Reply-To: <glcasey@gte.net>
Reply-To: lancair.list@olsusa.com
From: "Gary Casey" <glcasey@gte.net>
To: <lancair.list@olsusa.com>
Subject: engine vibration
Date: Sun, 24 Feb 2002 06:50:27 -0800
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<<I have been told by an aerospace engineer that with our big Continental
engines which historically have not been perfectly internally balanced, that
the use a heavier Hartzell prop may provide longer engine life than lighter
composite props because it acts as a dampening flywheel for many of the
internal vibrations in the engine.  The lighter props allow more "shake,
rattle, and roll" inside our engines, potentially leading to early failures
in many components/accessories.>>

I think you are talking about two different things here.  I know that
Continental (and probably Lycoming) doesn't have really tight requirements
on engine balance - and it probably doesn't make any difference.  They allow
you to replace a single all-aluminum piston with one that has an iron ring
belt, for example.  The "precision balancing" that some people advertise
probably won't result in any measurable improvement in smoothness or
durability.  The potential problem with the lighter prop is torsional
vibrations as the resonant frequency of the total system will go up and the
variation in crank speed will be greater than with the heavier aluminum
blades.  I'm assuming MT has done a vibration analysis and said it's okay??
The higher crank speed fluctuations will also effect the accessories, but
how much I don't know.

<<When I bring up that most aerobatic performers use the lighter props for
fast response, he points out that using the prop in that environment is
different, because those engines are usually torn down and reworked or
replaced every year anyway, so the early failures of components don't have a
chance to show up in that short a time frame.>>

I talked to Sean Tucker about this - he uses a Lycoming engine.  He tears
the engine down every few hundred hours and "always" finds cracks, which he
attributes to the high precession loads from the prop.  And then I saw a
video he took when the prop departed the plane (broken crank) during a snap
roll.  A lightweight prop would be the way to go...Unless your sponsor build
aluminum ones..

Gary Casey
ES project


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