Make static scale props

Model Airplane News, Jun 2002 by van Mourik, Dick

HOW TO

The ultimate final touch

Whenever I attend a scale meet, I wonder why so many scale modelers spend so much time installing features such as sliding canopies and retract doors, but no one feels the need to upgrade from a plain gray 2-blade 12x6 prop for static judging. The use of static props doesn't have to be limited to the highest competition models; scale propellers are major attractions on any aircraft, and they are more than worth the time it takes to create them.

Constructing static scale props does not require any special talent or equipment beyond that possessed by the average modeler. To help explain the techniques involved, this article outlines the steps I took in the production of the static scale prop for my 1-4-scale Zlin Z-50 LS. Give it a try; you'll be amazed by what you can create with some glass, epoxy and a little practice!

1 A sketch and several photos of the full-size aircraft at various angles provided the basis for an accurate plug.

In this case, I was really lucky; the manufacturer of the full-size plane supplied me with factory information as well as a logo and identification sticker.

2 The choice of building materials for the plug depends on the builder's individual preferences. I used a block of hard polyurethane foam. It is vital that you smooth out the foam as much as possible. Any imperfections will immediately show up in the moldings. Careful sanding (start with 400-grit and end with 1200) and polishing will pay off later.

3

A prop blade is not the easiest part to form a mold around. To allow for removal, the molding must be split I placed the blade on a bed of modeling clay, which I then smoothed out as much as possible. I placed the whole thing on a bearer plate made of plasticcoated fiberboard, onto which I formed the flange.

Be sure that you liberally treat both the plug and the support with release agents. I normally use a combination of two: first, I use a special mold-release wax that I allow to dry for a few minutes before I polish it out. Depending on the plug's finish, I may repeat this process six to 10 times. Second, I carefully apply some poly vinyl alcohol (PVA) to the plug/mold with a soft, wet sponge. (You could also thin the PVA to a milk-like consistency and apply it in a single layer with a dry cloth.) As the water evaporates, it forms a very thin film. Ultimately, it's the wax that ensures debonding from the resin and plug, but this film provides an extra measure of safety. I always use a very thin layer of PVA because any imperfections will clearly show.

4 When the release agents were completely dry, I used a soft brush to gently apply the first layer of molding compound. Molding compound has a high viscosity and should be carefully stirred to allow the hardener to be thoroughly mixed. To prevent air bubbles, I always use a small brush in the corners first. It is best to apply thick layers and avoid retouching the mixture with the brush where it has already been applied. I normally get a decent result from two layers. Brush the second one on after the first layer has hardened but before it has fully cured.

5

When the second coat had hardened to the stage where it is sticky, I poured a mixture of cotton fibers and chopped fiberglass strands over the compound. The strands and fibers ensure a firm bonding between the compound and resin. When it had cured, I used a vacuum cleaner to remove any excess material.

6

Glass cloth will not take

to sharp comers, so I filled these with a mixture of resin, chopped strands and a thickening agent. A generous layer of this mixture will prevent any cloth from showing through. It also helps to flatten out any irregularities in the compound. Use only aircraft-grade epoxy; it's much easier to work with and it leads to better results.

7 Immediately following the resin/fiber mixture, I applied the layers of glass cloth that would form the mold. I try to minimize the amount of resin; the cloth reaches its maximum strength once it's completely soaked. Any excess resin only adds weight. The first layer of cloth can be laid on the mixture with almost no resin at all. I added subsequent layers of cloth until it formed a rigid mold. For small items like this, I stick to a wall thickness of 3/32 to 1/8 inch; this requires approximately five layers of 1 -ounce cloth, I always let the cloth cover a fairly wide area (about 1 inch) around the plug, as the second half will be formed against this and will require trimming after the moldings have cured. This photo shows the final stage of the first half of the mold.

8 When the first half had fully hardened, I began the second half of the molding. Never remove the molding from the support or the first half of the molding from the plug in the early stages; patience is the key to this process! I carefully removed the blade from the support and cleaned off any excess clay. This photo shows part of the newly formed flange in black, with all of the clay still stuck to the blade and flange. Thanks to the release agents, it was easy to remove.