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 2461969 for flyrotary@lancaironline.net; Thu, 08 Nov 2007 13:27:45 -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-omta05.mail.rr.com with SMTP id <20071108182705.KJCG8432.cdptpa-omta05.mail.rr.com@edward2> for ; Thu, 8 Nov 2007 18:27:05 +0000 Message-ID: <001001c82234$b5144f30$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 13:25:14 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_000D_01C8220A.CBF927E0" 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_000D_01C8220A.CBF927E0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Even in the Naca studies they often 'fess up that theoretical = considerations must give way to practical installation considerations = {:>). From what I have recently read, theoretically if you could do = your exit the best way, you might even get a small thrust benefit - at = least enough to overcome the cooling drag. However, I think the best = most can do is simply provide an unimpeded exit flow and minimize = losses. =20 There is some interesting information on usefulness of cowl flaps and = why they some times do not seem to make any difference. I don't claim = to fully understand it all, but it appears that once your losses in the = duct exceed a certain limit - opening up or even creating a low pressure = region at the exit does not promote more air flow through the duct. = There is only so much energy in the air velocity to turn into dynamic = pressure and if your losses in the duct total up to your dynamic energy = limit then nothing you do at the exit will improve the flow. At least = that is the way it appears to this old brain. But, it sure keeps an old brain from freezing up completely trying to = understand some of this. I personally believe that all of the = literature is pretty clear that the best thing you can do with your duct = work is to prevent flow separation in the diffuser.=20 Cooling goes down and drag goes up - not what we are looking for. Its = now finally clear why some of the reports quote 7-11 deg as max = diffuser divergence angles (2theta) and others show good diffuser = performance up around 60 deg divergence. The reason for the two = (seemingly conflicting) different findings is two different diffuser = configurations. One with no resistance behind it and one with = resistance (radiator). Another important basic is to set down and figure out the air mass flow = you must have to handle your critical cooling regime (full power climb = out?). That then drives your inlet size, the size cooler you need - and = as they say - is the basis from which all else flows(pun intended). But = as you say how many of us do that. I find that it is often similar differences that can/do end up confusing = those of us who are ignorant but trying to understand and apparently = find conflicting findings in these reports. You reallllllyyy have to = read them carefully from end to end. Ed ----- Original Message -----=20 From: wrjjrs@aol.com=20 To: Rotary motors in aircraft=20 Sent: Thursday, November 08, 2007 10:28 AM Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Ed, It seems like a cogent discription Ed. I have been studying the = problem for some time. I like your no core example, much cheaper but it = will only fly once. (And for a short time!) The question I have been = pondering is, does it really help us to consider a exit ducting to = direct our exit flows. The data you presented seems to indicate that it = does. The dynamics of the pressure drop across the core contain = compromises related to the efficiency of the heat exchanger, flow of the = water in it and air through it. Many of the designs I see lately pay = very little attention to the exit and re-merging the flow. In = core-in-the-standard-inlet systems such as yours the exit ducting may = not be practical. This is a problem I have see with the Eggenfellner = Sabaru installations as well. At least the rotary can have some exit = area without the cylinders right there in the way! The exit question = tends to favor the chin scoop. The problem is that this has always = proven to be a high drag choice. Currently I'm favoring a vertical side = radiator (or radiators) ducted to the outside (cowl) blowing into the = engine area with a diversion duct to turn the air towards the normal = rear bottom exit. Possibly with a cowl flap for climb. These have never = been easy choices. Often we intend an elegant solution, only to be = rebuffed by the need for hoses, wires, and exhaust pipes and other = unimportant stuff like that. ;-)=20 Thanks for all your research, Bill Jepson -----Original Message----- From: Ed Anderson To: Rotary motors in aircraft Sent: Thu, 8 Nov 2007 5:05 am Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity???? 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. -------------------------------------------------------------------------= ----- Email and AIM finally together. You've gotta check out free AOL Mail! ------=_NextPart_000_000D_01C8220A.CBF927E0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Even in the Naca studies they often 'fess up = that=20 theoretical considerations must give way to practical installation=20 considerations {:>).  From what I have recently read, = theoretically if=20 you could do your exit the best way, you might even get a small thrust = benefit -=20 at least enough to overcome the cooling drag.  However, I think the = best=20 most can do is simply provide an unimpeded exit flow and minimize = losses. =20
 
There is some interesting information on = usefulness of=20 cowl  flaps and why they some times do not seem to make any=20 difference.  I don't claim to fully understand it all, but it = appears that=20 once your losses in the duct exceed a certain limit - opening up or even = creating a low pressure region at the exit does not promote more air = flow=20 through the duct.  There is only so much energy in the air velocity = to turn=20 into dynamic pressure and if your losses in the duct total up to your = dynamic=20 energy limit then nothing you do at the exit will improve the = flow.  At=20 least that is the way it appears to this old brain.
 
But, it sure keeps an old brain from freezing up = completely trying to understand some of this.  I personally believe = that=20 all of the literature is pretty clear that the best thing you can do = with your=20 duct work is to prevent flow separation in the = diffuser. 
 
 Cooling goes down and drag goes up - not = what we are=20 looking for.  Its now finally clear why some of the reports  = quote=20 7-11 deg as max diffuser divergence angles (2theta) and others show good = diffuser performance up around 60 deg divergence.  The reason for = the two=20 (seemingly conflicting) different findings is two different diffuser=20 configurations.  One with no resistance behind it and one with = resistance=20 (radiator).
 
 Another important basic is to set down and = figure=20 out the air mass flow you must have to handle your critical cooling = regime (full=20 power climb out?).  That then drives your inlet size, the size = cooler you=20 need - and as they say - is the basis from which all else flows(pun = intended).  But as you say how many of us do that.
 
I find that it is often similar differences that = can/do=20 end up confusing those of us who are ignorant but trying to understand = and=20 apparently find conflicting findings in these reports.  You = reallllllyyy=20 have to read them carefully from end to end.
 
Ed
----- Original Message -----
From:=20 wrjjrs@aol.com
Sent: Thursday, November 08, = 2007 10:28=20 AM
Subject: [FlyRotary] Re: = Total,duct,=20 Ambient or Velocity????

Ed,
It seems like a cogent discription Ed. I have been studying the = problem=20 for some time. I like your no core example, much cheaper but it will = only fly=20 once. (And for a short time!) The question I have been pondering is, = does it=20 really help us to consider a exit ducting to direct our exit flows. = The data=20 you presented seems to indicate that it does. The dynamics of the = pressure=20 drop across the core contain compromises related to the efficiency of = the heat=20 exchanger, flow of the water in it and air through it. Many of the = designs I=20 see lately pay very little attention to the exit and re-merging the = flow. In=20 core-in-the-standard-inlet systems such as yours the exit ducting may = not be=20 practical. This is a problem I have see with the Eggenfellner Sabaru=20 installations as well. At least the rotary can have some exit area = without the=20 cylinders right there in the way! The exit question tends to favor the = chin=20 scoop. The problem is that this has always proven to be a high drag = choice.=20 Currently I'm favoring a vertical side radiator (or radiators) ducted = to the=20 outside (cowl) blowing into the engine area with a diversion duct to = turn the=20 air towards the normal rear bottom exit. Possibly with a cowl flap for = climb.=20 These have never been easy choices. Often we intend an elegant = solution, only=20 to be rebuffed by the need for hoses, wires, and exhaust pipes and = other=20 unimportant stuff like that. ;-) 
Thanks for all your=20 research,
Bill Jepson

-----Original Message-----
From: Ed = Anderson <eanderson@carolina.rr.com>
To: Rotary motors in = aircraft=20 <flyrotary@lancaironline.net>
Sent: Thu, 8 Nov 2007 5:05=20 am
Subject: [FlyRotary] Re: Total,duct, Ambient or=20 Velocity????

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=20 need other types of pressure measurements to figure out the = problem.  In=20 fluid flow talk, they appear to refer to loss of energy through  = wall=20 friction as a major loss as it is not recoverable (but this is minor = at our=20 speeds) , while trades between dynamic and static in the duct result = in=20 "minor" losses which may 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 = turbulence for better heat transfer.
 
  However, there is a balancing point, more = pressure=20 drop generally means better heat transfer from metal to air, however, = it also=20 generally means less mass flow because of the resistance.  Too = much=20 pressure drop =3D too little mass flow and overheating, too little = pressure drop=20 =3D great mass flow but higher duct drag and less heat transfer per = unit time=20 which can also lead to overheating. 
 
I like to use this example  to emphasize the=20 point.  You would get maximum pressure drop by placing a solid = board=20 across the duct - however, the air flow would be nil and cooling=20 likewise.  On the other hand, if you remove all obstructions in = the duct=20 (including the core) , the pressure drop would be nil,  the = airflow would=20 be maximum but cooling would still be nil.  The only significant=20  difference is the  no core approach is cheaper and = causes less=20 drag {:>)
 
In any case, all the literature I have read seems = to=20 indicate that the difference in pressure between the inlet and out let = of the=20 duct is a (if not THE) key factor.  That dynamic pressure is the = only=20 thing (assuming no fans/blowers) that will move significant air = through the=20 duct.  Since this dynamic pressure is referenced to the dynamic = pressure=20 available in the freestream flow as that is what it starts out as, I=20 personally think referencing dynamic pressure measurements to ambient = air is=20 what we are mainly interested.  This is  rather than = referencing it=20 to the duct static pressure as shown in the diagram. =20 But, you have to remember this is all from  the guy who has not = done any=20 duct 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=20 your static pressure is up and vice versa. If you have dynamic = pressure then=20 you have 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=20 knowledge.
 
Ed
 
 
----- Original Message -----
From:=20 WRJJRS@aol.com=20
To: Rotary motors in = aircraft=20
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=20 still confused as to what will give you the "best" data. The static = in duct=20 pressure compared to the total, or to the velocity?  It = probably=20 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.
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