A very bright engineer, Martin Poettcker at our local EAA chapter, who
is building a Zenair with a Subaru engine has researched and designed a
torsional damper for his personal application. His opinion was that
engine-mount softness was not a contributing factor to gear-loading and
wear at idle. Interesting though is his analysis that at idle-speed the
forces on the gearbox can be over 10x the forces seen at full power.
Most folks in the aviation community attempt to mitigate the problem by
reducing tolerances (removing gear-lash or tightening belts), but from
this fellow's research and analysis it his opinion that all these
actions do is raise the Q of the system resulting in even higher gear
forces at idle speeds.
I wonder if on the Rotax design the slipper-clutch comes into play at
idle speeds, and if so how much power it dissipates in an effort to save
the gear-train. One reason to keep that idle speed up as previously
discussed on this forum.
Martin's presentation was most enlightening, and his one-off design,
although based on a variant of an existing textbook design is ingenious
in its effectiveness and simplicity. I especially like the rubber
expansion chamber to accommodate the oil expansion due to the oil
damping-induced power dissipation. I'll be following his success,
failures and tuning with much interest.
His slides can be seen at:
http://eaa245.dhs.org/Presentations/EA81Torsion/Torsion/sld001.htm
For the folks who enjoy prop dynamics and construction techniques there
was also another EAA member who put on an equally intriguing
presentation. It can be seen at:
http://eaa245.dhs.org/Presentations/CozyProp/sld001.htm
Cheers,
Pete
A239
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