Installed sheet
metal ‘baffle’ to form new upper wall of the diffuser as shown in the
photo. The idea was to assist in maintaining attached flow, and to block
leakage through the gap at the top. The baffle was done in 3 pieces in
order to insert past the divider/supports in the scoop; each piece is about 7
˝” wide. There are gaps between pieces of about 3/8 – ˝” inch. The
inlet pressure probe was placed at point “D”.
Test flight showed
no noticeable difference in delta T on the oil. The pressure measured at
“D” was 3” H2O. Pressure behind the exit fairing was again -3/4”.
The pressures at C and D (measured at different times) are at about 6” from
the inboard end of the 22” long cooler. There may be some variation
axially. Because of the gaps in the baffle, and fitting around the end
tanks, there is still some air bypassing the cooler; but I don’t know how
significant. Given the 9+” H2O dynamic pressure out in front of the
scoop still indicates not good pressure recovery.
Nonetheless; it is
certainly disappointing that there was no change (within the accuracy of the
temp measurements) in the effective cooling. This suggests that the wall
shape and the air leakage are not the problem.
Calculating back
from the temp changes in oil and air suggest there is only about 1000 cfm
going through the cooler core. The extrapolation of my measured data on air
flow vs pressure drop across the core suggests that at 3” H2O there should be
about 2000 cfm through the core. Because of the centrifugal blower I was
using for flow tests I was only able to get data up to about 0.6” H2O and 700
cfm. I fit the data to Y=ax+bx2 using regression analysis,
which gave a very good fit up to that point; but extrapolating out to 3” may
be stretching it. If I assume the pressure drop goes as the cube of the
flow velocity, the extrapolation is considerable different – about 1330 cfm at
3” H2O.
Al
-----Original
Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Al Gietzen
Sent: Friday, July 20,
2007 10:42
AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Oil cooler
inlet
Well;
I may end up with VGs and change in upper duct wall shape. My intention
yesterday was to install VGs as a first step, test fly, measure pressure and
temps; then proceed with installing sheet metal upper duct wall
change.
In
deciding where to put the VGs, I looked at things with the gear up (Photo 1).
The gear door has a bump, and there is some gap around the door.
Don’t know what all this does to BL. Ended up putting VG toward the left
side about 26” in font of scoop, and toward the right side right on the gear
door bump.
I then
spent a bunch of time trying to get the pressure measuring tube situated.
The only access is through the scoop opening, and I can’t get my hands
in there; so it is very tough. Plus the tube going in there, or along
the surface in front will affect the flow behavior, so what affect are we
going to measure. Having multiple measurements would be great; but very
difficult to achieve.
While
doing that, I spent some time looking in there with a small mirror. What
I noted was that initial gaps above and below the cooler (required to slide
the unit in and out) had changed a bit. The cooler is supported on pads
of ‘Cool-Mat’ insulation. Those have compressed just a little, so now
there is very little gap at the bottom, and 1/8”+ along the top. That is
a fairly substantial leak, and the loss of pressure at the top likely
exacerbates the flow separation. I decided it wasn’t worth going to test
the VGs as long as that leakage gap was there.
Taking
the wing off (mostly getting it back on because of next to impossible access
to nuts), and removing the cooler looks a bit much right now. I realized
then; that by putting in a sheet metal ‘false’ upper duct wall, I could extend
it up into the gap at the top (photo 2), thereby changing the shape, and
(mostly) closing the gap at the top.
The
false wall has to be in three parts for the three openings, and there will be
gaps between because of the supporting dividers; but it could make a
substantial difference. I made the piece for the center, and considered
testing just that; but the upper gap concerns me enough that I think I’ll try
to get all three fit in.
Then
go take a flight test. Unfortunately this combines three changes, VGs, closing
gap, and changing duct wall. I had hoped to test these one at a
time. If there is a substantial change; it will be easy to remove the
VGs to see what that effect was.
Of
course I’ll let you know when I get some results.
Oh,
the price of innovationJ.
Al
-----Original
Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Thomas Jakits
Sent: Thursday, July 19, 2007 10:31
AM
To: Rotary motors in
aircraft
Subject: [FlyRotary]
Re: Oil cooler inlet
Monty thinks the emphasis is on the
BL.
I believe (don't know), the
main-problem is the upper ductwall shape. Even if you have perfect BL flow,
the upper wall shape is still not good and will stall the flow.
At the end of the game you want good flow
at all speeds and be able to close any ducts to limit excess cooling (when you
hopefully get there).
Obviously BL will play a role in your
installation as the intake is rather narrow.
However BL or not - BL does not mean
there is no flow, just slower and more turbulent, but still generally going
towards the cooler.
Aerodynamics in the duct should be much
the same for laminar, turbulent, any flow, as long as there is
flow.
When things stall is when flow pretty
much ceases (in the stalled area ....), no matter how well things where at the
entrance.
The stall in this case is rather "easy"
to get, as the speed seems rather low already. Still may be good enough if you
can do away with the stall.
So I suggest to work on the duct wall
first and optimize it.
As suggested, with some kind of sheet,
alu, fiberglass, etc. You can curve it more and more until you
peak.
Maybe pinched ducts (copyright Ed!!) are
not working here, but it may as well - if they work a Ed's theory explains
(energizes the flow...)
If this works, modify according to the
best shape found.
Then try to improve with VGs or sanding
or turbolator tape.
Then go for the exit - after all it is a
differential pressure game....
On 7/18/07,
M Roberts <montyr2157@alltel.net>
wrote:
I think you need to do something
to energize the boundary layer. If you can't divert it you need to put some
energy into it. It is probably getting slow and separating from the face of
the duct. That is what your data seems to indicate to me.
I like the shape that Thomas
proposes better than what you have now, however, I still think you will need
some VG's in front of the inlet.
I know it may seem counter
intuitive, but turbulence may actually help in this case. You will not get
very efficient internal diffusion, but it will be a lot better than what you
have now. I don't think that putting a turning vane will help too much without
doing something to energize the boundary layer first. You'll just have a slow
thick low energy layer, and a high energy layer separated by a turning vane.
It is really easy to duct tape
some aluminum VG's in front of the inlet and see what it does.
You may need a combination of
Thomas' contour, VG's and a turning vane. Go with the easy fix and work your
way up in complexity.