Engine cowling

The building of the cowl started by mounting the spinner master model into position on the prop hub. as previously mentioned, I designed this master model with a 1' diameter hole that aligned with the 1" prop hub flange. After the spinner master was in position I constructed  4 templates and mounted them internally. These templates represented the side inboard profile, and the max half breadth of the plan view. Masonite templates were first fitted and then a splining stick was used to fair  the curves from the fuselage to the spinner. These will become sanding guides later in the process. as I mentioned earlier, I am using 1lb/ft cu floral urethane foam. Its important that you buy enough to finish the first time. My experience is you cannot mix manufactures or different production batches. I bought all of this foam from  Hobby Lobby. They had a new larger sheet product that I am using for the first time. I swear every time I buy a large batch of this foam the cashier thinks I am a gay florist!! I am outlining this process to show how I make these molds without the use of a NC router. These steps will not be necessary for a plans builder because I will have all of these molded parts available. The process is interesting however!!

foam being fitted

After the templates were mounted small gusset blocks were made to lock each of them together. In this picture you will notice a blue/green sheet of 1/4" PVC foam that was bonded to the back side of the spinner master. This is a spacer to ensure a gap between the cowl and the spinner.

 All of the supporting structure and the eventual sanded foam blocks are designed to slip off the front of the engine after they are shaped and a layer of glass is added. The surfacing of the cowl will be done while it is attached. This will all be later removed and plaster splashes will be taken of the plug master. the cowling will be split horizontally through the center line of the prop hub. In addition to the actual cowling, there will also be two "cheek" cowls added after the finishing operation.

Blocks of foam are fitted to the templates. Care is taken to ensure the internal foam is cut out to clear the engine when the plug is removed.

upper foam in place

 All of the foam for the upper half is in place. if you have any gaps, stuff wedges of the foam to fill these voids.

initial shaping begins

 After the foam is in place, the initial shaping begins. I try to cut within a 1/2" to the horizontal and vertical templates. any tool will do, I used a hack saw blade and a small hand saw. Do under cut into the foam.  I doesnt hurt to stand back and look before each cut.

Vertical template sanded to the edge

 The next step is to sand with a sanding stick down to the template edge. Hold the stick perfectly level when sanding the vertical template and hold the stick vertical when sanding the horizontal templates. This is so you avoid undercutting the contour.

initial shaping of the foam

After the templates are sanded to the edges, you can start to carve away the foam in between them. Use a saw or a butchers knife for this. Do not over cut, be very generous at this stage. Once the foam is trimmed, the real magic of this process happens. Scrap foam blocks are used to sand the foam. Both materials will equally abrade each other and transfer the contour back and forth. This creates a spherical interface and it is the same principal that Telescope hobbyists use to grind their lenses, they use glass to grind the glass.

L/H view

Here is the final outcome of the foam shaping. This is a beautiful wave and dimple free surface the cleanly blends between the firewall and the spinner. The "P-51" look of the nose is now evident.


Next is the bottom surface. That will be the next blog entry