Positioning the Nosebowl

I can’t make any more progress without getting the approximate position of the nosebowl pinned down. I spoke with the folks at American Propeller about the dimensions of the Whirlwind spinner, and they sent me some nice CAD pictures of two options. The short answer is that the aft edge of the spinner is pretty close to the front of the prop flange. Since the rubber baffle seal on the front of the baffle parts will allow for some adjustment, I only need to get a rough idea of where the nosebowl will fall. Step one: assemble the halves.

Bearhawk Nose Bowl

The two halves have a fairly rough fit, but I should be able to make it better with some filling and sanding.


Lycoming Prop Flange Layout

Since I don't have a spinner yet, I needed to make something to hold the nose bowl in place.


This piece of 1/4″ OSB should do nicely. I measured and drilled all 6 holes, then held it up to see how I did:
Not the Lycoming Prop Flange Layout

Hmmm... it looks like I measured to the center, but drilled to the edge of the hole.


Improved Prop Flange Layout

That's better, though perhaps a bit over-perforated.


I enlarged a couple of the holes so that I could get small c-clamps in to hold the OSB to the flange.
Simulated Spinner Backplate

The red circle is an aid for locating the nose bowl on the OSB.


Bearhawk Engine Side View

This is the location, plus or minus half an inch or so.


Limited Clearance

In this position, there isn't much (enough) room between the engine and nose bowl.


Nosebowl Alignment

This view is a handy way to align the nosebowl. From here I can compare the height of the seam to the "Lycoming" letters in the valve cover. As long as things line up the same way on both sides, I know that the nose bowl is level left-right.


With a rough idea of where the nose bowl will end up, I started working on the top for the air filter housing. I’m using the Vans Filtered Air Box (FAB) because it has several great design features. For instance, the filter element is reusable, and available from non-aviation sources. Air from the carburetor heat source is still filtered, eliminating the need to turn off the carb heat after landing at dusty/grassy places. And best of all, it eliminates the need for that expensive and troubled box from aircraft spruce that everyone else is using. I started by cutting a hole in the FAB top so to clear the bump on the bottom of the carb.
Vans FAB Top

I used the Vans template to drill these corner holes as a starting point.


The FAB Cutout

This was not enough of a hole, but it was a good starting point.


Using the Vans FAB on a Bearhawk

This was the cutout shape that it took to fit my carb. Some of the guys on the VAF site have written about removing the casting part on the right from the carb, thus strengthening the area around that cutout. I think I'll just put a doubler on the aluminum FAB top instead.


Vans Filtered Air Box Top Cutout

Here's another view from the top-ish