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Ed,
As usual, an outstanding summary. I am aware of the function of the speed of the cooling air to drag, hence the questions about the wedge duct. If the wedge duct doesn't slow the air down, then I need to alter my cooling design to reflect this. I need to talk to Bernie again about these issues. Al, anybody else???
Thanks,
Ken
> 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
>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
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