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.