X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from cdptpa-omtalb.mail.rr.com ([75.180.132.120] verified) by logan.com (CommuniGate Pro SMTP 5.2c1) with ESMTP id 2463148 for flyrotary@lancaironline.net; Fri, 09 Nov 2007 08:26:04 -0500 Received-SPF: pass receiver=logan.com; client-ip=75.180.132.120; envelope-from=eanderson@carolina.rr.com Received: from edward2 ([24.74.103.61]) by cdptpa-omta01.mail.rr.com with SMTP id <20071109132526.FTOM6760.cdptpa-omta01.mail.rr.com@edward2> for ; Fri, 9 Nov 2007 13:25:26 +0000 Message-ID: <001a01c822d3$bc480720$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Date: Fri, 9 Nov 2007 08:23:36 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0017_01C822A9.D3319AC0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3138 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3138 This is a multi-part message in MIME format. ------=_NextPart_000_0017_01C822A9.D3319AC0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable 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 ------=_NextPart_000_0017_01C822A9.D3319AC0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Hi Al,
 
Not picky - some good points as always .  = Yes, I=20 agree, generalization does have its  pit falls, but on = the other=20 hand I think they can  help promote a conceptual understanding = which can be=20 refined (through study and experiments) to meet a particular = situation.  As=20 we know, cooling airflow is attempting to balance conflicting = aerodynamic and=20 thermodynamic  principles. 
 
I also agree that   much of this stuff = addresses=20 the "Perfect theoretical duct" out of necessity as there is = only one=20 perfect duct but many, many implementations  that fall = short of=20 perfect.  So its more of a conceptual goal to be aimed = for =20 - it may never be achieved, but provides at = least guidelines. =20  But,this is  just my opinion of 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 we=20 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
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 - no=20 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 any=20 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=20 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 after=20 (and sometimes before) every flight with the fan I have on the back = side of=20 the radiator) So in effect if the straw is pinched you = can=20 "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 your=20 core provides little if any additional benefit.  That does not = mean cowl=20 flaps never work or provide benefit.  In fact it appears that the = better=20 your duct,  the more benefit the cowl flaps appear to have, the = worst=20 your duct, the lesser benefit - just the opposite of what you might = think.

Ed;

 

Don=92t = mean to be=20 picky, but some of these generalities are making me = 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. =20 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

------=_NextPart_000_0017_01C822A9.D3319AC0--