X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 73 [XX] (60%) SPAMTRICKS: long string of words (40%) RECEIVED: IP not found on home country list Return-Path: Received: from [201.225.225.167] (HELO cwpanama.net) by logan.com (CommuniGate Pro SMTP 5.1.7) with ESMTP id 1873718 for flyrotary@lancaironline.net; Wed, 28 Feb 2007 09:17:42 -0500 Received-SPF: none receiver=logan.com; client-ip=201.225.225.167; envelope-from=rijakits@cwpanama.net Received: from [201.224.94.164] (HELO usuario5ebe209) by frontend1.cwpanama.net (CommuniGate Pro SMTP 4.2.10) with SMTP id 104259264 for flyrotary@lancaironline.net; Wed, 28 Feb 2007 09:24:44 -0500 Message-ID: <00cf01c75b43$0a6053d0$a45ee0c9@usuario5ebe209> From: "Thomas y Reina Jakits" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl openings for water radiators Date: Wed, 28 Feb 2007 09:16:28 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_00CC_01C75B19.20C91EB0" 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 This is a multi-part message in MIME format. ------=_NextPart_000_00CC_01C75B19.20C91EB0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Ed, Okay, now I understand your reasoning!! Well, if it works, you definitely did something right!! Anyone out there with access to some CFD programms to run some tests? Like: a) optimal streamline b) Truncated streamline according to K&N c) modifed truncated according to Ed Difference being that b. would have a way bigger intake than c. Bob, I already saw your next answer to this email: If you are re-doing your ductwork, I suggest you pay extra-attention to = the to the "after the radiator" area. Over the last couple of years I = read various articles of builders finding success with creating low = pressure areas in the exit area, at least 2 used exhaust augmentation. Combined with the best doable intake ducting - efficiency should result! = :) Thomas PS: I am specifically interested in your "packaging", as the BD-4 is my = "plan" ----- Original Message -----=20 From: Ed Anderson=20 To: Rotary motors in aircraft=20 Sent: Tuesday, February 27, 2007 2:20 PM Subject: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl = openings for water radiators I agree, Thomas - if you are forced to truncate the "streamline duct" = you want to start at the core and go toward the inlet. However, I = decided that to see if pinching the duct before the bell shape would = work since I have such a short distance. In fact, when I calculated out the opening by simply extending the = streamline duct coordinate out 3-6" from the core - the opening was = 60-75% (I forget exactly) the size of my core. I decided that would up = the cooling drag quite a bit - so decided to try the pinched shape which = restricts the size of the opening to near that of the streamline duct = but then tries to compensate for the short length by speeding up the = boundary layer for great penetration toward the core. Again, my cooling = is fine and I can top out at 200 mph in my draggy RV-6A, so I think the = drag is down a bit.=20 But, again that is just my theorizing about why I choose to pinch the = curve. Ed ----- Original Message -----=20 From: Thomas y Reina Jakits=20 To: Rotary motors in aircraft=20 Sent: Tuesday, February 27, 2007 1:52 PM Subject: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl = openings for water radiators Hmm, if I remember right from reading what I have from K&N - if you do = not have the optimal lenght available you start to cut it from the = intake - take radiator and apply the optimum duct, measure from the = radiator towards the intake whatever distance you have available and cut = the duct there.... Dave, are you a Navy-flyer or a medic?? :) Thomas ----- Original Message -----=20 From: Ed Anderson=20 To: Rotary motors in aircraft=20 Sent: Tuesday, February 27, 2007 1:34 PM Subject: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl = openings for water radiators 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_00CC_01C75B19.20C91EB0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Ed,
Okay, now I understand your=20 reasoning!!
Well, if it works, you definitely = did=20 something right!!
Anyone out there with access to some = CFD programms=20 to run some tests?
Like:
 
a) optimal streamline
b) Truncated streamline according to=20 K&N
c) modifed truncated according to = Ed
 
Difference being that b. would have a = way bigger=20 intake than c.
 
Bob, I already saw your next answer to = this=20 email:
If you are re-doing your ductwork, I = suggest you=20 pay extra-attention to the to the "after the radiator" area. Over the = last=20 couple of years I read various articles of builders finding success with = creating low pressure areas in the exit area, at least 2 used exhaust=20 augmentation.
Combined with the best doable intake = ducting -=20 efficiency should result! :)
 
Thomas
PS: I am specifically interested in = your=20 "packaging", as the BD-4 is my "plan"
----- Original Message -----
From:=20 Ed=20 Anderson
Sent: Tuesday, February 27, = 2007 2:20=20 PM
Subject: [FlyRotary] Re: = Pinched ducts=20 was : [FlyRotary] Re: cowl openings for water radiators

I agree, Thomas - if you are forced to = truncate the=20 "streamline duct" you want to start at the core and go toward the=20 inlet.   However, I decided that to see if pinching the duct = before=20 the bell shape would work since I have such a short = distance.
 
  In fact, when I calculated out the = opening by=20 simply extending the streamline duct coordinate out 3-6" from the core = - the=20 opening was 60-75% (I forget exactly) the size of my core.  I = decided=20 that would up the cooling drag quite a bit - so decided to try the = pinched=20 shape which restricts the size of the opening to near that of the = streamline=20 duct but then tries to compensate for the short length by speeding up = the=20 boundary layer for great penetration toward the core.  Again, my = cooling=20 is fine and I can top out at 200 mph in my draggy RV-6A, so I think = the drag=20 is down a bit. 
 
But, again that is just my theorizing about = why I choose=20 to pinch the curve.
 
Ed
 
----- Original Message -----
From:=20 Thomas y=20 Reina Jakits
To: Rotary motors in = aircraft=20
Sent: Tuesday, February 27, = 2007 1:52=20 PM
Subject: [FlyRotary] Re: = Pinched ducts=20 was : [FlyRotary] Re: cowl openings for water radiators

Hmm,
 
if I remember right from = reading what I=20 have from K&N - if you do not have the optimal lenght available = you=20 start to cut it from the intake - take radiator and apply the = optimum duct,=20 measure from the radiator towards the intake whatever distance you = have=20 available and cut the duct there....
 
Dave,
are you a Navy-flyer or a medic??=20 :)
 
Thomas
----- Original Message ----- =
From:=20 Ed Anderson
To: Rotary motors in = aircraft=20
Sent: Tuesday, February 27, = 2007 1:34=20 PM
Subject: [FlyRotary] Re: = Pinched=20 ducts was : [FlyRotary] Re: cowl openings for water = radiators

Hummm, Dave, perhaps my understanding of = what it=20 takes to 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=20 the boundary layer's attachment to the duct=20 wall.  The pressure build up (area of slower = molecules)=20 tend to push and keep the boundary layer pushed = against the wall=20 of the duct as it curves out - at the same time it is slowing down = the=20 boundary layer.  So its the point of separation = is (at=20 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=20 ultimately separates.  The further the better is my=20 understanding.
 
  My understanding is that in a = duct - it=20 is the recovery pressure  which  builds in the expansion = area=20 just  before the core.  This "high" pressure area =  will=20 "push" back on the boundary layer causing it slow and eventually =  to=20 separate from the wall.  .  However, if you keep the = boundary=20 layer speed up it pushes further into the pressure recovery area = following=20 the duct curve before the "back pressure" slows it enough to cause = it to=20 separate. 
 
Also  the speed of a molecule in all = random=20 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=20 ft/sec in the duct.  So my interpretation is that (at least = in a=20 duct) its the back pressure of the recovered pressure that causes = the=20 separation - not necessary the curve of the duct alone although = that=20 certainly contributes to the pressure recovery.  That being = said,its=20 clear that  the three factors  (duct curve, expansion=20 area and separation)   really go hand in = hand.  The=20 greater the curve the more pressure recovery occurs and the = greater the=20 tendency for separation.   The higher the velocity of = the=20 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=20 improve the situation.  However given I only had 3 -6" = my take=20  was that speeding up the air (and boundary layer energy) = would=20 ensure it penetrated deeper into the bell shape before the = pressure=20 recovery caused separation.  But, as I have often stated - I = could be=20 completely wrong about what I think I understand.
 
  You are after-all the Navy flyer = and I know=20 they cram a lot of areo into Navy pilot's heads.  Me- I'm a=20 electrical engineer, so what  I know about aerodynamics is = what I=20 have read (and think I understand).
 
But, regardless these pinched ducts=20 have provided the  best cooling with the smallest = opening that I=20 have achieved - so, Dave,  if you stay quite it may not learn = the=20 truth {:>)
 
 
 
Ed
 
 
----- Original Message ----- =
From:=20 David=20 Leonard
To: Rotary motors in = aircraft=20
Sent: Tuesday, February = 27, 2007=20 11:46 AM
Subject: [FlyRotary] Re: = Pinched=20 ducts was : [FlyRotary] Re: cowl openings for water = radiators

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

--
David Leonard

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

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

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

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

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