X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 64 [XX] (50%) SPAMTRICK: obfuscated phone number (50%) RECEIVED: IP not found on home country list Return-Path: Sender: To: lml@lancaironline.net Date: Tue, 10 Apr 2007 10:45:09 -0400 Message-ID: X-Original-Return-Path: Received: from mail11.syd.optusnet.com.au ([211.29.132.192] verified) by logan.com (CommuniGate Pro SMTP 5.1.8) with ESMTPS id 1976867 for lml@lancaironline.net; Tue, 10 Apr 2007 00:37:45 -0400 Received-SPF: none receiver=logan.com; client-ip=211.29.132.192; envelope-from=fredmoreno@optusnet.com.au Received: from fred (optussatellitebintb.22bjipb002.optus.net.au [59.154.24.148]) (authenticated sender fredmoreno) by mail11.syd.optusnet.com.au (8.13.1/8.13.1) with ESMTP id l3A4aZmZ028090 for ; Tue, 10 Apr 2007 14:36:45 +1000 From: "Fred Moreno" X-Original-To: "'Lancair Mailing List'" Subject: FW: [LML] Heat drag and speed records - The Macchi-Cactold MC-72 X-Original-Date: Tue, 10 Apr 2007 12:36:58 +0800 X-Original-Message-ID: <018601c77b29$e7752c40$c211140a@fred> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0187_01C77B6C.F5986C40" X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.6822 Thread-Index: Acd6jum6ZiQSyLy+T5mf3DhFjCtXqgAmHldA X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3028 Importance: Normal This is a multi-part message in MIME format. ------=_NextPart_000_0187_01C77B6C.F5986C40 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Now that we know a lot more about boundary layer behaviour and heat transfer, we can see why the wing panel radiators were ineffective. =20 =20 To transfer heat to air (an insulator) you need very thin boundary = layers. Very thin boundary layers occur on and near the front of sharp leading edges, like those you see in your oil cooler and auto radiator. =20 =20 As the boundary layer grows thicker (as it moves downstream from the = leading edge), the thermal gradient between hot surface and colder flow drops dramatically, and the rate of heat transfer drops quickly. So to get excellent heat transfer, you need lots of knife edges facing forward, = and after the flow passes a short distance (fractions of an inch) you then terminate the surface (the fin) and start a new one with a new leading = edge to start a new, very thin boundary layer. The resulting heat transfer coefficients near sharp edges facing forward are many times greater than = for a well developed boundary layer that has passed over inches or feet of fuselage or wing surface.=20 =20 For fluids like air that have a Prandtl number near one (0.7 for air) = the thermal boundary layer and velocity/momentum boundary layer (the one we worry about for drag, stall and such) are nearly the same shape and thickness. (The Prandtl number is essentially a ratio of the ability of = a fluid to transfer momentum via viscosity compared to the ability to = transfer heat via thermal conductivity and specific heat.) So if you know one boundary layer, you more or less know the other. =20 =20 On a flat plate oriented parallel to the flow at the speeds we fly at, = the boundary layer thickness is roughly 1% of the distance from the leading = edge (gross over simplification, but communicates the idea). So two feet = back from a leading edge, the boundary layer is about a quarter of an inch = thick. That does not help heat transfer very much. =20 =20 If you take a magnifying glass and look into the passages of a modern radiator, you will see that the fins which are themselves not very long = are now themselves punched with tiny louvers to create still more leading = edges to get those thin boundary layers. The key is to balance heat transfer = and friction loss. Large truck radiators have gone far in this direction = since the cooling loads on a 600 HP diesel full throttle on a summer day = climbing the Rockies can be significant drag on fuel economy.=20 =20 Wing panel radiators have two other fundamental problems. First, they = get hot and cold, and thus they grow and shrink. It makes mechanical design = a bit of a nightmare especially when the panel is many square feet in = size. Second, heat addition to boundary layers is destabilizing, causing = premature transition from laminar to turbulent flow which then increases drag.=20 =20 Behold the humble modern radiator. It accomplishes so much in so little space.=20 =20 Fred __________________________________________ =20 =20 The Macchi-Castoldi can be seen at an Italian air museum on Lago di Bracciano near Rome. An interesting detour for aviation nuts. According to description in the museum, the wing radiators were = sufficient for speed run, but not for the Schneider Trophy. Probably to due to hydraulic flow questions. In any case the idea was not used again. Yakjock wrote:=20 With the discussion on speed and heat drag I thought I'd remind folks of = the Macchi-Castoldi MC-72. =20 ------=_NextPart_000_0187_01C77B6C.F5986C40 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

Now that we know a lot more about boundary = layer behaviour and heat transfer, we can see why the wing panel radiators were ineffective. 

 

To transfer heat to air (an insulator) you = need very thin boundary layers.  Very thin boundary layers occur on and near = the front of sharp leading edges, like those you see in your oil cooler and = auto radiator. 

 

As the boundary layer grows thicker (as it = moves downstream from the leading edge), the thermal gradient between hot = surface and colder flow drops dramatically, and the rate of heat transfer drops = quickly.   So to get excellent heat transfer, you need lots of knife edges facing = forward, and after the flow passes a short distance (fractions of an inch) you = then terminate the surface (the fin) and start a new one with a new leading = edge to start a new, very thin boundary layer.  The resulting heat transfer coefficients near sharp edges facing forward are many times greater than for a well developed = boundary layer that has passed over inches or feet of fuselage or wing surface. =

 

For fluids like air that have a Prandtl number = near one (0.7 for air) the thermal boundary layer and velocity/momentum = boundary layer (the one we worry about for drag, stall and such) are nearly the = same shape and thickness.  (The Prandtl number is essentially a ratio of = the ability of a fluid to transfer momentum via viscosity compared to the = ability to transfer heat via thermal conductivity and specific heat.)  So if = you know one boundary layer, you more or less know the other.  =

 

On a flat plate oriented parallel to the flow = at the speeds we fly at, the boundary layer thickness is roughly 1% of the = distance from the leading edge (gross over simplification, but communicates the idea).  So two feet back from a leading edge, the boundary layer is = about a quarter of an inch thick.  That does not help heat transfer very much.  =

 

If you take a magnifying glass and look into = the passages of a modern radiator, you will see that the fins which are = themselves not very long are now themselves punched with tiny louvers to create = still more leading edges to get those thin boundary layers.  The key is to = balance heat transfer and friction loss.  Large truck radiators have gone = far in this direction since the cooling loads on a 600 HP diesel full throttle = on a summer day climbing the Rockies can be significant drag on fuel economy.

 

Wing panel radiators have two other = fundamental problems.  First, they get hot and cold, and thus they grow and shrink.  It makes mechanical design a bit of a nightmare especially = when the panel is many square feet in = size.  Second, heat addition to boundary layers is destabilizing, causing premature = transition from laminar to turbulent flow which then increases drag. =

 

Behold the humble modern radiator.  It accomplishes so much in so little space.

 

Fred

__________________________________________

 

 

The Macchi-Castoldi can = be seen at an Italian air museum on Lago di Bracciano near Rome.  An interesting detour for aviation nuts.

According to description in the museum, the wing radiators were = sufficient for speed run, but not for the Schneider Trophy.  Probably to due to = hydraulic flow questions.  In any case the idea was not used again.


Yakjock wrote: 

With the discussion on speed and heat drag I =
thought I'd remind folks of the
Macchi-Castoldi MC-72…  =
------=_NextPart_000_0187_01C77B6C.F5986C40--