Earlier in the summer, I began the task of making top and bottom wing fairings for my not yet completed 601XL taildragger. I'm finished with the fairings now. I leaned a few things along the way. Some aspects of the job went ok and some not so good. I thought I'd pass along what I did and observations I made to help others avoid some of the pitfalls.
I have worked with composites for decades so none of this was particularly new, but I found a few new problems. I wanted to use carbon fiber with epoxy resin. The carbon fiber for a stiff piece with minimal weight. I also wanted to retain the clear resin for visual "cool factor" look. The airframe will be polished aluminum with minimal paint. That means I could use no filler since even micro balloons mixed with epoxy gives an opaque milky look filler. The finished piece should be given a coat of clear for UV protection. I used PPG clear urethane mixed for 80% gloss and 20% flat so it's a bit of a satin finish. It looks nice. I used epoxy rather than polyester resin for low shrinkage, better dimensional stability, and higher temperature resistance. The black CF will be in the sun and will heat to 200F or more.
I made my male forms from 1" thick styrene foam held to the airframe with double sided adhesive tape. That worked well. I used a small amount of epoxy adhesive to stick multiple sheets of the foam together. That caused some problem where I had to cut into the adhesive when shaping the foam. This is not a big problem but does require good filling of any voids created. The styrene foam can be coated with brushed on epoxy resin and then bondo over that. Sand to perfection and fill low spots with more bondo. Or, fill directly over the foam with clay. That's ultimately quicker and less messy. I used Jake Reyna's suggestion to cover the foam with duct tape overall with enough overlapping the aluminum so no resin will get on the aluminum. That method works fine. The resin separates from the duct tape so no multiple coats of mold release wax is needed. My big mistake on the forms was to not spend the time to get them smooth enough. Not being able to use filler requires that the forms be shaped perfectly and to be smooth. Every lap joint on the CF will make a bump. Every imperfection in the layup will later require sanding the surface smooth. That's using 150 or 180 grit at first and on down to 320 before putting on the final clear coat. Thin coats of resin can be brushed on during this phase and sanded off to fill the coarser sanding scratches. Due to not spending the extra hours getting the forms smooth my finished fairings have high and low spots and I'm disappointed in the outcome. The bottom fairings are better than the top. I can't take the time to do it over now so I'll use them. They do fit on perfectly and will be fastened with sheet metal screws.
It is way more difficult to do layups upside down as with the lower fairings. I found it best to use smaller pieces of CF cloth, no more than about 16" long, and let the resin begin to get tacky before applying the cloth. The resin can't be too tacky or it's difficult to slide the cloth around over the form. Smaller pieces of cloth means more lap joints and that means more sanding and smoothing later.
I used 6 oz./sq. yd. twill cloth. The twill weave squirms around better and is more "drapable" than normal weave. I thought I should use the twill because the shape is complex at the front where landing gear attachment, gear leg, and wing leading edge have to be dealt with. However, with the difficulty of upside down layups and needing smaller pieces, I didn't need the twill weave and could have used the closer/tighter weave of non-twill. I used 3 layers overall, but where overlap occurs it is thicker. Had I made my molds smoother, needing less sanding, and used tighter weave cloth, I could have used 2 layers. The 3 layer fairings are very stiff when mounted on the airframe and stiffen the fuselage sides considerably. All four fairings weigh 7 lbs. I don't need the rubber seals so that saves a couple of pounds. The fairings likely cost around 5 lbs overall. I may have been able to cut that a couple pounds by using two layers rather than 3.
I still have some more work to do to get better airflow at the front of the landing gear where it goes horizontally under the fuselage. This will be addressed when I work on the air outlet from the cowling. The TD get fits about 3/4" under the bottom of the fuselage and is a high drag item. I'll need to make a different entry step since the step is about 6" outboard of the original location.
If anyone wants to tackle a similar project and has a question, I'll try to answer.
Here are a few pics.
Foam form during construction. Clay filler added.
Foam form coated with duct tape ready for layup. Should be much smoother.
Top fairing layup done.
Top fairing installed with clecoes.
Detail of top fairing at front.
Side view of front of fairing.
Looking toward rear from front past the landing gear.
Under wing fairing from front behind landing gear looking toward rear.
Fairings in the flap area.