Re: Torsional Vibration and PSRU Design
- From: "Gordon Arnaut" <goarnaut@xxxxxxxxx>
- Date: Fri, 14 Apr 2006 11:34:21 -0400
Dan,
Thanks for the critique.
Your point about props not being a source of excitation except in disturbed
flow is technically correct. However, the prop is a spring mass that acts to
amplify the excitations that comes from other sources, such as combustion
pressure and imbalance.
That is why using a heavier prop with a bigger moment can cause torsional
vibration to become destructive, where previously, with a lighter prop, it
was acceptable. And usually it will be the crank or the gearbox that will
fail, not the prop, which, due to its springiness, can take much more
twisting divergence.
About the flywheel, again youare technically correct. It is not a damper,
which as you pointed out is a device that transfers kinetic energy into
another form of energy, such as heat. However, in the context of my
discussion about excitation forces and restraining forces it is a
restraining force that brings about a "diminishing" effect on vibrations.
And yes, it is the inertia produced by the centrifugal force, not the
centrifugal force itself that is the restraining force. Of course, the
amount of inertia is directly proportional to the centrifugal force...
And yes, the crank will have N-1 frequencies. I did say that there is more
than one resonant frequency, but for the sake of understandability I was
trying to simplify as much as possible. Perhaps I oversimplified too much.
All of your points fill in the technical details nicely. As Einstein once
remarked, the best scientific explanation is one that is stated "as simply
as possible, but no simpler."
You are right that it is useless to try to work out an actual solution to an
actual problem using this forum. That wasn't my point. I was just trying to
address the basics of TV, because there appears to be some fundamental
misconceptions that are always brought up in these discussions.
I am also not pushing a specific solution, ie. a large-inertia flywheel and
stiff shaft, as you gave me credit for. I don't know where you got that
from. I was simply trying to point out how mass and stiffness come into play
in the context of a basic understanding.
I agree with you on the potential pitfalls of the approach you outlined.
However I do not agree that the problem frequency will necessarily have to
fall within the operating range. Stiffness of the shaft will be largely a
function of its slenderness ratio, so using a material with a high modulus,
perhaps carbon, and a large diameter, could produce a shaft that is light
yet stiff enough to do the trick.
In any case, I would try to make the shaft as short as possible, hence my
suggestion about the very compact rotary engine.
Again, my goal was to give an understanding of the basics, not to offer
specific design advice.
Regards,
Gordon.
"Dan Horton" <danhorton@xxxxxxxxxxxxx> wrote in message
news:1144977033.833313.312210@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Gordon Arnaut wrote:
<<Another contributor of excitation is imbalance in a rotating or
reciprocating mass.>>
True, but it has limited power compared to gas pressure
oscillation. I was trying to keep things simple.
<<Yet another source of excitation in airplanes is the spring effect of
the prop>>
Naaaa. Propellers in disturbed flow can excite the system, but
usually the concern is the opposite. Might be a few rare causes. For
example, I've seen (with telemetry) a variation in torsional vibration
due to a propeller-powerplant whirl mode.
<<a crankshaft has its own natural resonant frequency and will vibrate
if it is disturbed with enough force.>>
Actually a crank has N-1 natural frequencies, where N equals # of
inertias. That would be four frequencies for a typical 4 cyl with 90
degree throws (4 crankthrows plus flywheel, 5 minus 1). However, to be
fair, when designing a PSRU you can model the crank and flywheel
assembly as a single mass moment of inertia, IF the crank is short and
stiff. The inaccuracy in F1 prediction will be small, like 200-300
RPM.
<<A flywheel is a simple example of a damping device. It uses
centrifugal force to counteract and overcome the twisting.....>>
A flywheel is an inertia. A damper is a device that removes energy
from the system, usually as heat. Think slipping clutch, slipping
v-belt, or viscous ring damper. Designed a viscous disk damper and ran
it parallel with a soft element in a drive a few projects back. It
shed a lot of heat, and telemetry said it damped resonant amplitudes
very well. The successful Raven drive for the 3 and 4 cyl Suzukis uses
a dry frictional damper.
Ahhhh, I'll let you correct the part about centrifugal force <g>
<<This shows that mass is important, unlike some erroneous comments
that mass doesn't matter.>>
Who said anything about mass? For the record, please note that the
previous comment was "Shaft weight is not a factor", the context being
ship propulsion. Shoot, I'm all for careful use of terms. In the
context of torsional vibration, what IS important is "mass moment of
inertia". And that ain't the same as mass or weight.
Ok, you argue that torsional problems can be eliminated through the
use of flywheel mass and stiff shafting. I argue that your approach
has severe drawbacks when applied to the subject at hand, a long shaft
aircraft system.
I agree that a large-inertia flywheel (which is not necessarily a
large-mass flywheel) always reduces vibratory amplitude. It may not
be reasonable to incorporate a huge flywheel inertia in an airplane
because of effect on (1) handling (remember the Sopwith Camel), as well
as (2) aircraft empty weight. You must use a moderate flywheel, a
compromise, not the infinite inertia you describe.
As for stiffness in the shafting that connects the inertias, what
magic did you have in mind? All practical shaft materials exhibit a
stress-strain relationship. I know of only one practical PSRU concept
that meets your goal of near infinite stiffness; it has no shafts at
all other than the crankshaft. Hardly the long shaft system under
consideration. With a shaft several feet long, some degree of twist is
physical reality.
Given that infinite stiffness is impossible in the long shaft
system, I'll tell you what you'll really get. A stiff shaft will raise
the system F1 so that it intersects the gas pressure oscillation order
somewhere up in the operating range close to peak torque. The system
will resonate into junk. The classic solution then tried by the
uninformed is to make it "stronger" (the result being stiffer), which
makes the problem worse. Near idle or below idle is where you want the
intersection of F1 and gas pressure frequency, because gas pressure
oscillation isn't very powerful at idle. You do that with a soft shaft
or rubber element, and note that it doesn't take a huge inertia to
smooth a small near-idle-speed oscillation. By tailoring frequencies,
we can get a practical, lightweight system.
Nobody can teach this subject in RAH posts. Hell, "Practical
Solution" is several volumes. What we can do is (1) direct folks to
quality reference material, and (2) quit telling them it is impossible.
I think I'll puke if I see one more guy reference the Hessenaur
article and declare "torsional vibration even beat Rutan". If somebody
had handed Burt the right books or introduced him to J.P. Den Hartog,
you can bet you wouldn't be reading that crap.
Dan
.
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