When building my Balsa USA Fokker Triplane, I switched from the included fiberglass cowling to one made from aluminum for some improved scale appearance, I replaced the one that comes with the kit with the Arizona Model Aircrafters aluminum cowl.
Start by installing the engine in your fuselage so you can center the face plate on the cowling.
Tape the face plate to the front of the cowling and measure the distance from the center prop-hub opening to the outer edges of the cowling ring. It should be equal in all dimensions top, bottom, and sides.
Mark the location on the face plate, and remove it from the outside and tape it to the inside of the cowl. Use your hash marks to set it in the correct position.
For my engine, The Zenoah GT-80 I had to enlarge the prop-hub opening so I used a half-round file and held the face plate in a vice.
Remove the mufflers and then slip the cowling into place. Tape the cowling into place with the opening centered around the engine’s prop hub.
Double check the cowling’s position and then mark the lower corners of the fuselage where it meets the aft edge of the engine cowl.
Transfer the corner marks to the inside surface of the engine cowling along the aft edge.
The cowling comes with a “Cut Template” and it is used to remove the bottom portion of the cowl ring. There is a side of the pie shaped template that has an arrow printed on as shown above. This should be aligned with the cowl’s aft edge.
With the template taped to the inside of the engine cowl, trace the cut line onto the inside edge cowling using the template. It should end with the bottom point resting on the corner of the faceplate.
Before cutting the cowling’s opening, use small button head sheet metal screws to attach the face plate. It is easier to do this while the cowling is still in one piece. after the cowl is cut, it will spring open and it’s diameter will change slightly throwing off the face plate’s positioning. I use screws from Micro Fasteners
Here the cowling has been reinstalled to again check the position of the face plate.
Remove the face plate, use a Dremel Moto-Tool and a cut-off disc to cut away the bottom portion of the cowling ring. Sand and file the cut edges smooth and position the cowl on the fuselage to check the bottom corner alignment.
To add strength to the fuselage face, I added a lite-ply doubler ring as shown above. This is what the attachment blcoks (there are 7,) will be glued to. Note that the middle wing alignment dowel has to clear the inside edge.
Before gluing the blocks into place, drill the holes in the center of the blocks and install the blind nuts as shown. Also, I sanded a very slight radius on the outer face of the block to match the curve of the cowling. I used ZAP CA Gel adhesive. Also, later the blocks will be cross-drilled and round wooden toothpicks will be used to strengthen the glue bond to the fuselage face/doubler ring.
Before drilling the holes for the attachment bolts, be sure to check the cowl positioning and draw reference lines on the fuselage to make sure the cowl remains straight.
Reattach the face place . All the screws should match up to the holes and when all tightened into place, they will pull the cowling back into shape. Check the fit of the cowling to the fuselage again. Make sure the bottom corners line up with the fuselage and the bottom of the faceplate is level.
When the cowling and faceplate are joined, the stress of the cowl ring with the bottom section removed causes the faceplate to bow outward. this is fixed by gluing a 1/4-inch carbon fiber tube to the inside edge of the faceplate.
To add some stiffness to the bottom edge of the faceplate, I also bend a curf into the bottom edge, and the CF tube will rest in the backside of this bend.
Here you see the CF tube glued into place with JB-Weld epoxy. Be sure to clean and sand the surfaces to make a strong bond.
Use more JB-Weld to glue and seal the face plate to the cowl ring. Also clip the screws down with a pair of wire cutters. Let the JB-Weld cure overnight.
Measure the position of the hole in the top block and transfer to the cowling then drill a pilot hole. Enlarge the hole to fit your screw (I use a 4-40 hex-head bolt with washer) and secure the cowling by snugging the screw into place.
Do the same for the two lower screws and snug them bolts into place.
Finally install the 4 side screws and snug into place. There are a total of 7 cowling attachment screws.
There you have it. A nice aluminum engine cowling in true Fokker Triplane fashion center and secured to the fuselage. The job takes a little time, but the end result is much better than using the weak, non-scale fiberglass cowling that comes with the kit.
All that’s left to do is clean and paint the cowling.