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Comments?
Subject: Inlet cooling article
I remember the Laboda article about enlarging their cooling inlets,
but not many of the details.
This:
The plenum
receives air through two circular air intake ducts behind the
propeller and squeezes it, Bernoulli-style, so that the air
accelerates across the cylinders and between their fins, carrying
the heat back, down and out an outflow "gate" at the back and
bottom of the engine area, forward of the firewall.
Is contrary to everything I've ever read about cooling efficiently.
Faster relative flow will always have higher drag, all else being
equal. Accelerating the air even faster than freestream just sounds
crazy. My understanding is that there's a balancing act between
having the room in an a/c to 'recover' (increase) differential
pressure across the heat exchanger (engine fins, in this case), and
causing too much drag from the air going through the fins too fast
(there's aerodynamic drag in the heat exchanger, just like over the
a/c itself). It's surprising to me that James made the plenum the
way he did. The rest sounds like putting bandaids on stuff. The
next-to-last image, of the final inlet, shows what appears to be a
*much* smaller plenum inlet than the cowl ring in front of it, and a
rather sharp edged lip where the plenum starts. It looks like the
air would accelerate until it hits that sharp lip, and immediately
go turbulent, which will kill any pressure recovery and actually
slow flow into the cylinder fins.
Most Lyc plenums I've seen (even the ones James made for the 4 cyl
engines) have significant volume above the cylinders with smoothly
expanding ducts feeding the plenum. That allows the air to slow in
an organized fashion, which increases *pressure*, which is what
actually makes the air move through the fins.
But what do I know; I have an Economics degree....
Charlie
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