REFLECTOR: updraft vs sidedraft vs updraft cooling

[gpoole]

I have been making the same deliberations as Doug for quite some time.
There are compromises with every option ..sigh! Scoops vs NACA ducts...

If pinching air before the prop with a scoop on the belly is considered to
affect prop efficiency then perhaps (surely?) Al G's & several other's
approach of putting radiators in the wing is the way to go.  I like the idea
of being able to tailor the amount of air to the radiators to balance the
compromise between drag and amount of cooling required by having variable
inlets. Only problem is that the air needs to do several sharp turns to get
to the cylinder heads....which would should slow it down considerably....

Would be interested in this thread continuing to see what others think....

Greg in Sydney.

gregpoole at saaachapter11.com


-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Douglas Holub
Sent: Sunday, 18 February 2007 9:13 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: updraft cooling

"I thought that using NACA intakes would 'theoretically' be more drag
effecient ?"
-------------------------------------------------------------------
I was just comparing updraft to downdraft. It looks like I could put one big

NACA underneath the rear seat. I was thinking of using that spot for ram 
air, though.

I'm a little confused about the benefits of a NACA scoop.  I need to read up

on them some more. I think that if the cowl lip were extended up a little to

catch the air it might be more efficient than the NACA scoops.  But even if 
it were more efficient, you would be making it a little harder for air to 
flow to the propeller because the cowl would be getting a little taller. But

then, you lose some head room in the back seats with the NACAs. Decisions, 
decisions.

Doug Holub

----- Original Message ----- 
From: "Scott Derrick" <scott at tnstaafl.net>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Saturday, February 17, 2007 2:57 PM
Subject: Re: REFLECTOR: updraft cooling


> Doug,
>
> I had updraft on my IO360 and it worked fine. I had to run LOP in the
> summer(I did all the time anyway)  to keep the engine cool enough.  There
> were times when I stopped for gas and during the following departure
> climbout I would have to level off at an intermediate altitude for awhile
> to get the oil temps back down below 230.
>
> Installing my 520 I conferred with Velocity(ScottB and Brendon) and was
> advised I would need to use downdraft as they had never successfully seen
> an  updraft system work for the big six cylinder engines.  So I did the
> conversion.
>
> I thought that using NACA intakes would "theoretically" be more drag
> effecient ?
>
> Scott
>
>
>> I can understand why Burt Rutan and Nat Puffer are proponents of updraft
>> cooling. From an engineering point of view, it has a lot going for it. 
>> You
>> need more cooling when the airplane is climbing. If the cooling intake is
>> below the wing, the pressure is higher during a climb so you 
>> automatically
>> get more cooling during a climb. Similarly, it would be nice if cooling
>> was minimized during descent. The pressure is reduced under the wing
>> during descent, and so there is less cooling to the engine. Also, you've
>> got convection working with you instead of against you with an updraft
>> system.
>>
>> That all adds up to more drag with down draft cooling, because the NACA
>> scoops have to be large enough so that there is adequate cooling during
>> climb out, when the pressure at the NACAs is at its minimum.
>>
>> But the down draft is a lot simpler to implement, and that's probably
>> going to be the deciding factor for me.
>>
>> Doug Holub_______________________________________________
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