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Hi Ken,
Good questions and no, I have not attempted to measure the air velocity
throught the ducts. Keep in mind that as long as there is sufficient air
mass flow through the radiator, it WILL cool at 0.1 to 0.4 and higher
ratios of duct velocity to airstream velocity. So adequate cooling is not
necessarily the only criteria for an "optimum" cooling system.
The only problem is at the higher velocities through the core, you have a
lot more cooling drag. So you can get "good" cooling even with an less than
optimum cooling setup - BUT, you won't get the minimum weight or cooling
drag possible. Air mass flow is the key, if you don't have sufficient then
you will not cool. Low velocity is important as that results in less cooling
drag.
Since we are basically talking about a constant air density situation at
our speeds, then consider an air mass that flows at 0.1 V through a radiator
of size X and provides adequate cooling. But, radiator of size X is too
large for your installation. If you reduce the size of the radiator then
the airflow at velocity 0.1 simply provides too little air mass flow to
conduct away the heat. BUT, if you increase the velocity through the
smaller radiator thereby increasing airmass flow to the point it carries
away adequate BTU for cooling, then you may find the velocity required
through the duct to be say 0.3. While that WILL increase the cooling drag
over the original size X radiator, at least in this example you will cool
and you have a radiator that fits your constraints. Cooling drag appears to
increase proprotional to area of the core but to the square of the air
velocity throught it. Larger radiators incure more frontal area
resistance - but, since they permit (but you have to make it so via good
ducting) a lower air velocity, the less drag due to the lower velocity more
than offsets the frontal drag of the larger frontal area.
The worst cooling drag situation would appear to be a large radiator with
HIGH air velocity through the core. There you would have great cooling but
also very high cooling drag. So it would appear that it becomes even more
important to get good ducting and diffuser action (lower velocity) with a
larger frontal area radiator than perhaps with a smaller radiator. Just my
opinion.
I have not studied the wedge shape duct so can't really comment on it.
But, again I see no reason why it would not cool - so long as there is
adequate air mass flow - it will cool. Whether you get the minimum possible
cooling drag with it, I simply do not know. I would presume it has some
merit - perhaps simplicity of ducting and installation in certain
configurations. Someone else may know of a source on Wedged Ducts
information - if so, I would like to know.
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
From: <kenpowell@comcast.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, January 28, 2004 6:01 PM
Subject: [FlyRotary] Streamline Ducts
> Hi Ed,
> Thanks for sharing your approach and Neal's response. Sometimes we seem
to forget that what we are trying to do is to convert the speed of the air
to PRESSURE. Your approach seems to be working well. Have you ever
measured the speed of the air moving though the radiator (where slower is
better)? I understand that this type of diffuser should reduce the speed of
the air to somewhere between .1 to .4 of the freestream velocity, so I
wonder how well your modified ducts work (I bet pretty well). Also, do you
know happen to know how well the wedge type duct (for radiators under the
engine) recover pressure? Should the wedge ducts also reduce the speed of
the air to somewhere between .1 to .4 of the freestream velocity or they
inherently less efficient? If anyone else knows the answers to these
questions, please chime in.
>
> Thanks,
> Ken Powell
>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
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