X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 50 [XX] Return-Path: Received: from ms-smtp-02.southeast.rr.com ([24.25.9.101] verified) by logan.com (CommuniGate Pro SMTP 5.1.8) with ESMTP id 2023665 for flyrotary@lancaironline.net; Thu, 03 May 2007 11:31:26 -0400 Received-SPF: pass receiver=logan.com; client-ip=24.25.9.101; envelope-from=eanderson@carolina.rr.com Received: from edward2 (cpe-024-074-103-061.carolina.res.rr.com [24.74.103.61]) by ms-smtp-02.southeast.rr.com (8.13.6/8.13.6) with SMTP id l43FUH1Z017360 for ; Thu, 3 May 2007 11:30:18 -0400 (EDT) Message-ID: <000d01c78d98$22e69270$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" Subject: Cooling area drag Date: Thu, 3 May 2007 11:31:33 -0400 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_000A_01C78D76.9A4B9350" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3028 X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2900.3028 X-Virus-Scanned: Symantec AntiVirus Scan Engine This is a multi-part message in MIME format. ------=_NextPart_000_000A_01C78D76.9A4B9350 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Less we forget how important drag is in our hobby, I took a formula for = calculating drag at different airspeeds and the Hp required to push the = given frontal area along at the stated airspeed. This is for two of our traditional GM evaporator cores using their = combined frontal area of 180 sq inch or 1.25 sq feet. This assumes = that airspeed shown represents the velocity through the cooling core = (which is not really likely to reach speeds above 80 mph if you have any = sort of ducting), but that is an assumption on my part since as Bill = keeps reminding me I have not instrumented my ducts {:>) Air Speed (MPH) HP=20 =20 40 0.533333 =20 60 1.80 =20 80 4.27 =20 120 14.40 =20 140 22.87 =20 160 34.13 =20 180 48.60 =20 200 66.67 =20 Clearly the faster your cruise speed the more important it is to = minimize cooling drag. Of course the airspeed the core sees should = normally not be over 10% of your cruise speed or 30% of your climb speed = (According to Horners rule of thumb). So slowing down your cooling = airflow to lessen drag is one reason for paying some attention to your = ducting. However, cooling again depends on many other variables, for = instance accepting a high velocity airflow through your core may permit = you to use a smaller frontal area core thereby offsetting to some = extent the higher drag. In fact, space constraints may force you to his = configuration regardless. Another factor to consider is trade off between frontal area drag and = thermal transfer efficiency. A large thin radiator is theoretical the = most efficient due to that factor. However, it disturbs a larger = segment of air (resulting in higher drag) - not really important in an = auto at 60 mph but very important in a Cozy at 200+ MPH. =20 A thicker core with smaller frontal area disturbs less air and while it = has more skin drag that is small compared to the frontal area drag. = Tracy refers to the approach of thicker cores as "... getting the most = cooling possible for the smallest column of air disturbed". So while = theoretically the thicker core is less thermodynamic efficient - it = turns out with sufficient dynamic pressure available it provides = definite benefits in our application. The average thickness of NASCAR = radiators is 3" and up to 7" for the longer high speed tracts. Since = they operate in speed regimes close to what most of us fly - they just = might know what they are doing given the $$ they will spend for even a = slight speed advantage. Ok, back to creating a company - boy, a lot to learn Ed =20 Ed Anderson Rv-6A N494BW Rotary Powered Matthews, NC eanderson@carolina.rr.com http://members.cox.net/rogersda/rotary/configs.htm#N494BW http://www.dmack.net/mazda/index.html ------=_NextPart_000_000A_01C78D76.9A4B9350 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Less we forget how important drag is in our = hobby, I took=20 a formula for calculating drag at different airspeeds and the Hp = required to=20 push the given frontal area along at the stated airspeed.
 
This is for two of our traditional GM evaporator = cores=20 using their combined frontal area of  180 sq inch or 1.25 sq = feet. =20 This assumes that airspeed shown represents the velocity through = the=20 cooling core (which is not really likely to reach speeds above 80 mph if = you=20 have any sort of ducting), but that is an assumption on my part since as = Bill=20 keeps reminding me I have not instrumented my ducts {:>)
 

Air Speed (MPH)

          HP=20

40

0.533333

60

1.80

80

4.27

120

14.40

140

22.87

160

34.13

180

48.60

200

66.67

 
 
Clearly the faster your cruise speed the more = important it=20 is to minimize cooling drag.  Of course the airspeed the core sees = should=20 normally not be over 10% of your cruise speed or 30% of your climb speed = (According to Horners rule of thumb).  So slowing down your cooling = airflow=20 to lessen drag is one reason for paying some attention to your = ducting. =20 However, cooling again depends on many other variables, for instance = accepting a=20 high velocity airflow through your core may permit you to use a smaller = frontal=20 area  core thereby offsetting to some extent the higher drag.  = In=20 fact, space constraints may force you to his configuration=20 regardless.
 
Another factor to consider is trade off between = frontal=20 area drag and thermal transfer efficiency.  A large thin radiator = is=20 theoretical the most efficient due to that factor.  However, it = disturbs a=20 larger segment of air (resulting in higher drag) - not really important = in an=20 auto at 60 mph but very important in a Cozy at 200+=20 MPH.  
 
A thicker core with smaller frontal area = disturbs=20 less air and while it has more skin drag that is small compared to the = frontal=20 area drag.  Tracy refers to the approach of thicker cores as=20 "... getting the most cooling possible for the smallest column of = air=20 disturbed".  So while theoretically the thicker core is less = thermodynamic=20 efficient - it turns out with sufficient dynamic pressure available it = provides=20 definite benefits in our application.   The average thickness = of=20 NASCAR radiators is 3" and up to 7" for the longer high speed = tracts. =20 Since they operate in speed regimes close to what most of us fly - they = just=20 might know what they are doing given the $$ they will spend for even a = slight=20 speed advantage.
 
Ok, back to creating a company - boy, a lot to=20 learn
 
Ed
 
 
 
  
 
 
Ed Anderson
Rv-6A N494BW Rotary = Powered
Matthews,=20 NC
eanderson@carolina.rr.comhttp:/= /members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.dmack.net/mazda= /index.html
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