Just a quick additional note to clarify my point about stiffness.

Stiffness is a restraining force that acts against an excitation force -- as
we see in the taut guitar string. Mass and damping are also restraining
forces.

So it is not just that it takes more force to displace a stiff object enough
to set it to vibrating -- it is technically correct to think of stiffness as
a restraining force.

Also, since we are talking about excitation versus restraint it should be
noted that most discussions of torsional resonance fixate on the engine
power pulses as a source of exciation.

This is quite erroneous because as soon as we add a propeller we have an
object with a very large moment arm and hence inertial mass that can -- and
does -- produce very powerful excitation.

The gearbox itself can also be a source of excitation because it too has
inertia and mass, although far less than a propeller.

Another source of excitation is imbalance. We see this in the
non-counterweighted Lycoming engine in which this imbalance creates enough
of an excitation that, when combined with the excitation of a propeller can
set the crank into resonance, resulting in a broken crank.

It should be added that no piston engine is perfectly balanced. That's why
even a V-8 needs a balancer. On an opposed engine, the opppoosing cylinders
do not balance each other perfectly either because they are not operating on
the same plane. The result is a rocking couple and second order imbalance.

Apparently this is where the rotary has a big advantage because it can be
brought into perfect dynamic balance.

But the important thng to remember is that imbalance is just one of the
excitation forces that can contribute to resonance -- although not usually
in a big way.

Also, it is useful to remember that if you add a gearbox to the engine, the
concern will be with excitations coming from both the engine and prop and
setting a gear shaft into resonance. This is why redrives have such a dismal
record.

Regards,

Gordon.




"Gordon Arnaut" wrote in message
...
Ernest,

I have read Tracy Crook's piece on torsional resonance, even before you
pointed to it.

A couple of thoughts. First, Tracy has devised a good solid gearbox that
has proven itself in service with a respectable number of flight hours.

He is absolutely correct in pointing out that the crankshaft is a spring
mass, as I have said earlier. So is the propeller, and the gear shaft of
the transmission. Any complex piece of machinery is a combination of a
number of spring masses, each with its own resonant characteristics.

But let's back up a little and try to really understand this. I don't
think my earlier explanation was completely satisfactory.

The key thing to understand first is that any object will vibrate if force
acts on it to displace it in some way. In astrophysics we know that the
biggest objects in the universe vibrate, and even the universe itself
vibrates -- and has left a trace of its vibrations as it expanded after
the big bang.

A guitar string vibrates if you displace it with a pick. An engine
vibrates from power pulses. Even an electric motor vibrates from the power
pulses of its magnets.

But vibration is not resonation. Resonation is when an object vibrates at
its particular resonant frequency, at which point the harmonics (which are
vibrations that are integer multiples of the fundamental frequency; the
second harmonic is twice the frequency of the fundamental; the third is
trhree times, etc.) build on top of one another and lead to ever greater
amplitude of the vibration (the string moves back and forth in an
ever-wider arc until it breaks).

We see this in a guitar string when it breaks unexpectedly while we are
tuning it. As we were turning the tuning knob we just happen to hit the
resonant frequency and the string suddenly hear an increae in volume (and
a much richer, almost howling sound) and the vibration of the string gets
visibly bigger unti it snaps.

But if you just try to break that string by turning the knob tighter while
keeping the string perfectly still, you would be surprised how much force
it would take to snap that string in tension. On thicker strings, like on
a bass, you would not have enough strength in your hand to do it, despite
the help from the mechanical advantage of the gear knob.

It's the same thing with a crankshaft, except that the vibration is a
twisting back and forth of the shaft, rather than a swinging side to side
like on a string or a tuning fork.

That crankshaft is going to be vibrating with every power pulse because
each power pulse exterts a force on the lever arm of the crankpin which
causes a twisting of the shaft. And in the split second after the power
pulse subsides, the shaft will swing back twisting back beyond neutral --
just like a guitar string when you displace it swings to both sides of its
neutral axis as it vibrates.

So the crankshaft will be flexing and vibrating at all times when the
engine is running. Is this bad? No. This has nothing to do with resonance.

Yet this is where all the confusion comes in. An earlier poster pointed
out that V-8 engines use balancers in order to smooth out imbalances and
lessen vibration and shaking. This is desirable because we would all
rather have an engine or any piece of equipment that vibrates less not
more.

But this does nothing to address the problem of resonance.

So what causes the crankshaft to get to the point where it starts
resonating? Well, just like our guitar string it needs to be displaced
with enough force and in the right way -- force applied at just the right
number of times per second, or its frequency. Once this is accomplished,
the crankshaft will begin to resonate. It will literally ring like a bell,
with the harmonic notes all coming out and all of them joining together to
cause the amplitude of the vibration to intensify (the crank will begin to
twist back and forth in a wider and wider arc, just like the guitar string
going berserk.)

At some point, the crank cannot twist anymore and it will break.

Now if we were concerned about avoiding this situation how would we
proceed? Well, we know that an object's resonant frequency is related to
its mass. If we make a bigger crankshaft it will resonate at a lower
frequency, hoepfully below the actual operating rpm of the engine.

But this isn't alway possible. Another approach is to make that shaft
stiffer, which will actually increase the shaft's resonant frequency, but
also has a much more important benefit. It now takes much more force to
displace it, or bend it from its neutral axis.

Think of a very thick bass guitar string that is tightened as tight as you
can get it. Now try to pluck that string. You can't get it to vibrate,
because you don't have enough force in your finger to displace it far
enough from its neutral axis that it will vibrate.

So if you can make the crankshaft stiffer, it will take more force to
cause vibration, perhaps more force than the power pulses of the engine
will produce and then resonance can never set in.

So the engine manufacturers do take this into consideration and design
crankshafts that will not fail from resonance. So what's the problem?

Well the problem is when we go to attach something to the engine. And
engine isn't much good unless it is hooked up to something and doing
something useful -- like driving a propeller.

And that's where things get complicated, because now we are adding another
spring mass to the system. We now have two possible problems, either the
engine can set the propeller into resonance, or the propeller can set the
crankshaft into resonance.

And we haven't even got to the gearbox yet. Just the prop and engine is
enough of a problem that each engine and prop must be tested and certified
as a combination. You can't just bolt on any certified prop any certified
engine.

This is also why we see homebuilt aircraft breaking crankshafts or props.

So now when we add a gearbox too, we have multiplied the possible
scenarios that can go wrong. Both the crankshaft and prop are vibrating
springs, with the gearbox in the middle.

What is required is a design approach similar to the powertrain approach
used in auto industry.

However, very few people in the homebuilding community are trained in the
nuances of this particular discipline (including me). So we have people of
various technical abilities trying to tackle this problem in a bootstrap,
eyball engineering kind of way.

For example, Crook talks in his article about how springs in a clutch
plate do not work satisfactorily becuase they would be tuned for only one
frequency, while the engine oeprates over a wide range.

Yes, this is true if you are concerned about elimintaing the harshness of
everyday vibration. But if you want to stop resonance, then all you need
to do is exactly that -- tune the damping device for that one frequency.

Here again we see the issue of harshness and vibration clouding the issue
of resonance.

In any case, it is not an easy problem and the automakers have a lot fo
very thoruoghly trained people working out drivetrain issues for every new
combination of engine and drivetrain -- and they don't even have a prop at
the other end.

Regards,

Gordon.




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



You don't listen to or read the work of others, and you like to read your
own writing way to much.


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)."