ADK wrote:
IF you had to design a PSRU, to drive a pusher propellor via shaft, what
would your experience dictate? Thinking along the lines of a gearbelt, chain
or gear. Please, I would appreciate the collective experience available on
this group. I have decided on the aircraft, but want to make it the most
reliable and safest it can be.


For the sake of Peter, IT DOESN'T MATTER!!

For the energy to transfer to the prop, you have to attach the engine to
the prop. The engine doesn't produce smooth even power. It produces a
series of pulses. If the frequency of the pulses resonates with the
prop or shaft, it will store a little bit of each pulse as "spring
energy". This type of energy is stored by deflecting (ie, bending) the
prop or shaft. The prop stores it and then immediately tries to release
it by unbending. If the next engine pulse comes along at just the right
time, the new "spring energy" will be added to the previous "spring
energy" and the prop will bend a little more. This continues until the
prop or shaft has as much "spring energy" as it can phyiscally hold, and
then the element just vibrates. Eventually, the prop or shaft gets
tired of all the bending and unbending and just gives up (ie, breaks).

Making the pulses smaller doesn't help for the most part. All that does
is cut down on the amount of "spring energy" added with each pulse. A
smaller pulse will take 2000 pulses to fill the prop with "spring
energy" vs 1000 with a unmodified pulse. Whoop-te-do! What difference
will that make with the engine turning 2000RPM and four pulses per round.

Any one of the gearboxes you mentioned made to work safely, and each has
a set of advantages and disadvantages that are well known and easily
engineered around. The type of gearbox has nothing to do with torsional
resonance. Will not mitigate torsional resonance. Will not
cure/alleviate/lesson or bypass torsioanl resonance. Torsional
resonance is a totally different issue.

You didn't tie gearbox type and torsional resonance together directly,
but many people have in the past, and it's just self-deception. Any of
the gearboxes you mentioned can be as safe and dependable as any of the
others, if engineered properly.


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."