Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Sat, 23 Nov 2002 19:36:38 -0500
Message-ID: <redirect-1885407@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: Engine care and feeding
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<<>  Piston ring flutter - the design of the compression ring/piston groove
presumes there will always be a downward load on the ring to keep it seated,
this load being generated by positive pressure in the cylinder.  Windmilling
the engine with a low power setting can reduce cylinder pressures such that
the compression rings are unloaded and begin to "flutter" in the ring
grooves.
...
>  Crankshaft counterweight detuning - this doesn't apply to us 4-cylinder
drivers, cuz we don't have counterweights.... If there is little or no power
in the power stroke and RPM is still in the normal operating range, the
counterweights suddenly become unemployed (or underemployed, actually) and
flop around with nothing to do

.. the Jake-Brake idea is probably not a good solution.  I guess we'll just
have to accept that we can't come screaming into the downwind at 175 knots
and expect to drop the rollers at the 180!

By the way, as several others on the list have said, the Sky Ranch
Engineering Manual is an excellent resource>>

I certainly agree that it is an excellent resource - but not perfect.  It
contains a combination of experienced-based and theoretical information,
sometimes combined inappropriately, in my opinion.  For instance,
"fluttering" in the ring grooves?  I doubt it, as the ring tension is what
holds the rings against the cylinder, and half the piston strokes are with
no pressure against them anyway.  Bottom line is that running the engine at
high rpm with no load really doesn't do anything bad - probably doesn't do
anything particularly good either.

The book has a good discussion about the pendulum-type tuned absorbers up to
a point.  Incidentally, the "6" in the Lycoming engine suffix indicates that
it has these devices and as far as I know all the spread-valve 4 cylinder
and all 6 cylinder engines have them.  They are better than a
fixed-frequency tuned absorber as used in car engines in that they can
absorb different frequencies at different rpms.  And his book is right in
that the frequency they absorb is dependant on the relative diameter of the
pins and bushings, so if either wears significantly the tuned frequency will
change, possibly precipitating a crank failure.  They don't bring the
torsional vibration amplitude to zero as the book implies, but they can take
out up to 90% of amplitude.  What's all this have to do with running the
engine at high rpm at no load?  Nothing, as the weights certainly don't
"flop around", but instead are held in place by centrifugal (centripetal to
be precise) loads just as they are with the engine under power.  The only
time these weights flop around is when turning the prop by hand or cranking
the engine.

There are only a couple of disadvantages of closing the throttle and running
the rpm up.  Aircraft engines have relatively loose piston and valve guide
clearances and the high vacuum during the intake stroke can suck in more oil
that you would like, possibly fouling the plugs and prevent the engine from
re-starting back when you open the throttle.  Also, the engine will cool
down a lot, creating high thermal stresses when you re-start it.  I say
"re-start it" because the engine will cease to fire with a closed throttle
above maybe 1500 rpm.  So what do you do if you want to make a max-rate
descent?  Depends on whether you want the engine available at the end of the
descent or not.  If you do, then pull the mixture to shut the engine off,
run the rpm to max and keep the throttle wide open.  This helps to prevent
too much oil from entering the cylinders and at the end of the descent just
push the mixture back in and it will re-start.  If you aren't going to need
the engine (??) then keep the throttle closed as the friction horsepower
(actually pumping loss) is higher with a closed throttle than with an open
one.  As for shock cooling damage, you can worry about that when safely on
the ground.  Of course, I would hesitate to pull the mixture in the air
regardless, but sometimes you do what you need to do - like if you had a
passenger with a heart attack.

Gary Casey