Torsional Vibration and PSRU Design

My original remarks and the quote was in response to the following
statement you made:

Gordon Arnaut wrote: "Yet another source of excitation in airplanes is
the spring effect of the prop, where the blade tips whipsaw back and
forth as they undergo acceleration and deceleration due to the torque
spikes of cylinder firing."

I had assumed that you were refering to the potential of the blades to
flex parallel to the axis of the hub due to variations of blade loading
created by the periodic variations of torque. I made that assumption
since you were describing the prop as the source of the excitation and
yet still made specific mention of the engine torque variations. I had
once thought axial loading and flexure might feed back into the system
at one time myself, but Ker states that it does not( except in
extraodrinary cases), which I took note of in my studies -- thus the
quote. If you were not refering to axial vibrations, then your
statement is silly since you identify the prop ( or its spring effect)
as the source of excitation and yet you end the statement with the
actual excitation source ( torque spikes ). Alter the frequency of the
"torque spikes" and the system returns to normal.

Dan is the one that brought up blade interactions with disturbed
airflow. And I quoted the passage regarding issues of a non axial
airstream (which is not the same as disturbed airflow).

Even via he prop, Wilson is relatively proscriptive:

Wilson wrote: "The airscrew is one source, of aerodynamic origin,
arising from the passage of the blades through a non-uniform airstream,
or due to the airstream entering the airscrew disc obliquely when the
aircraft is executing certain manouevres ...."

The phrase "of aerodynamic origin" when not cropped out is the
significant part. You may want to claim the prop as a source since it
is complicit in the translation of the vibrations, but the same could be
said of the crank, so whats the point? In any event you state:

Gordon Arnaut wrote: "Yet another source of excitation in airplanes is
the spring effect of the prop"

The Wilson quote mentions nothing of spring effectof the prop as a
source of excitations and in fact if you think about it you will see (
or at least most people would) that the vibrations "of aerodynamic
origin" do not require any springiness of the prop blade to impinge upon
the system.


And I didn't even bother to comment on this gem of yours:

Gordon Arnaut wrote: "My original point was that if you have a flywheel
with enough inertia, it will be an effective restraining force to act
against excitations that would otherwise produce vibration."

Which is just plain wrong. In the industry I work in there are machines
with moments of inertia measured in tons and they are still subject to
torsional vibration issues. Upping the moment of inertia just alters
the resonant frequency ranges. Without damping of some sort, excitation
in the critical range will still drive the system into resonance. If
you meant to say "if you have a flywheel with enough inertia, it will
lower the resonant frequency of the system to a range conducive to safe
operation" that would be a different thing.



Charles


Gordon Arnaut wrote:
Charles,

What exactly are you saying? Once again, I find myself scratching my head
trying to fathom your actual point. You are nothing if not a master of
obfuscation.

I said very plainly in my original post that the prop is a source of
excitations, as is the cylinder firing of the engine, as well as imbalance
in the system.

I don't know who said what, but your post contained this in quotation marks:
"In aero-engine/airscrew systems there are, in general, two series of
excitations. The airscrew is one source..."

So whoever said that, whether it is you or Wilson, it is quite plain and
quite true. The Prop IS a source of excitations, whether they are of
aerodynamic origin or whether they are due to resonance.

If you disagree with this, please say so plainly, otherwise do not try to
muddy the waters further -- it is only doing a disservice to the discussion.

Regards,

Gordon.








"Charles Vincent" wrote in message
et...

Actually Gordon, the words you quoted were my words not Wilson's. You
will notice there are no quotaion marks around them in my original post.
The text with the quotation marks is from Wilson. The excerpt was
actually confirming Dan's contention that the excitation source was
disturbed airflow, that it does not originate in the prop. Take the
disturbed airflow away and the natural hysterisis of the prop and rest of
the system will cause it to return to normal. So while excitations can
enter through the prop or they can enter through the crank, these two
components don't create the excitation, they react to them. There are
components in a redrive system that can originate excitations though,
which is why if you just want to fly it is easiest to pick a direct drive
wooden prop snd go fly. Not guaranteed, but much simpler.

Charles



Gordon Arnaut wrote:

Charles,

Thanks for that snippet from the Wilson book.

Please note the part where he says how the "flexural properties of the
propellor are key in determining how the SYSTEM (my emphasis) will
respond to the excitations since the prop will resonate."

Is this not exactly what I said about the prop beginning to resonate and
then cuasing something else in the system to break?

I have said all of this in my posts, with the exception of the part about
prop excitations arising from aircraft manouevers, which is really part
of the point about distubed flow.

Thank you for confirming the correctness of my position. For the record
now, I don't think there can be any serious question that the prop does
not contribute a very real component to the excitations side of the
equation.

Regards,

Gordon.




"Charles Vincent" wrote in message
y.net...


Dan Horton wrote:


Hello Charles,
According to Ker Wilson, prop flutter has no real impact on
torsional vibration. He could be wrong, but he devoted more than a
half century to the subject. Blade passing frequency, however,
apparently does come into play in some systems. So does whirl, but
that isn't the internet topic of the year.

Ahh, thank you, appreciate the confirmation.

Lucky dog, wish I had my own copy. I have to beg my local
librarian to get it from the UA library.

Dan


A quote:

"In most practical cases coupled axial/flexural modes occur independently
of coupled torsional-flexural modes since there is usually no appreciable
coupling whereby component harmonics of the shaft torque are able to
excite symmetrical blade vibration."

And to your earlier point:

"In aero-engine/airscrew systems there are, in general, two series of
excitations. The airscrew is one source, of aerodynamic origin, arising

from the passage of the blades through a non-uniform airstream, or due to

the airstream entering the airscrew disc obliquely when the aircraft is
executing certain manouevres ..... The other series originates from the
non uniform character of the engine torque."

Hence the blade passing frequency. Still the flexural properties of the
propellor are key in determining how the system will respond to the
excitations since the prop will resonate.

As far as modeling the propeller and determining its natural frequencies
(it has multiple as well) it appears to be a right bear. The shape is
complex and there are multiple modes of vibration and all of them have to
be adjusted for RPM because the stiffness varies with the centrifugal
force (the real kind). For an adjustable prop, the stiffness in the
plane of rotation changes with angle.

Charles