X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from ispmxmta06-srv.alltel.net ([166.102.165.167] verified) by logan.com (CommuniGate Pro SMTP 5.0c5) with ESMTP id 774182 for flyrotary@lancaironline.net; Wed, 19 Oct 2005 21:41:34 -0400 Received-SPF: pass receiver=logan.com; client-ip=166.102.165.167; envelope-from=montyr2157@alltel.net Received: from Thorstwin ([4.227.96.205]) by ispmxmta06-srv.alltel.net with SMTP id <20051020014047.RBHK12191.ispmxmta06-srv.alltel.net@Thorstwin> for ; Wed, 19 Oct 2005 20:40:47 -0500 Message-ID: <001301c5d517$4cf9f1a0$cd60e304@Thorstwin> From: "Monty Roberts" To: Subject: flyrotary NACA scoops Date: Wed, 19 Oct 2005 20:40:48 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0010_01C5D4ED.63521AB0" 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_0010_01C5D4ED.63521AB0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable I think it boils down to this.=20 You need a certain pressure difference across a given heat exchanger to = create the right convection coefficient at the right mass flow to move = enough heat to keep things cool.=20 What is the most efficient way to achieve this. Everybody wants a black and white answer.=20 There is not one.=20 The NACA works by making two vortices that curl down into the inlet. The = high speed air enters under the lip due to this. To create these = vortices there must be something "sucking" on the inlet. Once flow is established, you get about 80% of dynamic pressure = recovery from a properly designed NACA and downstream diffuser. A conventional diffuser slows and expands the air thus increasing the = pressure. If the air stacks up it can also create external diffusion, = something that the NACA cannot do. It just slips past with no vortices = being produced. A canard typically flies with some positive angle of attack. It also has = a bluff body at the back of the cylinders. I know this will offend the = canard guys, but the aft end of a canard is bad...really bad. And the = bluff body creates a negative pressure. Which is bad for drag, but good = for cooling. The suction here due to this and the prop disk can suck the = air past the cylinders and allow the vortex to form. So it works. Would a NACA in front of a thick 4 core dense radiator work...probably = not. Would a NACA in front of a thin radiator with exits in the right place = work....probably Would a P-51 scoop be better? maybe if it didn't add a lot of frontal = area, surface area, interference drag, and such that cancel out the 10% = increase in dynamic pressure recovery that is available. I have never understood the statement that the original paper made about = how the flush inlet was not suitable for a carb air inlet. Seems like it = would work fine if you only want 80% vs 90% pressure recovery. The = original report just says this without really saying why. The argument = about radiator inlets makes sense-most of the time. But induction air is = not as clear cut an issue. They also admonish you not to place it in a thick boundary layer. I would point out that the NACA inlet has not even been used for jet = engine inlets very often (other than APU's). The only non-research = example that comes to mind is the Caproni jet that used one over the = cockpit. Most jet engines are very sensitive to turbulence in the inlet, = and the NACA is not very good in this regard. The Caproni used two small = centrifugal type jets which can tolerate a lot of inlet turbulence. = Pressure recovery is also very important to a jet.=20 In short....it depends. If it works....it works. Monty ------=_NextPart_000_0010_01C5D4ED.63521AB0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
I think it boils down to this. =
 
You need a certain pressure difference = across=20 a given heat exchanger to create the right convection coefficient = at the=20 right mass flow to move enough heat to keep things cool.
 
What is the most efficient way to = achieve=20 this.
 
Everybody wants a black and white = answer.=20
 
There is not one.
 
The NACA works by making two vortices = that curl=20 down into the inlet. The high speed air enters under the lip due to = this. To=20 create these vortices there must be something "sucking" on the=20 inlet.
 
Once flow is established, you get about = 80% of=20 dynamic pressure  recovery from a properly designed NACA and = downstream=20 diffuser.
 
A conventional diffuser slows and = expands the air=20 thus increasing the pressure. If the air stacks up it can also create = external=20 diffusion, something that the NACA cannot do. It just slips past with no = vortices being produced.
 
A canard typically flies with some = positive angle=20 of attack. It also has a bluff body at the back of the cylinders. I know = this=20 will offend the canard guys, but the aft end of a canard is bad...really = bad.=20 And the bluff body creates a negative pressure. Which is bad for drag, = but good=20 for cooling. The suction here due to this and the prop disk can suck the = air=20 past the cylinders and allow the vortex to form. So it = works.
 
Would a NACA in front of a thick 4 core = dense=20 radiator work...probably not.
 
Would a NACA in front of a thin = radiator with exits=20 in the right place work....probably
 
Would a P-51 scoop be better? maybe if = it didn't=20 add a lot of frontal area, surface area, interference drag, and such = that cancel=20 out the 10% increase in dynamic pressure recovery that is=20 available.
 
I have never understood the statement = that the=20 original paper made about how the flush inlet was not suitable for a = carb air=20 inlet. Seems like it would work fine if you only want 80% vs 90% = pressure=20 recovery. The original report just says this without really saying why. = The=20 argument about radiator inlets makes sense-most of the time. But = induction air=20 is not as clear cut an issue.
 
They also admonish you not to place it = in a thick=20 boundary layer.
 
I would point out that the NACA inlet = has not=20 even been used for jet engine inlets very often (other than APU's). The = only=20 non-research example that comes to mind is the Caproni jet that used one = over=20 the cockpit. Most jet engines are very sensitive to turbulence in the = inlet, and=20 the NACA is not very good in this regard. The Caproni used two small = centrifugal=20 type jets which can tolerate a lot of inlet turbulence. Pressure = recovery is=20 also very important to a jet.
 
In short....it depends.
 
If it works....it works.
 
Monty
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