Torsional Vibration and PSRU Design

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