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I was never very happy with the aluminum gear fairings, and I need a new project, so I'm planning to make new fairings of carbon fiber (I could do fiberglass but I've already worked with that, so time for something new).

Since the fairings come together at the back, and I can't make a female mold, I've made a long positive mold from aluminum, in the shape of the fairing (longer than needed so I can cut it to length). I plan to put on the release agent, cover it with the carbon fiber layers, and then pull the fairing off the mold when cured. I'm looking into vacuum bagging it, but I'd have to make sure the "mold" didn't collapse under the pressure. This method creates more finish work since you don't get the ultra smooth exposed faces that you do when you use a female mold, but I don't see a traditional mold as a possibility.

If anyone has any insight on working with carbon fiber, including fabric choices and epoxies (only small quantities needed), I'd like to hear about it.

Mike, why not make the fairings in two halves? That way you could use a female mold. You would only need one mold for all the fairings. Carbon fiber is so still you may have trouble installing a one piece fairing.

radfordc wrote:Mike, why not make the fairings in two halves? That way you could use a female mold. You would only need one mold for all the fairings. Carbon fiber is so still you may have trouble installing a one piece fairing.

Two halves mean a connection of some kind. I'd like to try to make it a single piece. I'm kind of counting on the stiffness of the carbon fiber. I want them to come together at the trailing edge, for a tight fit, but not to actually be sealed at that joint. Pry the joint open enough to slip it on the gear leg, then let it naturally spring back to a tight fit. All ideas are on the table, though.

Why can't you use the inside of a metal gear fairing like thing as a female mold? Put the hinges on the outside or maybe just clamp it, use a balloon inside instead of vaccum bagging, and a piece of flashing to separate the CF trailing edges.

Bryan Cotton wrote:Why can't you use the inside of a metal gear fairing like thing as a female mold? Put the hinges on the outside or maybe just clamp it, use a balloon inside instead of vaccum bagging, and a piece of flashing to separate the CF trailing edges.

I thought about it, but I don't think it's physically possible to lay up fabric and epoxy on the inside of something that tight. You can't even get a long paint brush in very far. I could make a female mold out of two halves, and then join them together with a separate set of layups. But I'd still have to do a lot of surface prep at that layup joint, so I'm not sure it saves me anything.

There are examples of great looking composite gear leg fairings out there. I wonder how they've done it? I did a Google search but didn't find much.

Mike,
I have done very little with carbon but worked with some of the masters at Sikorsky. There was a lot of pre-preg used there. That would be the hot ticket. Even without prepreg I would think you could lay it out flat, work in resin, then drape it in. If you use a gear leg like thing as a mold it will be sprung open. Just an idea, but I do admit I am more of a metal guy!

Bryan Cotton wrote:Mike,
Just an idea, but I do admit I am more of a metal guy!

Before I built a Sonex I was more of a wood guy :-)

I've only ever done work with glass, never with carbon fiber. I reckon to using a female mould for this would be many times more difficult than using a male mould.

I want to someday glass over my home made prop, it would be a similar task. I plan to and use peel ply over the layup. Then work on it after curing to make it presentable. The peel ply gives better or smoother finish than the plain fabric weave.

Steve
Sonex 892

Hi Mike, I have thought of doing something like this too. I pretty much hate the leg fairings. I suggest you make a female mold, which could be just the standard metal part without the piano hinge. You can never fold the aluminium to the final Sonex shape because it springs back, so the leg fairings always have to be sprung into place, so your mold is not going to be completely closed over. The final molded part can be sprung into place just like the metal part. If you are determined to get the molded shape exactly right, you could fit the female mold into a couple of wooded "formers" to close it to the final shape after you have applied the goop to the inside. I guess I'm assuming that you are going to fit the fairings something like the originals. Carbon fiber is good for bragging rights, but for a non-structural fairing, pretty much a waste of money. The lower stiffness of glass should make the parts easier to fit and remove.

Peter Anson
Sonex 894
190 hours

Bryan Cotton wrote:Mike,
...Even without prepreg I would think you could lay it out flat, work in resin, then drape it in...

I agree. For all the glass work I do, I always pre-wet the glass between 2mil plastic sheets:

- Cut 2 pieces of 2mil larger than the glass you want to we out
- Place fiberglass (or carbon), as many layers as you need cut slightly oversized, on top of one piece of the 2 mil
- Pour epoxy onto glass
- Put top layer of 2mil over glass/epoxy
- Use a spreader to work the epoxy into the glass until glass is saturated
- Use a fabric rolling cutter to cut glass into whatever shapes you need

This technique is real clean an allows you to easily cut / place glass exactly where you need it and you can use it for large and small layups.

Just an Fyi, if I was going to do glass leg fairings, I'd probably just use blue foam from Home Depot, create the fairing shape using the foam, lay the glass on over the foam, then once cured use gasoline to disolve the foam. It's a pretty quick and simple technique for simple molds (this is how I made my naca inlets).