The inside radius is about 27" and the strip is 3" wide.
At the aft end of the cowl where the hot air exits, we will make a lip that will cause a venturi effect and enhance the low pressure area. This will create a better flow of cooling air through the engine.
I've seen a couple of variations of the aft end of the cowl including movable cowl flaps, and various flanges both straight and curved. Usually on the ones I've seen that have a lip or flange, the lip is at a near 90 degree angle to the flight path, which in my opinion creates a turbulent air flow and lots of drag. Mine will slope back at about 45 degrees.
I have decided that I want a curved piece with about a 2" lip that starts at one side of the fuselage tunnel, curves around and ends up lined up with the other side of the fuselage tunnel. Sort of a big half of an oval shaped circle. I also want the flange or lip to slope back at about a 45 degree angle for smoother air flow. Making a piece like this is a bit of a challenge, so I asked my friend Mickey Whittenburg, who is an excellent metal craftsman to help me.
Mickey started with some soft, 5052 aluminum sheet, .032" thick and cut a
large semi-circle out of it.
The inside radius is about 27" and the strip is 3" wide.
Next Mickey clamped together a couple of blocks of wood 1 1/2" x 2" with
a shim in between them, to create a little gap at the end so he could slide
the edge of the sheet in. Then sliding the aluminum strip in about
3/4", working it a little at a time back and forth, Mickey bent a lip down
until he had about a 35 degree angle on the strip's outer edge:
Sorry for the blurry picture.
Here is the initial fitting of the new piece to bottom of the cowl.
At this point it needs a little trimming on the sides, but all in all, it's
a beautiful piece of work, thanks Mickey. It blends perfectly with the fuselage
tunnel and the flange slopes back at about 45 degrees. HEY - I'll
bet we could make the bottom wiandshield fairing like this - - -hum???
Just food for thought, we'll see.
Here is a side view of the lip after trimming and tapering the sides a bit:
Here is a shot looking down inside the bottom cowl. This shows how
much area for exiting air I will have:
This could also be a speed enhancement. A nice smooth flow of exiting
air out the cowl creates less drag and therefore increased airspeed.
The final bit of business on the bottom cowl is to make a nice cover for
the air intake tunnel. Again, its important to leave a 1/2" or so clearance
around the air tunnel that is attached to carb airbox. This allows room for
the engine to shake in its mounts, especially during start-up and
shut-down.
Like most parts, we started by making a cardboard template to get the basic
shape. I want the aft end of the cover to extend about 1" down the
exit air flange at the aft end of the cowl, leaving about 1" of lip exposed.
The front of the cover will end just ahead of the air filter bracket on the
air intake tunnel.
Here is the template:
This represents the final shape of the side of the cover.
The template was placed on some 5052 aluminum .032" thick, and traced with a sharpie marker. While laying out the cut lines on the aluminum, the following flanges were added to the template shape: 2" at the top, 3" and the front, 5/8" at the bottom and 5/8" and the rear.
The piece was cut out and all edges smoothed. The inside corner of all flange bends were drilled with a 3/16" bit and deburred. The 2" top flange was bent outward at slightly less than 90 degrees to match the shape of the cowl bottom. The 5/8" bottom flange was bent inward at 90 degrees. The 5/8" flange at the aft end was bent until it fit the flange at the aft end of the cowl. The 3" flange at front was left untouched for now.
Here are the two side pieces of the cover installed on the bottom cowl:
As you can see, the top 2" flange was later trimmed down to 1". I just
wanted to make sure that I had plenty of room for adjustment to cover the
hole in the bottom cowl. The bottom of the cover is a separate piece that
will be fabricated later.