures occurred due to twist as it was very hard to ensure that the load was applied in only the vertical direction.

All in all, I was very pleased with the results. I think the spar is more than strong enough for my applications. It is not flexible in the x-y plane but I don't plan on cartwheeling the plane (famous last words). The plane that would have these spars installed would be around 20-21oz max. The spar/tube combo easily held 10lbs. It held 15lbs too but then the spar flipped on its side and snapped.

Pictures below! Thanks again for everyone's help and advice. Mbpetri - I am also interested in testing the spar design you mentioned. I don't think it would work well with the geometry of this particular wing, but I have other designs in which I may be able to utilize it.

…so the final version (for now) will have carbon caps that are as wide as the spar itself. In this case (5-6mm). Cap thickness will be .5mm. Spar material will be basswood. The entire spar will be wrapped in dental floss to secure caps with extra reinforcement in the tube-spar transition area. I guess "simple" is out the window, but it seems I now have the strength I desired.
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Old Nov 03, 2014, 09:03 PM
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...so the spar saga continues.

... I've been thinking and thinking and I realized that my option 3 spar is flawed. No matter how strong the caps are, the cut spar with the inset tube is weaker than a solid spar. Every failure of the spar occurred at the tip of the inset tube - the weakest point. Eliminating this weak spot would create an even stronger spar.

SO - we must revisit spar option 1! I reconstructed option one with the carbon caps along the length of the spar. This makes the spar itself super strong and there are not any obvious weak points. I think the inset bar was kind of acting like a pry bar in the middle of the wood spar. With the wood spar left in tact and the tube glued to the face of the spar, we are dealing with twisting shear forces between the spar and the tube. I hope this makes sense. In short - i retested the modified option one - and it outperformed option 3! In fact, the carbon joiner tube broke before the actual spar did!

...so I finally think I have a good candidate for a rigid and strong wing spar. When I have used carbon arrow shafts in the past, I get strong wings but they tend to be "bendy" and lack in rigidity.
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Old Nov 03, 2014, 10:21 PM
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I agreee with your observations.

Some questions:

1 - is this a straight wing or has dihedral?
2 - Are there any ribs? Especially where the 2 wings join together?
3 - How do you plan on locking the 2 wing halves together?
4 - what about the fuse, full bodied or profile?

Marty
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Old Nov 04, 2014, 09:00 AM
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Hello Marty,

To answer your questions:
1 - This is for a straight wing 3D aerobatic model - a Slick - no dihedral. However, I do intend to use this research in other builds (like my stalled A26 project) that do have dihedral. The Slick is completed and has been proven to be airworthy. I am just tweaking the design as I intend to use it as a 3D trainer. While we are on the subject of dihedral - the modified option 1 spar makes adding dihedral easy. It would only require rotation of the tube to the proper angle.

2- I typically build my wings with only two full ribs - a tip rib and a root rib. On longer wings, I may have an extra half rib or two in front of the spar. The spar is really the only internal structure. I also tend to use 5-6mm foam (DTF or MPF) to skin my wings - similar to Armin style wing. I do not like overly fat airfoils so a lot of times my spars are thin.

3 - In this particular application, I have a tab that extends from the root rib. Once the wing is inserted, the tab is screwed into a plate in the fuse.

4 - Full Fuse!
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Last edited by SNice; Nov 04, 2014 at 09:21 AM.
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