Repair of balsa thermal wing

[goofeyfoot]

Several years ago my young nephew and I built a thermal glider from balsa.

Unfortunately, a friend of mine landed the thing on the open door of a truck and bashed the leading edge in.

The wing is made of balsa leading edge, balsa ribs and some 1/16 balsa sheathing.

My nephew is not little any more and went into the air force just this month.

I thought I would try to repair the damaged part of the wing which is about 6 inches wide and located in the center of the wing.

I still have the original plans for all the parts and such.

Is there a recognized way to do this procedure? I wanted to do a good job and send the plane to my nephew. Don't want to do a sloppy job. I think my nephew will be quite surprised.

Michael

[Esprit2]

How about posting a photo of the damage. Strip off the covering and show us the damage to the framework.

[Guz]

Quick suggestion.

Cut the leading edge at an angle in front of a good rib from looking down from the top. i.e \ / (45 degree or more). Provides more surface area for gluing.

[goofeyfoot]

Guys:

Here are some pictures to give you an idea of what I have to fix.

Michael

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[raed5]

Start off by gluing together the split skins, then cut the damaged skin out leaving straight edges to make it easy to splice in new pieces. Make new sections of ribs and glue to the side of what is left of the broken ribs. Cut the edges of what is left of the leading edge at an angle (45 degrees is fine), glue on and shape a new section. Cut new skins to shape and glue on. Sand everything smooth and recover. It is a lot easier than it reads, I would use medium ca for everything.
Good luck.
Raed
Orlando Buzzards

[goofeyfoot]

Start off by gluing together the split skins, then cut the damaged skin out leaving straight edges to make it easy to splice in new pieces. Make new sections of ribs and glue to the side of what is left of the broken ribs. Cut the edges of what is left of the leading edge at an angle (45 degrees is fine), glue on and shape a new section. Cut new skins to shape and glue on. Sand everything smooth and recover. It is a lot easier than it reads, I would use medium ca for everything.
Good luck.
Raed
Orlando Buzzards

Orlando:

Thanks for the advice. That is how I will proceed. A few other questions.

1. When you cut out the sheeting, do you just cut that square-edged or do you bevel those edges too?

2. Also with regard to the sheeting, what are you gluing to? Do you make little gussets or tabs on either side or the repair so you have something to glue to?

2(a) Also with regard to the sheeting when you fit the new pieces in there must be some gaps because nothing is going to fit perfectly. Do you use some kind of filler or is there a way to fit the sheets together more precisely?

3.. Working from a paper plan, what is the most accurate way to refabricate the ribs? Hand cutting would seem to be pretty imprecise.

5. When you rebuild the leading edge would you reinforce the 45 degree joints or just rely on the glue, the ribs, and the sheeting to provide enough strength?

6. I am trying to make a nice flat building board to do the work on. I used to use acoustic ceiling tiles but I get the sense that those things can twist and then impart twist to the wing. What do you guys use for a building board?

Sorry to ask so many questions but I wanted to get the research done before going at it and making a mess.

Thanks again for your help.

Michael

[Wazmo]

1. When you cut out the sheeting, do you just cut that square-edged or do you bevel those edges too?

A beveled edge will be stronger, but it's harder to do with thin sheeting and not leave bumps.


2. Also with regard to the sheeting, what are you gluing to? Do you make little gussets or tabs on either side or the repair so you have something to glue to?

It'll help the strength if you can first glue a strip of balsa, maybe 1/2" wide to the underside of the existing sheeting so that half of it overhangs, so the other half will support the new sheeting. Then glue the new sheeting onto that exposed part of the strip. Make sure the grain direction of the strip is oriented the same as the sheeting, so it'll bend easily.


2(a) Also with regard to the sheeting when you fit the new pieces in there must be some gaps because nothing is going to fit perfectly. Do you use some kind of filler or is there a way to fit the sheets together more precisely?

Very often, if the balsa has just split and no pieces broke off, you can carefully squeeze the edges against each other and they'll pop back in line. If there are small broken holes, use a little lightweight filler. If the holes are larger and you can get to the back side, glue in a small piece of balsa sheet to the back side. Otherwise, try replacing the broken portion of sheeting.


3.. Working from a paper plan, what is the most accurate way to refabricate the ribs? Hand cutting would seem to be pretty imprecise.

It'll be good enough. That's how lots of people used to build. Just hand cut the pieces slightly oversize and carefully sand them down, frequently comparing the piece to the plan.


5. When you rebuild the leading edge would you reinforce the 45 degree joints or just rely on the glue, the ribs, and the sheeting to provide enough strength?

The 45 (scarf) joints will probably be just as strong as the wood. If you can make the angle shallower (longer joint - more gluing area), it will be stronger.


6. I am trying to make a nice flat building board to do the work on. I used to use acoustic ceiling tiles but I get the sense that those things can twist and then impart twist to the wing. What do you guys use for a building board?

A ceiling tile is just fine, so long as the table it sits on is flat. You can use a long straight-edge to check. If you can, use a light behind the table shining on the surface and lay the straight-edge across the table, in both directions and diagonally. Look for more light shining under the straight-edge at certain spots along it's length. If it's more than a little, you may need to tape down shims at the low spots. Or, find a flatter table. :)
Make sure you also check again for flatness after putting your building board on the table.

[raed5]

Wazmo answered all the questions for me, thanks. I would like to add a few things:
With regard to the sheeting, check the fit and sand as required to acheive no gaps.
If you have the plans then photo copy the section and glue to the wood then cut and sand to final shape.
My building board is 1/2 inch thick glass table top, I used the ceiling tile before and it worked great especially when pinning parts down.
Best of luck.
Raed