Engine cowl part 3 (Cheek cowls).

The next step in the construction of the master model was fabrication of the cheek cowl cores. The Robin is a loose copy of the Fournier RF4D Motor glider. This plane was originally powered by a  flat 4 cyl. VW Limbach conversion. as such it had two side cheek cowls that streamlined the cylinders. On the Robin, I am using an upright single cylinder 2 stroke. I decided to retain the cheek cowls because they serve two purposes, they will streamline the carburetor and the air filter and streamline the exhaust. At this point in the construction of the engine cowl, I have completed the main body of the cowl and fared the firewall to the spinner. The main cowl is shaped, a layer of fiberglass was added and a light milk coat of body filler was added. I began construction of the Cheek cowl by laying out the surface loft on my 3D cad system. I plotted the plan view and section cuts space exactly 3" apart. I transferred the loft to some Masonite and cut out all of the templates.      

R/H Cheek cowl Patterns.

        
1/8" Masonite was used for the templates. The original pattern was plotted on printer paper and 3M 777 spray adhesive was used to affix them. do not use water based adhesive for this step, the paper will swell and distort. at some point the contour reverses and the same template is used for the fore and aft position.

Cowl template in position

I trial fitted the template at this point to ensure I had enough excessive flange area. As shown in this view, more tape had to be added above and below the cheek cowl. The actual part will only be a 1 inch flange beyond the cheek cowl, but excessive edge is needed for the building of the lay up tool and plaster splashes.

foam being sanded flush on the belt sander

All of the templates were lightly tacked to the foam with 5 minute epoxy of bondo. just a small dab is all that is needed, because the template is removed later. The foam is rough cut on the band saw and then flush sanded on the belt sander .

templates being bonded

the foam had to be pieced together for the larger templates. a small bond was all that was needed and it was placed such that it was buried inside the final contour.

foam blocks in place on master template

The foam was bonded with a very small dab of bondo to the master template. I want to remove the foam later after it has been shaped. in this view, the plan view off the foam was sanded flush to the template.

Top surface was sanded tangent to the foam blocks

In this view the sanding stick was used with 36 grit paper to sand the top surface tangent to the foam. The surface is sanded until the edge becomes a line.

corners sanded tangent

the same sanding stick was used to sand the corners flush. At this point the foam can be final shaped using a block of scrap foam as the sanding block

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as you can see the results are excellent!!!

Foam shown during final shaping.

By sanding the foam with the foam, both pieces abrade each other. the result is a beautiful spherical interface. This surface is wave free and readily finishes after a layer of fiberglass is applied.

Another view of the final surface

After the foam was shaped I was able to peel the main template off without damaging the foam.

36 grit sandpaper tacked to the cowl

After the cowl was formed, it was necessary yo contour the base so it would fit flush to the engine cowl. I hit on the idea of sanding the contour into the foam cheek cowl cores. It worked like a charm!!!

cheek cowl bonded into position

After the cheek cowl was fit to the main cowl, it was bonded in place. The next step was to lay one layer of 8 oz fiber glass on the foam so I could complete the finishing. this is urethane foam, so any suitable laminating resin can be used. Because of cost and time considerations I used polyester resin. The cure was complete in 45 minutes.

L/H cheek cowl/inlet

The L/H cheek cowl was fabricated similar to the R/H cheek. The main difference is the addition of a cooling air inlet.

the cowl was also sanded into position and later attached.

L/H cowl in position

As I write this update i have completely coated and block sanded the cowls. I have added blend fillets between the cheeks and the main cowls.




The final finish and removal of the cowls is the subject of the next Blog.