Thanks..
I'll bet the exit is in a region of reasonably thick boundary layer. If
so, the flow on the surface may be very slow or even reversed in places
- leading to relatively high exit pressures and low flow through the
cooler. I think making the exit lip taller should help things, though
not pretty. Like a Gurney Flap, or the cowl exhaust on an older
Cessna..
Matt-
--------------
Matt;
My little boundary layer program says the BL at the exit position (not
considering the effects of the fairing) would be about 0.8"
One of the photos that Paul posted (first photo) is my original
configuration - just a curved ramping up to the exit. This never flew
because an "expert" told me that it would create a lot of turbulence
and
drag - that a better approach would be an airfoil shape that would
be the
flow attached. This supposedly would provide lower pressure at the
exit
and would also provide more spacing internally from the exit face of
the
core. Convinced; I re-did the fairing as shown in the second photo.
Based on the theory of low pressure above at the exit, and high pressure
below at the entrance, not much of a scoop was put at the entrance;
just an
airfoil shaped lip that extended below the surface about 1/2" (photo 3).
The oil cooler, and second coolant rad are in the wing root behind the
strakes (photo 4)
Early flights showed the setup was not effective. (BTW; custom oil
cooler
core dimensions of 5.5" x 21.5" x 3.3" thick.) It seemed the most
expedient
solution was to make the inlet into more of a ram scoop, which I did
initially with a temporary metal extension, the photo that Paul
attached
earlier. This improved things enough to allow flight testing to
proceed. I
then replaced it with a nicely glassed scoop of similar
configuration (1.25"
x 23" wide, sorry, no photo) and as we went into cooler winter
weather; all
was fine.
This scoop does ingest the boundary layer, but if it's overall
effectiveness
is better than 65%, it should provide the necessary air flow. I
have place
some VGs forward of the entrance to reduce BL effects.
I have managed to get tubes in place to measure pressure before and
after
the core to measure pressures there, and will then move them to get
pressures at the entrance and exit. It'll be a few days before I
have a
chance to get data.
I thought just cutting the exit fairing shorter would help, so I cut the
center section of the fairing back about 3" to see if it would help. No
detectable difference in the temperatures. I have also tried VGs on
the
upper surface of the exit fairing to no avail. Perhaps VGs further
forward
on the strake?
The other wing has a second parallel coolant radiator still with the
original inlet (no scoop), and it does very little cooling. But it is
interesting that the in-cowl rad with sort of p-51 style armpit scoop,
handles the cooling just fine - total core volume of about 400 cu. in.
Sorry to gone on here, but I appreciate any help I can get. A solution
(with minimal structure changes, of course) is necessary.
Al
Maybe a boundary layer rake would be useful to figure out what's going on..
I've been told that using an effective inlet scoop without an equally
effective exhaust scoop is like "pushing a rope".. Both (air and rope)
tend to pile up and tangle. Getting adequately low pressure on the
outlet side is at least as important as having high enough pressure on
the inlet side.
Since the shapes of the scoops appear to be roughly correct, that says
to me that the air pressures around the scoops are not favorable. My
guess was that the exit was more susceptible to problems than the inlet
- higher vs. lower pressure, boundary layer thickness leading to slow
local flow, leading to poor suction/vacuum effect.
To improve things, either the exit scoop needs to stick up high enough
to get above the boundary layer, or something needs to be done to
thin/energize the boundary layer - more/bigger VG's than you tried
before? Trimming away the exit scoop would seem to me to have a minimal
effect if the local flow is slow.
Here's another idea that might work (later).. I can't tell from the
pics which way the air flows while it's in the wing root - is it
basically vertical? Or is one scoop ahead of the other? I'm guessing
the inlet scoop is ahead of the exhaust? It might be advantageous to
have the exhaust as far forward (up against the back of the wing spar)
as possible in order to put it in an area of thinner boundary layer.
Here's another thought.. Is there any chance that there's high pressure
air passing through the joint between the wing and strake (wing root)?
If so, that might radically interfere with the airflow over the exit
scoop..
Regards,
Matt-
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