The current thread on combustion air, ram pressure, where to place the air
intake, etc. brings up the issue of flying in rain with the Europa equipped
with the 912S, having the air intake arranged as a NACA duct on top of the
cowling and the plenum box just below the opening. There are two 3.3 mm
(1/8") drain holes at the bottom of this plenum box, which will be able to
drain away some water intake (you must fly with the ball in dead center for
both to be effective). A certain air pressure build-up in the plenum makes
the drainage more effective.
One theory I have heard goes as follows: If the engine were stopped and
surrounding air passed over the cowling at a speed of say 120 knots, very
little rain would enter the air box. The air would be backed up in the NACA
duct and "new" air and rain drops would for the most part pass over it.
Given this starting scenario (the aircraft moving at a speed of 120 knots):
If you now have a running engine it will still not suck in so much
combustion air that the velocity through the NACA duct is anywhere close to
120 knots + propeller slip stream velocity. Air would therefore still be
backed up in front of the plenum intake, with a certain amount of the air
and rain drops flowing over the inlet.
Theory and reality are two different matters also in this case. What has
been experienced when it comes to flying this configuration in rain
(drizzle, light, heavy) - is it definitely to be avoided or is it more a
matter of vertical and horizontal visibility?
Regards,
Svein
LN-SKJ
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