X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from ispmxmta05-srv.alltel.net ([166.102.165.166] verified) by logan.com (CommuniGate Pro SMTP 5.0.4) with ESMTP id 882342 for flyrotary@lancaironline.net; Wed, 14 Dec 2005 17:01:47 -0500 Received-SPF: pass receiver=logan.com; client-ip=166.102.165.166; envelope-from=montyr2157@alltel.net Received: from Thorstwin ([4.89.246.195]) by ispmxmta05-srv.alltel.net with SMTP id <20051214220100.RDTW8731.ispmxmta05-srv.alltel.net@Thorstwin> for ; Wed, 14 Dec 2005 16:01:00 -0600 Message-ID: <002c01c600f9$e0e1f800$8ee1e204@Thorstwin> From: "Monty Roberts" To: Subject: Cowl Exit Date: Wed, 14 Dec 2005 16:01:03 -0600 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0029_01C600C7.95A30AA0" 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 This is a multi-part message in MIME format. ------=_NextPart_000_0029_01C600C7.95A30AA0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable 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_0029_01C600C7.95A30AA0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
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 to=20 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 have=20 seen that ratio up to 1.7. so that would indicate you would need 1.2*72 = =3D 86 to=20 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 the=20 exit openings, because the air is heated and thus presents more volume = than the=20 original intake air, should be 2.5 times the area of the intake opening. = With=20 good extraction of this heated air, allowed by the fact that it is = exhausted=20 into a low pressure area, the exit cross section can be as small as the = intake=20 in some cases. In others larger, but I have never seen this exit area = have to=20 exceed about 1.5 to twice the area of intake.
 
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=20 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 for=20 me.
 
Ed A
 
 
Ed,
 
The oft sited rules of thumb are fine. But you have = to keep in=20 mind it all depends on your flight conditions. If you size a radiator, = inlet and=20 exit for hot day climb, then it will be WAY too big for cruising at 75% = power at=20 12K feet. In this case you use a cowl flap to bring the exit size down. = This=20 restricts the flow of air through the radiator thereby reducing momentum = drag.=20 It also causes the pressure in the cowl to go up. The air stacks up and = you get=20 external diffusion. The high pressure heated and expanded air now = squirts out=20 through this restriction and you gain some of your drag back. In this = case, it=20 is possible that the exit may be smaller than the inlet (the inlet is = sized for=20 WOT climb, not cruise). Remember the engine is putting out way less than = rated=20 power because you are (or should be) running LOP at 75% minus the = power=20 loss with altitude. So the heat load is way less than what you need to = remove=20 at WOT climb on a hot day. You should have some way to = vary the=20 exit if you want to get the best out of the airplane in cruise, of = course=20 it is more 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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