From: kenpowell@comcast.net
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: Streamline Ducts
Date: Thu, 29 Jan 2004 03:18:05 +0000
Message-Id: <012920040318.9418.218f@comcast.net>

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