Workshop Tips — Wing Repairs for Built-up Airplanes

Jun 20, 2014 3 Comments by

Note: This Hangar 9 Piper Pawnee ARF was purposely damaged so we could demonstrate this repair technique.

LET’S FACE FACTS. If you fly RC airplanes, sooner or later, you’re going to damage your model. Knowing how to deal with common damage inflicted on our models saves you money and extends the life of your plane. So why buy a replacement wing when you can fix it yourself?

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Here’s our damaged wing. This is much like what would happen if you ran your plane into the safety fence at the flying field.

With so many ARFs available on the market today, few modelers are actually building up their models from kits. This has produced a whole generation of excellent flyers who simply don’t have the building skills needed to produce an RC model. Without this experience, trying to repair an ARF can be difficult. The techniques shown here are not limited to ARFs and can be used to fix any model you have.

MATERIALS

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The two most important things for a repair project are good glue and a sharp hobby knife. For most of my repairs, I use Pacer Technology’s Zap medium and thin CA, Zip Kicker and for high-stress areas, 20-minute Z-Poxy epoxy.

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A sharp hobby saw is perfect for making smooth cross-cuts in wood. When smoothing things out, you’ll need a couple of sanding blocks or bars equipped with medium and fine sandpaper. For removing large amounts of material, a good razor plane is also a good tool to have.

1 The first thing to do is to remove the covering material so you can see exactly what’s busted. Be like a doctor and cause no further harm! Don’t just cut deeply into the covering and balsa sheeting. Be careful to cut only through the covering without damaging balsa sheeting. If you do, it can weaken the area around your repair.

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2 To remove the sheeting around the damaged area, I used a long sanding bar as a straightedge to guide my hobby knife. The sandpaper prevents it from shifting while cutting the balsa. Once you remove the sheeting, you’ll be able to see if there are any internal parts that need replacement.

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3 For this wing repair, we needed to replace the leading edge (LE) and the front portion of the broken rib.

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4. Here you see the the repair area has been cleaned up and the replacement LE material and the two notches that will need to be cut to carry the LE past the open area are shown.

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5. A razor saw does this job quickly and easily.

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6. Here the replacement LE material has been fitted snugly into place. To produce the shape of the rib replacement part, trace an undamaged rib next to the damaged one. Cut the part to shape and then place it against the damaged one.

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7. Here you see the rib front and the LE replacement parts all glued into place. Tack glue the parts into place first, then lightly flow thin CA into the cracks and seams to make sure everything is secure.


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8. Before you can replace the top and bottom wing sheeting, you first have to add doubler strips under the edges of the undamaged sheeting so you have something to glue the new sheeting to. You may also need to glue some doublers to the side of the ribs to provide purchase to support the ends of the new replacement sheeting.

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9. Here the new sheeting has been glued in place. Before closing up the bottom of the wing, re-glue all of the inside seams to make sure you have strong bond everywhere.

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10. Once the wing structure has been closed up, start removing material from the leading edge and then shape and sand everything flush and smooth. A Balsa Razor Plane makes short work removing material from the LE stock.

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11. Filling the seams with a lightweight spackling compound is the next step. I use Red Devil “OneTime” filler for this. It is extremely lightweight, dries in 30 minutes and is very easy to sand smooth. (It’s available at the hardware store and home improvement department at Home Depot.) To make the filler easier to apply, use a damp sponge to lightly moisten the wood around the repair. Use a scrap piece of sheeting and apply the filler like you are frosting a cake. Press it firmly into all the seams and dents and then let dry.

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12. Use 220-grit sandpaper and sand everything smooth. If there are any starved areas needing more filler, just repeat the process and sand again until everything is level and smooth.

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13. Wipe the dust off the model and get some matching covering material, your covering tools and supplies. For the Hangar 9 Pawnee I used matching UltraCote, (from Horizonhobby.com.) Use some rubbing alcohol to degrease the covering all around your repair. This removes the oily residue from your fingers and fuel residue that will prevent a good bond.

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14. First apply the base white color. Cut the white covering about 1-inch larger all around and apply the patch in two pieces starting with the bottom side of the wing. Iron the covering down and smooth out any wrinkles and then apply the top piece.

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15.

Once the white has been applied, cut to shape and apply the trim color and overlap all the seams by about an inch. Be sure to seal all the edges down securely and, while you are at it, check all the other edges and covering seams on the wing and seal them down as well with your hot iron.

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That’s it! Don’t look now, but your wing panel is ready for flight again! If you kept everything neat, your repair will be hard to see.

To see my video showing this repair, follow this link. http://www.modelairplanenews.com/blog/2014/06/20/wing-repair-for-built-up-airplanes-video-from-the-workbench/

Tips & Tools

For good repairs (and building for that matter,) you need to have the proper tools. Here are some of the ones I used for this repair.

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Sharp razor saws are the tools of choice when cutting thick stock and for cross grain straight cuts.

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Sandpaper and sanding bars. You can’t have enough. I have long and short ones and I like the aluminum ones from Great Planes with stick-on sandpaper strips.

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My favorite tool of all is the Razor Plane. There are several available and I like this one from Master Airscrew. It is designed to used common hardware store razor blades. It is made of composite plastic and They last for years and years.

So that’s it. Go build or repair something and get back in the air!

 

Featured News, From the Magazine, Gerry Yarrish

About the author

Senior Technical Editor About Me: I have a lifelong passion for all things scale, and I love to design, build and fly scale RC airplanes. With 20 plus years as part of the Air Age family of magazines, I love producing Model Airplane News and Electric Flight.

3 Responses to “Workshop Tips — Wing Repairs for Built-up Airplanes”

  1. Gary says:

    Jerry, Here’s an alternate method I have used. For the leading edge material in photos 4-5, I would angle cut both the LE on the wing and my new material at about a 45 degree cut, or as close as you can free hand it. Making sure you have a good even fitting surface on both pieces before gluing. I feel this gives it added strength as you do not disturb the glue joints of the two wing ribs left and right of the new material.
    I always enjoy reading your tips and articles and have to say I have learned much from your experience as a Master model builder. Thanks again for a fine article.
    Gary

  2. Stuart Brooks says:

    Nice job. I’ve had to that one a number of times. Too bad ARFs and RTFs have taken over. I prefer to design and build my own. Once the knocking knees were gone after a few flights I wanted to do another but better or different. I used to start a simple 40 size fun flier on Sunday and flying it the next weekend. But then … I’m an old guy.

  3. Dennis says:

    Nice article, but I’ll go even farther than the first reply. Your repair provides a nice cosmetic repair, but does very little to bring the wing structure back to its original strength. Most of the bending loads of the wing are resisted by the wing spar, however the leading edge provides some bending strength, and the leading edge sheeting provides considerable bending strength, as well as torsional strength. A better repair would have been to make scarf joints on the leading edge replacement, as the end grain joints are very weak. Common full size aircraft practice is for a slope of 8-12:1, but on a model, 3:1 should be adequate. Backing up the joint with something like 1/32 or 1/16 ply would be a further improvement. Ideally, you would do the same for the leading edge sheeting, however, that is not practical. Another good procedure is to make two more false ribs from the same pattern as you used for the rib portion that you replaced and glue them the the other two ribs to give some glue area for the ends of the sheeting; using 3/32″ or 1/8″ thick balsa for these would be even better. However, this is still going to result in far less strength for the leading edge sheeting as there was before the wing was undamaged. The get the strength of the sheeting back to its undamaged state requires further work. A technique I have used is to add an exterior patch to transfer the loads through edge grain-edge grain. Since this is the outside of the wing and we want to maintain smoothness for aerodynamic and cosmetic reasons, a good procedure is to use 1/64″ thick plywood, which is approximately the same strength as 1/16″ thick balsa. If you use a strip 1/2″ wide, you have effectively made an 8:1 scarf joint. As 1/64″ ply has 3 plies, run the grain in the same direction as the balsa grain and patch all around the replacement balsa sheeting. Fair the patches with the filler as you did, and the repair won’t be very noticeable, and it should bring the repaired area back to its original undamaged strength.

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