X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [201.225.225.169] (HELO cwpanama.net) by logan.com (CommuniGate Pro SMTP 5.0.4) with ESMTP id 882598 for flyrotary@lancaironline.net; Wed, 14 Dec 2005 19:38:55 -0500 Received-SPF: none receiver=logan.com; client-ip=201.225.225.169; envelope-from=rijakits@cwpanama.net Received: from [201.224.93.110] (HELO usuarioq3efog0) by frontend3.cwpanama.net (CommuniGate Pro SMTP 4.2.10) with SMTP id 54151055 for flyrotary@lancaironline.net; Wed, 14 Dec 2005 20:07:53 -0500 Message-ID: <341d01c6010f$cc4e2b50$6e5de0c9@usuarioq3efog0> From: "rijakits" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Cowl Exit Date: Wed, 14 Dec 2005 19:37:58 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_3419_01C600E5.E3296710" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1437 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1441 This is a multi-part message in MIME format. ------=_NextPart_000_3419_01C600E5.E3296710 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit ----- Original Message ----- From: Monty Roberts To: Rotary motors in aircraft Sent: Wednesday, December 14, 2005 5:01 PM Subject: [FlyRotary] Cowl Exit John, It really depends on how much power/heat you are going to be producing. However, if you have --say 24 sq inch for each of three openings (two for GM cores and one for oil cooler) then you have a total of 72 sq inches for cooling air to come into the cowl. Now you are going to heat the air which will naturally cause it to expand in volume, and based on your opening size 4.25*11.25 = 47.8 sq inches for this expanded air to get out. Your exit area is way too small in my opinion. A "rule of thumb" for exit area that I have seen is no less than 1.2 ratio of exit to intake. I have seen that ratio up to 1.7. so that would indicate you would need 1.2*72 = 86 to 1.7*72 = 122 sq inches. Personally, I would lean toward the 122 myself. Here is a rule of thumb I just got off the internet You will hear rules of thumb that the exit openings, because the air is heated and thus presents more volume than the original intake air, should be 2.5 times the area of the intake opening. With good extraction of this heated air, allowed by the fact that it is exhausted into a low pressure area, the exit cross section can be as small as the intake in some cases. In others larger, but I have never seen this exit area have to exceed about 1.5 to twice the area of intake. From a different source Exit area totals 66.7 square inches which comes to about a 1 to 1.5 ratio inlet to exit area. Notice no indication that anything less than intake area would do the job. I used the solid pink foam from a lumberyard (had to glue several planks together). I taped it with duct tape to prevent the foam from dissolving when I put on the epoxy - work fine for me. Ed A Ed, The oft sited rules of thumb are fine. But you have to keep in mind it all depends on your flight conditions. If you size a radiator, inlet and exit for hot day climb, then it will be WAY too big for cruising at 75% power at 12K feet. In this case you use a cowl flap to bring the exit size down. This restricts the flow of air through the radiator thereby reducing momentum drag. It also causes the pressure in the cowl to go up. The air stacks up and you get external diffusion. The high pressure heated and expanded air now squirts out through this restriction and you gain some of your drag back. In this case, it is possible that the exit may be smaller than the inlet (the inlet is sized for WOT climb, not cruise). Remember the engine is putting out way less than rated power because you are (or should be) running LOP at 75% minus the power loss with altitude. So the heat load is way less than what you need to remove at WOT climb on a hot day. You should have some way to vary the exit if you want to get the best out of the airplane in cruise, of course it is more complex. It's all a trade off. I must admit I like the current post following the previous. One can only rebel for just so long ;-). Monty Monty, you are rebel!! Bottom posting it is, ....at least on this thread! Back to square 1!! We had a similar thread back in April! ROT's about exit/intake ratio where exit HAS to be bigger are gospel. If you want to reduce cooling drag, read up on the following: (I am cheap enough to just copy my past posts and replies here:)) copy from 06-April 2005: ...cut Note: the exhaust area requirement is greater than the inlet combination of oil and coolant due to the now considerable hotter air temperature. Rule of thumb: Coolant air inlet opening for 200 HP coolant cooled engine ~60 sq. in., oil inlet opening ~30 sq. in. equals a total opening of 90 sq. in. A good place to start with exhaust opening is 1.4 times the inlets or 126 sq. in. Close the exit area down with cowl flaps to as little as 80% of the inlet combination at cruise conditions! ...cut There was an excellent article about that in Kitplanes of Feb.2004, about Brian Schmidtbauer's Mustang II. Though Lyc powered it still goes 250 mph, just about par with his friends RV-4, same engine (Dave Anders of CAFE foundation fame). Of course Dave is part guilty for that Speed demon, as he gave some ideas to Brian about cooling. I do understand that an air-cooled installation is a different animal, but by the end of the day ( or at the cowl exit...for that matter) everything is air cooled. The interesting thing is that both run a intake/exit ratio of 78%, the intake being bigger - just about confirming the above statement! Both seem to use exhaust augmentation for the exit. Their main study material was "NACA reports back to the 1920's and Hoerner's book about Fluid Dynamic Drag" ( by the way that book is still available, but at around a 100 bucks rather pricey, but worth it - says hearsay. Out of my range for now....) I mentioned that on the other list, but I was dismissed as " you can't compare aircooled engines with a rotary". I don't think there is a great difference on the amout of cooling necessary, as the efficiency of both engines are fairly close. So some heat goes out the exhaust and the rest has to be cooled. For sure you need different ducting, but the amount of heat energy should be about the same and you want to get rid of it with the least drag, either way. Schmidtbauer mentiones the " rule of thumb" - exit about 150% of inlet. By fine tuning the ductwork he got rid of up to 30% of the total drag, just by eliminating most of the cooling drag. Anders beat John Harmon in his Harmon Rocket, by over 1000 points (CAFE system, Harmon 1316,45 - Anders 2381,24) Harmon around 300 hp, Anders 200hp (6mph faster than Harmon - 250+ mph for the RV-4) So much to "thumb-rules", and "not doing ones homework, because I don't like formulas, etc." I believe until there is a solid FWF- instalation for every popular airframe, you will HAVE to do your homework, or give away efficiency in a big way!! There are so many guys on this list that know their ways around formulas, being on the list to share info and trying to help on every corner, you don't even have to dig in that hard, just ask someone who enjoys formulas! Back lurking, Thomas J. PS: I have 3 pages of that article scanned, if anyone wants/needs them I can forward it. =00-300Kb each.... end of copy I got to direct replies - here the copies to complete my answer:) reply 1: Al, I did mean "intake"! On both their planes, the Mustang II and the =V-4 the intake area is biiger than the exit!! As mentioned, if you need I can email the scans =f the mag! You also might want to read up on: http://www.cafefound=tion.org/aprs/RV-4.pdf Thomas J. ----- Original Message ----- From: Al =ietzen To: Rotary motors in =ircraft Sent: Wednesday, April 06, 2005 =1:29 PM Subject: [FlyRotary] Re: rule =f thumb and RV-3 sizes- ..cut Note: the exhaust area requirement is greater than the inlet combination =f oil and coolant due to the now considerable hotter air =emperature. Rule of thumb: Coolant air inlet opening for 200 HP coolant cooled =ngine ~60 sq. in., oil inlet opening ~30 sq. in. equals a total opening of =0 sq. in. A good place to start with exhaust opening is 1.4 times =he inlets or 126 sq. in. Close the exit area down with cowl flaps to as =ittle as 80% of the inlet combination at cruise conditions! ...cut I do understand that an air-cooled installation is a different animal, =ut by the end of the day ( or at the cowl exit...for that matter) everything =s air cooled. The interesting thing is that both run a intake/exit ratio =f 78%, the intake (I think you meant 'exhaust) being bigger - just =bout confirming the above statement! Schmidtbauer mentiones the " rule of thumb" - exit about 150% of =nlet. I think =or our purposes, the 1.5 ratio is more applicable than 1.28. It can be =ower IF you have well shaped entrance and exit ducting. Typically we focus =n entry ducting, and then have rapid expansion at the exit from the =ore - sudden expansion pressure loss; losing all momentum and then, =sually a rather sudden acceleration out a fairly small opening. =efinitely less than optimum. I don't think there is a great difference on the amout of cooling =ecessary, as the efficiency of both engines are fairly close. So some heat =oes out the exhaust and the rest has to be cooled. For sure you need =ifferent ducting, but the amount of heat energy should be about the same and =ou want to get rid of it with the least drag, either way. You're right. The biggest difference is the higher rejection temp, =nd larger delta T of the air on the air-cooled engines allows for a lower air =low rate. Al reply 2: Hi Barry, I agree that augementation plays a serious role in these two planes. It was never mentioned or opposed or other, they just found the right combination/optimization of various areas in their system to make it work contrary to the "rule of thumb" of 150% (or whatever number) the exit area hat to be bigger than the intake. Obviously this ROthumb does not care too much about drag reduction. Thomas J. > Thomas J, > > The key line in that article followed the size of the exhaust: "is > augmented by exhaust flow." Exhaust augmenters really work according to > Contact magazine and others. This article is just more evidence of that. > Apparently the augmenter is so effective that it allows what would > ordinarily be a restrictive outlet on the back of the heat exchanger. > > Barry Gardner > Wheaton, IL OKAY, end of post: To recollect: If anyone cares for send me your email and I forward the scanned pages of the KItplane report. Also check out http://www.cafefound=tion.org/aprs/RV-4.pdf on Anders RV-4!! Thomas J. ------=_NextPart_000_3419_01C600E5.E3296710 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
----- Original Message -----
From:=20 Monty=20 Roberts
Sent: Wednesday, December 14, = 2005 5:01=20 PM
Subject: [FlyRotary] Cowl = Exit

John,  =20 It really depends on how  much = power/heat you are=20 going to be producing.  However, if you have --say 24 sq inch for = each of=20 three openings (two for GM cores and one for oil cooler) then you have = a total=20 of 72 sq inches for cooling air to come into the cowl.  Now = you are=20 going to heat the air which will naturally cause it to expand in = volume, and=20 based on your opening size 4.25*11.25 =3D 47.8 sq inches for this = expanded air=20 to get out.   Your exit  area is way too small in  = my=20 opinion. 
 
  A "rule of = thumb" for exit=20 area that I have seen is no less than 1.2 ratio of exit to = intake.  I=20 have seen that ratio up to 1.7. so that would indicate you would need = 1.2*72 =3D=20 86 to 1.7*72 =3D 122 sq inches.  Personally, I would lean toward = the 122=20 myself.
 
Here is a rule of = thumb I just got=20 off the internet
 
You will hear rules of = thumb that=20 the exit openings, because the air is heated and thus presents more = volume=20 than the original intake air, should be 2.5 times the area of the = intake=20 opening. With good extraction of this heated air, allowed by the fact = that it=20 is exhausted into a low pressure area, the exit cross section can be = as small=20 as the intake in some cases. In others larger, but I have never seen = this exit=20 area have to exceed about 1.5 to twice the area of intake.=20
 
From a = different=20 source  Exit = area totals=20 66.7 square inches which comes to about a 1 to 1.5 ratio inlet to exit = area.
 
Notice no indication = that anything=20 less than intake area would do the job.
 
 
I used the solid pink = foam from a=20 lumberyard (had to glue several planks together).  I taped it = with duct=20 tape to prevent the foam from dissolving when I put on the epoxy - = work fine=20 for me.
 
Ed A
 
 
Ed,
 
The oft sited rules of thumb are fine. But you = have to keep=20 in mind it all depends on your flight conditions. If you size a = radiator,=20 inlet and exit for hot day climb, then it will be WAY too big for = cruising at=20 75% power at 12K feet. In this case you use a cowl flap to bring the = exit size=20 down. This restricts the flow of air through the radiator thereby = reducing=20 momentum drag. It also causes the pressure in the cowl to go up. The = air=20 stacks up and you get external diffusion. The high pressure heated and = expanded air now squirts out through this restriction and you gain = some of=20 your drag back. In this case, it is possible that the exit may be = smaller than=20 the inlet (the inlet is sized for WOT climb, not cruise). Remember the = engine=20 is putting out way less than rated power because you are (or should = be)=20 running LOP at 75% minus the power loss with altitude. So the = heat load=20 is way less than what you need to remove at WOT climb on a hot=20 day. You should have some way to vary the exit if you = want to=20 get the best out of the airplane in cruise, of course it is more=20 complex. It's all a trade off.
 
I must admit I like the current post following the = previous.=20 One can only rebel for just so long ;-).
 
Monty
 
 
Monty, you are rebel!!
Bottom posting it is, ....at least on this=20 thread!
 
Back to square 1!!
 
We had a similar thread back in = April!
ROT's about exit/intake ratio where exit HAS = to be=20 bigger are gospel. If you want to reduce cooling drag, read up on the=20 following:
(I am cheap enough to just copy my past posts = and=20 replies here:))
 
copy from 06-April=20 2005:
...cut
Note:  the exhaust area requirement is greater = than the=20 inlet combination of oil and coolant due to the now considerable = hotter air=20 temperature.  Rule of thumb:  Coolant air inlet opening for = 200 HP=20 coolant cooled engine ~60 sq. in., oil inlet opening ~30 sq. in. = equals a=20 total opening of 90 sq. in.  A good place to start with exhaust = opening=20 is 1.4 times the inlets or 126 sq. in.  Close the exit area down = with=20 cowl flaps to as little as 80% of the inlet combination at cruise=20 conditions!
...cut
There was an=20 excellent article about that in Kitplanes of Feb.2004, about Brian=20 Schmidtbauer's Mustang II. Though Lyc powered it still goes 250 mph, = just=20 about par with his friends RV-4, same engine (Dave Anders of CAFE = foundation=20 fame). Of course Dave is part guilty for that Speed demon, as he = gave some=20 ideas to Brian about cooling.
I = do understand=20 that an air-cooled installation is a different animal, but by the = end of the=20 day ( or at the cowl exit...for that matter) everything is air = cooled. The=20 interesting thing is that both run a intake/exit ratio of 78%, the = intake=20 being bigger - just about confirming the above=20 statement!
Both seem to use=20 exhaust augmentation for the exit.
Their main study=20 material was "NACA reports back to the 1920's and Hoerner's book = about Fluid=20 Dynamic Drag" ( by the way that book is still available, but at = around a 100=20 bucks rather pricey, but worth it - says hearsay. Out of my range = for=20 now....)
 
I = mentioned that=20 on the other list, but I was dismissed as " you can't compare = aircooled=20 engines with a rotary".
I = don't think=20 there is a great difference on the amout of cooling necessary, as = the=20 efficiency of both engines are fairly close. So some heat goes out = the=20 exhaust and the rest has to be cooled. For sure you need = different=20 ducting, but the amount of heat energy should be about the same and = you want=20 to get rid of it with the least drag, either = way.
 
Schmidtbauer=20 mentiones the " rule of thumb" - exit about 150% of inlet. By fine = tuning=20 the ductwork he got rid of up to 30% of the total drag, just by = eliminating=20 most of the cooling drag.
Anders beat John=20 Harmon in his Harmon Rocket, by over 1000 points (CAFE system, = Harmon=20 1316,45 - Anders 2381,24)
Harmon around=20 300 hp, Anders 200hp (6mph faster than Harmon - 250+ mph for the=20 RV-4)
 
 
So = much to=20 "thumb-rules", and "not doing ones homework, because I don't like = formulas,=20 etc."
I = believe until=20 there is a solid FWF- instalation for every popular airframe, you = will HAVE=20 to do your homework, or give away efficiency in a big=20 way!!
 
There are so=20 many guys on this list that know their ways around formulas, being = on the=20 list to share info and trying to help on every corner, you don't = even have=20 to dig in that hard, just ask someone who enjoys=20 formulas!
 
Back=20 lurking,
 
Thomas=20 J.
 
PS: I have 3=20 pages of that article scanned, if anyone wants/needs them I can = forward it.=20 =3D00-300Kb each....
 
end=20 of copy
 
 
 
I = got to direct=20 replies - here the copies to complete my = answer:)
 
reply 1:
 
 
Al,
 
I did mean "intake"!
On both their planes, the Mustang II and the = =3DV-4 the=20 intake area is biiger than the exit!!
As mentioned, if you need I can email the = scans =3Df the=20 mag! You also might want to read up on: http://www.cafefound= =3Dtion.org/aprs/RV-4.pdf
 
Thomas J.
----- Original Message ----- =
From:=20 Al=20 =3Dietzen
To: Rotary motors in = =3Dircraft
Sent: Wednesday, April 06, = 2005 =3D1:29=20 PM
Subject: [FlyRotary] Re: = rule =3Df=20 thumb and RV-3 sizes-

..cut

Note: =20 the exhaust area requirement is greater than the inlet = combination=20 =3Df oil and coolant due to the now considerable hotter air=20 =3Demperature.  Rule of thumb:  Coolant air inlet = opening for=20 200 HP coolant cooled =3Dngine ~60 sq. in., oil inlet opening = ~30 sq. in.=20 equals a total opening of =3D0 sq. in.  A good place to = start with=20 exhaust opening is 1.4 times =3Dhe inlets or 126 sq. in.  = Close the=20 exit area down with cowl flaps to as =3Dittle as 80% of the = inlet=20 combination at cruise conditions!

 

...cut

 

I = do=20 understand that an air-cooled installation is a different = animal, =3Dut by=20 the end of the day ( or at the cowl exit...for that matter) = everything=20 =3Ds air cooled. The interesting thing is that both run a = intake/exit=20 ratio =3Df 78%, the intake (I think you meant=20 =91exhaust) being bigger - just = =3Dbout=20 confirming the above statement!

 

Schmidtbauer mentiones the " rule of = thumb" - exit=20 about 150% of =3Dnlet.

 

I = think =3Dor=20 our purposes, the 1.5 ratio is more applicable than 1.28. It can = be=20 =3Dower IF you have well shaped entrance and exit ducting.  = Typically=20 we focus =3Dn entry ducting, and then have rapid expansion at = the exit=20 from the =3Dore =96 sudden expansion pressure loss; losing all = momentum and=20 then, =3Dsually a rather sudden acceleration out a fairly small=20 opening.  =3Definitely less than optimum.

 

I = don't think=20 there is a great difference on the amout of cooling =3Decessary, = as the=20 efficiency of both engines are fairly close. So some heat =3Does = out the=20 exhaust and the rest has to be cooled. For sure you need = =3Different=20 ducting, but the amount of heat energy should be about the same = and =3Dou=20 want to get rid of it with the least drag, either = way.

 

You=92re=20 right.  The biggest difference is the higher rejection = temp, =3Dnd=20 larger delta T of the air on the air-cooled engines allows for a = lower=20 air =3Dlow rate.

 

Al

reply 2:
 
Hi Barry,

I agree that=20 augementation plays a serious role in these two planes.
It was = never=20 mentioned or opposed or other, they just found the=20 right
combination/optimization of various areas in their system = to make=20 it work
contrary to the "rule of thumb" of 150% (or whatever = number) the=20 exit area
hat to be bigger than the intake. Obviously this = ROthumb does=20 not care too
much about drag reduction.

Thomas = J.

>=20 Thomas J,
>
> The key line in that article followed the = size of=20 the exhaust: "is
> augmented by exhaust flow." Exhaust = augmenters=20 really work according to
> Contact magazine and others. This = article=20 is just more evidence of that.
> Apparently the augmenter is = so=20 effective that it allows what would
> ordinarily be a = restrictive=20 outlet on the back of the heat exchanger.
>
> Barry=20 Gardner
> Wheaton, IL
 
OKAY, end of=20 post:
 
To=20 recollect:
 
If anyone cares = for send me=20 your email and I forward the scanned pages of the KItplane=20 report.
Also check out http://www.cafefound= =3Dtion.org/aprs/RV-4.pdf on=20 Anders RV-4!!
 
Thomas=20 = J.
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