For those on the matronics list:
A good dimensional analysis of the =93NACA=94 duct for proper operation is
in the attached NACA document.
A ratio of 4 to one width to depth and a shape as shown in the NACA Ames R
esearch Center document 5121 attached gives the optimum shape of the inlet.
The NACA document has a lip that was changed over the series of tests but i
n reality just a flat thin inlet (thickness of the skin) works very well gi
ving a good pressure flow rate.
I did a paper on plenum inlet design some time ago due to poor fit and ques
tions on how large to make the plenum flange on the cowl and also some shap
es but the Europa install manual is good enough for most. I would recommen
d some changes as in my pdf attached on preventing water intrusion.
1. As a technique, try to have the NACA inlet into the cowl capture the
pressure by being semi sealed to the plenum and as we have found out, in ra
in, the water tends to =93stick=94 to the boundary layer or skin of the air
craft and will flow into the inlet filling the filter. Of course, when pa
rked rain will catch in the NACA duct and funnel into your inlet unless you
prevent that. One can make a slight deflector on the inlet lip that funne
ls the water from inflight and ground use out of the engine inlet. See Air
box water intrusion in the carb box pdf attached.
2. For those with home made plenums, be sure to install 1/8 inch drain h
oles in the plenum at a point lower than the air inlet to the carb (some so
rt of dip prior to the carb inlet would be nice) to prevent water from pool
ing in the plenum.
3. Every oil change, pull the filter and look into your plenum as in dam
p environments, water, will pool in the plenum, and from the drain holes, s
ome pollen or dust will form around the drain hole water drip hole and may
in fact plug the drain causing water to get into the carb/engine. Nasty.
4. For the 912/912S owners, in cold and rainy environments, you may be w
ell served to install a carb heat system and consider a water deflector or
siphon on the inlet to divert water from directly entering the air filter/p
lenum hole. On home made plenums I have seen the owner elevate the filter
to prevent water intrusion also.
I toyed with the idea of a Rotax 912S plenum in the Europa. It would work
with possibly less work when changing from two cone type air filter to a ca
rb heat and inlet setup.
See drawing:
[Diagram Description automatically generated]
Best Regards,
Bud Yerly
-----Original Message-----
From: owner-europa-list-server@matronics.com<mailto:owner-europa-list-serve
r@matronics.com> <owner-europa-list-server@matronics.com<mailto:owner-europ
a-list-server@matronics.com>> On Behalf Of n7188u
Sent: Wednesday, December 15, 2021 4:53 PM
Subject: Europa-List: Re: Induction cold air plenum
rb@gmail.com>>
Thanks "Spurpura",
It's good to hear that mine is like other XSs. I am probably going to do so
me surgery to this plenum anyway as I want to ensure enough clearance, smoo
th flow from the scoop into the plenum and some protection against water in
gress.
As a good airplane homebuilder friend of mine says: "There you go, adding a
nother two month of work to the project" :)
Chris
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