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

"Bill Daniels" bildan@comcast-dot-net wrote

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.)

Strange, but I am considering a system, very much like that. Flying boat,
with the engine in the fuselage, instead of on a pylon.

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.

On that, I would agree. I believe you will have a new problem, though.

I believe that you will have a problem with the belt vibrating, like a
guitar string. At some speeds, it will get to the same resonance of the
belt, and slap like the devil. I have a 20" band saw that does this every
time, as it slows down, right before it stops.

One way to deal with this is to make the string (belt) shorter, with some
idler pullies. At least that is my plan. It would be a good idea to make
the distances between the plies a little different, or it will have a strong
tendency to vibrate as a complete string, at a higher fundamental (lowest)
frequency.

It is the same (in a way) as muffler design. The volume in the different
chambers is a little different, so when the frequency is resonating in one
chamber, it will not be resonating in the other chamber, thus more
frequencies are muffled.
--
Jim in NC

"Morgans" wrote in message
...

"Bill Daniels" bildan@comcast-dot-net wrote

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.)

Strange, but I am considering a system, very much like that. Flying
boat, with the engine in the fuselage, instead of on a pylon.

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.

On that, I would agree. I believe you will have a new problem, though.

I believe that you will have a problem with the belt vibrating, like a
guitar string. At some speeds, it will get to the same resonance of the
belt, and slap like the devil. I have a 20" band saw that does this every
time, as it slows down, right before it stops.

One way to deal with this is to make the string (belt) shorter, with some
idler pullies. At least that is my plan. It would be a good idea to
make the distances between the plies a little different, or it will have a
strong tendency to vibrate as a complete string, at a higher fundamental
(lowest) frequency.

It is the same (in a way) as muffler design. The volume in the different
chambers is a little different, so when the frequency is resonating in one
chamber, it will not be resonating in the other chamber, thus more
frequencies are muffled.
--
Jim in NC

A low RPM high thrust prop on a high thrust line would be ideal for a
seaplane.

Actually this is not too different than some motorgliders with the engine
buried in the fuselage and the prop on a retractable pylon. The tooth belts
have only one or two idler pulleys. I figgured on at least two idlers to
maintain belt tension and to damp belt vibrations.

I want the engine in an external conformal pod below the fuselage for
accessability and to locate it below the pilot for survivability reasons.
I'd want the whole engine/cooling system in this pod so it could be removed
as a unit. The prop and drive belt would remain with the airframe.

Bill D

Morgans wrote:


"Bill Daniels" bildan@comcast-dot-net wrote

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.)


Strange, but I am considering a system, very much like that. Flying
boat, with the engine in the fuselage, instead of on a pylon.


Curious...
Tell me why?
What is the advantage?
(not argumentative, please - just curious)

If I absolutely *had* to do that, I'd think two stages.

HTD belt on the first stage from the engine, and either HTD, or Vopar type
chain for the final. Come to think of it, probably have to be chain for the
final drive due to the length of the thing...

But I'm back to the original question - why?

Wouldn't that put the engine where you would want to put people?

Richard

ADK wrote:
This is probably going to open old wounds. What I would like is experienced
input on the advantages, for economic, efficiency and longevity etc. of
different types of redrives.

I am leaning towards a cog-belt reducer in a 6 cylinder, liquid cooled,
configuration driving a long drive shaft to the prop.

If it means anything, having the redrive and the 4 cyclinder ahead of
the drive shaft probably helps. This means the system would be seeing
a large number of small impulses rather than the small number of large
impulses with a direct drive 4.

On the other hand, the longer that drive shaft is, the lower its
natural frquency, and the larger the gear reduction of your redrive,
the higher the impulse frequency, so those might be coming into range
of each other.