Chris,
I saw your plenum setup when we were both at ADA and I was impressed -
especially with the preliminary data you had collected. Most importantly,
I liked the gentle sloped and long (about 10"?) diffusers into the "chamber" in
front of the cylinders and one other feature that is very important. Your
cowl rework matches the only really successful plenums I have seen - that is,
the cooling air inlets are built into the lower cowling so that the inlets can
be efficiently connected to the plenum (See Lee Behel's Legacy if you get a
chance), thus the removal of the plenum top is easy for access to the engine and
the top cowl is "merely" for fuselage aerodynamics. On our
Lancairs, pressurizing the upper cowling at 180 Kts IAS tends to puff it
out and this may alter the airflow over the cowl, including early separation -
the plenum may eliminate this as a drag issue.
Nice work!
Scott Krueger
AKA Grayhawk
N92EX IO320 Aurora, IL (KARR)
Some Assembly Required
Using Common Hand Tools
and some heavy breathing.
In a message dated 10/21/2004 1:21:33 AM Central Standard Time,
Christopher.Zavatson@udlp.com writes:
I
actually did consider using the cowl as the plenum top. There
were
two issues. The first was how to transition the top of the
diffusers to
the cowl without creating a step that would cause flow
separation given
sealing and engine movement considerations.
The
second issue was controlling leakage in a conventional baffle
system.
NASA did a study pertaining to the cooling of horizontally
opposed aircraft
engines that produced some very enlightening data.
They took an Aztec
engine installation and found 55% of the air entering
the inlets was being
lost to leakage. 38% was attributed to the
flexible baffle seals and
the rest was traced down to various sheet
metal to engine interfaces.
A hardtop plenum and RTV got things sealed
up. Accepting a method of
sealing that is so inherently lossy is not an
issue if you have an
oversized cooling system. I was going in the
opposite direction by
reducing inlet and exit areas. System
efficiency was now of
greater importance and so I did not want to go
down the path of accepting a
sealing method that has been shown to have
a very high leak
potential. Do leaks really have an impact? If you are
already
losing half your cooling air, you could probably drill a few
more holes in
the baffling and not see a difference in cooling. On my
first flight
with the plenum, I actually had a leak. One of the SCAT
ducts was not
pushed up onto the diffuser quite far enough. There was
no visible
gap, but the internal spiral wire was not entirely up on the
diffuser so
there was a short section of unsupported silicone creating
the seal.
You could just barely squeeze a finger between the diffuser
and the SCAT
duct in that area. On that first flight, the cylinder
closest to that
leak ran 20 degrees hotter. Do leaks really make
a
difference? In this case it did.
While a conventional
baffle systems will work ( I used one for 6
years), it is, IMHO, much
easier to achieve a well sealed and efficient
system with an enclosed upper
plenum. The benefits of this efficiency
become more apparent, the
faster we want to fly.