light twins?

I'm going to write out that equation for resonant frequency because the
symbols did not translate over properly.

It should read:

Omega sub-n (resonant frequency) = the square root of K (elasticity) divided
by m (mass)

If you plug any numbers into this equation you see that resonant frequency
goes down as stiffness goes up (elasticity goes down = stiffness going up).

We also see the same result if we increase mass: resonant frequency again
goes down.

This is important because we want to design an engine-gearbox-prop system
that resonates at an rpm below actual operation, if possible.

Regards,

Gordon.



"Gordon Arnaut" wrote in message
...
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!?

Ernest,

There is no gibberish in any of what I said. And I wasn't talking about
"many" subjects, only one: resonance.

My point was to illustrate the difference between simple vibration and
resonance.

Since you didn't get it, I will try again. Think of a guy on a diving board
bouncing up and down. He is vibrating that board at a certain frequency (the
number of times per second he goes up and down). This is vibration.

Now when he jumps off that board, the board will continue vibrating, only
now it is resonating, because it is vibrating at its natural frequency. If
you count the number of times per second that the board bounces up and down,
you will have discovered its resonant frequency. (It's usually higher than
the frequency of the guy jumping up and down, which is why it starts
flapping).

However, the resonation does not continue or increase, because the
excitation force stopped. Only if the excitation force were to continue at
the same frequency, would the board go into HARMONIC resonance, which causes
the oscillations to increase in amplitude.

Here is an experiment you can try at home: take a think metal shaft and hold
it horizontally. Now attach one end to a wall or other solid surface so it
can't move, and provide a means of holding the other end up so the shaft
remains in the horizontal position, but free at this other end.

Now add a round disk or wheel to this free end. Grasp the disk in your hand
and turn it, causing the shaft to twist. This is what happens when the
torque pulses of the engine act on the crankshaft. When you have twisted as
far as you can, let go and count that number of times the wheel swings back
and forth per second -- if you can; it will probably be too fast to time
without special equipment.

You will now know the torsional (twisting) resonance of your shaft.

All you need to do is twist it once and let go. by definition resonance is
the vibrational frequency that results from only a SINGLE excitation.

Again, if you were somehow able to twist that shaft back and forth at
precisely the timing required to maintain its resonant frequency, you would
induce harmonic resonance.

I daresay that if you have read and considered what I have said, you will
have improved you understanding of resonance by a fair margin.

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)."

("Gordon Arnaut" wrote)\
[snips]
A key concept here is tranmissibility,

Like being in college again. 'Transmissibility' ...ouch, it hurts when I
pee!


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

Site is interesting reading. Worth a look.


Montblack

"Gordon Arnaut" wrote

I daresay that if you have read and considered what I have said, you will
have improved you understanding of resonance by a fair margin.

Regards,

Gordon.

Oh, good Lord...
--
Jim in NC