Sailplane Wing Design Round II

[Nodd]

Hey folks,

This is a continuation of my Scratch Designed/Built Sailplane build log. My design flew beautifully but...



...unfortunately my wing could use a rethink. Basically it folded in half mid-flight on the fourth or fifth flight...



The wing was of built-up balsa & spruce construction with carbon fiber reinforcing here & there...




Thankfully the damage was limited to the wing, not sure how the fuselage survived...



So my plan is to design & build a new stronger wing. This thread will document that process.

So to get started here's a fun YouTube video featuring the old wing...

YouTube Video - Wing Failure


I'll post details about my new design shortly.

[Larry3215]

Id suggest you take a look at how the center sections on other sailplanes are designed and follow a more typical practice as far as the carbon reinforcements go.

Placing the carbon tubes outside the spar is your basic problem. They dont do you any good out there and in fact will contribute to the failure. When the wing flexes under load, they will cause the ribs to split. The ribs are unable to carry or transfer that type of load. Once they split, you no longer have the top and bottom sheeting acting together and the failure will cascade from there.

I think your basic design - other than the carbon tube placements - was sound. You might consider a better shear web technique or perhaps running a thin carbon strip on the top spar cap and wrapping the spar in the center section - similar to the Bubble Dancer spar and many many other bullet proof designs.

Or just up-size the spar caps slightly. The top one is actually the only one you probably have to worry about. The caps typically fail in compression.

Good luck!

[Larry3215]

I just noticed something else Id suggest you change.

It looks like you only applied the shear webbing on one side of the spar? Also, it looks like your sheeting butts into the side of the spar caps rather than running over the top?

Id web both sides of the spar in the center section.

Id also run the sheeting over the top of the top spar cap and the the bottom sheeting over the bottom of the bottom spar cap. In other words, recess the spar caps fully into the ribs and let the sheeting run over the tops completely. The structure will be a lot stronger that way.

[JetPlaneFlyer]

+1 on the shear web.. This is an absolute 'must have' feature to prevent the top spar from buckling when placed in compression. Usual practice on sailplanes would be to have ply webbing in the inner part of the wing and balsa (with grain vertical) on the outer.

Good luck with the wing re-build.

Steve

[Nodd]

Thanks! I totally agree with everything you suggested. I've done a bunch more reading up on wing design, learning all sorts of good stuff. I don't agree this occurred however...

Originally Posted by Larry3215

Placing the carbon tubes outside the spar is your basic problem. They dont do you any good out there and in fact will contribute to the failure. When the wing flexes under load, they will cause the ribs to split. The ribs are unable to carry or transfer that type of load. Once they split, you no longer have the top and bottom sheeting acting together and the failure will cascade from there.

Yeah my wing rods were a little unorthodox but those weren't the problem. Actually I couldn't brake that section of the wing even when I tried. It was the weaker spruce only section that failed.

Other than that I'm liking the suggestions. Much appreciated, keep em coming :-)

[HO-229]

Nodd,
Sorry about the wing… I am sure you will quickly sort it out

Cheers,
Dave

[soarrich]

Nodd

Sorry to see the wing broke.

The load test with the knee was invalid, it will break at the knee 99% of the time. Better to hold the center section, then add weight at the tip.

It looks like you wing joiners end, carbon spar end, and your servo cut out all are at the same place, probably not the best design practice.

Your right about CF being the way to go, Mark Drela was once asked how big the spurce spars would have to be to make a BubbleDancer's wing with out the CF, he did the math and came back with "Carve the wing out of solid spruce."

I'm looking forward to your redesign.

Great looking plane!

[Nodd]

Yeah the knee test was just to see how hard it would be to break the wing in the area where it failed. Not very scientific but it gave me a good feel for how strong the spruce/sheer web only section actually was.

I didn't like my transition from super strong joinery to whimpiness either. The new design hopefully will avoid that by "tapering" the strength gradually.

That's pretty funny about the solid spruce BubbleDancer wing. I'm fast becoming a fan of CF.

Anyway let me get some design ideas posted so you folks can let me know if I'm on the right track. Thanks a bunch.

[Nodd]

Just so we're all on the same page, here's a few amazing articles about shear loads, spars & wing design...

Shear Loads (look at pages 20 - 21)
http://www.rcsoaringdigest.com/pdfs/...SD-2005-04.pdf
http://www.rcsoaringdigest.com/pdfs/...SD-2005-05.pdf

Compression Load at Spar or Joiner Bend (look at pages 20 - 21)
http://www.rcsoaringdigest.com/pdfs/...SD-2005-06.pdf

I found those in this very informative thread, well worth a read too...

http://www.rcgroups.com/forums/showt...00#post6164407

That's just some of what I've been reading recently. Based on my new found understanding of wing spar science I'll have a few nifty design concepts posted shortly.

[JetPlaneFlyer]

Nodd,

Just watched the video and i see you did have sheer webs. I think the problem may have been that the web was just glued onto the rear face of the spar rather than fitted between the spruce spar caps.

If you go with carbon spar caps the usual method is to have solid balsa wood infill between the caps with the balsa as thick as the spar cap is wide and with it's grain vertical. The balsa infil is tricky to make without a jig. Then the entire assembly is bound with kevlar thread.

Check this build page: http://aviatorstudio.net/bubbledancer/bd_center.html

[Nodd]

Thanks for the BubbleDancer link, that's a pretty good article. I'm not sure I have enough cans of tomato though :-P

I've read a lot about the pros & cons of attaching traditional shear webbing either between or to the rear of the spar caps. Here's a brief summery of what I learned...

Shear Web Between Spar Caps
Pros: Very resistant to compressive forces.
Cons: More challenging to install precisely Vs rear mounted webbing & its important that it be precise if its to do its job, gaps are bad. Less resistant to pealing forces than side mounted webs.

Shear Web on Side of Spar Caps
Pros: Easy to install. Has more contact area with spars for the glue to do its job. More resistant to pealing forces.
Cons: Not quite as resistant to compressive forces.

Based on that, for regular shear webbing I'm leaning more towards mounting it to the rear or even possibly to both the front & rear of the spar caps forming a box. Of course if I take the BubbleDancer approach with full width webbing then that'd be mounted between. I'm in the process of drawing up a few designs so we can see what's what. Thanks again for the input.

[JetPlaneFlyer]

All good points of the pros and cons on in or out type sheer web. However a straight wing under bending should I think never see any forces that try to pull the spar away from the web.. all forces are (in theory) compressive.

The reason for this as I see it is that a wing (especially a wood wing) will always bend a bit when stressed, so the spars form a arc. The bottom spar cap is in tensions so tries to straighten out and so pushes up against the web. The top cap is in compression so tries to buckle downward, also pushing against the web... The cap to web joint is therefore always in compression.

[JetPlaneFlyer]

I was shooting from the hip on the above statement based a mixture of intuition and distant memories or collage courses. But it looks like that is indeed the way it works which is why webs inbetween the spar caps are better:

Assuming an upward flex, the upper spar cap is compressed, while the lower cap is stretched. To compensate, both caps try to move towards the centerline of the spar. This is inhibited by the spar’s shear web, resulting in a primarily compressing force within the shear web

http://www.dg-flugzeugbau.de/holm-aufbau-e.html

[Nodd]

Yeah I'm not a 100% clear on peeling either. Sounds like it occurs when the rectangular shaped webbing experiences shearing as its forced into a parallelogram during wing flex (shear). I guess that's why its called shear webbing *grin*. You're right though, its also under compression too.

[quorneng]

JPF
If the web is only taking compressive forces why is it called a shear web?

The answer of course is the web takes a shear load as well.
When the wing bends the top and bottom surfaces try to move relative to each other putting the web in..er....shear.
In a built up spar the web to spar glue is the only thing resisting this force so it is important to design sufficient glue area.

A carbon tube spar does get over this problem as it carries all the stresses within its structure but a tube is not the most efficient shape to resist the loads in a wing spar.

[JetPlaneFlyer]

yep, obviously the shear web usually takes shear loads as well.. i was refering to pure bending in the spar, in which case providing the spar caps are tied at the root and tip then the shear web only sees compression. In the real world you have shear forces as well as pure bending on the wing, and that's why the shear web is called the shear web.

The important point i was trying to make is that you should not see much in the way of forces trying to pull the spar caps away from the web, because the joint is held in compression.

[Nodd]

The Old Wing


The New Wing


Okay so the obvious difference here is the 12mm x 2mm CF spar caps, no more wimpy spruce. After looking at a load of shear web options I've chosen to go with full spar width, vertical gain, balsa shear webbing. The CF & webbing will be epoxied together & then wrapped with Kevlar ribbon (not shown). That should produce an exceedingly strong spar.

I've eliminated the sheeting aft of the spar as I don't think its necessary, will save some weight there. Will probably fully sheet the first few bays around the center though. Speaking of the sheeting, it now overlaps the spar caps instead of butting up to them, should provide a much larger bonding surface. The holes in the ribs are for the servo wires. Lastly I doubled up the thickness of the TE material that the flaps/ailerons hinge against. The old 1/8" TE scolloped after I shrank the covering.

I'm looking into wing joinery options, have a couple of ideas in mind. Anyway that's the plan for now. Questions or suggestions? Please fire away.

[Nodd]

Here's a picture I stole of a BubbleDancer's spar wrapped in Kevlar, using full width shear webbing. Hopefully that's a little clearer than my diagram...

[JetPlaneFlyer]

Nodd,

2mm x 12mm is massively over the top for the spars. The bubble dancer is 3m span and the wing is stressed to take 150lb load when on tow.. And it only uses a 12mmx1.5mm spar and that's only for the centre section, the outer panels go down to 6mmx0.375mm thick

For your model that doesn't require to take the huge loads of being towed then 12mm x 1mm on the centre and 6mm x 1mm deep in the outer panels would be still well in excess if what's really needed.

Steve

[soarrich]

Originally Posted by JetPlaneFlyer

Nodd,

2mm x 12mm is massively over the top for the spars.

Steve

Once bitten, twice shy! It is way over kill though.

[Nodd]

Well I'm not sure I'd class an extra 0.5mm as "massively over the top" but for obvious reasons I'm inclined to build this extra strong. The other reason I went with 12mm x 2mm is that's what my CF supplier has in stock. Their other 12mm sized strips are 3mm. I could look elsewhere but I've use RCfoam.com before, like their products & that seemed like as good a place as any to start.

As for the outer sections, Yes I was planing to either taper the spar or drop down to 10mm or 7.5mm.

Okay here's a question... What is the downside to using 12mm x 2mm CF for the center section?

[JetPlaneFlyer]

Originally Posted by Nodd

Okay here's a question... What is the downside to using 12mm x 2mm CF for the center section?

Not much i guess.. a few grams of weight. 12x1.5mm is still way over the top to be honest , you could easily go lighter.. But got the sake of a few grams then I might well take the 'way over the top' solution too

Looks like a bullet proof design whatever anyway.

Steve

[John Thanner]

Where do you fly in Aderdeen?

[JetPlaneFlyer]

A sports field in the Bridge of Don area... do you know Aberdeen?

http://maps.google.co.uk/?ll=57.1810...13937&t=h&z=17

Steve

[rcers]

Originally Posted by Larry3215

Placing the carbon tubes outside the spar is your basic problem. They dont do you any good out there and in fact will contribute to the failure. When the wing flexes under load, they will cause the ribs to split. The ribs are unable to carry or transfer that type of load. Once they split, you no longer have the top and bottom sheeting acting together and the failure will cascade from there.

Good luck!

Don't discount this statement. You really need the main tube inside the spar caps. This will be your next failure if you don't address it!

You must transfer the load directly to the spar. Other designs stress this, in fact I have never seen a main tube outside the spar.

Mike