Ernest,

You are right that springs and elastomers do not technically dampen kinetic
energy, they simply store and relase it at a later time (how much later
depends on the frequency at which it is tuned).

However, both springs and elastomers can achieve our objective of clipping
of destructive harmonics if they are tuned to the resonant frequency of the
object that we want to protect from resonance.

Also, fluid dampers do exist for both engines and propellers which actually
do dampen kinetic forces by turning them into heat, just like the shock
absorber on your car.

Drive belts do this as well, including the synchronous (cog) belts which
don't actually slip but which do absorb some energy as heat.

In fact, even a propeller can be tuned efectively to do this, because it is
kind of like a spring -- it flexes at its tips with the power pulses and
then whipsaws back during the lulls. A propeller thus tuned can do the same
thing as a harmonic balancer on an engine.

Of course the trick is in the tuning. More often we are concerned about the
opposite, where they propeller may in fact add to the excitations of the
crankshaft at its resonant frequency. We see this in non-counterweighted
Lycoming four-cylinder engines, which have operating restrictions in certain
rpm ranges.

So when we talk about torsional resonance issues, there are two aspects: one
is the engine itself, and specifically the crankshaft, which shoulders the
torsional forces. The other aspect is what the engine is connected to,
whether a propeller, a gearbox, or both.

We know of course that each object has a natural frequency at which it
resonates -- the point at which external excitations cause harmonics to
build. The object's mass, stiffness and damping determine how it will
respond to an excitation force.

Obviously having smoother excitations -- such as cylinders or rotors firing
at more frequent intervals -- will result in less extreme instantaneous
torque excursions, and the engine will run smoother. Dow that mean it will
be free of torsional vibration? No. Even an electric motor has torsional
vibration because the magnets arranged around its circumference which propel
the rotor are like firing pulses -- a motor with 12 magnets will be smoother
than a motor with two magnets.

However the engine can be as smooth as butter, but if the shaft on which it
is riding is very thin, it will still flex -- and at some point will reach
its resonant frequency, and if left there for the harmonics to build up --
will break.

So that's where mass and stiffness comes into play. Look at tuning forks,
the smaller ones will vibrate at a higher frequency. Even a small excitation
will cause a lot of flex in the thin tuning fork.

It's the same with crankshafts or other shafts -- such as those in a
gearbox, or a propeller -- which are exposed to torsional forces.

For example, most V-8 engines come with a harmonic balancer, even though
they have four power pulses for each crankshaft rotation. That's because
there is enough flex in the crankshaft that the crank can begin to resonate
at some rpm within the operational range.

But look at the four-cylinder Subaru opposed engine. It has never needed a
harmonic balancer. Why? It's crankshaft is quite massive and very stiff, so
its resonant frequency is below the engine's operating range. (It is stiff
because it has five main bearings, so each crank throw is supported by two
main bearings, one on each side; plus the crank journals are quite massive).

Although I'm not yet fully up to speed on the rotary, it is quite clear that
it is similar to the Subaru in that it does not need a torsional dampening
device. That e-shaft is quite rigid and it's throw is short enough that it
will not flex the way a crankshaft will flex. So more stiffness equals a
lower resonating frequency.

So why be concerned with torsional vibration? It's because we are putting a
gearbox and propeller on this engine. Just because the rotary does not
resonate itself does not mean it won't set a gear shaft into resonance.
That's the point of the damping at the engine-gearbox coupling.

When I asked what the source was of torsional issues with the rotary, I did
not have a full grasp of the dynamics with the rotary engine -- until I
watched some helpful animations.

However, as you can see, torsional vibration is not strictly a function of
power pulses. It is a function of the many things inside the engine --
including moments of inertia, stiffness of torque shafts, etc -- plus just
as many variable in whatever it is the engine may be connected to.

Regards,

Gordon.




"Ernest Christley" wrote in message
. com...
Gordon Arnaut wrote:

Still, as you pointed out, the cure to torsional excitation is to dampen
it, not to build a stronger transmission. It should also be noted that
one way to avoid torsional vibration is not to run the engine
continuously at the rpm where excitation occurs (as with the
non-counterweighted Lycoming).

No. The only cure for torsional excitation is the move the system's
harmonic into a range that will only be seen in low power operations and
then won't be used for very long. Tracy's PSRU moved the excitation range
down to between 500 and 800 rpm (working off the top of my head here, so
the numbers may be off). A well tuned rotary will idle in the 900-1000rpm
range (prop at about 1/3rd of that).

Dampening doesn't exist. Elasticity in the system may shorten the peaks,
but you'll be left with a fatter mountain. The energy has to go
somewhere. The problem is that you'll still be hitting the shaft at it's
resonant frequency, causing a vibration in it. Each hit adds a little to
the vibration. If you repeatedly hit anything at it's resonant frequency,
each hit will add to the system vibration until it either wears out very
quickly or comes apart catastrophically. That applies to reduction units,
crankshaft, and control surface skins (yes, flutter is a form of resonant
vibration). Adding some elasticity to the shaft may make the gearbox last
longer, but you won't be able to do much in those few seconds 8*)

Here's someone who says it much better than I:

http://rotaryaviation.com/PSRU%20Zen%20Part%202.html


--
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)."