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Posted for "Carl La Rue" <c.larue@verizon.net>:
Scott and others;
I will tell you what I did and why, re alternate air on my Lancair IVP. Some
of you may find it useful and others may think I'm nuts. It takes a story.
At one time Lancair recommended removing the alternate air doors because of
failures that trashed the turbos with metal parts, and the temporary
solution was to glass the openings shut while they reengineered new parts.
I had less than 150 hours on the plane and was building performance data in
climbs to 25,000 feet (and others) in preparation for my first point-to-
point speed record. I noticed that after I had glassed the openings shut I
began to run out of manifold pressure, i.e. began to decrease below 31" at
about 22,000 feet in the climb. I surmised that I was running out of air
because when I had the alternate air doors in place I had no trouble holding
31" at 25,000 feet and I figured they were opening without me knowing it.
So.........I reestablished a couple of temporary small openings in the same
area, with no doors, and tried it again. I got to about 23,500 before
manifold pressure began to fall off, so I cut the openings a little bigger
and was rewarded with the manifold pressure I wanted at 25,000 feet, 31". It was only temporary, since the fix was coming (Lancair said), but I wanted
to fly my record profile. I worried some about foreign object damage (FOD)
so I found a very stiff screen material with what I thought was a
sufficiently small mesh opening and RTV'd it in place over each opening
(where the alternate air doors originally were). I happily began flying to
all altitudes, using all manner of power settings, and after about 300 hours
TT, 85% or so of all my cruise was done at LOP. I have never experienced
high CHT's before or after this "fix", or any noticeable difference pre-fix
or post-fix in CHT's. When the Lancair-reengineered alternate air doors were sent to me I looked
at them and decided that there was very little improvement and still posed a
risk of failing due to vibration, so I left my screens in place and the
doors on a shelf. I always send my oil out for analysis and started to pay
very close attention to what I was told might be the result of the
unfiltered opening at low level, the silicon content. To my relief, it has
always remained about the same, averaging about 8 ppm at each oil change,
about the same as when I had the doors installed. The only other change I
have made along the way was to change my air filter enclosure slightly to
accommodate a racing style air filter that allows more air through it than
the original type I started with. I have inadvertently been in icing
conditions a few times in the 1334 hours I have on my plane, and did not
notice high CHT's at those times, but that is not something that I want to
go back and test.
I have my original turbochargers on the plane, in great shape to the eye,
and they have never been off for maintenance. I take pains to eliminate
potential FOD items, like safety wire clippings, and closely inspect my
screens (actually wire grates) at each oil change, seeing no deterioration
to date. I'll go out on a limb and opine that I actually have a safer setup
from a flying standpoint because it is practically foolproof (has no moving
parts) but I can't yet opine that from a maintenance longevity standpoint
(possible dirty engine). I'll take THAT risk though as opposed to having a
turbocharger come apart at a gazillion RPM. (I lost some compressor blades
off from the engine of my T-33 back in air force days and the aircraft
structure doesn't even begin to slow down that fast-moving debris).
Anyway, flame suit on and ready. I can take and send pics at my next oil
change later this month if anybody is interested.
Carl La Rue
c.larue@verizon.net Ph 740-439-4988
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