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
et...

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