Introduction: Building a Composite Airplane Wing
This is a relatively simple guide to composite airplane wing construction. As simple as I can make it anyway... This requires quite a bit of time and work, but the finished wing is very light and has very good performance characteristics. This project also requires quite few tools and materials that can be difficult to gather. I would only recommend this project to an more advanced builder. This particular wing is intended for an Discus Launch Glider that meets F3k rules and requirements. The process for building the stabilizers is almost exactly the same as building a wing, except its smaller and uses a symmetrical airfoil shape.
Step 1: Gather Materials
The kevlar is used to create a hinges for the aileron and rudder. This design is going to have a v-tail, so the elevator has no hinge. The carbon fiber is used to add strength where its needed. The wingtips are nice and strong, ready for a launch peg.
I bought my bagging materials as a kit from CST composites, which is an online composites dealer. I bought my fabrics and epoxy from ACP Composites, which is also an online composites dealer.
List of Materials
- Vacuum Bag System
- Release Wax
- PVA (Release Film)
- Epoxy Resin
- Lowe’s Foam
- Peel Ply
- Breather Cloth
- Spray Paint (for your color scheme)
- Paper Towels
- Butcher Paper
List of Tools
- Exact O Knife
- Cutting Bow(Hot Wire)
- Epoxy Roller
- Mixing Cups and Sticks
- Paint Brush
- Sanding Block
- Very Very Sharp Scissors
Step 2: Cut Wing Cores
I cut my airfoil templates using my CNC machine. I used a four template setup: one template for the top center, one for the bottom center, one for the top tip, and one for the bottom tip. I cut the templates from 1/16” aircraft ply because I had scraps around, a much better material for hot wire cutting is Formica, which is what is sometimes used to cover kitchen countertops.
Cutting Wing Core Blanks
Wing Core Blanks must be cut from foam. I used Lowe’s foam which was a cheaper alternative to rohacell foam. After the blanks are made, the wing templates can be aligned on them and the airfoil can be cut.
Using a Hot Wire Bow the wings are cut based on the template shape. The hot wire follows along the template edge and cuts out the wing. I used a gravity fed system, which pulls the wire through the wider chord of the wing slightly faster than at the tip. This achieves a much more accurate wing shape, because it reaches the highest point on the chord, leading edge, and tailing edge at the same time on both templates. Here is an example of such a system, this one uses a drop bar: http://www.acpsales.com/Feather-Cut.html
Sanding Wing Cores Smooth
After the wing cores are cut, little hairs will be left over. These need to be sanded away so the wings are nice and smooth. Also, the "negatives" should be sanded as well, and be sure not to loose them. The "negatives" can be used to store the completed wing perfectly.
Step 3: Prepare Bagging Materials
Trimming Breather Cloth and Peel Ply
The breather cloth and peel ply are trimmed to a size slightly larger than the finished part on all sides. I left about an inch or two of space all the way around, to ensure everything was covered.
Setting Up Bag Length
I bought a length of vacuum bag off a roll, it needed to be trimmed to the proper length to bag the wings. I left about 6-8 inches excess on both sides for the bag clips, which are used to seal the bag after the parts are inserted.
Inserting Hose & Sealing Bag
A small slit is cut into the side of the bag for the vacuum pump hose to be inserted. Some bag sealant tape is used to ensure the vacuum bag is sealed.
Step 4: Waxing & Painting Mylar Templates
Waxing the Templates
The mylar templates must be prepared before they can be used to make parts to ensure that they do not stick to the wing. The first step in this process is to cover them with a release wax. Several coats of wax are applied to build up a release surface on the mylar templates. With the Hi-Temp release wax I used, I waxed on, let dry for about 30-60 seconds, then buffed off. 3-6 coats should do it. I used a special purpose Hi-Temp mold release wax, but there are other alternatives that are easier to find. One of these, which comes very highly recommended by several experienced builders, is Carnauba wax.
After the mylars have been waxed, a thin layer of PVA is added to help ensure a nice mold release. Use a high quality brush to get the smoothest finish. Some people elect not to use PVA and only use wax. I haven’t tried it personally, but I have heard that things can sometimes stick.
This is the fun part of the mylar prep, spraying on the colors of your finished airplane. I used regular Krylon Indoor/Outdoor spray paint. Its important to let the spray paint completely dry for a day before bagging your wings. If the paint causes the PVA layer to crack and peel, you might have problems pulling your mylars after bagging. Be careful not to add too much spray paint, because it will add weight to the finished wing. I used the template for the wingtip to create a unpainted pattern at the tip.
Taping Together Mylars
To ensure the seam lines up perfectly, I taped the mylars leading edge together. This ensures that everything lines up when you place the foam cores on the mylars during the bagging process.
Step 5: Prepairing Fabric
Paper templates are made in all the shapes of the fabric that need to be cut out, card stock or index makes for the best tracing paper.
Trace Templates on the Fabric
Tracing on the fiberglass and kevlar is pretty straight foreword. I use a silver sharpie on my carbon fiber and it works okay for marking things out.
The hinges need to be cut on a bias, this is to align the fibers in the best possible way to create a hinge. The kevlar will create a very nice lightweight hinge in the completed wing, but its also very strong.
Cutting and Storing Fabric
I cut all the straight lines with an exacto, some of the curves too. This part gets kind of tricky, I used mostly exacto, but I found I wanted scissors for a couple of things. The Kevlar all has to be cut with special scissors that are extra extra sharp, its really tough stuff! I stored my cut templates on butcher paper to keep them from being messed up before they were used. Bias cut fabric is particularly fragile.
Step 6: Bagging
Preparing Everything as much as Possible
Before you mix your epoxy, make sure and prep everything possible to ensure you have enough working time. This means laying out all the tools and materials you need, making strips of tape, laying out cloth, etc.
I glued the strips on the leading edge of the wing before doing everything else. I sprayed down the 1" carbon fiber bias cut strips I was using with 3M then carefully wrapped them onto the leading edge.
I used 30 minute epoxy, which means the epoxy has a 30 minute working time from when you mix it to when your part should be under vacuum. Make sure you measure out your epoxy precisely and mix it thoroughly to ensure the strongest finished structure. The EZ Lam Epoxy I used required a 2:1 mixing ratio.
Spread the epoxy evenly over the fiberglass, Carbon, and Kevlar parts as carefully as possible. Make sure everything stays in place. I found it quite useful to squeegee the glue over some of the smaller parts so they wouldn't stick to the roller. After the epoxy is evenly spread and has thoroughly impregnated the fabric, I lay out paper towels over it and roll out any excess epoxy. This greatly helps to reduce the finished weight of the part.
After the epoxy is evenly spread, the foam cores are placed on the bottom of the wing, then I flip the top mylar over on-top of the foam core and carefully place in the vacuum bag.
The order of the layers placed in the bag
Carbon Parts/Kevlar Hinge
This is where the table would be.
Make sure all the layers are in the right order, then turn on the vacuum and set the pressure. The pressure depends mostly on the foam core that you use. Some foams can withstand more pressure than others. White bead foam can only handle a couple bars before it is crushed. For the Lowe’s Foam I used, I pulled about 10 bars of pressure.
Step 7: Un-bagging
Don’t touch for 24 Hours
Leave it alone for 24 hours. Don’t touch it. This is the hardest part, don’t believe me? Try it.
Carefully peel away bagging materials
This is the best part, pulling out the nearly finished product of your hard work. Carefully peel away the breather cloth, peel ply, and mylars, any you are left with your composite wing!
Step 8: Trimming Wings
Trim Excess Away from Trailing Edge
The excess must be trimmed away with a exacto and a straight edge.
Cut Hinge Line
To cut the hinge line, carefully line up a straight edge on the top of the wing and slice down to the kevlar strip on the bottom surface on the wing. Then, work the hinge until the aileron moves freely. Sometimes, depending on the layup schedule, scoring the bottom surface of the wing might be required.
Sand Hinge Line
I used a foam sanding block to create the space for negative aileron deflection. I bent the aileron open to about 70 degrees then ran the sanding block up and down the gap until I created the appropriate amount of space.
Sand Leading Edge Seam
Depending on how well you lined up your seam, the leading edge will most likely need a little sanding to make it nice and smooth.
Step 9: Conclusion & Coming Soon
The finished weight on this set of wings is 129.9 grams. The Wingspan is 38.5 inches. I haven't calculated the precise wing loading but its more than low enough for this airplane. I haven't taken them for their maiden flight yet, I will have to update this later. The finished weight for the set is 150.2 grams. Im guessing the finished weight of this model will be around 375-400g depending on the battery I choose.
I'll be posting an instructable soon on rolling carbon fiber tail-booms. Its actually pretty easy and you don't need a vacuum or many tools.
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