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Sorry, Ken
Didn't mean to belabor the obvious. I will have to see what research I
can find on Wedge ducts. While K&W doesn't specifically address "Wedged"
ducts, they do discuss "oblique" ducts which has the core at an angle to the
airstream, so that at the extremes it appears similar to a Wedge to me. As
best I understand their discussion of the topic, there appears to be some
critical angle (depends on the core characteristics) which up to that point
obliqueness doesn't seem to have much adverse effect - but which after that
point cooling drag goes up considerably.
The question is what portion of your total drag is your cooling drag? In
fast, streamlined airframes, I have read that cooling drag could constitue
as much as 1/3 of your total drag. Perhaps on something more draggy such as
a biplane, cooling drag is a much smaller precentage. So perhaps it depends
on what type airframe you have would indicate how much concern you should
have about cooling drag.
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 10:18 PM
Subject: [FlyRotary] Re: Streamline Ducts
> 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
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
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