X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [24.25.9.101] (HELO ms-smtp-02-eri0.southeast.rr.com) by logan.com (CommuniGate Pro SMTP 5.0.4) with ESMTP id 882693 for flyrotary@lancaironline.net; Wed, 14 Dec 2005 21:37:17 -0500 Received-SPF: pass receiver=logan.com; client-ip=24.25.9.101; envelope-from=eanderson@carolina.rr.com Received: from edward2 (cpe-024-074-025-165.carolina.res.rr.com [24.74.25.165]) by ms-smtp-02-eri0.southeast.rr.com (8.12.10/8.12.7) with SMTP id jBF2aU4p022871 for ; Wed, 14 Dec 2005 21:36:30 -0500 (EST) Message-ID: <003601c60120$5eb8fa00$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Cowl Exit Date: Wed, 14 Dec 2005 21:36:36 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0033_01C600F6.75A4FEA0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2180 X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 X-Virus-Scanned: Symantec AntiVirus Scan Engine This is a multi-part message in MIME format. ------=_NextPart_000_0033_01C600F6.75A4FEA0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Yes, you are absolutely correct, Monty. The entire cooling system = depends on how much heat you need to get rid of and for what flight = regime you wish to optimize the cooling system. A system that keeps it = cool on take off and climbout will carry excessive cooling capacity (and = cooling drag) at higher cruise airspeeds. A design that focuses on = optimizing for cruise will undoubtedly pay the penalty on take off and = climb out. Unless like Todd and Perry you only fly in sub zero weather = {:>) But, I think its fairly safe to say that for most of us your exit should = be larger than your intake area - unless perhaps you have some sort of = exhaust augmentation to push it out Ed A ----- Original Message -----=20 From: Monty Roberts=20 To: Rotary motors in aircraft=20 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 =3D 47.8 sq inches for this = expanded air to get out. Your exit area is way too small in my = opinion.=20 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 =3D 86 to 1.7*72 =3D 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.=20 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 ------=_NextPart_000_0033_01C600F6.75A4FEA0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Yes, you are absolutely correct, Monty.  = The entire=20 cooling system depends on how much heat you need to get rid of and for = what=20 flight regime you wish to optimize the cooling system.  A system = that keeps=20 it cool on take off and climbout will carry excessive cooling capacity = (and=20 cooling drag) at higher cruise airspeeds.   A design that = focuses on=20 optimizing for cruise will undoubtedly pay the penalty on take off and = climb=20 out.  Unless like Todd and Perry you only fly in sub zero weather=20 {:>)
 
But, I think its fairly safe to say that for = most of us=20 your exit should be larger than your intake area - unless perhaps you = have some=20 sort of exhaust augmentation to push it out
 
Ed A
----- 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
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