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I'm a mechanical engineer and I say don't worry about it, just dump it
out in the low pressure area of the cowl. Here's why... The vacuum pump
pulls a regulated vacuum of about 5.4 inches of mercury. One atmosphere
is about 30 inches of mercury or about 15 psig, so one psi is roughly
equal to two inches of mercury. At about 200 knots indicated, the
total ram pressure "felt" by the pitot tube is about one psi, or about
2 inches of mercury. Your cowl recovers about 75% of this (if it is
fairly good) above the engine. The pressure drop across the engine is
most of the incoming pressure at high cruise speeds. The resulting
pressure in the low pressure (hot air) portion of the cowl is therefore
at most only a bit above ambient. Do the math, and the highest pressure
you can expect in the low pressure region of the cowl is about 0.5
inches of mercury, probably less. And this is at about 200 knots
indicated. If you are indicating 150 knots the total ram pressure is
0.56 psi. (It goes as the square of the indicated air speed.)
therefore the pressure in the lower portion of the cowl represents
only a tiny fraction of the total pressure rise across the vacuum pump
(10% or less) so that venting the pump to a low pressure location for
discharge is not worth it.
A better place to exert your efforts is in making sure that your cowl
sealing is leak free. This goes especially for the high pressure side
of your oil cooler ducting. NASA work by Smiley in the early 80's
showed that with a new Piper Aztec (540 Lycoming engines) in addition to
the air needed to cool the engine, an additional 50% more leaked around
the engine through baffles, leaks, cracks, and poor sealing around the
nose of the engine. That means that one-third of the air going through
the cowl was creating drag, decreasing pressure drop across the engine
proper, and not contributing to cooling. On a large 6 cylinder engine,
the entire engine represents the equivalent of a sharp edged orifice about
4-5 inches in diameter through which all cooling air passes. Viewed this
way, you can see that a square inch here and a square inch there quickly
adds up to substantial leakage. You should have zero tolerance
for leaks, and then you will minimize drag and maximize cooling.
Fred Moreno
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