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Old Jun 14, 2014, 07:59 AM
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Suggestions: Repair Delaminating Balsa Skins

Hey guys! Found and picked up a S/RC Bootlegger last week. A bit rougher than anticipated; however, the engine turned out to be an OPS Red Head (the Ursus I believe) and it cleaned up very nicely.

Pulled all of the gear out and began cleaning it up. I did notice the stab had some delamination at the trailing edge. Further cleaning revealed quite bit more than initially seen, and it extends about a third of the way forward of the T.E. The same condition exist on a much smaller section of the wing trailing edge. Pretty sure the skins were bonded with Sorgum (sic).

Need some advice or suggestions on how to approach fixing this. Last resort is to cut the stab out and replace it entirely. I need to order some Saturn cores from Don so I could get the Boot cores too. Overall, the fuse with the pipe floor is in great condition! The elevators, rudder, and ailerons are all hinged with pinned hinges and withstood a darned good pull test!

Plan is to put a R.E Rossi in her, refit with mechanical retracts ( or redo the Rhom Airs), and use JR 821s (it had JR NES 2001s, those gotta be old!) I work slower than a one-armed paper hangar so plenty of time to work on it. Plan to have it done for next summer as it will have been 35 years since I flew my Boot at the "80 Nats". Appreciate any and all input guys. Thanks.

Mark
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Old Jun 14, 2014, 08:42 AM
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What about either contact cement? Hard to tell without a photo. I have a kit at home (T2A-40) with the trailing edges needed for re gluing too on the wing and stabilizer sections. This had a plywood veneer over the foam. I am thinking of water based contact cement.

Jim
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Old Jun 14, 2014, 10:34 AM
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The easiest way to repair would be to drill some 1/6 holes in the sheeting in a 1 inch grid then working from the outside moving inward inject some thin foam safe CA in the holes as you apply a little pressure. The way I would do it is to fabricate a vacuum bag on the surface having an injection hole and a vent hole. The bag with breather goes over the affected area and the vent hole. When vacuum is pulled it will bring the sheeting back in contact with the foam then as you inject epoxy resin into the injection hole the vacuum will draw it into the delam. You are finished injecting resin when it starts coming through the vent hole. Now all that being said, if the original adhesive is giving up in spots is it safe to trust on the remainder of the surfaces?
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Old Jun 14, 2014, 11:41 AM
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Appreciate the comments. I'll post some pics as soon as I get a chance JP. I think though that the delamination is more the common type, nothing unusual in the separation of the skin and foam. Exf, the grid idea is good, but the delam starts at the trailing edge spar so injecting whatever adhesive is not a problem. Yes, I am also concerned that the balsa may still separate elsewhere. Although I did reach a point under the skin where the adhesion felt more durable. I'll check back in after our boat ride.
Mark
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Old Jun 15, 2014, 10:35 AM
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Mark,

I had a similar situation with a WK Tiporare that I built many years ago. I used Southern RC Sorgum to attach the skins to the wings as it was the fastest way to skin a wing and needing a plane a plane right away.... It fit the bill. After roughly 3 months of hard core flying I started to notice stress cracks around my landing gear blocks for the retracts ( glass and painted finish ). The sheeting had a "floating" feeling to it in many spots. After further investigation..... the only thing holding the skins to the foam was the glass work for the finish. Needless to say it was the last time I used any kind of contact cement for sheeting over the tried and true method of epoxy.

After trying many things to repair it such as light heat and hold, syringe with epoxy, etc.... I finally gave up. After a few flights the repair gave out. The problem you are going to have is the contact cement leaves a film that prevents new adhesives to penetrate the foam and/ or the skin for a good bond. The only thing I found to re-bond the cement style glues is the cement used to due the original skinning as it bonds to itself. Unfortunately you are no better off then you were before. After I gave up on chasing the separation of the skins from the foam.... I literally was able to de-skin the wings with very little effort.

Mark, I hate to say it..... but I think you are going to find the same thing I did with your delaminating problem. See if you can fix the stab but I would build a new wing. The time I spent doing the repairs on my above problem... I could of built a new wing and then some.... which I ended up doing anyways.

Mark K.
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Old Jun 15, 2014, 11:31 AM
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Looking forward to the rework Mark.

I like the foam safe CA idea. Thinned epoxy under vacuum might work too although be careful of the amount of vacuum because any tunnels for wires will cave in (doubt there are any because the servos are probably in the middle).

Tim
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Old Jun 15, 2014, 12:31 PM
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I've been thinking about this and I suspect Mark (Tiporare) is right. The contact cement (reason for which its called that) is a bond that takes place between the two layers of the same adhesive. In time, contact cement is well known for "surfacing" off non porous materials and I have the feeling that the side that lets go in this case is the cement/foam layer. The cement/balsa bond is probably still good but once the cement layer on the foam has released, good luck getting anything to penetrate the "two" fused layers of cement into the balsa.

Maybe the foam safe CA will work although I've never found that type of CA to be particularly effective especially if its not kicked. Epoxy (maybe PU too but I wouldn't bet on it) might just actually hold on to the contact cement/balsa laminate and ahdere the skins back down on to the foam. Vacuum and/or heat to draw the thin epoxy in might also be needed.

Another option might be to slice the LE of the surfaces and see if the entire paint/glass/skin composite comes off cleanly and then clean up the underside of the skins and the foam and re-attach with epoxy. Like Mark said though, it might just be faster to build a new wing. The finish is another matter though.

In any case, I look forward to hearing about the outcome Mark!

David
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Old Jun 15, 2014, 04:31 PM
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I suspected that might be the case with it Mark (Tipo). I was hoping to save it but, as you noted, I can separate the skin if I push a thin stick further towards the L.E. I'm certain the skins were bonded with sorghum too. There's also a slight separation on the main wing, but much more confined so I'm hoping I can repair that.

David, not a bad idea separating the skin from the L.E. and reattaching with epoxy, might just try that. Since I need to order some cores for a Saturn fuse I'm going to order the Boot cores too. I won't spend too much time on the stabs.

Any thoughts on the best method for cutting out the stabs? Here's a few pics.
Mark

So anybody need an OPS Red Head with pipe and header!
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Old Jun 15, 2014, 08:43 PM
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Mark,

This is how I did my stab on my first Bootlegger after a accident that required replacing the stab.

First thing I would do is determine if the stab incidence is correct while the plane is still together. Either way if it does or does not check out.... removing the stab will be the same. One of the strong points of the Bootlegger is the built up vertical fin. However.... since yours has the the flat stab mod that moves the horizontal stabilizer up in the fuse..... Chances are the vertical fin is tied into it.

I would cut the stabs as close to the fuse as you could. IF there is a nice fillet there, just cut it on the outside of it. (Saves a little work later) Next step would be remove all the foam from the remainder of the stab in the fuse. I would leave as much of the top sheeting for the stab in the fuse do to the above about the vert. fin. Just get it cleaned enough to make a good bond to the new stab. Sand the top fillet at a angle that you can blend in later to the the new stab. Do not remove this part of the fillet as you are going to rely on it for for added strength
as it is already bonded to the old sheeting and fuse already.

The bottom part, leading edge and trailing edge, bottom fillet of the stab is a different story. Use a Dremel with various bits to clean out the reminder of the stab. Remove as much of the bottom fillet as you can safely with out getting into the fuse. If you half to make any changes to the incidence or open it up a little to fit the new stab in. Do it in these areas. Dropping the stab the thickness of the sheeting left in the fuse from the top sheeting will not hurt the plane as far as flying goes.

Next step would be remove the rudder and cut out roughly 1" to 1 1/2" of the vertical post from the bottom of the fuse. This will be your access point to the bottom part of the stab for gluing in the new stab. I would not cut away anymore then the above mentioned as the tail post should be part of the built up vert. fin.

As for cleaning up the inside of the fuse for the new stab. I would take a piece of thin flat stock aluminum or steel and put a 90 degree bend in it so you have roughly a 3/4" tongue hanging down. (Make a couple of these at different widths). Glue some sand paper on the front side of the bend. Now you have a sanding block that you can insert through the stab opening and remove/rough up the opposite side of the fuse to ready it for the new stab. A Dremel tool with the right bit works too to remove the bulk of the epoxy used to glue in the stab, but I like to finish things by hand.

Stab install:
If your stab needs a adjustment to get it lined up correctly as far as incidence goes.... Now is the time to do so with the bottom part of the fuse of the stab opening. In my case the stab was right on. All I did was make the opening wide enough to work in some epoxy on the top part of the stab to glue it to the old sheeting. Cut some small balsa wedges and use them to apply pressure between the bottom part of the stab and fuse side. Measure and make adjustments as need before epoxy dries.

The bottom part was done through the tail post opening via a syringe with a small diameter straw to apply the epoxy/micro balloon mix. Followed by an artists brush with alcohol to smooth out. (hint: Cut the brush handle down and tape it to a metal coat hanger so you can bend the wire to work in the epoxy mix into the areas that otherwise can not be reached.) Smooth out the epoxy mixture that comes out the bottom of the stab on the outside which will later become the fillet. In my case, the top fillet only needed to be sanded (tape the stab so you do not sand into the sheeting) and quick epoxy/micro mix to re-blend the fillet in was needed.

Replace the tail post that you cut out to gain access to the stab via angle sanding the existing post and new replacement piece ( A small tongue and grove would work better but as usual one thinks of this after the work is done). Epoxy in the replacement piece and clamp it til the epoxy cures. Then it is off to the finishing stages of the rebuild.

This repair lasted 2 years until I dumb thumbed into the ground at full bore. Ironically the tail survived.

Hope this gives you some incite on how to do a stab repair on the Boot.

MK
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Old Jun 16, 2014, 07:11 AM
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Excellent directions Mark. I think all said this what I'll do. It does have a very well done fillet around the stab so I'll preserve that on the top. The main wing appears to have separated (about 3" in length span wise) along the T.E. only so I'm going to pin it with tooth pics when I rebond it. Still going to order a new set of wing and stab cores. Really appreciate the detailed "how to"!
Mark
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