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Creating Custom Wheelpant Molds

By Chris Boultinghouse, EAA 575512

Wheel pants
When Chris wrote this article for
CONTACT! Magazine,
he was working on a Sonex. Since then, he has
sold the project and moved on to other things.

I have been an airplane nut since the age of 2 when my mother took me to the airport to pick up my father. Being the only one in the family with such an obsession was difficult as a child! By high school, I was flying radio-controlled (RC) models. First sailplanes, then powered, next helicopters, and back to sailplanes and electric-powered scale models. During that time I designed and marketed several all-composite model designs, where I learned a lot about mold-making and vacuum-bagged wing-building techniques. I think it was this experience that led me away from composites when I decided to build a full-scale airplane.

I got my private certificate in 1999 and a glider rating in 2000. I’ve been married to my wonderful (and understanding!) wife, Liz, for 15 years. She’s always ready to lend a hand with my projects and, in fact, took most of the photos for this article.

The first question one might ask, since I’m building a metal airplane, is, “Why would you mess with fiberglass?” The answer? I just can’t say no, and I always seem to do things the hard way.

This was the case when Fred Hulen, Zodiac builder extraordinaire, asked if I could be commissioned to craft a set of wheelpants for his incredible Jabiru 3300-powered Zodiac HDS. I’d met Fred several years ago, and we have kept in touch during the gestation of both our aircraft projects. His Zodiac is without a doubt one of the finest examples you’ll find of that breed, and he wanted wheelpants that were different from those offered by Zenith. I suppose he figured that a fellow builder and longtime RC modeler might be able to provide the goods. And so the saga began! Fred measured various wheelpants until he had a good idea what would fit the somewhat-unusual landing gear arrangement of the Zodiac HDS and created full-size top and side view templates for me to use.

When Fred’s templates arrived, I formed a plan of action. The paper top and side view templates were used to define the shape of the plug, which was made from 1-inch thick foam blanks (of alternating colors) laminated together using 3M Super 77 spray adhesive.

Wheel pants

It was at this point I encountered my first problem: If I glued enough foam together to accommodate the shape of the plug, it would not fit in my band saw for cutting!

I decided to make the plug one half at a time, and it actually worked out for the best since I could then make a 0.025-inch aluminum “spine” to sandwich between the left and right halves. This would help to keep the correct profile during shaping. The next task was deciding exactly how to cut the top-view shape. Once again it was too tall to fit in the band saw, so I decided to revert to my old RC model-making days and use the hot-wire method. A pair of templates were made from scrap aluminum, and the hot-wire bow was retrieved from the attic and pressed into service.

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The bow is one of two homemade units that I’ve used over the years to cut hundreds of foam wing cores for RC models. The stainless-steel wire is heated by an ancient transformer, hidden behind me on the floor. A push-button switch on the bow allows me to get everything in position before starting the current flow to the wire.

Wheel pants

Hot wire cutting (done properly) is a rather slow process with a feed rate of perhaps ½-inch per second. I’ve never actually measured it. I just set the bow temp to get a feed rate that experience tells me is right. I’m sure there is someone in your local EAA chapter who has built a canard pusher and is familiar with the methods. Also, be cautious when using a hot-wire bow cutter. Other than the obvious safety issues with handling electricity and hot things, there are some foams that give off toxic fumes when heated or melted.

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The process was repeated for both plug halves. They were then joined (with the spine in the middle) with the 3M adhesive.

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And now for the fun (and messy) part: shaping the beast! It is at this point you will see the reason for using alternating colors; it allows you to easily see the form of the plug as you start removing material. It’s much more difficult if you use foam that is all the same color! If you cannot find foam of different colors in your area, an acceptable substitute is to lightly spray paint the pieces a dark color before joining. This will give you a defined line when shaping. Just make sure the paint is foam-compatible. Remove large bits with a sharp razor or box knife, and use wood rasps and sanding blocks until you are satisfied with the shape.

Wheel pants

Try not to take off too much, though it isn’t the end of the world if you do. Fill any minor dents and dings with wall spackle or build up low spots with wallboard compound. It’s soft and sands easily when dry.

Wheel pants

Even though I knew better, I tried to shortcut the laborious plug finishing process and use only a light layer of glass and then prime and paint. Big mistake! The glass didn’t give a firm enough surface, and when primed and painted one could clearly see the demarcations where the foam layers met.

Wheel pants

Multiple treatments with filler and re-sanding didn’t fix it, so the paint was rough-sanded and a layer of heavy (8 ounce) glass and resin was laid up over it. This was then rough-sanded to knock off the high spots, and a batch of thick “pookie” (resin and West Systems 407 filler, which is a blended microballoon-based filler) was spread over the surface. I actually prefer the West Systems 410 filler, but 407 was all I could find locally on short notice.

Wheel pants
This mixture is put on quite thick,
but most of it will be removed later.

Wheel pants

Remember, this plug is not a “flying part” so weight control is not required. The rest of the finishing process is just like any composite project. You know, wet sanding, spot filling, wet sanding, spot filling, priming, wet sanding, priming, wet sanding, and painting. Did I mention wet sanding?

Wheel pants

Take your time and do it right, because your finished parts will look just as good (or bad) as your plug!

Now that the plug is perfect, it’s time to fabricate a female mold in which you will make the actual parts. This is broken down into several steps and processes.

The first step is to make a “parting tray.” This is simply a divider that defines where the mold will split into halves. Think carefully about the placement of your parting line, since you need a negative draft angle in order for the parts to release from the mold later. In this case, the part is symmetric so we will split the mold directly down the vertical centerline. As such, we can use our side view template to make a divider from ¼-inch tempered hardboard.

Wheel pants

The next step is to put some supports and legs on this divider tray so the plug is suspended at the midpoint.

Wheel pants

Nestle the plug in place and use oil-based modeling clay to close up any gaps between the tray and plug.

Wheel pants

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Clay is used to seal the gaps between the plug
and the parting tray, as well as creating a “dam”
at the area where the tire will protrude from the pant.

Wheel pants
The four raised white clay alignment pins will be
used for aligning the mold halves. With a liberal
coat of PVA mold release, you are ready for the next step.

Don’t skimp here! Use a good quality mold release wax, which should contain no silicone. Apply at least six coats, buffing between each. Also at this time you can use clay to build “dams” for any openings you want in the mold. Use clay to form index marks in the mold for alignment purposes. Finally, use an inexpensive paint gun to fog on several coats of PVA (polyvinyl alcohol) mold release. You might get by with brushing the PVA on, but I was able to get a much nicer surface with the spray gun.

The tooling coat is a thickened resin that coats the plug, providing a smooth interior surface that can be polished. It is typically a dark color so that air bubbles in the part lay-up will be easier to spot. I purchased some tooling resin from a supplier but was not pleased with the results. The consistency and color were fine, but it was very difficult to mix properly. As a result, I ended up with a few small blemishes in the mold where the mixture did not harden properly. I would recommend using my home-brew tooling coat, made from West Systems 406 filler and West Systems 420 Aluminum Powder mixed with epoxy to a thick sour cream consistency. This makes for a dark gray mold surface that is very hard and polishes out almost like metal.

Once you have your tooling coat mixed, apply it to the plug and parting tray flange and allow it to cure at least partially. You don’t want to disturb the mixture during the next steps, but if you allow it to fully cure you may not get a good bond with subsequent layers.

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Black tool coat is in place, ready for reinforcement.

Mix some resin and milled glass fiber to a peanut-butter consistency, and put it in all of the “corners” of the mold and around the alignment pin bumps. This will prevent voids when you apply the glass reinforcement layers. Next are strips of heavy fiberglass and epoxy on the mold where the plug meets the parting tray. And finally, heavy glass and epoxy over the entire plug and tray.

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It’s at this point that you can use one of two different methods for bulking up the plug. I chose to use an epoxy tooling dough, available from Resin Services.

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This product was topped off with another layer of cloth and resin. This is an expensive option but, in my opinion, it produces a superior mold that is warp-resistant, strong, and doesn’t weigh a ton. The cheaper and more common method is just to continue adding layers of heavy cloth and epoxy until the mold is 1/8-inch to ¼-inch thick, then bond angle extrusion around the perimeter to tie it together and prevent warping. The choice is yours.

When the first half has cured, remove the parting tray and dig the clay out of the alignment pin areas. Scrape away the modeling clay, being careful not to damage the plug. Trim the rough edges from the mold using metal snips or a cut-off disk. Wax the plug and mold flange, then use clay to form the opening on the other half. Spray the plug and mold flange with PVA. Now repeat the process with the tooling coat, corner fill, and reinforcements.

When cured, it’s time to pry the halves apart and retrieve your plug. Use a metal scraper, screwdriver, or other thin tool to get between the halves. Apply steady pressure, and with luck it will crack open to reveal your new mold! Depending upon the shape of your plug, you may need to use thin strips of stiff plastic (Mylar) to work under the plug and get it out of the mold.

Wheel pants

The hard work is done. Now it’s just a matter of polishing and waxing the insides of your molds, and you are ready to make parts! The actual lay-up schedule varies depending on what part you are creating, how strong it needs to be, and what materials you use. That could be covered in another article!

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