PSRU design advantages

"Ernest Christley" wrote in message
...
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)."

Let me try a different idea.

Suppose the prop shaft is to be just long enough for the gear belt pulley
and the neccessary bearings - say 10 inches. But the engine flywheel pulley
is to be 4 - 6 feet below the prop shaft. The idea is to use a very large
multi-blade carbon fiber prop turning 800 - 1000 RPM driven by a 4 cyl Soob
turning at best power RPM. The idea is to get best thrust in the 0 - 60
knot range. The airframe configuration is a prop over tail boom pusher - an
ultralight on steroids. (BTW, I'm not looking for a long engine life under
these conditions. I'll treat the Soob as a throwaway power plant.)

I'm thinking there isn't too much torsional vibration concern with very
short shafts, high reving engine and a stiff carbon fiber prop. The prop
will be seeing 6 - 7 power pulses per rev from the high reving Soob.

Bill