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 ½” wide. There are gaps between pieces of about 3/8 –
½” 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.