Message-ID: <001d01c38b43$1d7b5ec0$1702a8c0@WorkGroup>
From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-2624468@logan.com>
Subject: Re: [FlyRotary] Re: Thick Vs Thin Radiators - NASCAR
Date: Sun, 5 Oct 2003 09:18:06 -0400
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  ----- Original Message -----=20
  From: Jim Sower=20
  To: Rotary motors in aircraft=20
  Sent: Saturday, October 04, 2003 9:48 PM
  Subject: [FlyRotary] Re: Thick Vs Thin Radiators - NASCAR


  <... The key appears to be the dynamic pressure available to produce =
the Mass flow through the radiator ....>=20
  I was talking for a while at the Rough River fly-in last week with an =
engineer who argued pretty forcefully that it's the STATIC pressure that =
forces the air through the radiator core.  He said if you want to =
succeed, you need to have a big plenum.  The inlet needs to expand into =
the plenum so the kinetic energy of the air is converted to pressure =
which then forces the air through the core.  He was adamant that your =
ramp from the intake expanding into the plenum can't exceed 7 deg until =
the x-section of the plenum is twice, and preferably three times the =
inlet area.  This is so the flow will stay attached to the ramp and =
expand (kind of like an airfoil - too much curvature (as in chamber) and =
the flow separates (stalls) and you have a great big eddy of dead air =
behind the too-radical curve.  If your ramp "stalls", the flow pretty =
much stops.  If you ramp up "gently" the flow stays attached and expands =
uniformly and totally into high pressure air in the plenum.  I read =
stuff that sounded like this from PL's College or Convoluted Rocket =
Science a few years ago.  As I recall, the P-51 scoop on the P-51 ramped =
relatively slowly up into a largish plenum and a very thick radiator.  =
So the effectiveness of 7" thick radiators would seem to turn on the =
internal aerodynamics of the plenum.=20
  But I've already told you more than I know .... Jim S..=20



  Hi Jim,

  Thanks for bring this up because I think the issue does confuse folks.

  I really think we are talking "Apples and Apples" here.  Static =
pressure for us fliers amounts to the ambient atmospheric pressure.  The =
equation Pt =3D Pa+1/2pV^2 gives the total pressure which is a =
combination of ambient pressure (the Pa term) and dynamic pressure (the =
1/2pV^2 term) IF there is any.   The contribution from the second term =
will only exist if there is moving air (dynamic) otherwise it is ZERO =
leaving only the ambient pressure term Pa The second term is composed of =
the density of the air (p) and the velocity squared of the air (V^2)

  What the second term really means is that - it is the contribution to =
total pressure Pt (which is static or not moving) that moving air makes =
when its kinetic energy is converted to static pressure.  However, if =
you do not have any dynamic pressure then the pressure is the same on =
both sides of your radiator (Pa). So technically the pressure that =
remains once the energy of the moving area is converted to pressure is =
static.  However, the pressure differential or increased pressure on the =
face of your radiator depends totally on this contribution to this total =
pressure (Pt)  by the dynamic factor (1/2pV^2).  If you plane is sitting =
still there is no pressure differential (unless your prop is blowing it =
into your duct) because there is no dynamic pressure.

  Therefore most folks refer to the pressure build up (which in the end =
is static) caused by the kinetic energy of the moving air (which is =
dynamic) as the Dynamic Pressure.  Technically, I guess we should call =
it the "Dynamic Pressure Contribution to the Total Pressure Equation", =
but most folks just refer to it as "Dynamic Pressure"

  The engineer is quite correct about the purpose of the plenum.  It's =
purpose is to convert the dynamic kinetic energy of the air into an =
localized pressure increase in front of the face of the radiator.  =
Studies have indeed show that (depending on the type of duct) that a =
maximum 7 degree divergence is the optimum to preclude separation and =
turbulence in the plenum.=20

  My most recent experimentation was to take my old plenum inlet which =
diverged very sharply at many/most points from the optimum.  Basically =
the inlet simply opened into a box without any attempt at making the =
transition smooth.  Yet, despite these deficiencies it provided adequate =
cooling.  However, once I filled in the plenum with foam and shaped this =
foam to provide smooth curves from the inlet to radiator face - trying =
but certainly failing to maintain anywhere close to the 7 Deg optimum =
(the distance from radiator to inlet is simply too short to achieve that =
optimum) I found I could reduce the inlet from 24 square inches to 8-9 =
square inches (a 33 % reduction in my overall radiator intake area - =
both combined of 48 square inches)  and still had adequate cooling.  So =
the smoothing of the transition did have a benefital effect.  I actually =
reduced the plenum volume by over 50 percent (filling it with expanding =
foam and shaping it after it dried)

  So regarding the engineer's statement. =20

  1.  Technically it is static pressure - however, it results from the =
dynamic airflow conversion to pressure and is commonly (if perhaps =
inaccurately) referred to as Dynamic Pressure.  No dynamic air =3D No =
moving air =3D no pressure increase =3D no pressure differential =3D no =
cooling (well maybe some radiant and convection cooling, but nothing =
close to the cooling provided by the airflow).

  2.  The K&M duct flow studies, do indicate that 7Deg is optimum, =
however, they also have two different duct types and it dependents on =
which duct type you use.  But, the point here, is while 7 degs appears =
to be the OPTIMUM, that does not in any way mean that there is no =
benefit to pressure recovery by smoothing the airflow even if less than =
optimum.  Besides who is going to tell your duct its not 7 Deg {:>)

  So I do not see any significant difference between my statement and =
what the engineer told you, however you want to refer to the pressure =
increase due to the moving (dynamic) air, that is the part that pushes =
air through your radiator.  Since it is the contribution caused by =
dynamic (or moving) air, I will (as do most folks) continue to refer to =
it as the dynamic pressure.

  Did this help or did I just make it more confusing?

  Best Regards

  Ed Anderson
  RV-6A N494BW Rotary Powered
  Matthews, NC
  eanderson@carolina.rr.com



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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<META http-equiv=3DContent-Type content=3D"text/html; =
charset=3Diso-8859-1">
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<STYLE></STYLE>
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<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2></FONT><A=20
href=3D"mailto:eanderson@carolina.rr.com"></A>&nbsp;</DIV>
<BLOCKQUOTE dir=3Dltr=20
style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
  <DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
  <DIV=20
  style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>=20
  <A title=3Dcanarder@frontiernet.net =
href=3D"mailto:canarder@frontiernet.net">Jim=20
  Sower</A> </DIV>
  <DIV style=3D"FONT: 10pt arial"><B>To:</B> <A =
title=3Dflyrotary@lancaironline.net=20
  href=3D"mailto:flyrotary@lancaironline.net">Rotary motors in =
aircraft</A> </DIV>
  <DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Saturday, October 04, =
2003 9:48=20
  PM</DIV>
  <DIV style=3D"FONT: 10pt arial"><B>Subject:</B> [FlyRotary] Re: Thick =
Vs Thin=20
  Radiators - NASCAR</DIV>
  <DIV><BR></DIV>&lt;... <FONT face=3DArial><FONT size=3D-1>The key =
appears to be=20
  the <B><I>dynamic </I></B>pressure available to produce the Mass flow =
through=20
  the radiator ....&gt;</FONT></FONT> <BR><FONT face=3DArial><FONT =
size=3D-1>I was=20
  talking for a while at the Rough River fly-in last week with an =
engineer who=20
  argued pretty forcefully that it's the STATIC pressure that forces the =
air=20
  through the radiator core.&nbsp; He said if you want to succeed, you =
need to=20
  have a big plenum.&nbsp; The inlet needs to expand into the plenum so =
the=20
  kinetic energy of the air is converted to pressure which then forces =
the air=20
  through the core.&nbsp; He was adamant that your ramp from the intake=20
  expanding into the plenum can't exceed 7 deg until the x-section of =
the plenum=20
  is twice, and preferably three times the inlet area.&nbsp; This is so =
the flow=20
  will stay attached to the ramp and expand (kind of like an airfoil - =
too much=20
  curvature (as in chamber) and the flow separates (stalls) and you have =
a great=20
  big eddy of dead air behind the too-radical curve.&nbsp; If your ramp=20
  "stalls", the flow pretty much stops.&nbsp; If you ramp up "gently" =
the flow=20
  stays attached and expands uniformly and totally into high pressure =
air in the=20
  plenum.&nbsp; I read stuff that sounded like this from PL's College or =

  Convoluted Rocket Science a few years ago.&nbsp; As I recall, the P-51 =
scoop=20
  on the P-51 ramped relatively slowly up into a largish plenum and a =
very thick=20
  radiator.&nbsp; So the effectiveness of 7" thick radiators would seem =
to turn=20
  on the internal aerodynamics of the plenum.</FONT></FONT><FONT=20
  face=3DArial><FONT size=3D-1></FONT></FONT>=20
  <P><FONT face=3DArial><FONT size=3D-1>But I've already told you more =
than I know=20
  .... Jim S..</FONT></FONT>=20
  <P>&nbsp;
  <P><FONT face=3DArial size=3D2>Hi Jim,</FONT>
  <P><FONT face=3DArial size=3D2>Thanks for bring this up because I =
think the issue=20
  does confuse folks.</FONT>
  <P><FONT face=3DArial size=3D2>I really think we are talking "Apples =
and Apples"=20
  here.&nbsp; Static pressure for us fliers amounts to the ambient =
atmospheric=20
  pressure.&nbsp; The equation Pt =3D Pa+1/2<EM>p</EM>V^2 gives the =
total pressure=20
  which is a combination of ambient pressure (the Pa term) and dynamic =
pressure=20
  (the 1/2pV^2 term) IF there is any.&nbsp;&nbsp; The&nbsp;contribution =
from the=20
  second term will only exist if there is moving air (dynamic) otherwise =
it is=20
  ZERO leaving only the ambient pressure term Pa&nbsp;The second term is =

  composed of the density of the air (<EM>p</EM>)&nbsp;and the velocity =
squared=20
  of the air (V^2)</FONT>
  <P><FONT face=3DArial size=3D2>What the second term really means is =
that=20
  -&nbsp;it&nbsp;is the contribution to total pressure Pt&nbsp;(which is =
static=20
  or not moving) that moving air makes when its kinetic energy is =
converted to=20
  static pressure.&nbsp; However, if you do not have any dynamic =
pressure then=20
  the pressure is the same on both sides of your radiator (Pa). So =
technically=20
  the pressure that remains once the energy of the moving area is =
converted to=20
  pressure is static.&nbsp; However, the pressure differential or =
increased=20
  pressure on the face of your radiator&nbsp;depends totally on this=20
  contribution to this&nbsp;total pressure (Pt) &nbsp;by the dynamic =
factor=20
  (1/2<EM>p</EM>V^2).&nbsp; If you plane is sitting still there is no =
pressure=20
  differential (unless your prop is blowing it into your duct) because =
there is=20
  no dynamic pressure.</FONT>
  <P><FONT face=3DArial size=3D2>Therefore most folks refer to the =
pressure build up=20
  (which in the end is static) caused by the kinetic energy of the =
moving air=20
  (which is dynamic) as the Dynamic Pressure.&nbsp; Technically, I guess =
we=20
  should call it the "Dynamic Pressure Contribution to the Total =
Pressure=20
  Equation", but most folks just refer to it as "Dynamic =
Pressure"</FONT>
  <P><FONT face=3DArial size=3D2>The engineer is quite correct about the =
purpose of=20
  the plenum.&nbsp; It's purpose is to convert the dynamic kinetic =
energy of the=20
  air into an localized pressure increase in front of the face of the=20
  radiator.&nbsp; Studies have indeed show that (depending on the type =
of duct)=20
  that a maximum 7 degree divergence is the optimum to preclude =
separation and=20
  turbulence in the plenum.&nbsp;</FONT>
  <P><FONT face=3DArial size=3D2>My most recent experimentation was to =
take my old=20
  plenum inlet which diverged very sharply at many/most points from the=20
  optimum.&nbsp; Basically the inlet simply opened into a box without =
any=20
  attempt at making the transition smooth.&nbsp; Yet, despite these =
deficiencies=20
  it provided adequate cooling.&nbsp; However, once I filled in the =
plenum with=20
  foam and shaped this foam to provide smooth curves from the inlet to =
radiator=20
  face - trying but certainly failing to maintain anywhere close to the =
7 Deg=20
  optimum (the distance from radiator to inlet is simply too short to =
achieve=20
  that optimum) I found I could reduce the inlet from 24 square inches =
to 8-9=20
  square inches (a 33 % reduction in my overall radiator intake area - =
both=20
  combined of 48 square inches)&nbsp; and still had adequate =
cooling.&nbsp; So=20
  the smoothing of the transition did have a benefital effect.&nbsp; I =
actually=20
  reduced the plenum volume by over 50 percent (filling it with =
expanding foam=20
  and shaping it after it dried)</FONT>
  <P><FONT face=3DArial size=3D2>So regarding the engineer's =
statement.&nbsp;=20
</FONT>
  <P><FONT face=3DArial size=3D2>1.&nbsp; Technically it is static =
pressure -=20
  however, it results from the dynamic airflow conversion to pressure =
and is=20
  commonly (if perhaps inaccurately) referred to as Dynamic =
Pressure.&nbsp; No=20
  dynamic air =3D No moving air =3D no pressure increase =3D no pressure =
differential=20
  =3D no cooling (well maybe some radiant and convection cooling, but =
nothing=20
  close to the cooling provided by the airflow).</FONT>
  <P><FONT face=3DArial size=3D2>2.&nbsp; The K&amp;M duct flow studies, =
do indicate=20
  that 7Deg is optimum, however, they also have two different duct types =
and it=20
  dependents on which duct type you use.&nbsp; But, the point here, is =
while 7=20
  degs appears to be the OPTIMUM, that does not in any way mean that =
there is no=20
  benefit to pressure recovery by smoothing the airflow even if less =
than=20
  optimum.&nbsp; Besides who is going to tell your duct its not 7 Deg=20
  {:&gt;)</FONT>
  <P><FONT face=3DArial size=3D2>So I do not see any significant =
difference between=20
  my statement and what the engineer told you, however you want to refer =
to the=20
  pressure increase due to the moving (dynamic) air, that is the part =
that=20
  pushes air through your radiator.&nbsp; Since it is the contribution =
caused by=20
  dynamic (or moving) air, I will (as do most folks) continue to refer =
to it as=20
  the dynamic pressure.</FONT>
  <P><FONT face=3DArial size=3D2>Did this help or did I just make it =
more=20
  confusing?</FONT>
  <P><FONT face=3DArial size=3D2>Best Regards</FONT>
  <P>Ed Anderson<BR>RV-6A N494BW Rotary Powered<BR>Matthews, NC<BR><A=20
  =
href=3D"mailto:eanderson@carolina.rr.com">eanderson@carolina.rr.com</A><F=
ONT=20
  face=3DArial size=3D2></FONT>
  <P><FONT face=3DArial =
size=3D2></FONT>&nbsp;</P></BLOCKQUOTE></BODY></HTML>

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