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Date: Tue, 19 Oct 2004 21:53:05 -0400
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Subject: RE:Plenum Cooling
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From: "Halle, John" <JJHALLE@stoel.com>

I am certainly no expert but I think the Bernoulli principle (an =
expanding stream of air shows a reduction in velocity and an increase in =
pressure and vice versa) applies if the expansion is generally =
non-turbulent, which happens if the rate of expansion is around 5 =
degrees or less.  In my Legacy with a Lycoming engine, air intake on =
both sides was turbulent.  On the left much more on the right because =
the baffling angle is much steeper to accommodate an oil cooler (which =
we installed elsewhere.)  It is fairly clear that no one did any airflow =
engineering on the intakes.  We got some significant temperature =
reductions (about 35 degrees CHT) from making some minor alterations to =
the right side baffling (basically putting in a small ramp that blocked =
about an inch of the #1 cylinder), installing about a 4" deep cone on =
the left intake and a deflector to direct left intake air down toward =
the #2 cylinder.

Once the cross sectional area through which a stream of air passes is =
increasing at a rate greater than 5 degrees, the flow becomes turbulent =
and the Bernoulli principle ceases to apply.  Therefore the rapid =
increase in area entering the plenum chamber should have unpredictable =
effects on pressure (but good intake geometry can increase pressure =
before the air gets to the plenum chamber.)

George Braly's observations (as usual based on actual data for which I =
am most grateful) were very interesting.  Based on them, I think I am =
going to stop work on the plenum I was building and concentrate on =
seeing if I can make further improvements to the intake flow.

The turbulence problem also applies to outflow and all the junk hanging =
from the engine and engine mount make outflow wildly turbulent.  If you =
can find a way to channel outflow, there should be a noticeable increase =
in speed and decrease in temps.  I have looked in the engine compartment =
and concluded that that is easier to say than to do but, it might be a =
nice project for someone who is looking for a challenge.

The person who really has this stuff down "cold" is Dave Anders who has =
the world's fastest RV-4.  He used to live in Visalia and now, I think, =
has moved to somewhere near Redding.  I spent a few hours with him a =
couple years ago and he could not have been more helpful.