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.