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Ed,
Thanks for the analysis. We'll have to continue this conversation at Tracy's next fall.
Ken P.
> 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
>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
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