PSRU design advantages

Dan Horton wrote:
There is nothing that eliminates a long shaft from the design of a
PSRU. Nonbelievers might be advised to consider ship propulsion; long
shafts, low cylinder counts, propellers operating in uneven flow, often
via a gearbox. Sound familiar?

Yes, but they use mass for damping like solid shafts. Weight is
nowhere near the issue it is for aircraft.

Dan, U.S. Air Force, retired

Ian Stirling wrote:
Ernest Christley wrote:

Ian Stirling wrote:


Best tool available to the amateur is a variable speed strobe - Party Light!

That way you can actually look and SEE what's happening.


That'll spot ordinary vibrations.
Torsional ones are a little bit harder.

Especially if you want, as you probably should, a graph of maximum stress
anywhere in the shaft/PSRU/Prop system vs RPM.

A few fine white lines down the length of the shaft will clear up that
problem.


Will they?
It'll obviously show huge torsional movement, but many, especially
shorter shafts may fail before it becomes visible.


Ian, if it won't stay together long enough to turn on the strobe,
it probably needs a little more work...

Richard

Jim in NC writes:
I think you will find that they do it on ships, with pure weight. A
big,
heavy, solid steel shaft. Very heavy! That is how they get the
stiffness.

Sheesh. Shaft weight is not a factor. And more stiffness may or
may not be desired.

Shaft stiffness is one of the parameters that may be adjusted (up OR
down) so that the system has a natural frequency not matched by a
significant exciting frequency. If the system is driven by internal
combustion, identifying the most significant exciting frequency is dead
simple. It is (RPM x #cyls)/120 = hertz for a 4-stroke and (RPM x
#cyls)/60 = hertz for a two stroke. Designing a system with a natural
frequency that does not match the exciting frequency identified by this
equation is easy IF the engine runs at one RPM only. It gets a lot
harder if you expect to use a wide RPM range.

Also, the shaft turns very slowly, so there are many pulses per
revolution;
more than you will get with a 4 or 6 cylinder, 4 cycle airplane engine,
in
most cases.

Sheesh again. Shaft rotational speed alone is not a factor.

Shaft speed AND number of propeller blades may be of interest if
disturbed flow is the source of an exciting frequency. "Pulses per
revolution" sort of defines the term ''order" as it is used in
rotordynamics (the number of times anything happens per revolution). A
handy term, nothing more. For example, a ship's shaft at 90 RPM turns
1.5 times per second. If it has a four-blade prop and a single source
of disturbed flow (perhaps a strut supporting the shaft), then the
disturbance is a 4th order event. Order times rotational speed per
second (4 x 1.5) means an exciting frequency of 6 hertz. In this case
let us hope the engineer designed a system with no natural frequencies
between 4 and 8 hertz.

Wnat another example of speed x order? Consider the cardan joint.

I agree with the rest of your post; dig into the engineering text
books.

I wish you well with them.

Dan

Ian Stirling wrote:
Richard Lamb wrote:

Ian Stirling wrote:

Ernest Christley wrote:


Ian Stirling wrote:



Best tool available to the amateur is a variable speed strobe - Party Light!

That way you can actually look and SEE what's happening.


That'll spot ordinary vibrations.
Torsional ones are a little bit harder.

Especially if you want, as you probably should, a graph of maximum stress
anywhere in the shaft/PSRU/Prop system vs RPM.

A few fine white lines down the length of the shaft will clear up that
problem.


Will they?
It'll obviously show huge torsional movement, but many, especially
shorter shafts may fail before it becomes visible.


Ian, if it won't stay together long enough to turn on the strobe,
it probably needs a little more work...




I suspect you know what I mean.
But to clarify in any case, any shaft will have a given torque/torsional
bending ratio.
Say a half a degree at maximum safe load (or whatever) per 10 diameters.
If the shaft is half a diameter long, it's going to be much harder to
observe, than if it's 100.

The discussion was about long shafts. Rear mounted engine on tractor
airplane. No, it won't do much good on a 6" shaft.


--
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)."

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.