How to Make Scale Engine Cowls the Easy Way
By Gerry Yarrish
When it comes to scale modeling, there’s one task that often stops most projects from even getting started. That being making a scale engine cowling. To produce a fiberglass unit, you have to make a master plug then layup a female mold so you can then lay up a cowl using fiberglass cloth and epoxy resin or polyester resin. The task is time consuming at the very least. Here’s a way to make a one-off cowl and the technique and materials can be used for any type of scale model you want to build.
1. Here’s what you can make with basic tools and materials. I built this “one-off” balsa and foam engine cowl for my Staudacher 300GS featured as a Model Airplane News construction article.
2. Here’s the stuff you need! Sandpaper rough and fine 150 , 220 and 320 grits, Pacer Z-Poxy Finishing Resin, 2oz. and 1oz. fiberglass cloth, basic hobby tools, balsa sheet, blue foam and a clean place to work.
3. Start by attaching your engine and engine mount to the model’s firewall/engine mount box assembly. Be sure to properly position the engine so the propeller and spinner are at the correct spaced and positioned from the firewall.
4. If you are using one, prepare your spinner by making a plywood spinner spacer disc the same diameter of your spinner’s back-plate. Sand smooth and stick it to the back-plate with double-sided sticky foam tape.
5. Make the forward face of the cowl out of two layers of 1/4-inch or one layer of 1/2 inch balsa. Mark the air inlet openings but don’t cut them out yet. Use your plans and photos to determine the proper size and spacing.
6. Tack glue the aft plywood backing pieces for the cowling to the firewall. The top piece is cut flush with the outer edge of the fuselage. The two sides are inset slightly so the side pieces can be glued to them and still be flush with the side of the fuselage. Once the cowl has been made, screws will hold these backing pieces to the firewall securing the cowl in place.
7. Tack glue the spinner disc to the face piece and then bolt the spinner back-plate to the engine. Make sure the face piece is level. Cut out and glue the two 1/8-inch balsa side pieces to connect the face piece to the cowl backing pieces at the firewall.
8.Add the corner doublers to the inside edges of the side pieces. Here I used ¼-inch sheet balsa (bottom) and 1/4 inch square balsa pieces (top.)
9.Plank the bottom of the engine cowl with 3/32 inch balsa sheet. Notice that the grain runs across the fuselage from left to right. This adds stiffness to the sheeting. Use Zap CA and kicker to speed construction.
10. Now is a good time to make the needle-valve and glow plug driver holes in the side of the cowl. Before the foam top is added, you can see how things line up.
11.Cut a piece of 2-inch thick blue insulation foam to size and place on top of the balsa sides. You’ll have to carve and route the foam out to fit around the engine and engine mount. Another piece of foam will be added to the fill in the rest of the space behind the balsa face piece.
12. Using a hobby saw, roughly cut and trim the foam blocks to the shape of the engine cowl sides. Once this is done, you can start to shape the entire foam section! But don’t glue the foam into place just yet!
13. I used a piece of balsa sheeting as a template to accurately mark the side-view shape of the cowl onto the foam block.
14. After cutting foam to the side shape, start removing the corner material to round the block. This saves a lot of sandpaper.
15. Once you’ve roughly shaped the foam, epoxy it into place against the balsa side pieces and the aft plywood stiffener. Notice that the fuselage has been wrapped with plastic sheet to protect if from the glue.
16. Here the foam block has been smoothed to shape and it blends nicely into the balsa sides and the front of the fuselage. I used a lot of blue tape to protect the fuselage from the sanding bar.
17. Here the plywood spinner ring and foam fairing block behind the ring have been glued in place and shaped to blend together. Use a very small amount of glue to hold the foam block in place. Notice that the balsa face piece has been shaped and sanded to blend back into the foam.
18. To add strength to the underside of the cowl, glue a balsa spinner ring fairing to the bottom of cowl and sand it to shape to blend into the spinner.
19. Here the cowl has been removed from fuselage so I could fix any dents and seams with filler. Use a lightweight filler and allow to dry before sanding with 22o grit sandpaper. Repeat the process until all defects are eliminated.
20. After sanding all the filler smooth, apply the first layer of 2oz. fiberglass cloth and coat with a thinned mixture of Epoxy resin and Alcohol (60:40 mix) using Pacer Finishing Resin.
21. After the resin cures, cut and trim the cloth to shape and sand the edges smooth. Now apply 2oz. cloth to balsa sides and two layers over the top of the foam. Once everything has cured, sand smooth and apply another layer of Cloth and Resin (Use 1oz. this time.)
22. Sand all the cloth smooth again and apply a final layer of finishing resin. Sand smooth with 320 grit sandpaper.
23. After sanding the resin smooth with 320 grit sandpaper, apply glazing putty to fill any weave and pinholes.
24. If your cowl has one, use balsa to make the basic scoop and then sand smooth and apply fiberglass cloth and resin to seal the grain. Cut the opening in the bottom of the cowl and glue the scoop in place. Fill the gaps and seams and sand smooth.
25. Here the cowling has been sprayed with the first of several coats of sandable primer. The primer is available at most department stores and Automotive supply shops.
26. Sand each coat of primer smooth and then fill any defects and pinholes with red glazing putty.
27. Here you see the glazing putty sanded smooth. You have to sand almost all of the putty off to properly prepare the cowling for paint.
28. Using your favorite brand, apply a coat of white primer, sand smooth and apply several light mist coats of gloss white paint.
29. Allow the white base coat to dry over night and the mask off and spray on your trim colors of paint. Lightly sand the paint with 400 grit sandpaper and then rub down with some 0000 steel wool. Wipe down with a tack-cloth and apply a couple of coats of your favorite gloss clear.
Attach the cowl to your firewall with suitable screws and brackets and your cowl is done. For an even nicer scale look apply paint and scale markings to your spinner and propeller blades. Add a few scale decals and you’re good to go.
I hope this “How To Article” will inspire you to try the technique for yourself. You can make complex shapes as well as round radial engine cowls with this easy step by step system. Experiment and above all, keep building! Have fun…