Sailplane Wing Design Round II - RC Groups
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May 29, 2012, 12:44 AM
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Nodd's Avatar
Build Log

Sailplane Wing Design Round II

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...

Nodd RC - 013 - Wing Failure (4 min 18 sec)

I'll post details about my new design shortly & will give you folks an opportunity to see if I'm headed in the right direction. Thanks.
Last edited by Nodd; Nov 12, 2013 at 12:07 AM.
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May 29, 2012, 01:31 PM
Registered User
That's a shame, Nodd. Looking at the pictures I would think that a good first step would be rotating your main spars 90 degrees or going to something like 5/16 to 3/8 square. The way that your spars are oriented gives you no more strength than a 1/8 inch spar.
May 29, 2012, 01:43 PM
volare est vivere
ray foley's Avatar
hey nodd

Vertical grain or 45 degree crossed shear webs epoxied between the spar caps( spruce or cf sticks) and kevlar or CF tow wrap will dramatically enhance your otherwise delightful design. It doesn't add a lot of weight and the strength increase is truly dramatic.

Remember, that is what prototypes are for...learning and improvement. From your video it looks like a new center gull wing panel is all you need. That should be a fairly quick rebuild.

Don't give up! You are so close to complete success.

Total encouragement -rjf
Last edited by ray foley; May 29, 2012 at 01:50 PM.
May 29, 2012, 01:46 PM
Registered User
What ray said. I would still go for more vertical thickness of the spars but sheer webs are a must.
May 29, 2012, 02:47 PM
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Nodd's Avatar
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)

Compression Load at Spar or Joiner Bend (look at pages 20 - 21)

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

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.
Last edited by Nodd; May 29, 2012 at 02:56 PM.
May 29, 2012, 08:16 PM
volare est vivere
ray foley's Avatar
hi there from Toledo

Check out the builds of the Olympic III, the Houston Hawk, and the Bubble Dancer for real good designs and spar assembly techniques right here on RCG. They are a remarkable resource and at your finger tips.

ciao -rjf
May 29, 2012, 09:13 PM
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Nodd's Avatar
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.
May 29, 2012, 09:46 PM
Registered User
I'm still a little doubtful about the total vertical thickness of the spars. I would think you would be better served by using, let's say, 4X6 spars set vertically. Another option would be to use the top spar as is but form an inverted "L" with a 4X6 or even a 4X4 or 2X6 bottom spar.
May 29, 2012, 10:15 PM
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Nodd's Avatar
I'm not sure where you're getting this vertical Vs horizontal thing from but built-up wooden wings generally have the spar caps installed horizontally. A spar of this type gets its strength not from the vertical thickness of its spar caps but from the compressive force acting on the top spar & the tensile force trying to stretch the bottom spar. The shear webbing between them keeps the two from sliding past each other & thus keeps the wing rigid.

If this was a foam wing & you were adding just one stiffening spar then yes, mounting it vertically might be the way to go. This system is different from that though.
Last edited by Nodd; May 29, 2012 at 10:42 PM.
May 29, 2012, 10:40 PM
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Nodd's Avatar
Originally Posted by ray foley
hi there from Toledo

Check out the builds of the Olympic III, the Houston Hawk, and the Bubble Dancer for real good designs and spar assembly techniques right here on RCG. They are a remarkable resource and at your finger tips.

ciao -rjf
Much obliged Ray. Yeah my current design is heavily influenced by the BubbleDancer. I'll check out the other two, thanks.
May 29, 2012, 11:21 PM
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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...

May 30, 2012, 08:42 PM
volare est vivere
ray foley's Avatar
hi there from Toledo

Dr Drela designed his spars for serious gorilla full pedal ping launches from a 12V winch. For epower this fully wrapped spar may be overkill but still economical and seriously strong. For the gull wing panel, go for it. The tip panels are probably fine but shear webs "couldn't hoit".

again, total encouragement -rjf
Last edited by ray foley; May 31, 2012 at 09:04 AM.
May 30, 2012, 09:55 PM
Red Merle ALES VI
Curtis Suter's Avatar
The first thread is good and although different building techniques it is well worth the read.
Some very great minds sharing with us mere mortals.

Post #6 may pertain to this somewhat.

PS. I like building wings with foam so I am somewhat more educated in layups and this is one of my favorite testing threads too; by Phil Barnes.

These will take some time to fully understand, well for me anyway.

May 31, 2012, 12:09 AM
Registered User
My point, Nodd, is that the weakest and most flexible axis is also the axis of greatest stress. Even with CF 2mm just strikes me as being a possible concern. I would worry that the flex of the spars might exceed the strength and flexibility of the supporting structure.

Many many years ago I built a glider with 1/8X1/2 spruce spars and, while it didn't break, it sure did flap; especially on the hi-start! I built another wing with 1/4X3/8 spars and it was much, much, more rigid.
May 31, 2012, 03:12 AM
vespa's Avatar
Great looking plane and excellent CAD and Sim work! Damn!

Your spruce spars may have been strong enough but your shear web was very poor. With only one web glued to the back of the caps the C-channel shaped spar is asymmetrically loaded so the caps will twist and then easily buckle. A somewhat better design is front/rear webs to form a complete box beam, but even better (and lighter) is a center web to form an I-beam. Kevlar wrapping is not usually very helpful in this design other than around the joiner sections.

Of course carbon caps can be far stronger and lighter. If you go this route you can use much thinner material and it should be lighter. You don't need a solid balsa core though, just weakly bond thin carbon caps to a lightweight blue/pink styrofoam core and wrap a single thin layer of fiberglass or carbon around the whole thing with a +/-45 fiber orientation. That Bubble Dancer spar can handle the tightest loops you've ever seen at 100mph, you don't need anything even close to that. The problem with a composite spar like this is that you can't put ribs thru it.

A good compromise may be to use the same spruce spar caps, with centered webbing, no wrapping, and just a paper-thin (.005") strip of carbon bonded to the outer faces of the spruce caps. Spruce will generally fail in tension and carbon generally in compression/buckling, so a hybrid spar can be very effective.

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