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I believe they are havingTORSIONAL RESONANCE problems.
The following was extracted from the references below.
Many of the individuals and/or companies involved in the development of
aircraft engines do not seem to have an understanding of the problem of
torsional vibration. This is not true in every case. Some appear to have a
profound knowledge of torsional vibration but others seem to dismiss it
as a minor problem. Experience shows that torsional vibration just
doesn't go away. It can be the life or death of an entire project, not only
technically, but it can also lead to a financial black hole for the
individuals or company involved! The potential for success in such a
project would be much higher if the individuals involved knew what they
were dealing with and would use valid aircraft engineering procedures
during the design and development of an engine. The same laws of
physics, dealing with torsional vibration, are still in effect today, as they
were 20 to 50 years ago.
To explain without going into a lengthy engineering dissertation, the
propeller is a form of spring, being acted upon by the ignition impulses of
the engine (compression impulses at shutdown). As the impulse strikes,
the propeller bends back (loads up like a compressed spring), then
swings forward (unloads) past neutral position, momentarily
accelerating the output shaft, then swings back past neutral, gets
further loaded by the next impulse, swings forward again, etc. If the
system is operating at an rpm where the next impulse comes in just as
the prop swings backward past neutral again, additional energy is
added to the system and the deflections grow. The timing of the
oscillations at that instance is the natural frequency of the system.
As the reactions feed on themselves and the deflections grow, the
magnitude of the torsional feedback(torque) also grows, sometimes
much higher than even the worst-case design condition for normal
operation. Since the load application is quite sudden, the effect also acts
similarly to an impact loading, again needing higher service factors for
safe design. How high can these loads go? As much as 25 times the
operational load, although theoretically, given enough time, the
magnitude can be virtually infinite. The actual value depends on how
much dampening there is within the system, and how long the condition
is allowed to persist.The torsional loads going through the drive system
appear to be much higher than the original analytical numbers indicate.
This is due to the torque reversals that occur at resonance point. Any
engine that produces 1 power stroke per revolution is bad news when
it comes to torsional vibration.This vibration is evident on some aircraft
on shut down or at very low RPM when the RPM matches the stiffness
of the engine mounts and the whole aeroplane shutters. However a
similar vibration can occur at higher RPM involving primarily the inertia of
the prop, flywheel and the crankshaft. The dynamic equations for this
case involve the inertia of the components, the stiffness of the shafts,
and the RPM of the engine. The problem is very complex from an
analytical point of view, since it is further influenced by damping from
friction and viscous sources. The stiffness of the components is not a
piece of cake to establish either, since the geometry is constantly
changing. It would take a very complex analysis to determine the
magnitude of this problem. This is best analysed empirically on an engine
test stand; even this is beyond more than a cursory evaluation by most
small manufacturers.
References:
PROPELLER DRIVE SYSTEMS AND TORSIONAL VIBRATION : By Donald
P. Hessenaur
Reduction Drives . . .An Engineering Perspective: By Bill Husa,
Reconsideration of Gyroscopic Forces and Torsional Vibration in Auto
Conversions :Keith Spreuer
See www.prime-mover.org
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Date: Sat, 20 Jun 1998 09:06:36 +0200
From: "Europa Club Internet Rep" <europaclub@avnet.co.uk>
Subject: BMW aircraft conversion designer
From: Thom Besch <tbesch@netgsi.com>
Dear Europa builders,
I'm an Avid Flyer builder in the US and from searching the net I have
been hearing alot about a a designer who is (or was) involved in
building a redrive to convert the BMW 1100 engine for aircraft use.
Although several people have been very helpful providing information
about this engine, I still haven't been able to locate the name,
telephone number and if possible an e-mail address for this individual.
I have an engine, and I need to locate a redrive. I believe this engine
holds great promise as an alternative engine for our size airplanes.
I would very much appreciate any help locating this designer and any
impressions or information about this engine. Please e-mail it to my
address at tbesch@netgsi.com
Thank you for your help!
Thom M. Besch
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