X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from cdptpa-omtalb.mail.rr.com ([75.180.132.121] verified) by logan.com (CommuniGate Pro SMTP 5.2c1) with ESMTP id 2461422 for flyrotary@lancaironline.net; Thu, 08 Nov 2007 08:08:15 -0500 Received-SPF: pass receiver=logan.com; client-ip=75.180.132.121; envelope-from=eanderson@carolina.rr.com Received: from edward2 ([24.74.103.61]) by cdptpa-omta06.mail.rr.com with SMTP id <20071108130736.EPQW507.cdptpa-omta06.mail.rr.com@edward2> for ; Thu, 8 Nov 2007 13:07:36 +0000 Message-ID: <000f01c82208$130e34c0$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Date: Thu, 8 Nov 2007 08:05:45 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_000C_01C821DE.29F7C860" 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_000C_01C821DE.29F7C860 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Hi Bill, It is my opinion, based on my limited knowledge of the topic, that = dynamic pressure in the duct is the most significant factor. If you = don't have it - you have no flow. If you do have it you will have flow = but you could have significant Major losses - that's why you may need = other types of pressure measurements to figure out the problem. In = fluid flow talk, they appear to refer to loss of energy through wall = friction as a major loss as it is not recoverable (but this is minor at = our speeds) , while trades between dynamic and static in the duct result = in "minor" losses which may or may not really be minor. Here is my understanding, you would like to convert dynamic energy to = static pressure increase in front of the core as that slows down the = velocity reducing drag and tends to give you more even velocity = distribution across the core (assuming little or no separation of flow = from the duct walls). You would like the greatest pressure drop across = the core which results in the highest velocity through the core tubes = generating turbulence for better heat transfer. However, there is a balancing point, more pressure drop generally = means better heat transfer from metal to air, however, it also generally = means less mass flow because of the resistance. Too much pressure drop = =3D too little mass flow and overheating, too little pressure drop =3D = great mass flow but higher duct drag and less heat transfer per unit = time which can also lead to overheating. =20 I like to use this example to emphasize the point. You would get = maximum pressure drop by placing a solid board across the duct - = however, the air flow would be nil and cooling likewise. On the other = hand, if you remove all obstructions in the duct (including the core) , = the pressure drop would be nil, the airflow would be maximum but = cooling would still be nil. The only significant difference is the no = core approach is cheaper and causes less drag {:>) In any case, all the literature I have read seems to indicate that the = difference in pressure between the inlet and out let of the duct is a = (if not THE) key factor. That dynamic pressure is the only thing = (assuming no fans/blowers) that will move significant air through the = duct. Since this dynamic pressure is referenced to the dynamic pressure = available in the freestream flow as that is what it starts out as, I = personally think referencing dynamic pressure measurements to ambient = air is what we are mainly interested. This is rather than referencing = it to the duct static pressure as shown in the diagram. But, you have = to remember this is all from the guy who has not done any duct = instrumentation. But, my reason for focusing on dynamic pressure is that you can infer = a lot from your duct dynamic pressure readings about what is going on in = the duct. If your dynamic pressure is down, then your static pressure = is up and vice versa. If you have dynamic pressure then you have flow = while static pressure does not necessarily tell you that. =20 However, it all really depends on what you are trying to figure out on = what measurements you take. It would appear if you know how to interpret what you are measuring then = all provide some useful information. That's about the extent of my limited knowledge. Ed ----- Original Message -----=20 From: WRJJRS@aol.com=20 To: Rotary motors in aircraft=20 Sent: Thursday, November 08, 2007 12:28 AM Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Ed, The slide is a good way to explain the various references. I am = still confused as to what will give you the "best" data. The static in = duct pressure compared to the total, or to the velocity? It probably = doesn't matter if you use the same method all the time. Bill Jepson -------------------------------------------------------------------------= ----- See what's new at AOL.com and Make AOL Your Homepage. ------=_NextPart_000_000C_01C821DE.29F7C860 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Hi Bill,
 
It is my opinion, based on my limited knowledge = of the=20 topic, that dynamic pressure in the duct is the most significant = factor. =20 If you don't have it - you have no flow.  If you do have it you = will have=20 flow but you could have significant Major losses - that's why you = may need=20 other types of pressure measurements to figure out the problem.  In = fluid=20 flow talk, they appear to refer to loss of energy through  wall = friction as=20 a major loss as it is not recoverable (but this is minor at our speeds) = , while=20 trades between dynamic and static in the duct result in "minor" losses = which may=20 or may not really be minor.
 
Here is my understanding, you would like to convert = dynamic=20 energy to static pressure increase in front of the core as that slows = down the=20 velocity reducing drag and tends to give you more even velocity = distribution=20 across the core (assuming little or no separation of flow from the duct=20 walls).  You would like the greatest pressure drop across the core = which=20 results in the highest velocity through the core tubes generating=20 turbulence for better heat transfer.
 
  However, there is a balancing point, more = pressure drop=20 generally means better heat transfer from metal to air, however, it also = generally means less mass flow because of the resistance.  Too much = pressure drop =3D too little mass flow and overheating, too little = pressure drop =3D=20 great mass flow but higher duct drag and less heat transfer per unit = time which=20 can also lead to overheating. 
 
I like to use this example  to emphasize the = point. =20 You would get maximum pressure drop by placing a solid board across the = duct -=20 however, the air flow would be nil and cooling likewise.  On the = other=20 hand, if you remove all obstructions in the duct (including the core) , = the=20 pressure drop would be nil,  the airflow would be maximum but = cooling would=20 still be nil.  The only significant  difference is the =  no core=20 approach is cheaper and causes less drag {:>)
 
In any case, all the literature I have read seems to = indicate=20 that the difference in pressure between the inlet and out let of the = duct is a=20 (if not THE) key factor.  That dynamic pressure is the only thing = (assuming=20 no fans/blowers) that will move significant air through the duct.  = Since=20 this dynamic pressure is referenced to the dynamic pressure available in = the=20 freestream flow as that is what it starts out as, I personally think = referencing=20 dynamic pressure measurements to ambient air is what we are mainly=20 interested.  This is  rather than referencing it to the duct = static=20 pressure as shown in the diagram.  But, you = have to=20 remember this is all from  the guy who has not done any duct=20 instrumentation.
 
But, my reason for focusing on dynamic = pressure  is=20 that  you can infer a lot from your duct dynamic pressure readings = about=20 what is going on in the duct.  If your dynamic pressure is down, = then your=20 static pressure is up and vice versa. If you have dynamic pressure then = you have=20 flow while static pressure does not necessarily tell you that. =20
 
However, it all really depends on what you are = trying to=20 figure out on what measurements you take.
It would appear if you know how to interpret what = you are=20 measuring then all provide some useful information.
 
That's about the extent of my limited = knowledge.
 
Ed
 
 
----- Original Message -----
From:=20 WRJJRS@aol.com
Sent: Thursday, November 08, = 2007 12:28=20 AM
Subject: [FlyRotary] Re: = Total,duct,=20 Ambient or Velocity????

Ed, The slide is a good way to explain the various references. I = am still=20 confused as to what will give you the "best" data. The static in duct = pressure=20 compared to the total, or to the velocity?  It probably doesn't = matter if=20 you use the same method all the time.
Bill Jepson
 



See what's new at AOL.com=20 and Make AOL Your=20 Homepage.
------=_NextPart_000_000C_01C821DE.29F7C860--