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Here's the formula I use to determine induction aperture
area, sq. in.: RPM/60 x CD!/2 x 1.25 / (TAS, mph, x 22/15 x 12) The
1.25 gives a 25% increase to take care of inlet losses and the need
for increased area at low speed. My MAP always calculates out to PSTATIC + PDYNAMIC - PCARB DROP. For the O-235, the
carb drop ranges from 1.5" at sea-level to about 1.2" at 12,500'. At 8000' dalt,
250mphTAS, dynamic pressure should be 1.68". If you see more MAP rise than this,
you might have a source of pixie dust handy! My induction air inlet is the
"chin" style to take advantage of the slight velocity increase from the air
displaced by the spinner; my cooling air inlets also try to take advantage
of this by having the inside edge in-line with the edge of the spinner. I expand
the air into a 10" x 4" K&N filter, with a nose-gear operated bypass around
the filter. This doesn't seem to be necessary, as I see no MAP change when the
bypass opens. That's probably due to the air being slowed down so much through
the filter, so there is minimal pressure drop. One thing to mention,
however. Jeff's/Chris' "Miss Gianna" is using the ELIPPSE prop I designed
for them and for the Tom Aberle/Andrew Buehler "Phantom", the 2004 and 2005
Gold Championship winner, which is somewhat similar in planform to the
three-blade I designed for my Lancair. This design has true airfoil and helix
angle all the way into the spinner, and promotes, rather than blocks
the airflow in the hub region, as opposed to almost all fixed-pitch and the
majority of CS props with their god-awful hub shapes! This prop-induced blocking
of airflow is the reason for mounting cooling inlets farther out from center
where the prop is actually inducing airflow. At least, so said John
Roncz!
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