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.