If you’ve been following my online Build-Along Series on the MAN website for my Sopwith Camel, then you know I have been featuring little mini projects as they pop up in the construction. Well, as master scale modeler Dave Platt has mentioned numerous times, “A scale model is never truly finished–you simply stop working in it.” Here as well, I first test flew the Sopwith Camel back in June and have flown it several times over the summer. All the while with my Zenoah G38 engine fully exposed in that big radial cowling. So, to help dress up the Camel, I have begun to install a dummy rotary engine. I hope you find the techniques involved interesting as they can be used on any similar round nosed airplane you might be building.
Balsa USA is a great source for all things WW1 and they have been offering nice molded fiberglass dummy engines for a long time. I picked up a 9-cylinder molding at the recent Rhinebeck Jamboree and wasted no time in getting it detailed out and ready to add the Camel’s engine compartment.
Nicely molded from several layers of fiberglass cloth, the BUSA molding is nicely detailed. You would spend a lot more time trying to make your own. This is an older photo, the one I just picked up has a nice gel coat finish on the front.
The first thing to do with any molded fiberglass part is to use a grinding wheel and a Moto-tool and remove any sharp edges or areas starved of resin. After this, throw it in the kitchen sink and scrub it with dish washing detergent so you can remove any left over parting agent that might still be on the surfaces.
Here you see the open end of the cowling, it is important to use a molded engine that will fit properly. The 1/4-scale BUSA rotary engine is 7 3/4 inches in diameter, a perfect fit.
With the cowling removed you will notice I installed a ballast box filled with lead shot and resin right over the engine. I made it’s dimensions with the use of a dummy engine in mind.
Here’s the front end of the G38 with it flywheel and magnet for the magneto ignition showing proudly in front of the cylinder head. The engine is installed upside-down with the cylinder and muffler both pointing downward. The first thing to do is to open up the molded engine to clear the ignition block. But more importantly, the opening is also for cooling airflow to enter the engine compartment.
Making the opening is easy with a cutoff disc and a Dremel. I simply removed one of the cylinders and opened up the area between the other cylinders as shown here. Also, the center opening for the crankcase, has to be big enough to fit over the G-38’s propeller hub.
Here I am test fitting the molding over the G-38. The opening should provide about 1/8 inch of clearance all around so vibration won’t cause any chaffing of parts. You can also see that cylinder is right in the line of fire for airflow.
So the molding fits nicely centered over the G38 while contacting the front of the ballast box. Another good thing about the box structure is that it helps channel airflow down toward and around the engine and not up in the empty space about the engine.
Before painting, I drilled holes and added nuts and screws to add detail to the engine case. Also, I drilled offset holes in the engine case for the bushing (from short sections of brass tubes), for the pushrods which will be added later. Medium and Thick ZAP CA glue is used throughout. Be light with the kicker as it can cause the glue to foam up, which will not help the finish.
You might also have noticed the lite-ply guide/supports I glued to the engine molding. These have 1/16 inch holes in them set at the proper spacing to hold the pushrod wires. Here is the simple cut job loaded into my laser cutter.
Using my 40W Full Spectrum Laser Hobby Laser I was able to import some simple CAD drawing and cut out the eight are required for the dummy engine. You can see the lite-ply guide plates in detail here below.
To begin the paint job, I start with a coat of metallic silver from Décor. This paint is fast drying and it gives a real metallic almost chrome look to the part. I like using this bright coat as the first layer which I will build the weathering coats onto later in the process.
The paint really makes all the bits and pieces come together for a scale appearance. The 16 pushrods are made from pieces of 1/16 inch welding wire.
So here the cylinders, and the back webbing have been spray painted flat black. I have found that there is no need to carefully mask off the engine case while spraying the cylinders. Again here I use Decor spray paint which dries very quickly and is gasoline proof when dry.
As you can see, there is a slight amount of over spray on the edges of the crankcase, and that’s what we want. Additional detailing and weathering will be added to the engine to help bring out the finer detail and overall realism a scale model requires.
The next step is to add the pushrods. For the Sopwith Camel, I want a more “used” appearance.
Here above, we have the 16 pushrod wires for the rotary engine. I used 1/16 inch welding wire and sanded them smooth and clean. They has a natural metal finish so no painting is required. I also bent one end 90 degrees. The length is about 1/4 inch.
Before installing the pushrods, I took some 320 grit sandpaper and glued it to some wood to form sanding sticks. A wide one for the top of the cylinder and a narrow one for the base. As you can see, by carefully sanding away the black paint, you expose the underlying silver and gray colors. This adds depth and kills some of the shine of the black paint.
As with everything else about scale modeling and weathering, it takes a subtle touch. Don’t get heavy handed as you will only make the silver lines (tops of the cooling fins), wider.
After sanding the fins, I go around the engine case and apply a very thin layer of watered down acrylic black to all the details. When dry, this black will being out the surface details.
Here you see two of the pushrod wires inserted into the wood guide plate and inserted into the brass bushing tubes at the crankcase. A little ZAP will hold them in place. These top plates will be covered over by the lip of the engine cowling and not seen.
Here all the pushrods are in place. They are glued at the top bent ends as well as inside the bushing tubes from the inside of the engine molding.
Next I mist a very light coat of light gray over the chrome silver base color of the crankcase and let dry. I also use a thin brush and add a gray wash to flat areas and in corners where the real full-size engine would gather dust and oil residue.
Here you see the gray and black wash still puddled around some of the nuts and screws around the engine and the base of the cylinders. Now let it dry. You can speed the process with a heat gun, but I find that letting the washes evaporate naturally produces the best looking staining.
The over all project took only about 10 hours and spread over a few days, that’s no time at all. I finally decided to attach the molded engine to the inside of the engine cowling. This is the typical way to affixing it to a model, but if you have the room, you can also install standoffs to the firewall around the G38 and attach the dummy rotary that way. Here’s how I did it.
You’ll notice the wood guide plates I installed above each cylinder. These both support the pushrod wire ends and, give excellent attachment points for the molding to be glued to the inside of the engine cowling.
After sanding the paint off of the edges of the guide plates, set the dummy engine in position over the model’s real engine and then you reattach the engine cowling. Make sure the dummy engine is centered around the prop hub and that nothing comes in contact with the G38 when the propeller shaft is turned. Now tack glue a couple of the guide plates to the inside of the cowling using Thick Zap and kicker. Remember, before applying any glue, it is very important to clean off the inner surface of the cowling to ensure a strong glue bond. I use MEK to remove all the paint and primer from the inner surface of the fiberglass cowling.
After the glue sets, tack glue a couple more plates to the inside of the cowling. Once you have three or four plates glued in place, you can remove the cowling.
Here’s a close up of one of the plates glued to the cowling. Finish gluing the rest of the guide plates in place being sure to build up a strong fillet around the guide plates.
After all the attachment points have been secured, be sure to make some openings in the webs between the cylinders. These are to get to any screws or other attachment points for securing your engine cowling in place.
All you need is an opening large enough to fit a screwdriver . With my magnetically secured cowling, I also installed two screws to prevent the cowling from rotating due to engine vibration. The screws go through the rear plywood cowling ring and thread into hard points in the front of the firewall.
Here you see the backside of the dummy engine. Notice that there’s plenty of space around the dummy engine for airflow to cool the engine. Most of the airflow will be through the large opening just in front of the cylinder. Also if there is a gap between the cowling and any of the plates, (as shown here to the right), add a scrap of lite ply to fill in the space. Use plenty of Zap and kicker.
Reinstall the engine cowling and check the clearances around the model’s engine. Try to have at least 1/8 inch all around. When everything lines up properly, remove the engine one more time and give it a few light coats of honey colored clear Urethane spray finish. This will seal the wood guide plates and coat the pushrods so they don’t corrode. Also the color give the engine a semi wet look like it just ran and is covered with caster oil.
So, that’s it! The molded fiberglass dummy rotary engine from Balsa USA is an excellent starting point for any round nose airplane. It certainly is a lot less work than scratch building your own WW1 cover-up. After a couple of flight, check the clearances between the scale engine and the prop hub. If anything is rubbing, use a Dremel Moto-Tool and grind away any interference areas to increase the clearance. Also pay attention to how your engine runs. If it seems to be running hotter than before, adjust the carb., to richen the top end a little. And recheck the idle transition. Have fun!