X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 2 [X] Return-Path: Received: from imo-d21.mx.aol.com ([205.188.144.207] verified) by logan.com (CommuniGate Pro SMTP 5.1.8) with ESMTP id 2030482 for flyrotary@lancaironline.net; Mon, 07 May 2007 13:28:49 -0400 Received-SPF: pass receiver=logan.com; client-ip=205.188.144.207; envelope-from=WRJJRS@aol.com Received: from WRJJRS@aol.com by imo-d21.mx.aol.com (mail_out_v38_r9.2.) id q.d14.a153fe5 (60448) for ; Mon, 7 May 2007 13:27:35 -0400 (EDT) Received: from webmail-db15 (webmail-db15.webmail.aol.com [205.188.105.80]) by ciaaol-r01.mx.aol.com (v115.11) with ESMTP id MAILCIAAOLR011-ec20463f6186b5; Mon, 07 May 2007 13:27:35 -0400 References: To: flyrotary@lancaironline.net Subject: Re: [FlyRotary] Re: Pinched Duct Date: Mon, 07 May 2007 13:27:34 -0400 In-Reply-To: X-MB-Message-Source: WebUI MIME-Version: 1.0 From: wrjjrs@aol.com X-MB-Message-Type: User Content-Type: multipart/alternative; boundary="--------MB_8C95EE4229DBA57_AA0_4FBD_webmail-db15.sysops.aol.com" X-Mailer: AOL WebMail 25698 Received: from 65.161.241.3 by webmail-db15.sysops.aol.com (205.188.105.80) with HTTP (WebMailUI); Mon, 07 May 2007 13:27:34 -0400 Message-Id: <8C95EE4229DBA57-AA0-29D4@webmail-db15.sysops.aol.com> X-AOL-IP: 205.188.105.80 X-Spam-Flag: NO ----------MB_8C95EE4229DBA57_AA0_4FBD_webmail-db15.sysops.aol.com Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii" Ed, An interesting factor here is that when considering duct inlet diameter/opening size is usually calculated from the high point of the opening. Provided the opening is smoothly configured. Since your pinched duct uses a smooth turn from the high point on the cowl, most of the time you would calculate the area as the highpoint on the opening for the purpose of calculating drag. I was talking to Peter Garrison about this when he visited our EAA chapter. I know the cooling improved, Was there any increase in speed? Assuming that same throttle position of course. Bill Jepson -----Original Message----- From: eanderson@carolina.rr.com To: flyrotary@lancaironline.net Sent: Mon, 7 May 2007 5:08 AM Subject: [FlyRotary] Re: Pinched Duct I understand, Al. I always appreciate your input and viewpoint. One reason I throw out the information and my "logic" is so that others can examine and critique - it would be nice if I knew it all, but, sigh! I am under no delusion that I do. You certainly bring up food for thought and I would suggest any who may considered adopting the "Pinched" duct give serious consideration of your viewpoint. External diffusion is undoubted a factor - as it is in most forward facing ducts . I do agree that any time you accelerate air your incur greater loses, my take was that accelerating the boundary layer delayed separation and the adverse effects so caused. That increased skin friction was a far lesser evil than flow separation as indicated by the NACA study. As to whether the boundary layer penetrates further due to increased velocity is probably dependent on the influence of the rate of pressure rise vs the rate of boundary layer deceleration. Is that relationship linear or non-linear? Given the number of variables, I suspect non-linear but don't know. The rapid expansion of the streamline duct configuration would tend to indicate (to me at least) delaying separation, by keeping the airstream energy high, until just before the core pays dividends. Now whether my ratio of inlet, pinched and core area successfully accomplish that is simply unknown at this point. Using this approach, I have been able to reduce my inlet area and make use of a very constrained space for a duct. Again, I make no pretense that it could possibly be as effective as a full up streamline duct for instance, but on the other hand I believe it creates less cooling drag than a streamline duct truncated sufficiently to meet my space constraints. It might well be that its success is largely due to external diffusion as you suggest , but I would have expected that effect on all my ducts. The pinched duct does cools my installation better than my previous approaches to a duct and has enabled me to reduce my radiator inlet area by 33% less area than previous duct attempts. But, your may be correct, I may have just gotten lucky on this duct shape and it is simply "less-bad" than my previous attempts. I do agree that all of this is conjecture from both view points and only instrumentation, data collection and analysis might clarify what is actually happening. As always, appreciate your viewpoint. Thanks again, Al Ed ________________________________________________________________________ AOL now offers free email to everyone. Find out more about what's free from AOL at AOL.com. ----------MB_8C95EE4229DBA57_AA0_4FBD_webmail-db15.sysops.aol.com Content-Transfer-Encoding: 7bit Content-Type: text/html; charset="us-ascii"
Ed,
 An interesting factor here is that when considering duct inlet diameter/opening size is usually calculated from the high point of the opening. Provided the opening is smoothly configured. Since your pinched duct uses a smooth turn from the high point on the cowl, most of the time you would calculate the area as the highpoint on the opening for the purpose of calculating drag. I was talking to Peter Garrison about this when he visited our EAA chapter. I know the cooling improved, Was there any increase in speed? Assuming that same throttle position of course.
Bill Jepson 
 
 
-----Original Message-----
From: eanderson@carolina.rr.com
To: flyrotary@lancaironline.net
Sent: Mon, 7 May 2007 5:08 AM
Subject: [FlyRotary] Re: Pinched Duct

I understand, Al.  I always appreciate your input and viewpoint. 
 
One reason I throw out the information and my "logic" is so that others can examine and critique - it would be nice if I knew it all, but, sigh! I am under no delusion that I do.  You certainly bring up food for thought and I would suggest any who may considered adopting the "Pinched" duct give serious consideration of your viewpoint.
 
External diffusion is undoubted a factor - as it is in most forward facing ducts .  I do agree that any time you accelerate air your incur greater loses, my take was that  accelerating the boundary layer delayed separation and the adverse effects so caused.  That  increased skin friction was a far lesser evil than flow separation as indicated by the NACA study.
 
  As to whether the boundary layer penetrates further due to increased velocity is probably dependent on the influence of the  rate of pressure rise vs the rate of boundary layer deceleration.  Is that relationship linear or non-linear?   Given the number of variables, I suspect non-linear but don't know.
 
The rapid expansion of the streamline duct configuration would tend to indicate (to me at least) delaying separation, by keeping the airstream energy high, until just before the core pays dividends.  Now whether my ratio of inlet, pinched and core area successfully accomplish that is simply unknown at this point. 
 
Using this approach, I have been able to reduce my inlet area and make use of a very constrained space for a duct.  Again, I make no pretense that it could possibly be as effective as a full up streamline duct for instance, but on the other hand I believe it creates less cooling drag than a streamline duct truncated sufficiently to meet my space constraints.  It might well be that its success is largely due to external diffusion as you suggest , but I would have expected that effect on all my ducts.
 
The pinched duct does cools my installation better than my previous approaches to a duct and has enabled me to reduce my radiator inlet area by 33% less area than previous duct attempts.   But, your may be correct, I may have just gotten lucky on this duct shape and it is simply "less-bad" than my  previous attempts. 
 
 I do agree that all of this is conjecture from both view points and only instrumentation, data collection and analysis might clarify what is actually happening. 
 
As always, appreciate your viewpoint.  Thanks again, Al
 
Ed
    
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