Return-Path: Received: from ms-smtp-03.southeast.rr.com ([24.93.67.84] verified) by logan.com (CommuniGate Pro SMTP 4.1.5) with ESMTP id 2624808 for flyrotary@lancaironline.net; Sun, 05 Oct 2003 09:20:20 -0400 Received: from o7y6b5 (clt78-020.carolina.rr.com [24.93.78.20]) by ms-smtp-03.southeast.rr.com (8.12.10/8.12.2) with SMTP id h95DHvW0005570 for ; Sun, 5 Oct 2003 09:17:58 -0400 (EDT) Message-ID: <001d01c38b43$1d7b5ec0$1702a8c0@WorkGroup> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Thick Vs Thin Radiators - NASCAR Date: Sun, 5 Oct 2003 09:18:06 -0400 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_001A_01C38B21.962821E0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1106 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 This is a multi-part message in MIME format. ------=_NextPart_000_001A_01C38B21.962821E0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable ----- 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 ------=_NextPart_000_001A_01C38B21.962821E0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
 
 
----- Original Message -----
From:=20 Jim=20 Sower
Sent: Saturday, October 04, = 2003 9:48=20 PM
Subject: [FlyRotary] Re: Thick = Vs Thin=20 Radiators - NASCAR

<... The key = appears to be=20 the dynamic pressure available to produce the Mass flow = through=20 the radiator ....>
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.  He said if you want to succeed, you = need to=20 have a big plenum.  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.  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.  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.  If your ramp=20 "stalls", the flow pretty much stops.  If you ramp up "gently" = the flow=20 stays attached and expands uniformly and totally into high pressure = air in the=20 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=20 on the P-51 ramped relatively slowly up into a largish plenum and a = very thick=20 radiator.  So the effectiveness of 7" thick radiators would seem = to turn=20 on the internal aerodynamics of the plenum.=20

But I've already told you more = than I know=20 .... Jim S..=20

 

Hi Jim,

Thanks for bring this up because I = think the issue=20 does confuse folks.

I really think we are talking "Apples = and Apples"=20 here.  Static pressure for us fliers amounts to the ambient = atmospheric=20 pressure.  The equation Pt =3D Pa+1/2pV^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.   The 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 The second term is = composed of the density of the air (p) and the velocity = squared=20 of the air (V^2)

What the second term really means is = that=20 - it is the contribution to total pressure Pt (which is = static=20 or not moving) that moving air makes when its kinetic energy is = converted to=20 static pressure.  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.  However, the pressure differential or = increased=20 pressure on the face of your radiator depends totally on this=20 contribution to this total pressure (Pt)  by the dynamic = factor=20 (1/2pV^2).  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.

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.  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"

The engineer is quite correct about the = purpose of=20 the plenum.  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.  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. 

My most recent experimentation was to = take my old=20 plenum inlet which diverged very sharply at many/most points from the=20 optimum.  Basically the inlet simply opened into a box without = any=20 attempt at making the transition smooth.  Yet, despite these = deficiencies=20 it provided adequate cooling.  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)  and still had adequate = cooling.  So=20 the smoothing of the transition did have a benefital effect.  I = actually=20 reduced the plenum volume by over 50 percent (filling it with = expanding foam=20 and shaping it after it dried)

So regarding the engineer's = statement. =20

1.  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.  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).

2.  The K&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.  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.  Besides who is going to tell your duct its not 7 Deg=20 {:>)

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.  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.

Did this help or did I just make it = more=20 confusing?

Best Regards

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

 

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