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Old 09-14-2008, 02:01 AM
Steve parkins's Avatar
Steve parkins Steve parkins is offline
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Location: wa state
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Default bad part

copyed from marc
Continued from Part 3:

So what's happening here? As Mr. Cottner and Mr. Read have stated,
the Pmag/Emag units lose timing information, firing at arbitrary and
sometimes random times. Clearly, this screws up the workings of the
engine, sometimes to the point of having no power whatsoever. Mr.
Cottner had four failures and lost his airplane because of this
failure mode - Mr. Read had to make a precautionary landing, and I had
three failures and one precautionary landing.

Emagair has issued a SB on both the 113 and 114 units. If you recall,
I had timing issues with my 113 units as well. They have instituted a
"fix" for the issue, notwithstanding that the 114 mechanical redesign
was in part already a "fix" for the problems with the 113 versions.

The Emagair units use a small magnet and a hall effect sensor to
determine crankshaft position. It's an ingenious solution, because it
uses a sensor that can determine crank position within 1 degree, not
just when at TDC or within 10 - 30 degrees, as some other EI's do.
However, the mounting of the sensor magnet has been flawed from
version 113 onward. In version 113, the magnet was epoxied into a
metal cup at the end of a shaft. The other end of the shaft had the
magneto gear on it and was in intimate contact with the accessory case
gears inside the engine, along with the engine oil. This guaranteed
that the shaft would get just about as hot as the oil in the accessory
case - about 200 F, if not more.

Due to the very thin bond line of the epoxy holding the magnet in the
cup and the differential CTE's (coefficients of thermal expansion) of
the magnet and cup, the thermal stresses in the epoxy can be very
large, cracking the epoxy. This would allow the magnet to rotate,
thereby losing positional accuracy and timing.

After having this failure mode pointed out to them (by me) a year and
a half ago, and after having refused my offers to assist them in
redesigning the mechanical portion of the units gratis (I am a
mechanical engineer with 27 years of experience), Emagair, with an EE
but no ME on staff, redesigned the magnet holding portion of the units
for the version 114's. They soldered the magnet into a large brass
holder, and then glued the holder onto the same shaft that the cup had
previously been attached to. They then added two locktited set screws
to the holder which applied force to the shaft. There was no flat on
the shaft where the set screws touched it.

This "fix" was far worse than the disease it was attempting to solve,
and is the root cause of the horrible performance of the version 114's
under heat and vibration loading. Now that the mass of the brass
holder has been made much larger than the mass of the magnet alone,
the stresses in the glued bond-line are far higher than previously,
and under heat and vibration is guaranteed to fail eventually. The
set screws do absolutely nothing, since the CTE of brass is much
higher than the CTE of the steel shaft, so as the system gets hot, the
set screws don't even touch the shaft. Failure of the glue bond line
is sufficient to cause the holder to be able to rotate, with the same
mis-timing issues as with a magnet disbond in the version 113 units.

Although Emagair has issued an SB, describing a "fix", I do not have
ANY confidence in this "fix"'s actually working in the long term. As
with Mr. Cotner's warning, I don't believe that the keyway and
roll-pin are anything resembling an adequate solution to the magnet
mounting problem, which is obviously NOT specific to my engine and/or
installation. As more hours are put on these units (I fly 120
hours/year - more than twice the homebuilt average), more will fail,
and I have no confidence that this fix will substantially change this
situation.


Recommendations:

My recommendations, based on my opinions and my examination of MANY
Emagair unit disassemblies are these:

1) If you are flying with a version 113 or 114 product from Emagair,
ground your plane immediately and remove the units. Do NOT fly with
them, or with the "fix" described in the SB. It will work for a
while, but for how long?

2) If you have a unit that you have purchased but haven't used yet,
return it for a refund - do NOT put it on your aircraft and fly it.

3) If you were considering Emagair products for your engine, consider
something else - do NOT purchase one and put in on your aircraft.

When Emagair have hired a competent mechanical engineering firm to
redesign their systems, have FULLY tested the units under O-360 type
vibration loads for thousands of hours, and publicly published the
results, THEN in may be appropriate to consider these units. Until
then, it is not.


If Mr. Cotner, Mr. Read, and my experiences with these units put
Emagair out of business, and it costs some of you the opportunity to
get a refund on your units, I'm sorry for that, but Mr. Cotner was
lucky, and relying on luck to keep people alive is not acceptable.


Everything that I have written here is either my personal experience,
my opinion, or my recommendation based on my opinion.


I know that many of you are saying (to yourselves or to others) "I
told you so", and you're right - you told me so, but my desire for
Emagair to succeed overrode what should have been enough evidence to
the contrary. Feel free to write me with "I told you so" messages, if
it makes you feel better.

I'm happy to address any issues, comments, or questions that anyone
may have. Since Emagair never responded to my entreaties to sign an
non-disclosure agreement with them, I do not have any responsibility
not to explain the inner workings of the units or the problems therein.

PLEASE, PLEASE, PLEASE - distribute these messages (and Mr. Cotner's
and Mr. Read's) to any and all aviation related mailing lists, fora,
and printed newsletters.

--
Marc J. Zeitlin mailto:marc_zeitlin@alum.mit.edu
http://www.cozybuilders.org/
Copyright (c) 2008 http://www.mdzeitlin.com/Marc/
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edited by steve for a good reason
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