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.