Hog Bipe–Cowl Design

I haven’t posted any progress on the Hog Bipe in quite some time, but I have been picking away at it and I am almost ready to start final assembly. I’ll try to catch up and post some of the key build steps.

The biggest deviation from the stock kit was the creation of a custom cowl. A while back, I posted a Sketchup model of the Cobra 4130 motor to be used in this plane. I then went ahead and built a Sktechup model of the nose of the Bipe including my new cowl.

I started by scanning front section of the plans to use as a guide to the fuselage shape. I then modeled the fuselage for the first few inches back from the firewall and placed the model of the motor in place with the prop plane at the stock location.

Cowl Step 1 Cowl Step 2

Next step was to try different section shapes and use the Curviloft Sketchup extension to generate the final form of the cowl. This was a bit of a trial and error process until I came up with a pleasing shape that blended to the fuselage and cleared the motor.

Cowl Step 4a Cowl Step 3

Once again, I ran into the basic limitations of Sketchup where tiny computational rounding errors would add up and cause small gaps and imperfections in the generated surfaces. I had to blow up the cowl to 10x size to work with the design, then shrink it back down once I was done. While this helped quite a bit, I still had to do a lot of fussing and tweaking to stich surfaces together and close up gaps.

Here is the final shape.

Cowl Step 4

Next step was to design the structure of the cowl itself. I decided to use standard formers and sheeting for the back half of the cowl, but with all of the compound curves, I decided to carve the front half from balsa rings stacked together like a wedding cake.

Cowl Final Cowl Step 5b

To generate the “layers” I created sections in Sketchup and then projected them forward. In order to minimize end grain in the final cowl, instead of cutting each layer out of a single sheet, the base form of the layer would be made by gluing  together square strips arranged with as much of the grain running lengthwise around the perimeter as possible. A horizontal and vertical line is used to align the layers.

Cowl Step 6

I then exported the final design to DXF and used a free DXF CAD editor to create a “blueprint” for each layer.

On to actual assembly. Each layer was built up from balsa stock. The outer edge was cut to the final shape using a scroll saw using the printed out layers.

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Here are the layers ready for assembly. Note that I did not yet cut out the center of the last layer, which is cut from a a single sheet  of 1/8” thick balsa, as I was afraid it would be a bit too flimsy. It will be cut out as the last step in shaping the cowl.

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The rear section was then built and sheeted using stringers and formers.

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Then each layer of the “wedding cake” is assembled using the horizontal and vertical tick marks for alignment.

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Then with a bit of carving and sanding, the final shape emerges.

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Using a Dremel with a sanding drum, I smoothed out the interior. I was afraid to get too aggressive with this step and sand through the wall. Note, I forgot to snap a picture of the cowl inside until right before I was ready to assemble it to the back section of the cowl.

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Here is the cowl assembled to the fuselage, ready for final sanding. I use three 1/4” dia rare earth magnets to hold the cowl to the fuselage along with a pair of 1/8” dowels as locating pins.

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