A aviation & planes forum. AviationBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » AviationBanter forum » rec.aviation newsgroups » Home Built
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

light twins?



 
 
Thread Tools Display Modes
  #1  
Old August 8th 05, 03:52 AM
Gordon Arnaut
external usenet poster
 
Posts: n/a
Default

Thanks for pointing out my typo.

As stiffness increases so does the frequency at which it resonates, as we
see on a taut guitar string, or drum, which increase in pitch as they are
tightened.


Regards,

Gordon.


"Bashir" wrote in message
oups.com...
Gordon Arnaut wrote:

So more stiffness equals a
lower resonating frequency.


Do you want to think that one through again, Gordon?



  #2  
Old August 8th 05, 04:13 AM
Bashir
external usenet poster
 
Posts: n/a
Default

He can be taught!! Who would have thought it!?

  #3  
Old August 8th 05, 05:06 AM
Morgans
external usenet poster
 
Posts: n/a
Default


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


Heaven forbid you should have written that what he said was crap! chuckle
--
Jim in NC


  #4  
Old August 9th 05, 12:53 AM
Richard Lamb
external usenet poster
 
Posts: n/a
Default

Well, MY last post on this subject was accused of being pornography...

Some people have NO sense of humor

Richard

  #5  
Old August 11th 05, 04:53 AM
Gordon Arnaut
external usenet poster
 
Posts: n/a
Default

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!?



  #6  
Old August 11th 05, 05:00 AM
Gordon Arnaut
external usenet poster
 
Posts: n/a
Default

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!?





  #7  
Old August 11th 05, 06:05 AM
Montblack
external usenet poster
 
Posts: n/a
Default

("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

 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Diesel aircraft engines and are the light jets pushing out the twins? Dude Owning 5 October 7th 04 03:14 AM
The light bulb Greasy Rider Military Aviation 6 March 2nd 04 12:07 PM
Light Twins - Again - Why is the insurance so high? Doodybutch Owning 7 February 11th 04 08:13 PM
Light Twins. How soft??? Montblack Owning 19 December 3rd 03 10:38 PM
Light Twins. How soft??? Montblack Piloting 19 December 3rd 03 10:38 PM


All times are GMT +1. The time now is 02:40 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
Copyright ©2004-2025 AviationBanter.
The comments are property of their posters.