X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from mail18.syd.optusnet.com.au ([211.29.132.199] verified) by logan.com (CommuniGate Pro SMTP 5.2c1) with ESMTPS id 2464000 for flyrotary@lancaironline.net; Fri, 09 Nov 2007 16:53:14 -0500 Received-SPF: none receiver=logan.com; client-ip=211.29.132.199; envelope-from=lendich@optusnet.com.au Received: from george (d211-31-228-51.dsl.nsw.optusnet.com.au [211.31.228.51]) by mail18.syd.optusnet.com.au (8.13.1/8.13.1) with SMTP id lA9LqTQb031113 for ; Sat, 10 Nov 2007 08:52:30 +1100 Message-ID: <001e01c8231a$d7c5de90$33e41fd3@george> From: "George Lendich" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Date: Sat, 10 Nov 2007 07:52:36 +1000 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_001B_01C8236E.A898F3B0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2180 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 X-Antivirus: avast! (VPS 0657-0, 12/12/2006), Outbound message X-Antivirus-Status: Clean This is a multi-part message in MIME format. ------=_NextPart_000_001B_01C8236E.A898F3B0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Ed and Al,=20 This is all good info me, it either confirms, clarifies or informs. The straw concept is a timely reminder of pressure differentials, a good = example IMHO. One thing I would really love to know is - at what point in the inlet = duct does the dynamic flow change to static pressure. I would assume = this would vary with different shaped ducts and different dynamic flow ( = airflow speed).=20 Your opinions on this or guesstimates ie 1", 2" or 3" from the face of = the rad, would be of great interest to me. George (down under) Hi Al, Not picky - some good points as always . Yes, I agree, generalization = does have its pit falls, but on the other hand I think they can help = promote a conceptual understanding which can be refined (through study = and experiments) to meet a particular situation. As we know, cooling = airflow is attempting to balance conflicting aerodynamic and = thermodynamic principles. =20 I also agree that much of this stuff addresses the "Perfect = theoretical duct" out of necessity as there is only one perfect duct but = many, many implementations that fall short of perfect. So its more of = a conceptual goal to be aimed for - it may never be achieved, but = provides at least guidelines. But,this is just my opinion of course. Actually, I disagree, you can not "suck" air though anything. You may = create a partial pressure difference with the fan, but it is the higher = pressure air on the other end of the duct that pushes or "blows" air = through the duct into the area of lower pressure {:>) . =20 But, semantics aside, yes, I agree, lower exit pressure is what you = are after and that does not always equate to larger exit duct area. In = fact, if the air heated by the core flows through a nozzle it might even = produce thrust and lower exit pressure using a smaller exit. But, in = general, I still believe that in most of our cases, we are short of the = level of duct design that would reliably permit that. What we need is = someone to invest in one of those $$$$ Computer Fluid Flow software = programs and see what they would reveal. Ed ----- Original Message -----=20 From: Al Gietzen=20 To: Rotary motors in aircraft=20 Sent: Friday, November 09, 2007 1:09 AM Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity???? It would seem "reasonable" that a low pressure area at the exit = will help flow through a duct - no argument on that point. What the = report appeared to say is that the after a certain point opening the = exit area wider does not appear to have any additional benefit. (Exit = "area" and exit "pressure" are not interchangeable terms) That if the = duct is capable of "using up" all of the kinetic energy in your air flow = by obstructions, pressure drops and friction losses then enlarging the = exit does not necessarily add to the flow. Remember you can not suck air through a duct, you can only blow it = through. (Of course you can suck air through a duct - I do it after (and = sometimes before) every flight with the fan I have on the back side of = the radiator) So in effect if the straw is pinched you can "suck" on it = all you want but it won't increase flow {:>). =20 If I understood the report, it appears that enlarging the exit area = beyond the frontal area of your core provides little if any additional = benefit. That does not mean cowl flaps never work or provide benefit. = In fact it appears that the better your duct, the more benefit the cowl = flaps appear to have, the worst your duct, the lesser benefit - just the = opposite of what you might think. Ed; Don't mean to be picky, but some of these generalities are making me = nervousJ. These things are applicable only when the duct/diffuser is = operating at max efficiency - which is rarely the case. Lot's of good info. Thanks. You're right; it's some kind of magic, = and you don't know for sure until you built it and try it. Al -------------------------------------------------------------------------= ----- No virus found in this incoming message. Checked by AVG Free Edition.=20 Version: 7.5.503 / Virus Database: 269.15.24/1115 - Release Date: = 7/11/2007 9:21 AM ------=_NextPart_000_001B_01C8236E.A898F3B0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
 Ed and Al,
This is all good info me, it either = confirms,=20 clarifies or informs.
The straw concept is a timely reminder = of pressure=20 differentials, a good example IMHO.
 
One thing I would really love to know = is - at what=20 point in the inlet duct does the dynamic flow change to static pressure. = I would=20 assume this would vary with different shaped ducts and different dynamic = flow (=20 airflow speed). 
Your opinions on this = or guesstimates ie 1",=20 2" or 3" from the face of the rad, would be of great interest = to=20 me.
 
George (down under)
Hi Al,
 
Not picky - some good points as always .  = Yes, I=20 agree, generalization does have its  pit falls, but on = the=20 other hand I think they can  help promote a conceptual = understanding=20 which can be refined (through study and experiments) to meet a = particular=20 situation.  As we know, cooling airflow is attempting to balance=20 conflicting aerodynamic and thermodynamic  principles. =20
 
I also agree that   much of this = stuff=20 addresses the "Perfect theoretical duct" out of necessity as there is=20 only one perfect duct but many, many implementations =  that=20 fall short of perfect.  So its more of a conceptual goal = to be=20 aimed for  - it may never be achieved, but provides at=20 least guidelines.   But,this is  just my opinion = of=20 course.
 
Actually, I disagree, you can not "suck" air = though=20 anything.  You may create a partial pressure difference with the = fan, but=20 it is the higher pressure air on the other end of the duct that pushes = or=20 "blows" air through the duct into the area of lower pressure =20 {:>)  .  
 
 But, semantics aside, yes, I agree, = lower exit=20 pressure is what you are after and that does not always equate to = larger exit=20 duct area.  In fact, if the air heated by the core flows through = a nozzle=20 it might even produce thrust and lower exit pressure using a = smaller=20 exit.  But, in general, I still believe that in most of our = cases, we are=20 short of the level of duct design that would reliably permit = that.  What=20 we need is someone to invest in one of those $$$$ Computer Fluid Flow = software=20 programs and see what they would reveal.
 
Ed
----- Original Message -----
From:=20 Al = Gietzen=20
To: Rotary motors in = aircraft=20
Sent: Friday, November 09, = 2007 1:09=20 AM
Subject: [FlyRotary] Re: = Total,duct,=20 Ambient or Velocity????

 

It  would seem = "reasonable"=20 that a low pressure area at the exit  will help flow through a = duct -=20 no argument on that point.  What the report appeared to say is = that the=20 after a certain point opening the exit area wider does not appear to = have=20 any additional benefit. (Exit =93area=94 and exit =93pressure=94 are not = interchangeable=20 terms)=20 That if the duct is capable of "using up" all of the = kinetic=20 energy in your air flow by obstructions, pressure drops  and = friction=20 losses then enlarging the exit does not necessarily  add to the = flow.

 

Remember you can not = suck air=20 through a duct, you can only blow it through. (Of course you can suck air through a duct =96 = I do it=20 after (and sometimes before) every flight with the fan I have on the = back=20 side of the radiator) So in effect if the straw is = pinched you=20 can "suck" on it all you want but it won't increase flow=20 {:>).  

 

If I understood the = report,=20  it appears that enlarging the exit area beyond the frontal = area of=20 your core provides little if any additional benefit.  That does = not=20 mean cowl flaps never work or provide benefit.  In fact it = appears that=20 the better your duct,  the more benefit the cowl flaps appear = to have,=20 the worst your duct, the lesser benefit - just the opposite of what = you=20 might think.

Ed;

 

Don=92t = mean to be=20 picky, but some of these generalities are making me=20 nervousJ.  = These=20 things are applicable only when the duct/diffuser is operating at = max=20 efficiency =96 which is rarely the case.

Lot=92s of = good=20 info.  = Thanks.  You=92re right; it=92s some kind of magic, and you = don=92t know for=20 sure until you built it and try it.

Al

No virus found in this incoming message.
Checked by AVG Free = Edition.
Version: 7.5.503 / Virus Database: 269.15.24/1115 - = Release Date:=20 7/11/2007 9:21 AM
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