light twins?

Thanks for the generous words, Richard.

Keep in mind I'm not an expert in this field, but this process of thinking
the issues through logically is also helping me to understand things better.

I may well be wrong on some points, and I encourage those with more
experience in this field to chip in and make corrections or contributions.

Regards,

Gordon.




"Richard Lamb" wrote in message
oups.com...
Very impressive, Gordon.
Probably the most understandable description of harmonics and
resonance, and how they can destroy stuff that I've ever seen here.

Thanks

Richard

Just one more post script about the Lycoming engine. It's interesting to
note that it is the prop that is the source of the excitations that set the
crankshaft into resonance.

The fact that the engine lacks counterweights (unbalanced) does not hurt it
one bit -- except for the unpleasantness of the shaking.

However, when you add the spring mass of that heavy prop, then that is
enough to set the crank into resonance, but only at a certain rpm -- between
2000 rpm and 2300 rpm, approximately, and only if run there for some length
of time to let the resonance build up.

It's interesting also that the problem can be solved by two ways, either
adding balance weights to the crankshafts, which some models of the same
engine employ, or using a lighter prop, like the two-blade MT, which is
STC'd without rpm restrictions.

So we see plainly that the resonance problem is not just an issue of engine
firing pulses, which is what a lot discussion seems to fixate upon.

Regards,

Gordon.




"Gordon Arnaut" wrote in message
...
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)."

Gordon Arnaut wrote:
Just one more post script about the Lycoming engine. It's interesting to
note that it is the prop that is the source of the excitations that set the
crankshaft into resonance.

Just like it is the wheels that push an engine around so that the car
can move.

--
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)."

Gordon Arnaut wrote:
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.


That should be and indication to you to shut up and learn from your elders.


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


Now your getting it.

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.


So, is the goal here to talk gibberish about as many subjects as
possible in the forlorn hope that there is an outside chance that you
might be right about SOMETHING!!

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.

Case in point. You obviously have no idea what you're talking about.
You read a science book once in high school and now consider yourself a
scholar. Here's a clue. Resonation requires TWO objects. You need
something to vibrate, and something to cause the vibration.

from: http://en.wikipedia.org/wiki/Resonate

In physics, resonance is an increase in the oscillatory energy absorbed
by a system when the frequency of the oscillations matches the system's
natural frequency of vibration (its resonant frequency).


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 close, yet so far away. If you'd shut up and read what you wrote up
to this point, you might get it. Dampeners do nothing to help the
destruction of an engine that is running at its resonant frequency.
They lower the harshness of the pulses, but the point of resonancy is
that the energy from each pulse is stored in the system until the next
pulse arrives. So you get 3sec run time vs 0.3sec? Big whup!

--
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)."

Ernest,

First off all, I described resonance perfectly accurately: You just pulled
out an encyclopedia definition that worded it another way and said exactly
what I said, that an object will resonate at its natural frequency, which
causes its oscillations to increase in amplitude.

Your saying that I am wrong is simply not true, and shows that you are
simply acting out of spite -- as demonstrated by the bitter and combative
tone of your message.

Just for the record, my discussion of resonance in engines is light years
beyond your silly encyclopedia blurb.

I explained that in order to understand resonance, it is useful to think in
terms of excitation forces -- power pulses, imbalance and outside spring
mass systems, such as a propeller, for example -- and restraining forces,
such as mass, stiffness and dampening.

You are wrong on several fundamental points: first, a damper is a
restraingin force and will do just that, it will dampen the oscillations
resulting from resonation. It does this by clipping the harmonics, which are
what cause the amplitutde of the oscillations to grow.

A damper can be a spring or an elastomer tuned to the resonant frequency, or
even a flywheel, which relies on inertia to dampen the oscillations. A sprag
clutch which allows freedom of movement in one direction but not the other,
also clips oscillations. All of these dampers work by counteracting the
energy of the harmonics. Without the energy of the harmonics, the
oscillations cannot increase.

Point blank question: Are you are saying that if you bring an object to
resonance, there is nothing that damping can do to restrain the
oscillations? I am asking again because that is exactly what you said. I
just want to confirm because this is an elementary point of understanding
resonance and if you don't understand this, then...well, I think we can draw
our own conclusions...

Also plain wrong is that resonance requires two objects. Resonance requres
only one object and an excitation force acting on it. Period. An object is
not a force.

Also, I see now that your are unable to discuss politely --throwing around
personal insults like confetti. Nice. Also, what right do you have to demand
that I "shut up?" Why don't you shut up?

Regards,

Gordon.




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


That should be and indication to you to shut up and learn from your
elders.


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


Now your getting it.

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.


So, is the goal here to talk gibberish about as many subjects as possible
in the forlorn hope that there is an outside chance that you might be
right about SOMETHING!!

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.

Case in point. You obviously have no idea what you're talking about. You
read a science book once in high school and now consider yourself a
scholar. Here's a clue. Resonation requires TWO objects. You need
something to vibrate, and something to cause the vibration.

from: http://en.wikipedia.org/wiki/Resonate

In physics, resonance is an increase in the oscillatory energy absorbed by
a system when the frequency of the oscillations matches the system's
natural frequency of vibration (its resonant frequency).


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 close, yet so far away. If you'd shut up and read what you wrote up to
this point, you might get it. Dampeners do nothing to help the
destruction of an engine that is running at its resonant frequency. They
lower the harshness of the pulses, but the point of resonancy is that the
energy from each pulse is stored in the system until the next pulse
arrives. So you get 3sec run time vs 0.3sec? Big whup!

--
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)."

Just one more note, about Ernest's talk of an engine "running at its
resonant frequency."

Just where do we see engine's running at their resonant frequency? I have to
wonder because if you are driving down the highway and holding a steady
rpm -- perhaps with the aid of cruise control -- I think you will want to be
sure not to run at this "resonant frequency."

How laughable. If this were true there would be cars beside the highway
every couple of miles or so.

The fact is that engines are designed not to resonate. Designers do this by
taking into account excitation forces and applying countering restraining
forces.

Mathematically the relationship is represented by :
M a + D v + K x = Me ² e sin( t - )

For simplification, the above equation can be written as:
Mass term + Damping term + Stiffness Term = Restraining Force

The restraining forces, represented by the various terms in the equation,
are what determines how a rotor behaves throughout its operating range. Any
excitation force, such as imbalance, is always in equilibrium with the
restraining forces of mass, damping, and stiffness. The amount of measured
vibration, as a result of these combined forces, will depend upon the
combined effect of all three terms in the equation.

Regards,

Gordon.



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

First off all, I described resonance perfectly accurately: You just pulled
out an encyclopedia definition that worded it another way and said exactly
what I said, that an object will resonate at its natural frequency, which
causes its oscillations to increase in amplitude.

Your saying that I am wrong is simply not true, and shows that you are
simply acting out of spite -- as demonstrated by the bitter and combative
tone of your message.

Just for the record, my discussion of resonance in engines is light years
beyond your silly encyclopedia blurb.

I explained that in order to understand resonance, it is useful to think
in terms of excitation forces -- power pulses, imbalance and outside
spring mass systems, such as a propeller, for example -- and restraining
forces, such as mass, stiffness and dampening.

You are wrong on several fundamental points: first, a damper is a
restraingin force and will do just that, it will dampen the oscillations
resulting from resonation. It does this by clipping the harmonics, which
are what cause the amplitutde of the oscillations to grow.

A damper can be a spring or an elastomer tuned to the resonant frequency,
or even a flywheel, which relies on inertia to dampen the oscillations. A
sprag clutch which allows freedom of movement in one direction but not the
other, also clips oscillations. All of these dampers work by counteracting
the energy of the harmonics. Without the energy of the harmonics, the
oscillations cannot increase.

Point blank question: Are you are saying that if you bring an object to
resonance, there is nothing that damping can do to restrain the
oscillations? I am asking again because that is exactly what you said. I
just want to confirm because this is an elementary point of understanding
resonance and if you don't understand this, then...well, I think we can
draw our own conclusions...

Also plain wrong is that resonance requires two objects. Resonance requres
only one object and an excitation force acting on it. Period. An object is
not a force.

Also, I see now that your are unable to discuss politely --throwing around
personal insults like confetti. Nice. Also, what right do you have to
demand that I "shut up?" Why don't you shut up?

Regards,

Gordon.




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


That should be and indication to you to shut up and learn from your
elders.


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


Now your getting it.

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.


So, is the goal here to talk gibberish about as many subjects as possible
in the forlorn hope that there is an outside chance that you might be
right about SOMETHING!!

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.

Case in point. You obviously have no idea what you're talking about. You
read a science book once in high school and now consider yourself a
scholar. Here's a clue. Resonation requires TWO objects. You need
something to vibrate, and something to cause the vibration.

from: http://en.wikipedia.org/wiki/Resonate

In physics, resonance is an increase in the oscillatory energy absorbed
by a system when the frequency of the oscillations matches the system's
natural frequency of vibration (its resonant frequency).


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 close, yet so far away. If you'd shut up and read what you wrote up
to this point, you might get it. Dampeners do nothing to help the
destruction of an engine that is running at its resonant frequency. They
lower the harshness of the pulses, but the point of resonancy is that the
energy from each pulse is stored in the system until the next pulse
arrives. So you get 3sec run time vs 0.3sec? Big whup!

--
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)."

And just one more thing.

I said previously that auto designers take resonance into account and design
powertrain systems not to resonate. They do this by applying restraining
forces as I mentioned in my previous message.

The key thing for us in the amateur-built aircraft community is that we are
putting together a powertrain piecemel. We take an auto engine, a prop, a
gearbox and put them all together. Sometimes this can result in resonation
of one or more pieces in the system.

What I have been doing is just trying to think through the problem and apply
known facts about resonance and known design principles.

Regards,

Gordon.


"Gordon Arnaut" wrote in message
...
Just one more note, about Ernest's talk of an engine "running at its
resonant frequency."

Just where do we see engine's running at their resonant frequency? I have
to wonder because if you are driving down the highway and holding a steady
rpm -- perhaps with the aid of cruise control -- I think you will want to
be sure not to run at this "resonant frequency."

How laughable. If this were true there would be cars beside the highway
every couple of miles or so.

The fact is that engines are designed not to resonate. Designers do this
by taking into account excitation forces and applying countering
restraining forces.

Mathematically the relationship is represented by :
M a + D v + K x = Me ² e sin( t - )

For simplification, the above equation can be written as:
Mass term + Damping term + Stiffness Term = Restraining Force

The restraining forces, represented by the various terms in the equation,
are what determines how a rotor behaves throughout its operating range.
Any excitation force, such as imbalance, is always in equilibrium with the
restraining forces of mass, damping, and stiffness. The amount of measured
vibration, as a result of these combined forces, will depend upon the
combined effect of all three terms in the equation.

Regards,

Gordon.



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

First off all, I described resonance perfectly accurately: You just
pulled out an encyclopedia definition that worded it another way and said
exactly what I said, that an object will resonate at its natural
frequency, which causes its oscillations to increase in amplitude.

Your saying that I am wrong is simply not true, and shows that you are
simply acting out of spite -- as demonstrated by the bitter and combative
tone of your message.

Just for the record, my discussion of resonance in engines is light years
beyond your silly encyclopedia blurb.

I explained that in order to understand resonance, it is useful to think
in terms of excitation forces -- power pulses, imbalance and outside
spring mass systems, such as a propeller, for example -- and restraining
forces, such as mass, stiffness and dampening.

You are wrong on several fundamental points: first, a damper is a
restraingin force and will do just that, it will dampen the oscillations
resulting from resonation. It does this by clipping the harmonics, which
are what cause the amplitutde of the oscillations to grow.

A damper can be a spring or an elastomer tuned to the resonant frequency,
or even a flywheel, which relies on inertia to dampen the oscillations. A
sprag clutch which allows freedom of movement in one direction but not
the other, also clips oscillations. All of these dampers work by
counteracting the energy of the harmonics. Without the energy of the
harmonics, the oscillations cannot increase.

Point blank question: Are you are saying that if you bring an object to
resonance, there is nothing that damping can do to restrain the
oscillations? I am asking again because that is exactly what you said. I
just want to confirm because this is an elementary point of understanding
resonance and if you don't understand this, then...well, I think we can
draw our own conclusions...

Also plain wrong is that resonance requires two objects. Resonance
requres only one object and an excitation force acting on it. Period. An
object is not a force.

Also, I see now that your are unable to discuss politely --throwing
around personal insults like confetti. Nice. Also, what right do you have
to demand that I "shut up?" Why don't you shut up?

Regards,

Gordon.




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


That should be and indication to you to shut up and learn from your
elders.


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


Now your getting it.

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.


So, is the goal here to talk gibberish about as many subjects as
possible in the forlorn hope that there is an outside chance that you
might be right about SOMETHING!!

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.

Case in point. You obviously have no idea what you're talking about.
You read a science book once in high school and now consider yourself a
scholar. Here's a clue. Resonation requires TWO objects. You need
something to vibrate, and something to cause the vibration.

from: http://en.wikipedia.org/wiki/Resonate

In physics, resonance is an increase in the oscillatory energy absorbed
by a system when the frequency of the oscillations matches the system's
natural frequency of vibration (its resonant frequency).


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 close, yet so far away. If you'd shut up and read what you wrote up
to this point, you might get it. Dampeners do nothing to help the
destruction of an engine that is running at its resonant frequency. They
lower the harshness of the pulses, but the point of resonancy is that
the energy from each pulse is stored in the system until the next pulse
arrives. So you get 3sec run time vs 0.3sec? Big whup!

--
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)."

Just one more point about dampers:

Ernest said: "Dampeners do nothing to help the
destruction of an engine that is running at its resonant frequency. They
lower the harshness of the pulses, but the point of resonancy is that the
energy from each pulse is stored in the system until the next pulse arrives.
So you get 3sec run time vs 0.3sec? Big whup!"

Here are the facts: dampers do not just give you an extra few seconds of
protection. They nip resonance in the bud very effectively.

Yes elastomer or spring-type dampers do delay the release of energy -- both
a spring or a piece of rubber store and relase energy at a later time,
usually a split second, as I mentioned earlier in this discussion.

However, the significance of this is not that it merely delays the onset of
resonance -- and the destruction of the engine -- by three seconds, as
Ernest would have us believe. The signficance is that by absorbing, then
releasing the energy from resonation a split second later, the timing of the
resonation is disrupted and the object cannot continue to resonate. It stops
resonation in its tracks.

Here's how it works: the first harmonic that appears with resonation is
dampened by the spring or rubber, or sprag clutch or whatever, so that
subsequent harmonics cannot be set into vibration, so the oscillation does
not build. The destructive chain is broken by disrupting the timing of the
first harmonic.

If you look at a harmonic damper of a car engine, you will see an inner
metal disk and an outer metal ring, and a strip of rubber in between. That
rubber is tuned to precisely disrupt -- or clip -- harmonics at the
crankshaft's resonant frequency. (This is a function of the density of the
rubber compound).

And this is why when you have your harmonic damper overhauled, it will come
with a new strip of rubber, replacing the old strip which has hardened and
aged and no longer absorbs energy at the intended frequency.

Springs in a clutch plate can do the same thing if they are tuned at the
same frequency as the rubber compound. A sprag clutch is another approach,
so is a flywheel (which dampens out the energy from the harmonic by the
force of its spinning inertial mass). The Eggenfellner gearbox uses uses
this approach and has the best record so far.

Ernest I am amazed that you don't even know this much, yet you have the
brass to hurl unprovoked personal insults.

Regards,

Gordon.






"Gordon Arnaut" wrote in message
...
And just one more thing.

I said previously that auto designers take resonance into account and
design powertrain systems not to resonate. They do this by applying
restraining forces as I mentioned in my previous message.

The key thing for us in the amateur-built aircraft community is that we
are putting together a powertrain piecemel. We take an auto engine, a
prop, a gearbox and put them all together. Sometimes this can result in
resonation of one or more pieces in the system.

What I have been doing is just trying to think through the problem and
apply known facts about resonance and known design principles.

Regards,

Gordon.


"Gordon Arnaut" wrote in message
...
Just one more note, about Ernest's talk of an engine "running at its
resonant frequency."

Just where do we see engine's running at their resonant frequency? I have
to wonder because if you are driving down the highway and holding a
steady rpm -- perhaps with the aid of cruise control -- I think you will
want to be sure not to run at this "resonant frequency."

How laughable. If this were true there would be cars beside the highway
every couple of miles or so.

The fact is that engines are designed not to resonate. Designers do this
by taking into account excitation forces and applying countering
restraining forces.

Mathematically the relationship is represented by :
M a + D v + K x = Me ² e sin( t - )

For simplification, the above equation can be written as:
Mass term + Damping term + Stiffness Term = Restraining Force

The restraining forces, represented by the various terms in the equation,
are what determines how a rotor behaves throughout its operating range.
Any excitation force, such as imbalance, is always in equilibrium with
the restraining forces of mass, damping, and stiffness. The amount of
measured vibration, as a result of these combined forces, will depend
upon the combined effect of all three terms in the equation.

Regards,

Gordon.



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

First off all, I described resonance perfectly accurately: You just
pulled out an encyclopedia definition that worded it another way and
said exactly what I said, that an object will resonate at its natural
frequency, which causes its oscillations to increase in amplitude.

Your saying that I am wrong is simply not true, and shows that you are
simply acting out of spite -- as demonstrated by the bitter and
combative tone of your message.

Just for the record, my discussion of resonance in engines is light
years beyond your silly encyclopedia blurb.

I explained that in order to understand resonance, it is useful to think
in terms of excitation forces -- power pulses, imbalance and outside
spring mass systems, such as a propeller, for example -- and restraining
forces, such as mass, stiffness and dampening.

You are wrong on several fundamental points: first, a damper is a
restraingin force and will do just that, it will dampen the oscillations
resulting from resonation. It does this by clipping the harmonics, which
are what cause the amplitutde of the oscillations to grow.

A damper can be a spring or an elastomer tuned to the resonant
frequency, or even a flywheel, which relies on inertia to dampen the
oscillations. A sprag clutch which allows freedom of movement in one
direction but not the other, also clips oscillations. All of these
dampers work by counteracting the energy of the harmonics. Without the
energy of the harmonics, the oscillations cannot increase.

Point blank question: Are you are saying that if you bring an object to
resonance, there is nothing that damping can do to restrain the
oscillations? I am asking again because that is exactly what you said. I
just want to confirm because this is an elementary point of
understanding resonance and if you don't understand this, then...well, I
think we can draw our own conclusions...

Also plain wrong is that resonance requires two objects. Resonance
requres only one object and an excitation force acting on it. Period. An
object is not a force.

Also, I see now that your are unable to discuss politely --throwing
around personal insults like confetti. Nice. Also, what right do you
have to demand that I "shut up?" Why don't you shut up?

Regards,

Gordon.




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


That should be and indication to you to shut up and learn from your
elders.


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


Now your getting it.

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.


So, is the goal here to talk gibberish about as many subjects as
possible in the forlorn hope that there is an outside chance that you
might be right about SOMETHING!!

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.

Case in point. You obviously have no idea what you're talking about.
You read a science book once in high school and now consider yourself a
scholar. Here's a clue. Resonation requires TWO objects. You need
something to vibrate, and something to cause the vibration.

from: http://en.wikipedia.org/wiki/Resonate

In physics, resonance is an increase in the oscillatory energy absorbed
by a system when the frequency of the oscillations matches the system's
natural frequency of vibration (its resonant frequency).


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 close, yet so far away. If you'd shut up and read what you wrote up
to this point, you might get it. Dampeners do nothing to help the
destruction of an engine that is running at its resonant frequency.
They lower the harshness of the pulses, but the point of resonancy is
that the energy from each pulse is stored in the system until the next
pulse arrives. So you get 3sec run time vs 0.3sec? Big whup!

--
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)."

"Ernest Christley" wrote

So, is the goal here to talk gibberish about as many subjects as
possible in the forlorn hope that there is an outside chance that you
might be right about SOMETHING!!

gordon likes to see himself type and post. He is so totally clues, it is
hard to know where to start.

My solution? Just let himself talk to himself. It is solution I wish
everyone here would adopt, and after a while with nobody playing with him,
he might go away.
--
Jim in NC

Bashir,

Actually, I spoke too quickly when I conceded a mistake.

Tautness and stiffness are two different things. A taut string will vibrate
at a higher frequency than a loose string, but we have not changed its
inherent stiffness or elasticity (e).

If you increase the stiffness (decrease the elasticity) of an object, you
will decrease its resonant frequency, as I first stated.

The resonant frequency of a system is symbolized by "w n"

and pronounced "Omega-sub-n". An object's mass and elasticity determines its
resonant frequency, and is expressed mathematically as:





wn = ?(k/m)





K is the value for elasticity, while m is the value for mass. So we see that
lower elasticity (greater stiffness) results in a lower frequency of
resonation.

So making a crankshaft stiffer does decrease the rpm at which it will
resonate. It also increases the value of restraining force acting against
excitaiton. So the benefits are cumulative.

We can see a real-world example of this in V-8 engines which would not last
very long without a harmonic damper, even though they have much smoother
torque pulses than a 4-cylinder. The reason is that the crankshaft has to be
much longer and thereby less stiff -- or more elastic.

On most four-cylinder engines, dampers are not needed because the short,
stout crank actually resonates at a frequency below the oeprating range.
Hence resonance will never be encountered.

It's useful at this point to back up and define what resonant frequency of
an object -- or system -- really means. Stated most simply it is the
frequency at the object or system will vibrate if it is excited by a single
pulse.

The actual torsional resonance of an engine can be calculated if you know
the torsional rate of the crankshaft (which is its spring value) and its
mass moment of inertia, which is a function of crank stroke and weight,
number of journals, dimensions of the flywheel, torsional absorber,
accessories.

So now we know a little about resonance and how it affects a crankshaft. But
what happens when we attach a propeller or gearbox-propeller combination to
that engine?

Well, now we are dealing with not just an object but a system. And this
system has its own torsional resonance frequency, which is different from
that of the single object itself, like the crankshaft.

A key concept here is tranmissibility, which is the ratio between the
amplitude of the excitation torque, and the amplitude of the output torque.
In simple terms, this means that the gearbox and propeller can be subjected
to vibratory forces many times higher than the torque peaks produced by the
engine.

Here is where damping comes in. But even with damping there will be some
amplification of vibratory forces transmitted from the gearbox to the
gearbox and prop.

There is some good reading at this website, with specific info on how
torsional resonance is dealt with in designing aircraft PSRU systems:
http://www.epi-eng.com/BAS-VibBasics.htm

Regards,

Gordon.





"Bashir" wrote in message
oups.com...
He can be taught!! Who would have thought it!?