Hi Randy,
I read an article once about a Velocity builder (canard pusher) who
used NACA ducts behind the cabin to cool his whole engine. As you can
imagine, he had some trouble getting adequate air flow through the
cowl. He tried many different modifications (short of bonding on some
big scoops) but nothing worked. He finally determined that the
boundary layer air was just skipping over the NACA ducts and very
little was being drawn into the cowl.

His solution ended up being very simple. He bonded a small triangular
ridge (think similar to a stall strip) on the cowl surface just ahead
of the NACA inlet to "trip" the boundary layer so that it would tumble
into the duct opening. IIRC, the final shape that worked best for him
was a "V" shaped (like a chevron) 3/8" ridge placed about an inch
forward of the ducts pointing toward the nose. This might not work in
your case but it would be easy to test. Just tape on some balsa strips
and go fly. I wish I had a picture for you but I can't remember where
I read it.

BTW, I'm still very impressed with the unbelievable quality of your work.

RonL



On Mon, Mar 9, 2009 at 12:41 PM,  <marv@lancair.net> wrote:
> Posted for randy snarr <randylsnarr@yahoo.com>:
>
>  All,
>  Thanks for the good suggestions on the oil cooler.
>  I flew again today and my oil temps are at 208 F flying in 35 f air.
>  They will be 215 or 220 this summer.
>  I will try a few solutions that do not need to cut the cowl. I will most
> likely make a duct to carry the exhaust air from the oil cooler down to the
> lower aft part of the cowl. I have also heard that the aft vertical edge of
> the naca opening should be shaped like an air foil. Mine is flat and fairly
> sharp. The easiest way to test will to be construct a piece to give it a
> little airfoil made from balsa or foam taped in place and flight tested. If
> that does not work, I will make a duct from the cooler to the low pressure
> area on the bottom of the cowl and lastly would be a small lip on the bottom
> of the cowl aft.
>  every producton airplane I have seen with an opening on the bottom of the
> cowl has 45 deg lip to create a low pressure area. I should have done that
> before I painted....
>
>
>  I will post my findings when I get them.
>
>  Randy Snarr
>  N694RS
>
>
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>
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