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 2462591 for flyrotary@lancaironline.net; Thu, 08 Nov 2007 20:43:45 -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-omta06.mail.rr.com with SMTP id <20071109014307.OIGH507.cdptpa-omta06.mail.rr.com@edward2> for ; Fri, 9 Nov 2007 01:43:07 +0000 Message-ID: <001a01c82271$9f6f00c0$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 20:41:17 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0017_01C82247.B6516870" 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_01C82247.B6516870 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Well, George, I'd say your understanding of the duct air flow essentials = are as good as mine. 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. 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. 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. But, Hey!, I'm only an electrical engineer, so all this airflow stuff = is magic as far as I am concerned. Ed From: George Lendich=20 To: Rotary motors in aircraft=20 Sent: Thursday, November 08, 2007 5:11 PM Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Ed,=20 If I'm understanding you correctly, it appears that you need dynamic = pressure ( flow) that turns into a high static pressure (at the Rad = face). To maintain this high static pressure, the dynamic flow must be free = of turbulence, which is associated with flow separation from the duct = walls. Hence the need for proper divergent angles. There must be good pressure drop across the Rad, not too high or you = lose heat transfer, not too low as to create excess drag. There must be = some turbulence within the duct fins to enhance heat transfer, but not = too much as to create restrictions. I still feel a low pressure area behind the rad would be beneficial.=20 George ( down under) 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! -------------------------------------------------------------------------= --- 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_0017_01C82247.B6516870 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Well, George, I'd say your understanding of the = duct air=20 flow essentials are as good as mine.
 
It  would seem "reasonable" that a low = pressure area=20 at the exit  will help flow through a duct - no argument on that=20 point.  What the report appeared to say is that the after a certain = point=20 opening the exit area wider does not appear to have any=20 additional benefit.  That if the duct is capable of "using up" = all of=20 the kinetic energy in your air flow by obstructions, pressure drops =  and=20 friction losses then enlarging the exit does not necessarily  add = to the=20 flow.
 
Remember you can not suck air through a duct, = you can only=20 blow it through.  So in effect if the straw is pinched you can = "suck" on it=20 all you want but it won't increase flow {:>).   =
 
If I understood the report,  it appears = that=20 enlarging the exit area beyond the frontal area of your core provides = little if=20 any additional benefit.  That does not mean cowl flaps never work = or=20 provide benefit.  In fact it appears that the better your duct, =  the=20 more benefit the cowl flaps appear to have, the worst your duct, the = lesser=20 benefit - just the opposite of what you might think.
 
 But, Hey!, I'm only an electrical = engineer, so all=20 this airflow stuff is magic as far as I am concerned.
 
  Ed
From:=20 George=20 Lendich
Sent: Thursday, November 08, = 2007 5:11=20 PM
Subject: [FlyRotary] Re: = Total,duct,=20 Ambient or Velocity????

 Ed,
If I'm understanding you correctly, = it appears=20 that you need dynamic pressure ( flow) that turns into a high = static=20 pressure (at the Rad face).
To maintain this high static = pressure, the=20 dynamic flow must be free of turbulence, which is associated with flow = separation from the duct walls. Hence the need for proper divergent=20 angles.
 
There must be good pressure drop = across the=20 Rad,  not too high or you lose heat transfer, not too low as to = create=20 excess drag. There must be some turbulence within the duct fins to = enhance=20 heat transfer, but not too much as to create = restrictions.
 
I still feel a low pressure area = behind the rad=20 would be beneficial. 
George ( down under)
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=20 if you could do your exit the best way, you might even get a small = thrust=20 benefit - at least enough to overcome the cooling drag.  = However, I=20 think the best most can do is simply provide an unimpeded exit flow = and=20 minimize losses. 
 
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=20 that once your losses in the duct exceed a certain limit - opening = up or=20 even creating a low pressure region at the exit does not promote = more air=20 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=20 up to your dynamic energy limit then nothing you do at the exit will = improve=20 the flow.  At least that is the way it appears to this old=20 brain.
 
But, it sure keeps an old brain from = freezing up=20 completely trying to understand some of this.  I personally = believe=20 that all of the literature is pretty clear that the best thing you = can do=20 with your duct work is to prevent flow separation in the=20 diffuser. 
 
 Cooling goes down and drag goes up - = not what we=20 are looking for.  Its now finally clear why some of the = reports =20 quote 7-11 deg as max diffuser divergence angles (2theta) and others = show=20 good diffuser performance up around 60 deg divergence.  The = reason for=20 the two (seemingly conflicting) different findings is two different = diffuser=20 configurations.  One with no resistance behind it and one with=20 resistance (radiator).
 
 Another important basic is to set down = and=20 figure out the air mass flow you must have to handle your critical = cooling=20 regime (full power climb out?).  That then drives your inlet = size, the=20 size cooler you need - and as they say - is the basis from = which all=20 else flows(pun intended).  But as you say how many of us do=20 that.
 
I find that it is often similar differences = that=20 can/do end up confusing those of us who are ignorant but trying to=20 understand and apparently find conflicting findings in these = reports. =20 You reallllllyyy have to read them carefully from end to = end.
 
Ed
----- Original Message ----- =
From:=20 wrjjrs@aol.com=20
To: Rotary motors in = aircraft=20
Sent: Thursday, November = 08, 2007=20 10:28 AM
Subject: [FlyRotary] Re: = Total,duct,=20 Ambient or Velocity????

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

-----Original = Message-----
From: Ed=20 Anderson <eanderson@carolina.rr.com>
To: Rotary motors in = aircraft <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=20 the topic, that dynamic pressure in the duct is the most = significant=20 factor.  If you don't have it - you have no flow.  If = you do=20 have it you will have flow but you could have significant = Major=20 losses - that's why you may need other types of pressure = measurements to=20 figure out the problem.  In fluid flow talk, they appear to = refer to=20 loss of energy through  wall friction as a major loss as it = is not=20 recoverable (but this is minor at our speeds) , while trades = between=20 dynamic and static in the duct result in "minor" losses which may = or may=20 not really be minor.
 
Here is my understanding, you would like to = convert=20 dynamic energy to static pressure increase in front of the core as = that=20 slows down the velocity reducing drag and tends to give you more = even=20 velocity distribution across the core (assuming little or no = separation of=20 flow from the duct walls).  You would like the greatest = pressure drop=20 across the core which results in the highest velocity through the=20 core tubes generating turbulence for better heat=20 transfer.
 
  However, there is a balancing point, = more=20 pressure drop generally means better heat transfer from metal to = air,=20 however, it also generally means less mass flow because of the=20 resistance.  Too much pressure drop =3D too little mass flow = and=20 overheating, too little pressure drop =3D great mass flow but = higher duct=20 drag and less heat transfer per unit time which can also lead to=20 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=20 duct (including the core) , the pressure drop would be nil, =  the=20 airflow would be maximum but cooling would still be nil.  The = only=20 significant  difference is the  no core approach is = cheaper=20 and causes less 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=20 the duct is a (if not THE) key factor.  That dynamic pressure = is the=20 only thing (assuming no fans/blowers) that will move significant = air=20 through the duct.  Since this dynamic pressure is referenced = to the=20 dynamic pressure available in the freestream flow as that is what = it=20 starts out as, I personally think referencing dynamic pressure=20 measurements to ambient air is what we are mainly = interested.  This=20 is  rather than referencing it to the duct static pressure as = shown=20 in the diagram.  But, you have to = remember this=20 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=20 about what is going on in the duct.  If your dynamic pressure = is=20 down, then your static pressure is up and vice versa. If you have = dynamic=20 pressure then you have flow while static pressure does not = necessarily=20 tell you that. 
 
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=20 are 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=20 12:28 AM
Subject: [FlyRotary] Re:=20 Total,duct, 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=20 duct pressure compared to the total, or to the velocity?  = It=20 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.
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