REFLECTOR: updraft cooling

[Dave Philipsen]

Where were the CHT thermocouples on his engine?  Did he move the 
thermocouples when he converted to downdraft?


Ron Brown wrote:
> And, the downdraft NACA cooling for some unexplainable reason, runs 
> about 40 degrees cooler than the updraft cooling.  Mark Machado 
> converted what is now the factory trainer from updraft to downdraft 
> and says the heads ran 30-40 degrees cooler.  My 173 Elite RG runs 
> 360-370 max on a long climb out and 320-340 degrees during a 2600 
> rpm/155 kt cruise.  I highly recommend the NACA cooling system.
>  
> Couple of other recommendations to get good cooling:
>  
> 1.  Make sure you make the extended wraps to go on the plenums that 
> wrap all the way under the cylinders and heads - such that cooling air 
> is forced to go through the fins - and exit through a gap which is 
> 1.5" to 2" wide at the bottom.  If you look at the lycoming supplied 
> inter cylinder baffles, you want to match them on them on the outside.
>  
> 2. Close up the factory plenum to the crank case gaps.  I put duct 
> tape on the crankcase, then laid up two BID on the plenum onto the 
> duct tape.
>  
> 3. Keep the gap at the rear of the cowl to prop clearance small - Alan 
> Shaw recommended 1". That is what I did - with reliefs at the top and 
> bottom to allow the cowl to slide by the prop (with a rag to protect 
> the prop finish).
>  
> My cooling concerns while building wound up being absolute non 
> issues.  Even on the ground - with long taxiing - like Oshkosh are no 
> problem at all.  The only temperature issue I have is a landing - shut 
> down - and start back up and climb out (like young eagles flights) on 
> a hot day, I'll get 360 degree CHTs on climb out.  I put the nose down 
> and pull back on the throttle a bit - temps start coming back down.
>  
> Ronnie Brown
>  
>   
>
>     ----- Original Message -----
>     *From:* Douglas Holub <mailto:doug.holub at tx.rr.com>
>     *To:* reflector at tvbf.org <mailto:reflector at tvbf.org>
>     *Sent:* Saturday, February 17, 2007 1:28 PM
>     *Subject:* REFLECTOR: updraft cooling
>
>     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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-- 
Dave Philipsen
Velocity STD-FG
N83DP