REFLECTOR: updraft cooling

[Ron Brown]

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 
  To: 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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