Flew with the new duct this morning for 45
minutes. Cooling was much better but not as good as I had expected
with the enlarged inlet - after I landed and examined the duct I discovered
why.
I glue foam sheets together to build up the
depth of the duct I need, shape the foam to get the contour I want and then
fiberglass the out side and put a protective layer of resin over the foam
inside the duct.. Well, this morning before going out to the hangar,
I found out a seam had opened between two sheets of foam - so I filled
it with some bondo type stuff, sanded it down, painted it and took it out
and installed in on the aircraft and went flying.
After leveling off the coolant stayed between
190F and 200F - a bit higher than I expected. So I limited the
rpm to 6500 rpm. Throttling back to 5000 rpm produced
170F.
After landing I took the duct off and found
what you see in Separation1s.jpg photo. The seam had opened up again
and the filler material and not helped either. Can you spell FLOW
SEPARATION? The red lines enclose the area I suspect that I had
flow separation due to the obstruction. White arrow indicates direction
of air flow.
My research indicates that slowing down
the boundary layer (such as these obstructions would) has a much more
significant impact on cooling than you might first think. The slowing
of the boundary layer as it encounters the positive pressure gradient
(created as the duct expands radically) can lead to a portion of the main
flow "tumbling" over the slower moving boundary layer forming "eddies"
and effecting a much larger area of flow than the small ridges would
indicate.
So I think a sizable area of the right half of
the core(looking at it from the front) may have been blocked or at least had
its cooling impaired by flow separation. Will fix the problem and fly
again to see if the cooling changes.