REFLECTOR: updraft cooling

[Douglas Holub]

"I thought that using NACA intakes would 'theoretically' be more drag
effecient ?"
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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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