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fklein(at)orcasonline.com
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 Posted: Wed Sep 26, 2007 11:20 am Post subject: Jabiropa cooling |
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| Quote: | Something is causing a high pressure bubble to form just outside the
mouth of the intake and the thread is showing that bubble
is preventing air flow from entering the intake. In fact, it appears
to be rejecting flow without the zigzag or vortex generators on about
half the diameter of the intake on the left, particularly on the
outboard edge.
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John,
As I look at your pix posted Mar. 11 (the one w/ you giving the "thumbs
up"), I'm noticing the cowling shape immediately below the 4" inlet. It
may just be a function of the light in the pix but it appears that the
cowl shape forms something of a ramp for air moving vertically from the
prop rotation. Could this be a factor in creating the high pressure
bubble you speak of? Might it be that the cowl shape tends to sweep air
vertically across the inlet which interferes with airflow into the
inlet?
In comparing the two inlets, the portion of your cowl below the left
inlet (appearing as bright white in your pix) would be the principal
difference in the cowling geometry which the air from the propeller
"sees" as it rotates as it approaches the inlets left and right.
I'm wondering if a concentric lip projecting forward of the inlet...say
maybe as much as an inch (?)...could act in such a way as to divert
that vertically rising air around the inlet, reducing pressure in front
of the inlet, and increasing flow into your duct...? (I'm aware that
you've already tweaked the cowling geometry around the inlets.)
At the very least, it appears to me that it would lessen the degree to
which prop rotation influences flow into the inlets.
Just a thought...
Fred
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TELEDYNMCS(at)aol.com
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| Posted: Thu Sep 27, 2007 5:35 am Post subject: Jabiropa cooling |
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In a message dated 9/27/2007 2:59:02 AM Eastern Daylight Time, europa-list(at)matronics.com writes:
| Quote: | I'm wondering if a concentric lip projecting forward of the inlet...say
maybe as much as an inch (?)...could act in such a way as to divert
that vertically rising air around the inlet, reducing pressure in front
of the inlet, and increasing flow into your duct...? (I'm aware that
you've already tweaked the cowling geometry around the inlets.) |
Hey Fred,
I tried using a thin mylar strip inside the intake hole to achieve exactly what you mention and it had no effect. If anything it made it worse.
I think what is happening is a pressure wave is setting up in front of the hole. It's caused by the spherical shape of the cowl intake and it's not at all dissimilar to what happens when a smooth sphere is moved through a fluid. Turbulating the flow on the edge of the sphere and the intake, much like dimples on a golf ball, zigzag tape or vortex generators, should significantly alter the airflow in a positive direction.
I've thought about dimpling the round parts of both intake areas like a golf ball in lieu of the zigzag tape. That would make for an "interesting" look, wouldn't it? It's tough to get the zigzag tape to lay flat in a curve like that. I might wind up making the zigzag turbulator out of paint. I think I can make a mold of sorts out of several layers of masking tape, then cut a ring out with pinking shears, then shoot over it. The result should be the same zigzag pattern, only made from paint. We've done this sort of thing before on gliders, but never in a circle.
The real puzzle is why the right side doesn't exhibit the same flow characteristics as the left. The right side sucks the string in with no turbulation, while the left side rejects the string on the outboard half of the intake ring unless the flow is turbulated as the air approaches the intake hole. I've placed the fan in such a way as to simulate the flow on the right side, but on the left, i.e., fan blade rising relative to the hole, as opposed to falling, and it made no difference. Granted, this is considerably lower flow than with the engine running at flight velocities, but I think the simulation should be valid. I would think the pressure wave would only be stronger at higher velocities. The flow behind the intakes above the top cowl, along the sides of the cowl, and down the front of the cowl is very laminar, in fact a lot more laminar than I expected. The only place I've seen the flow break is as it gets to the junction of the windshield. I'm going to tuft that area in front of the windshield on the top cowl prior to the next flight so I can see what is going on there, too. There might be an advantage to be gained by a few well placed vortex generators in that area.
Regards,
John Lawton
Whitwell, TN (TN89)
N245E - Grounded until Friday
See what's new="_blank">Make AOL Your Homepage.
[quote][b]
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paul.the.aviator(at)gmail
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| Posted: Sat Sep 29, 2007 3:24 pm Post subject: Jabiropa cooling |
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Hi John,
I don't know if this is a silly idea or not, but I remember a physics
experiment once where we emptied 5 gallon jugs. One jug we set up a
swirl action and compared the time to the jug that just "glugged
away"... the swirling action was much faster.
So, I was wondering if on your inlet if you could encourage the air to
spin a little if you would stop the pressure wave, and possibly get
more air into the inlet.
Hope this makes sense.
Paul
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Gilles.Thesee(at)ac-greno
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| Posted: Sun Sep 30, 2007 2:19 am Post subject: Jabiropa cooling |
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Paul McAllister a écrit :
| Quote: | One jug we set up a
swirl action and compared the time to the jug that just "glugged
away"... the swirling action was much faster.
So, I was wondering if on your inlet if you could encourage the air to
spin a little if you would stop the pressure wave, and possibly get
more air into the inlet.
Paul and all,
|
The swirl is for letting water out and air in at the same time.
Hopefully in a cooling duct there is a way out for the air !
The laws of engine or radiator aerodynamics are now well established.
You need a well designed inlet and diffuser at a location when air is
not too disturbed and you can take advantage of its velocity to convert
most of the dynamic pressure into static pressure.
Behind the engine you need a convenient exit with a converging duct to
convert the remaining pressure into velocity so that drag is as limited
as possible.
"The lip" often seen at the exit doesn't help much if the inlet is
misplaced or wrongly shaped.
Concerning the Jabiru, there is little room between the prop and the
front cylinders to provide a correct diffuser, so the inlet must be of
larger size, and the efficiency will be lower.
But the engine can be adequately cooled all the same, at the price of a
little more drag, of course.
Some info on a well cooled Jab cowling and principles of cooling at :
http://contrails.free.fr/diap/phpslideshow.php?directory=diapo_capot_rt
http://contrails.free.fr/engine_aerodyn_radia_en.php
http://contrails.free.fr/engine_air_inlets.php
FWIW,
Best regards,
--
Gilles
http://contrails.free.fr
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grahamsingleton(at)btinte
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| Posted: Sun Sep 30, 2007 2:35 am Post subject: Jabiropa cooling |
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Paul
I'm puzzled by your reference to a "jug" !? We used to wash lemonade
bottles for "return and collect 3 pence", swirling then pouring out was
very effective because the air going in to replace the water going out
has a smooth round hole to go through, instead of as you say, glugging.
It is an interesting thought though, swirling the air going in might
keep it attached to the duct and avoid the turbulence that causes the
blocking. This assumes you have a nice smooth duct of course
Graham
Paul McAllister wrote:
| Quote: |
Hi John,
I don't know if this is a silly idea or not, but I remember a physics
experiment once where we emptied 5 gallon jugs. One jug we set up a
swirl action and compared the time to the jug that just "glugged
away"... the swirling action was much faster.
So, I was wondering if on your inlet if you could encourage the air to
spin a little if you would stop the pressure wave, and possibly get
more air into the inlet.
Hope this makes sense.
Paul
|
--
Graham Singleton
Tel: +441629820187
Mob: +447739582005
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paul.the.aviator(at)gmail
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| Posted: Sun Sep 30, 2007 7:17 am Post subject: Jabiropa cooling |
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Gilles,
Of course !. Had I thought about it a little more I would not have
posed the question. Really nice photos of the Jab installation by the
way.
Paul
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