A simple jig system for making radial engine cowlings

I often had an issue lining up the front and rear discs when making a sheet wrapped radial engine cowling so that everything remained square, so I worked out a simple jig system which I use all the time now. I don't remember seeing anything similar on a plan before so thought it might be helpful to share it.

Here are the sheet parts for the cowling for my Open Scale curtiss SOC Seagull. The two jigs and the rear plate are 1/16" balsa and the rest 3/32".

The two jigs are interlocked and glued to the 1/16" sheet backplate, locating into the four slots. The grain on the jigs runs fore and aft.

Two 3/32" reinforcing discs were added behind the front cowling plate to give a secure location for the nose plug.

Next the 3/32" front plate is located and glued onto the other side of the jig.

The cowling is then wrapped with 1/16" sheet, locating any joins at one of the four supports. On the right are the four nose rings laminated together. It is easier to sand the inside diamter of the rings smooth before gluing them to the front of the cowling - I used sandpaper wrapped round a small bottle.

When dry, cut out the centre cross from the middle of the cowling, accessing it from the rear. The cuts will be easy to make because of the grain direction.

Finally, the front face of the cowling is sanded flat and the front rings glued into position. when dry the correct outer profile is shaped with sandpaper.

The same procedure was used to produce the cowling on my peanut scale Nakajima Ki.27 "Nate" As you can see, in this case the sheet wrap (1/32" balsa) does not extend behind the rear plate, but is sanded flush.

One advantage of this construction method is that it works just as well with tapered and oval sectioned cowlings.

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