By Gordon Buckland | [email protected]
I have spent a lot of time after contests retrieving models in trees, swamps, and fences, and they are often damaged. We are fortunate to have an avid builder and repair expert who has taken the time to document how to repair damaged wings. Thank you, Dennis Harvey, for this fantastic contribution about the skin graft technique.
Damage to the hollow wing of an older Vixen 2. The damage goes back to the spar, which is typical for hollow wings.Figure 1 shows the damage to an older Vixen 2, which is used as an example for this article. This is a hollow wing with a spar. This grafting technique also applies to foam-core wings, which will be mentioned later.
It is disheartening when you find that your beautiful carbon-fiber sailplane has some serious wing damage. It’s not uncommon after a midair collision at an F5J contest or after misjudging the distance to the tree line. Often, the damage seems so bad that you fear it isn’t repairable or, if it is, that it will gain a lot of weight. Fear not! The technique in this article could solve both problems. I have used it on dozens of repairs for both my sailplanes and those of others.
The first step is to assess the damage. If there is spar damage, it should be repaired first to ensure that the wing is strong and true. For now, let’s assume that the spar is intact and jump into an ugly damage situation where there is a significant gouge in the wing.
The gouge should be cut out and trued up. On the spar side of the cut, you can see that the left side reveals the ragged joiner box exterior.
Figure 2 shows the gouge trued up to a rectangular hole. There will be more preparation of this opening, but for now, I know how large of a hole I am dealing with.
Let’s turn our attention to making the skin graft. Because the damage has been trued to a rectangular hole, I know the size of the repair area. I am going to mold a skin patch—top and bottom continuously—that will be spliced into the damaged area.
Choose an undamaged part of your wing set that is roughly the same chord as the damaged area. You’re going to use this clean section of wing as a mold surface for making the graft. For this Vixen 2, I used the other wing, near the wing root. I measured the damaged area and added 30mm to the spanwise width of the graft.
This is the area on an undamaged wing that will serve as the mold surface for molding the skin graft. The mold surface is protected with clear packaging tape.
Figure 3 shows the location of where I will mold the skin graft. First, I carefully cover the wing skin with packaging tape, making sure that there are no trapped air bubbles. Epoxy will not adhere to clear packaging tape, so you will not need a release agent. The perimeter of my work area is protected with masking tape. Once I am finished making the graft, there will be no damage to the wing.
The graft can be made using 5.7-ounce per sq. yd. carbon-fiber cloth, which is the most economical weave. The graft should have the fibers oriented at 45° to facilitate forming around the leading edge (LE). I add an additional two layers of 0.7-ounce fiberglass (most of which will be sanded away) to get a smooth surface finish. If your damaged wing is made with straight tow or spread tow for the surface, you could add a layer of this over the 5.7-ounce carbon fiber for a cosmetically nicer, finished repair.
The carbon-fiber cloth is wetted out in place on the wing. I use West System 105 Epoxy because of its excellent properties and long work time. After wetting, I cover the matrix with Saran Wrap so that I can gently work out air bubbles and excess epoxy.
The carbon fiber is draped over the work area and epoxy is applied. Saran Wrap covers the matrix, so excess epoxy can be gently pushed to the perimeter.
In Figure 4, you can see that I use the handle end of a single-edge razor blade and work the epoxy to the edges of the work area. I might go over the matrix a dozen gentle times to make sure that I don’t break the Saran Wrap in the process. Let this cure hard overnight.
Once the graft is cured, the Saran Wrap peels off easily, as does the molded skin graft. The graft will have a thickness of approximately 0.3mm.
This shows the skin graft trimmed to fit perfectly. Read the text to understand how the flanges were added to the perimeter of the hole.
The next step is to prepare the hole with flanges to receive the skin graft. Look carefully at Figure 5 and you can see that on the sides of the opening there are flanges upon which the graft will be glued. These flanges are made with a 10mm wide piece of graft. To get this to fit precisely, you might have to trim away any foam core on the inside of the wing so that the flange bonds skin to skin. These flanges are structural, so you want to make sure that the joints are robust.
In this example, the joiner box and the wing spar shear web (made of balsa) served as a sturdy foundation to make an end-grain balsa flange running parallel to the spar. This flange was trimmed precisely to step down the thickness of the graft (remember 0.3mm?). Lastly, the graft is trimmed to fit precisely in the hole. If you did everything carefully, you shouldn’t feel a step where the old wing meets the graft. If you feel the graft is a little too squishy for your liking, add balsa ribs on the inside. Now, bond into place.
If you’re repairing a foam-core wing, I recommend cutting out the damaged core and not replacing it. When you are ready to put the side flanges into place, use an X-Acto blade to make a slit into which you can add epoxy to secure the flange. Finally, for those of you who like field fixes, you can always bond the graft directly over the damage.
There are a few things that you can do now to ensure an even more remarkable repair. Remember that I recommended adding two layers of fine fiberglass cloth (0.7 ounces per sq. yd., or thereabouts) over the carbon fiber. The purpose was to fill the valleys in between the carbon-fiber weave.
You should now block-sand this layer down to get a perfectly flat surface. You want to sand so that the sawdust is white. It will be gray if you start sanding the carbon fiber, and you don’t want to compromise that structure. I use 220-grit to get the surface flat then switch to 400-grit or higher. After all of the sanding marks are gone, I smear a thin coat of epoxy over the surface to shine and set it aside to hard cure.
The next step before bonding the graft into place is to trim it to the opening so that it fits with a minimum gap. Place the graft on the flanges and check for flushness. If it’s not perfect, you can either build up the flange surface with 5-minute epoxy or slightly file down the surface with a jeweler’s file. Once you’re happy, bond the graft in place, making sure that you clamp and tape it so that the flushness is maintained.
Compound Curve Example
Let’s look at another example of the skin graft technique that is applied to a compound curve, such as what you’d encounter in a gouged wingtip. The example I am using is a damaged Explorer wingtip. It is fortunate that the owner painted the wingtip because the finished work can be painted to mask all of the underlying repair work. Also note that there is some skin separation in the blue area.
The damage is trued up to a rectangular hole and the edges are beveled to match the bevel edge of the graft.
As with the Vixen 2 repair, the next step after assessing the damage is to square up the damaged area and create a rectangular hole (see Figure 8). Also notice that in this figure, instead of choosing to use flanges for the repair, I decided to bevel the edges of the repair area, so there will be an approximately 2mm overlap between the damaged edge and the graft.
For this repair, the owner had an identical sister aircraft from which I could use a good left wing as a mold surface for the graft. If you don’t have a sister aircraft, one of your friends might. Remember, this molding process causes no damage to the intact airplane.
Take a careful look at Figure 9. On this wing, instead of using packaging tape to protect it, I chose to stretch Saran Wrap around the wing surface. The wrap is held in place with masking tape, which also forms a perimeter for the work area. To make this graft, I chose to use fine fiberglass cloth so that it could form around the curves (especially the LE), and I would use multiple layers to achieve the desired stiffness. In this case, I used eight layers of 0.7-ounce per sq. yd. cloth to build up the equivalent of 5.6 ounces per sq. yd. I did not cover the matrix to work out the air bubbles. Instead, I just used an acid brush and brushed out the excess epoxy and potential air bubbles. It worked fine.
Figure 10 shows the graft installed. The graft was sized properly but with overlap and a bevel on the inner surface to match the bevel that was filed on the damaged wing. I used a small amount of epoxy and microballoons (brown) to form a smooth surface. Additionally, the adjoining wing surface was repaired with CA glue and sanded with 400-grit sandpaper. What you see in Figure 10 is the wing ready for primer paint. You can see that blue masking tape has already been installed on the white aileron wiper.
Figure 11 shows the finished repair. Both the blue stripe and the one white stripe had to be painted. The all-up weight gain was approximately 7 grams.
With this second example, you can see that the skin grafting technique has some versatility. Next time you have a bad bang-up wing accident, maybe the salvage and repair won’t seem so impossible.
Continued Support for the USA F3J Team
Please support Team USA F3J’s trip to Norway by purchasing tickets or apparel. The link is listed in “Sources.”
Go downwind and soar!
SOURCES:
League of Silent Flight (LSF)
www.silentfight.org
West System
866-937-8797
www.westsystem.com
Team USA F3J
www.teamusaf3j.com
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