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Date: Sun, 20 Feb 2005 10:02:30 -0500
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From: "Gary Casey" <glcasey@adelphia.net>
Subject: RE: Air intake formula
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<<My spreadsheet does not recommend a specific scoop diameter (velocity
ratio). The spreadsheet simply shows, based upon the scoop diameter,
the extra percentage of air that you, the builder, have chosen to
incorporate into your design.
--
Lorn H. 'Feathers' Olsen>>

Yes, I was thinking that your spreadsheet resulted in a "recommended" inlet
area.  Getting the best overall compromise is a bigger task.  The way I look
at it the goal is to gently transition the air velocity from the free
airstream to the velocity required in the engine air inlet (throttle body or
servo).  It depends on what operating configuration is the prime goal.  In
my case at design max cruise condition the airstream velocity is 327 ft/sec
and the air inlet velocity is estimated at 125.  I am (today) sizing the
inlet for a velocity of 160 ft/sec, or just less than 50% of the aircraft
velocity.  Therefore about 75% of the possible pressure recovery will be
accomplished ahead of the inlet (like a pitot tube) with the rest being
accomplished via a diverging section after the inlet.  There are two reasons
for doing it this way - one is that pressure recovery ahead of the inlet is
very efficient and the other is that most other important operating
conditions, like a full-power climb, are at lower airspeeds.  The important
thing is to avoid changing velocities unnecessarily, like requiring the air
to slow down and then speed back up again.  Unfortunately, in my case there
will be an air filter in the path, which requires a very low velocity (23
ft/sec) through the element.  There is no room for an efficient diffuser, so
I will have to be content with about 75% of the possible recovery.  To make
the system more complex the air velocity into the engine is cyclic in nature
as is the "free" airstream velocity just aft of the propeller.  Close the to
hub the propeller blades actually obstruct flow while further outboard they
enhance it.  For a 6-cylinder engine with a 3-blade prop the engine and
propeller effects are synchronous so does the pulsating flow from the
propeller increase engine power or reduce it?  The propeller could be
clocked to provide the best effect (Mooney tinkered with this idea), but
what is the best angle?  More questions than answers.

Gary Casey