X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 50 [XX] (100%) SPAMTRICKS: long string of words Return-Path: Received: from ms-smtp-05.southeast.rr.com ([24.25.9.104] verified) by logan.com (CommuniGate Pro SMTP 5.1.7) with ESMTP id 1872000 for flyrotary@lancaironline.net; Tue, 27 Feb 2007 13:35:10 -0500 Received-SPF: pass receiver=logan.com; client-ip=24.25.9.104; 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-05.southeast.rr.com (8.13.6/8.13.6) with SMTP id l1RIY8Ou001885 for ; Tue, 27 Feb 2007 13:34:08 -0500 (EST) Message-ID: <000901c75a9d$e430d7c0$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl openings for water radiators Date: Tue, 27 Feb 2007 13:34:18 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0006_01C75A73.FAF900B0" 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_0006_01C75A73.FAF900B0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Hummm, Dave, perhaps my understanding of what it takes to keep the = boundary layer attached to the duct wall is flawed. =20 From what I believe I understood regarding airflow in a duct is that the = pressure recovery both aids and hinders the boundary layer's attachment = to the duct wall. The pressure build up (area of slower molecules) tend = to push and keep the boundary layer pushed against the wall of the duct = as it curves out - at the same time it is slowing down the boundary = layer. So its the point of separation is (at least in part) contingent = on how much speed the boundary layer has enabling it to push how far = into the pressure recovery area - before it ultimately separates. The = further the better is my understanding. My understanding is that in a duct - it is the recovery pressure = which builds in the expansion area just before the core. This "high" = pressure area will "push" back on the boundary layer causing it slow = and eventually to separate from the wall. . However, if you keep the = boundary layer speed up it pushes further into the pressure recovery = area following the duct curve before the "back pressure" slows it enough = to cause it to separate. =20 Also the speed of a molecule in all random directions is much, much = higher than the component imparted by the airspeed - about 1100 ft/sec = at sea level as I recall compared to about 40 ft/sec in the duct. So my = interpretation is that (at least in a duct) its the back pressure of the = recovered pressure that causes the separation - not necessary the curve = of the duct alone although that certainly contributes to the pressure = recovery. That being said,its clear that the three factors (duct = curve, expansion area and separation) really go hand in hand. The = greater the curve the more pressure recovery occurs and the greater the = tendency for separation. The higher the velocity of the boundary layer = the further it can penetrate into the higher pressure area before being = slowed and separation occurs. =20 There is NO doubt that having a longer duct would improve the situation. = However given I only had 3 -6" my take was that speeding up the air = (and boundary layer energy) would ensure it penetrated deeper into the = bell shape before the pressure recovery caused separation. But, as I = have often stated - I could be completely wrong about what I think I = understand. You are after-all the Navy flyer and I know they cram a lot of areo = into Navy pilot's heads. Me- I'm a electrical engineer, so what I know = about aerodynamics is what I have read (and think I understand). But, regardless these pinched ducts have provided the best cooling with = the smallest opening that I have achieved - so, Dave, if you stay quite = it may not learn the truth {:>) Ed ----- Original Message -----=20 From: David Leonard=20 To: Rotary motors in aircraft=20 Sent: Tuesday, February 27, 2007 11:46 AM Subject: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl = openings for water radiators Ed, Good discussion about streamline ducts. No doubt that they are = superior although I have a slightly different take on what quality makes = them work best. I also agree that it is wall separation that we are = trying to avoid.=20 But IMHO the important way to get there is "avoiding sharp turns." I = think of the air molecules as little race cars coming in the duct. The = less turning they do, the better. If they need to turn, the radius of = the turn needs to be as large as possible. And just as important, the = turn radius is distributed so that more of the turn is done after the = air has started to slow down (near the face of the radiator). In other = words, the turn radius is a function of speed. Just like with a car, = don't turn it much before slowing down or it will separate.=20 With that in mind, see why the "conventional duct" is so terrible. = There is a single sharp turn right at the end of the straight-a-way. = Separation occurs there and the whole plenum becomes turbulent. With the bell shaped duct (K&N), it is easy to see why we need length. = The longer the duct, the larger the turn radius can be throughout the = whole distance. =20 Given our limited space however, there will undoubtedly be a point = where the necessary turn radius becomes too small for the speed of the = air and it separates. But at least get the air to expand as much as = possible before that happens.=20 With your restriction in the neck you are setting yourself back before = you start the necessary expansion. You have created less distance over = which to average the turning radius, you have increased the speed - = meaning the air can tolerate less of a turning radius before separating = (lower velocities are known to maintain laminar flow much better than = high velocity), and you have increased the total amount of expansion the = streamlines need to undergo (narrower starting point). So my guess is a = rather dismal effect on cooling compared to what you could have.=20 BUT, since your cooling is still adequate I am sure you have made a = very nice overall drag reduction. There is no way a conventional duct = with that amount of area would work well. In other works, while I am = very skeptical that the restriction actually helped cooling, big kudos = to you for absolutely minimizing drag and duct area while maintaining = sufficient cooling.=20 In fact, seeing as how you have proven that it works I am considering = doing that for my oil and intercooler ducts as they are currently = getting more air than they need... =20 JMHO --=20 David Leonard Turbo Rotary RV-6 N4VY My websites at: http://members.aol.com/_ht_a/rotaryroster/index.html http://members.aol.com/_ht_a/vp4skydoc/index.html http://leonardiniraq.blogspot.com=20 =20 On 2/26/07, Ed Anderson wrote:=20 Actually, there is, Joe. But, you are going to be sorry you asked = {:>). I spent quite a hit of time studying a tome (Kuchuman and Weber = better know as K&W) on air cooling of liquid cooled engines written = back in the hey day of high speed mustangs lightenings, spitfires, etc. = Sort of the liquid cooling bible. Chapter 12 (the one of most interest = to us) showed a duct that reportedly had the best pressure recovery (84% = or thereabouts) around for a subsonic duct that they had found. It was = called a "StreamLine Duct" (See attached graph - the graph a of the top = graph shows the shape of the duct (or at least 1/2 around the center = line - sorry for the poor quality). =20 After quite a bit of studying and thinking about what I had read = about cooling ducts, it finally became clear to me that the perhaps top = thing that is clearly detrimental to good cooling is having flow = separation in the duct. Most of the old drawings of a cooling duct = shape followed a sinusoidal shape - rapid expansion right after the = opening. It turns out that "traditional" shape is probably one of the = worst shapes for a cooling duct (the story why is too long to get into = here).=20 Anyhow, Flow separation leads to eddies and turbulence which casts = a "shadow" of turbulent air on the cooling core. Like a shadow, the = further away from the core the separation occurs (like near the entrance = of the duct) the larger the shadow it casts on the core area. This = "shadow" adversely interferes with the flow of air through the core and = reduces the effectiveness of the core.=20 What causes this separation is that as pressure is recovered by = the expansion of the duct, the build up of the very pressure recover we = want - starts to hinder the boundary layer flow near the wall of the = duct. It slows it down and causes it to lose energy and attachment to = the duct wall. At a certain point the flow separates and starts to = tumble/rotate and depending where (near the duct entrance or near the = core) the flow separates, determines how much of the core area is = adversely affected. So if the boundary layer's energy level (air speed = of its molecules) is maintained at a high level separation is less = likely.=20 So ideally, you would like to prevent any separation during pressure = recovery. The Streamline Duct is the so called "Trumpet" duct or "Bell" = duct . After the opening, there is a long section of non-expanding duct = followed by a rapid expansion into the "bell" shape just before the = core. The long non-expanding part of the duct maintains the energy (air = flow) of the boundary layer and separation does not occur until well = into the "bell" shape expansion. =20 In fact, it happens way up in the corner of the bell/core interface = and affects a very small area of the core. For full effectiveness the "Streamline duct" from K&W needs a length = of 12-17". Well, that's way more distance than I had. So I got to = thinking that if keeping the speed of the air molecules near the duct = wall helps prevent boundary layer separation and the cooling killing = eddy of turbulent air - what could I do with my short 3 - 6" (no jokes = you guys). We all know from Bernoulli that if an area is squeezed down = that the velocity of the air flow increases - right? =20 So I decided to try to maintain or increase the energy of the air by = pitching down the neck just before it goes into the bell shape = expansions in hopes that the increased energy will help the boundary = layer stay adhered to the duct wall until well into the corner of the = bell shape. So that's the story of the pinched ducts. There is no = question in my mind that this is not as effective as if I could have had = the 16" to build the duct - but, in this hobby, you work with what = you've got - right?=20 Does it work? Who knows - but I seem to fly with less opening area = than most folks and have no cooling problems. So that's my 0.02 on the = topic - see told you, you would regret asking {:>).=20 Ed ----- Original Message -----=20 From: John Downing=20 To: Rotary motors in aircraft=20 Sent: Monday, February 26, 2007 8:53 PM Subject: [FlyRotary] Re: cowl openings for water radiators =20 Ed, is there some particular reason that you necked the inlet down = small, then enlarged it again. Thankyou for the pictures. JohnD ----- Original Message -----=20 From: Ed Anderson=20 To: Rotary motors in aircraft=20 Sent: Monday, February 26, 2007 3:39 PM Subject: [FlyRotary] Re: cowl openings for water radiators =20 John, don't know if these photos will help. But, like you I = only have between 3 and 6" of duct distance on the radiators. You = should do Ok with 20 sq inch on each opening with a good diffuser/duct. = Attached are some photos of my current ducts. The openings are 18 sq = inches each. I have had one opening down to as little as 10 square = inches - but that was a bit marginal - so opened it back up. I have a = generous exit area for the hot air including a larger 4" x 12" bottom = opening as well as louvers on each side of the cowl. So you mileage = could vary - but Tracy has essentially the same size opening as well as = several others.=20 Ed ----- Original Message -----=20 From: John Downing=20 To: Rotary motors in aircraft=20 Sent: Monday, February 26, 2007 12:12 PM Subject: [FlyRotary] cowl openings for water radiators =20 What size openings do I need for the water radiators? The = Wittman Tailwind cowl I have has postal slots of 3' x 7 3/4" , which is = approx. 22 1/4 sq in. on each side. Sam James for the 160 Lycoming is = using 4 3/4' round holes which are 17.6 sq. inches on each side. My = radiators are quite close to the opening and I plan on making the = diffusers trumpet shaped, will the openings be large enough if I can = stay over 20 sq. inches on each side with a decent trumpet shape. JohnD = hushpowere II on order - hope to start in 2 weeks if weather = cooperates.=20 ---------------------------------------------------------------------- -- Homepage: http://www.flyrotary.com/ Archive and UnSub: = http://mail.lancaironline.net/lists/flyrotary/ ------------------------------------------------------------------------ -- Homepage: http://www.flyrotary.com/ Archive and UnSub: = http://mail.lancaironline.net/lists/flyrotary/ -- Homepage: http://www.flyrotary.com/ Archive and UnSub: http://mail.lancaironline.net/lists/flyrotary/ ------=_NextPart_000_0006_01C75A73.FAF900B0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Hummm, Dave, perhaps my understanding of what it = takes to=20 keep the boundary layer attached to the duct wall is=20 flawed.  
 
From what I believe I = understood regarding=20 airflow in a duct is that the pressure recovery both aids and = hinders the=20 boundary layer's attachment to the duct = wall.  The pressure build=20 up (area of slower molecules) tend to push and keep the boundary = layer=20 pushed against the wall of the duct as it curves out - at the same = time it=20 is slowing down the boundary layer.  So its the point of = separation=20 is (at least in part) contingent on how much speed the boundary = layer has=20 enabling it to push how far into the pressure recovery area - before it=20 ultimately separates.  The further the better is my=20 understanding.
 
  My understanding is that in a duct - = it is the=20 recovery pressure  which  builds in the expansion area just=20  before the core.  This "high" pressure area  will "push" = back on=20 the boundary layer causing it slow and eventually  to separate from = the=20 wall.  .  However, if you keep the boundary layer speed up it = pushes=20 further into the pressure recovery area following the duct curve before = the=20 "back pressure" slows it enough to cause it to separate.  =
 
Also  the speed of a molecule in all random = directions is much, much higher than the component imparted by the = airspeed -=20 about 1100 ft/sec at sea level as I recall compared to about 40 ft/sec = in the=20 duct.  So my interpretation is that (at least in a duct) its the = back=20 pressure of the recovered pressure that causes the separation - not = necessary=20 the curve of the duct alone although that certainly contributes to the = pressure=20 recovery.  That being said,its clear that  the three factors=20  (duct curve, expansion area and separation) =   really go=20 hand in hand.  The greater the curve the more pressure recovery = occurs and=20 the greater the tendency for separation.   The higher the = velocity of=20 the boundary layer the further it can penetrate into the higher pressure = area=20 before being slowed and separation occurs. 
 
There is NO doubt that having a longer duct = would improve=20 the situation.  However given I only had 3 -6" my take =  was that=20 speeding up the air (and boundary layer energy) would ensure it = penetrated=20 deeper into the bell shape before the pressure recovery caused = separation. =20 But, as I have often stated - I could be completely wrong about what I = think I=20 understand.
 
  You are after-all the Navy flyer and I = know they=20 cram a lot of areo into Navy pilot's heads.  Me- I'm a electrical = engineer,=20 so what  I know about aerodynamics is what I have read (and think I = understand).
 
But, regardless these pinched ducts = have provided the=20  best cooling with the smallest opening that I have achieved - so, = Dave,=20  if you stay quite it may not learn the truth {:>)
 
 
 
Ed
 
 
----- Original Message -----
From:=20 David=20 Leonard
Sent: Tuesday, February 27, = 2007 11:46=20 AM
Subject: [FlyRotary] Re: = Pinched ducts=20 was : [FlyRotary] Re: cowl openings for water radiators

Ed,
 
Good discussion about streamline ducts.  No doubt that they = are=20 superior although I have a slightly different take on what quality = makes them=20 work best.  I also agree that it is wall separation that we are = trying to=20 avoid.
 
But IMHO the important way to get there is "avoiding sharp=20 turns."   I think of the air molecules as little race cars = coming in=20 the duct.  The less turning they do, the better.  If they = need to=20 turn, the radius of the turn needs to be as large as possible.  = And just=20 as important, the turn radius is distributed so that more of the turn = is done=20 after the air has started to slow down (near the face of the = radiator). =20 In other words, the turn radius is a function of speed.  Just = like with a=20 car, don't turn it much before slowing down or it will separate. =
 
With that in mind, see why the "conventional duct" is so = terrible. =20 There is a single sharp turn right at the end of the = straight-a-way. =20 Separation occurs there and the whole plenum becomes turbulent.
 
With the bell shaped duct (K&N), it is easy to see why we = need=20 length.  The longer the duct, the larger the turn radius can=20 be throughout the whole distance. 
 
Given our limited space however, there will undoubtedly be a = point where=20 the necessary turn radius becomes too small for the speed of the air = and it=20 separates.  But at least get the air to expand as much as = possible before=20 that happens.
 
With your restriction in the neck you are setting yourself back = before=20 you start the necessary expansion.   You have created less = distance=20 over which to average the turning radius, you have increased the speed = - meaning the air can tolerate less of a turning radius before = separating=20 (lower velocities are known to maintain laminar flow much better than = high=20 velocity), and you have increased the total amount of = expansion the=20 streamlines need to undergo (narrower starting point).  So=20 my guess is a rather dismal effect on cooling compared to = what you=20 could have.
 
BUT, since your cooling is still adequate I am sure you have made = a very=20 nice overall drag reduction. There is no way a conventional = duct=20 with that amount of area would work well.  In other works, while = I am=20 very skeptical that the restriction actually helped cooling, big = kudos to=20 you for absolutely minimizing drag and duct area while=20 maintaining sufficient cooling.
 
In fact, seeing as how you have proven that it works I am = considering=20 doing that for my oil and intercooler ducts as they are currently = getting more=20 air than they need...
 
JMHO

--
David Leonard

Turbo Rotary RV-6 N4VY
My = websites=20 at:
http://memb= ers.aol.com/_ht_a/rotaryroster/index.html
http://members= .aol.com/_ht_a/vp4skydoc/index.html
http://leonardiniraq.blogspot.= com=20
 
On 2/26/07, Ed=20 Anderson <eanderson@carolina.rr.com&g= t;=20 wrote:=20
Actually, there is, Joe.  But, you are = going to=20 be sorry you asked {:>).
 
  I spent quite a hit of time studying = a tome=20 (Kuchuman and Weber better know as K&W)  on air cooling of = liquid=20 cooled engines written back in the hey day of high speed mustangs=20 lightenings, spitfires, etc. Sort of the liquid cooling = bible. =20  Chapter 12 (the one of most interest to us) showed a duct that = reportedly had the best pressure recovery (84% or thereabouts) = around for a=20 subsonic duct that they had found.  It was called a "StreamLine = Duct"=20 (See attached graph - the graph a of the top graph shows the shape = of the=20 duct (or at least 1/2 around the center line - sorry for the poor=20 quality).  
 
 After quite a bit of studying and = thinking about=20 what I had read about cooling ducts, it finally became clear to me = that the=20 perhaps top thing that is clearly detrimental to good cooling is = having flow=20 separation in the duct.   Most of the old drawings of a = cooling=20 duct shape followed a sinusoidal shape - rapid expansion right after = the=20 opening.  It turns out that "traditional" shape is probably one = of the=20 worst shapes for a cooling duct (the story why is too long to get = into=20 here).
 
Anyhow,  Flow separation leads to = eddies and=20 turbulence which casts a "shadow" of turbulent air on the cooling=20 core.  Like a shadow, the further away from the core the = separation=20 occurs (like near the entrance of the duct) the larger the shadow it = casts=20 on the core area.  This "shadow"  adversely = interferes with=20 the flow of air through the core and reduces the effectiveness of = the core.=20
 
  What causes this separation is that = as pressure=20 is recovered by the expansion of the duct, the build up of the very = pressure=20 recover we want -  starts to hinder the boundary layer flow = near the=20 wall of the duct.  It slows it down and causes it to lose = energy and=20 attachment to the duct wall.  At a certain point the flow = separates and=20 starts to tumble/rotate and depending where (near the duct entrance = or near=20 the core) the flow separates, determines how much of the core area = is=20 adversely affected.  So if the boundary layer's energy level = (air speed=20 of its molecules) is maintained at a high level separation is less = likely.=20
 
So ideally, you would like to prevent any = separation=20 during pressure recovery.  The Streamline Duct is the so called = "Trumpet" duct or "Bell" duct .  After the opening, there is a = long=20 section of non-expanding duct followed by a rapid expansion into the = "bell"=20 shape just before the core.  The long non-expanding part of the = duct=20 maintains the energy (air flow) of the boundary layer and separation = does=20 not occur until well into the "bell" shape expansion.  =
 
 In fact, it happens way up in the = corner of the=20 bell/core interface and affects a very small area of the = core.
For full effectiveness the "Streamline duct" = from=20 K&W needs a length of 12-17".  Well, that's way more = distance than=20 I had.  So I got to thinking that if keeping the speed of the = air=20 molecules near the duct wall helps prevent boundary layer separation = and the=20 cooling killing eddy of turbulent air -  what could I do with = my short=20 3 - 6" (no jokes you guys).  We all know from Bernoulli that if = an area=20 is squeezed down that the velocity of the air flow increases - = right? =20
 
So I decided to try to maintain or increase = the energy=20 of the air by pitching down the neck just before it goes into the = bell shape=20 expansions in hopes that the increased energy will help the boundary = layer=20 stay adhered to the duct wall until well into the corner of the bell = shape.  So that's the story of the pinched ducts.  There = is no=20 question in my mind that this is not as effective as if I could have = had the=20 16" to build the duct - but, in this hobby, you work with what = you've got -=20 right?
 
Does it work?  Who knows - but I seem = to fly with=20 less opening area than most folks and have no cooling = problems. =20 So that's my 0.02 on the topic - see told you, you would regret = asking=20 {:>).
 
Ed
 
 
----- Original Message ----- =
From: = John = Downing=20
To: Rotary = motors in=20 aircraft
Sent: Monday, February 26, = 2007 8:53=20 PM
Subject: [FlyRotary] Re: = cowl=20 openings for water radiators

 
Ed, is there some particular = reason that you=20 necked the inlet down small, then enlarged it again.  = Thankyou for=20 the pictures.  JohnD
----- Original Message ----- =
From: Ed = Anderson=20
To: Rotary motors in=20 aircraft
Sent: Monday, February = 26, 2007=20 3:39 PM
Subject: [FlyRotary] Re: = cowl=20 openings for water radiators

 
John, don't know if these photos will = help. =20 But, like you I only have between 3 and 6" of duct distance on = the=20 radiators.  You should do Ok with 20 sq inch on each = opening with a=20 good diffuser/duct.  Attached are some photos of my current = ducts.  The openings are 18 sq inches each.  I have = had one=20 opening down to as little as 10 square inches - but that was a = bit=20 marginal - so opened it back up.  I have a generous exit = area for=20 the hot air including a larger 4" x 12" bottom opening as well = as=20 louvers on each side of the cowl.  So you mileage could = vary - but=20 Tracy has essentially the same size opening as well as several = others.=20
 
Ed
----- Original Message ----- =
From: John = Downing=20
To: Rotary motors=20 in aircraft
Sent: Monday, February = 26, 2007=20 12:12 PM
Subject: [FlyRotary] = cowl=20 openings for water radiators

 
What size openings do I need = for the=20 water radiators?   The Wittman Tailwind cowl I have = has=20 postal slots of 3' x 7 3/4" , which is   approx. 22 = 1/4 sq=20 in. on each side.  Sam James for the 160 Lycoming is = using 4 3/4'=20 round holes which are 17.6 sq. inches on each side.  My = radiators=20 are quite close to the opening and I plan on making the = diffusers=20 trumpet shaped, will the openings be large enough if I can = stay over=20 20 sq. inches on each side with a decent trumpet shape. =20 JohnD       hushpowere II = on order=20 - hope to start in 2 weeks if weather cooperates. =

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