Fixing a Foam Wing

All,

I’ve got a Gieseke Nobler with a foam wing, sheeted with no internal spar. After only 100 flights, the inboard wing started to show signs of cracking in the sheeting right along the edge of the fuselage fillet, on the topside high point of the wing. Various expedients of using heated epoxy and then foam compatible CA held the wing together for another 30 flights or so. However, every time I flew in heavy winds (15-20) the glue repair would let go and the cracked area would enlarge, untill I landed from a practice flight at the KOI and discovered that the wing had cracked an inch from the leading edge on the bottom of the wing all the way around to the trailing edge on the top of the wing. You can put two fingers on the wingtip and wiggle the wing easily.
I had thought about grinding out the fillet with a dremel and then putting a strip of .5 ounce glass cloth and epoxy over the crack like a fillet of sorts, then putting in a new fillet and refinishing that area of the plane. The other option would be to somehow remove the whole fuselage over the left wing and try to glass over the whole cracked area of the wing from the wing centre section to an inch or so past the broken area, but this seems like an awfully involved process to fit a new section of fuselage back into place and still have a good wing/fuse joint in the area.
The first solution seems easiest, but I’m fishing for ideas on whether it would be strong enough. This is a very good flying plane and I don’t want to fold a wing if I can avoid it Hints from somebody who has had to do a fix like this will make the repair a lot easier, so I’m sending this out there to see what happens.

Thanks guys,

LAST EDITED ON Jan-22-04 AT 10:12 AM (CDT)
 
Steve,
If you want it to last, remove the fuselage! use ellipses of glass cloth or CF mat that will wrap top to bottom around the leading edge.. I would use two. First extending about 2 inches out from the fuse/wing joint, second one a little bit less. Make your cuts in the fuse at an angle so that realignment of the parts will be easier.
Did you use 3" fiberglass cloth to wrap the center section when it was first constructed?
Better yet, go ahead and get to trimmimg the Time Machine! You will listen one day, Grasshopper.

Bill <><

EDIT: We have had to a couple similar repairs. The method of construction used has a lot to do with how long it will last.

I, too, would probably say (Ok, I will say) to cut into the fuselage and wrap the joint, extending the wrap (well, now you have to) beyond the fuselage.

And for future reference, we, also, had a wing break that way. Our second Das Ugly Stunter prototype. And we had to repair it. And did it the same way. Yes, we used 3 ounce fiberglass just around the center joint, and all inside the fuselage. That puts a stress riser right at the fuselage and, yes, can (and did) lead to cracking.

I now suggest using three layers of lighter fiberglass to cover the center joint on a foam wing. All can be half or three quarter ounce fiberglass which keeps the weight down and helps with the finishing. First layer about 6 inches wide. Second layer about 4 1/2 inches wide. Third layer about 3 inches wide. Each layer extends beyond the edge of the fuselage, and each layer is cut with rounded or jagged edges so there is no sharp edge that can become a new shear point there. The problem you encountered was the shear point at the fuselage and the key is to bring it out farther.

I am putting the link to the picture I posted elsewhere of Matt's Rabe Mustang that he is a buildin'. This is a built up sheeted wing, not foam, but he has glassed the center section of this one anyway. This not being a foam wing, he didn't use the three layer process, but you can see the extensions of the fiberglass center section extending beyond the fuselage. And you can also see the rounding of one section and the jagged edges of another, all with the intent of spreading that shear point. Hope this helps.

[photo not recovered: 400d65ca31e3075d.jpg]

>Steve,
>If you want it to last, remove the fuselage! use ellipses
>of glass cloth or CF mat that will wrap top to bottom around
>the leading edge.. I would use two. First extending about
>2 inches out from the fuse/wing joint, second one a little
>bit less. Make your cuts in the fuse at an angle so that
>realignment of the parts will be easier.
>Did you use 3" fiberglass cloth to wrap the center section
>when it was first constructed?
>Better yet, go ahead and get to trimmimg the Time Machine!
>You will listen one day, Grasshopper.
>
>Bill <><
>
>EDIT: We have had to a couple similar
>repairs. The method of construction used has a lot to do
>with how long it will last.
>
>
>Disclaimer: I am neither a NATS Champion (yet), professional
>engine reworker, engine producer, column writer, Hobby Shop
>owner, *rockeet* scientist, nor otherwise. Just an old
>Advanced flier.

Steve I think this is what Bill is trying to discribe..
this is off Windy's 92 Cardinal plans

Bill's idea is good if you can manage a wing removal.

If the foam isnt oil soaked, you might think about a variant. The foaming urethane glue (Gorilla) will do a good job of restoring the foam core. To get a structural tensile resistant repair over the skin crack is harder. You might be able to create several 1/4" wide slots through the fillets and fuse sides flush with the top of the skin. Thread some kevlar thread or 12k carbon tow through the slots and out a couple of inches on the wing. Bond those down with CA or epoxy.

The approach also will depend upon how much effort you want to expend to make the repair invisible.

FWIW

I would start by cutting off the bottom block along the glue line. Then I would put glass cloth and epoxy from one side of the fuse to the other covering the wing completely. Then heat the epoxt to help penitrate. You may be able to do the same thing on the top of the plane as long as you watch out for the pushrod. Also I would by a can of K2R to remove any oil or fuel that may be in the inside of the plane after so many flights. Use the K2R before the glass cloth and epoxy. Other than that I really have no idea other than saving the wing and stab and building a new fuse around them.

Thanks guys for the advice from January. I tried the layered glass method spelled out my our host among others, but I think the wing suffered too much internal damage, as the repair failed on the first flight. It took awhile to clean up the oil soaked wood, but bonding to it was not the problem. The layered glass cracked clean through on the high point of the wing where the original crack started before the repair. I had even taken the expedient of removing the fillet on that side and leaving it off, thinking if nothing else I could move the stress riser inboard of the damaged area, but the wing still broke at the site of the original problem.

The lesson from this seems to be to make sure the wing has adequate strength from the very beginning. Thinner wings like a Nobler airfoil must have less resistance to bending under load than thick airfoils, and that forces it to bend repeatedly against the wing/fuse joint. I don't know if balsa/foam has fatigue issues or yield points like metal, but whatever the limit I certainly exceeded it on this model. Moving elipses of glass out to the 6" or so point, as mentioned by Leonard, is what I'll try on the next wing I build.

If I get really motivated, I'll build a new wing for the Nobler incorporating the improvements. The repair was just strong enough to let the plane come down safely, so I have something to rebuild instead of a pile of toothpicks.

I had a similar crack recently. I cut an access hole in the bottom of the center of the wing inside the pipe tunnel. I stuck a finger in the hole and scratched out the foam from the inside of the wing from the center to about an inch outboard of the fuselage side. Then I put carbon cloth onto the inside surface of the wood wing skin, the surface that had been against the foam. I haven't flown it yet.

As for building a new wing, you should probably have adequate shear webs as well as strong skins.

>I had a similar crack recently. I cut an access hole in the
>bottom of the center of the wing inside the pipe tunnel. I
>stuck a finger in the hole and scratched out the foam from
>the inside of the wing from the center to about an inch
>outboard of the fuselage side. Then I put carbon cloth onto
>the inside surface of the wood wing skin, the surface that
>had been against the foam. I haven't flown it yet.
>
>As for building a new wing, you should probably have
>adequate shear webs as well as strong skins.

Ron B.
F4Fguy

AMEN!
The problem is not tensile strength in the skin nor is it stress risers,it's buckling of the compressive member(the skin opposite the load)due to inadequate support.This puts excessive load on the tensile member(loaded side and,eventually,fatigue cracking of the skin from excessive load reversals. Howard's right,adequate shear webs will go a long way to solve the problem.This is one of the reasons I've gone to Spider Foam for cores,it has twice the compressive strength of blue foam,and better than four times 1# bead foam.
Even with Spider Foam,I still use either a stub spar or I glass the inside of the main spar core space to about 3" or 4"out*.
* this is easily done by making a razor saw cut flush with the foam spar on both sides,then sliding in a piece of 1 or 1-1/2oz glass into the saw cut.Finish by bleeding in thin GOLD cyo to saturation.Do NOT use kicker as the resulting heat will melt the foam.To ensure good contact wrap the core pieces in saran and reinsert while the cyo is fluid.