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Hi guys,
I know we've had this discussion before, so I'll keep it brief. Could you please limit your quotebacks? The following message has four lines in it. It quotes back an entire discussion that has gone on for awhile, the total message size is like 29kb. All that to pass along a 16 word reply. The FlyRotary archive contains almost 21,000 msgs. The Lancair Mail List contains just over 22,000 messages. The FlyRotary archive file size is almost 1.6 times the size of the LML archive. There's a message there. A little common sense and courtesy is in order. Thanks for your future cooperation in this regard.
<marv>
Posted for "Bill Schertz" <wschertz@comcast.net>:
Joe,
No problem on size, I would like a copy also.
Bill Schertz
KIS Cruiser # 4045
----- Original Message -----
From: Joe Ewen
To: Rotary motors in aircraft
Sent: Wednesday, February 28, 2007 12:04 PM
Subject: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl openings
for water radiators
I have a pdf of the K & W if you need it. I found the Excel sheet Ed made
reduced the K&W info into a manageable form.
Joe
----- Original Message -----
From: Ed Anderson
To: Rotary motors in aircraft
Sent: Tuesday, February 27, 2007 7:46 AM
Subject: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl openings
for water radiators
Thomas is right, Joe.
Lots of good stuff on cooling on Paul's site - if you want your own copy
of K&W they are long out of print. However, there was (may still be there) a
place in the Philippines that would make you a bound Xerox copy of the book
for somewhere around $30. Unless you like pouring over a lot of math thought
it makes for heavy reading {:>).
Ed
----- Original Message -----
From: Thomas y Reina Jakits
To: Rotary motors in aircraft
Sent: Monday, February 26, 2007 10:46 PM
Subject: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl
openings for water radiators
Joe,
I hate to do that, but if you need even more study stuff, go to Paul
Lamar's site and read up on the cooling chapter. He put an impressive
collection of data there and a lot of ideas - at your own risk. There some
ways that work well too (obviously, as they are used...), but you will not
find it on his site, as these ideas do not agree with his findings.
If you need something like Ed has, then you will get a lot of info here:
http://www.rotaryeng.net/cooling.html
(The quoted site belongs to Paul Lamar, all info on it was collected by
him and is provid to the public by him. Where applicable Paul Lamar holds
copyrights as noted on his site...)
Best Regards,
Thomas Jakits,
"Slim ball parasite and low life jerk"
PS: Whatever one thinks about him, he does have an impressive collection
of info!
----- Original Message -----
From: Ed Anderson
To: Rotary motors in aircraft
Sent: Monday, February 26, 2007 9:38 PM
Subject: [FlyRotary] Pinched ducts was : [FlyRotary] Re: cowl openings
for water radiators
Actually, there is, Joe. But, you are going to be sorry you asked
{:>).
I spent quite a hit of time studying a tome (Kuchuman and Weber
better know as K&W) on air cooling of liquid cooled engines written back in
the hey day of high speed mustangs lightenings, spitfires, etc. Sort of the
liquid cooling bible. Chapter 12 (the one of most interest to us) showed a
duct that reportedly had the best pressure recovery (84% or thereabouts)
around for a subsonic duct that they had found. It was called a "StreamLine
Duct" (See attached graph - the graph a of the top graph shows the shape of
the duct (or at least 1/2 around the center line - sorry for the poor
quality).
After quite a bit of studying and thinking about what I had read
about cooling ducts, it finally became clear to me that the perhaps top thing
that is clearly detrimental to good cooling is having flow separation in the
duct. Most of the old drawings of a cooling duct shape followed a sinusoidal
shape - rapid expansion right after the opening. It turns out that
"traditional" shape is probably one of the worst shapes for a cooling duct
(the story why is too long to get into here).
Anyhow, Flow separation leads to eddies and turbulence which casts a
"shadow" of turbulent air on the cooling core. Like a shadow, the further
away from the core the separation occurs (like near the entrance of the duct)
the larger the shadow it casts on the core area. This "shadow" adversely
interferes with the flow of air through the core and reduces the effectiveness
of the core.
What causes this separation is that as pressure is recovered by the
expansion of the duct, the build up of the very pressure recover we want -
starts to hinder the boundary layer flow near the wall of the duct. It slows
it down and causes it to lose energy and attachment to the duct wall. At a
certain point the flow separates and starts to tumble/rotate and depending
where (near the duct entrance or near the core) the flow separates, determines
how much of the core area is adversely affected. So if the boundary layer's
energy level (air speed of its molecules) is maintained at a high level
separation is less likely.
So ideally, you would like to prevent any separation during pressure
recovery. The Streamline Duct is the so called "Trumpet" duct or "Bell" duct
. After the opening, there is a long section of non-expanding duct followed
by a rapid expansion into the "bell" shape just before the core. The long
non-expanding part of the duct maintains the energy (air flow) of the boundary
layer and separation does not occur until well into the "bell" shape
expansion.
In fact, it happens way up in the corner of the bell/core interface
and affects a very small area of the core.
For full effectiveness the "Streamline duct" from K&W needs a length
of 12-17". Well, that's way more distance than I had. So I got to thinking
that if keeping the speed of the air molecules near the duct wall helps
prevent boundary layer separation and the cooling killing eddy of turbulent
air - what could I do with my short 3 - 6" (no jokes you guys). We all know
from Bernoulli that if an area is squeezed down that the velocity of the air
flow increases - right?
So I decided to try to maintain or increase the energy of the air by
pitching down the neck just before it goes into the bell shape expansions in
hopes that the increased energy will help the boundary layer stay adhered to
the duct wall until well into the corner of the bell shape. So that's the
story of the pinched ducts. There is no question in my mind that this is not
as effective as if I could have had the 16" to build the duct - but, in this
hobby, you work with what you've got - right?
Does it work? Who knows - but I seem to fly with less opening area
than most folks and have no cooling problems. So that's my 0.02 on the topic
- see told you, you would regret asking {:>).
Ed
----- Original Message -----
From: John Downing
To: Rotary motors in aircraft
Sent: Monday, February 26, 2007 8:53 PM
Subject: [FlyRotary] Re: cowl openings for water radiators
Ed, is there some particular reason that you necked the inlet down
small, then enlarged it again. Thankyou for the pictures. JohnD
----- Original Message -----
From: Ed Anderson
To: Rotary motors in aircraft
Sent: Monday, February 26, 2007 3:39 PM
Subject: [FlyRotary] Re: cowl openings for water radiators
John, don't know if these photos will help. But, like you I only
have between 3 and 6" of duct distance on the radiators. You should do Ok
with 20 sq inch on each opening with a good diffuser/duct. Attached are some
photos of my current ducts. The openings are 18 sq inches each. I have had
one opening down to as little as 10 square inches - but that was a bit
marginal - so opened it back up. I have a generous exit area for the hot air
including a larger 4" x 12" bottom opening as well as louvers on each side of
the cowl. So you mileage could vary - but Tracy has essentially the same size
opening as well as several others.
Ed
----- Original Message -----
From: John Downing
To: Rotary motors in aircraft
Sent: Monday, February 26, 2007 12:12 PM
Subject: [FlyRotary] cowl openings for water radiators
What size openings do I need for the water radiators? The
Wittman Tailwind cowl I have has postal slots of 3' x 7 3/4" , which is
approx. 22 1/4 sq in. on each side. Sam James for the 160 Lycoming is using
4 3/4' round holes which are 17.6 sq. inches on each side. My radiators are
quite close to the opening and I plan on making the diffusers trumpet shaped,
will the openings be large enough if I can stay over 20 sq. inches on each
side with a decent trumpet shape. JohnD hushpowere II on order - hope
to start in 2 weeks if weather cooperates.
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