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<< Lancair Builders' Mail List >>
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In a message dated 2/26/2002 5:38:02 AM Pacific Standard Time,
StarAerospace@aol.com writes:
<<<...On props, a heavier prop will have higher inertia and dampen torsional
vibration by mass; a composite prop will tend to dampen torsional vibration
by virtue of the superior dampening of the wood/composite blade structure...>>
>
Well, not really. Again, we have evidence of a misunderstanding of the
physical entity known as DAMPING.
( For those interested, an introduction to the basics of vibration can be
found at:
http://www.epi-eng.com/VibrationBasics.htm )
"...to dampen torsional vibration by mass..." is a concept with which I am
unfamiliar. A heavier prop (actually, a prop with a higher
mass-moment-of-inertia) does nothing to alter the damping of the engine
system. All it does is reduce the torsional resonant frequency of the
crankshaft system. Therefore, reducing the resonant frequency of the engine
system lowers the transmissibility of the mounting system and it's ability to
transfer vibration into the airframe.
Fortunately for those who like to experiment, the mechanism for absorption of
torsional excitation of the crankshaft system in larger LycoNentals is a
pendulous system which has the remarkable feature of being able to absorb a
particluar ORDER of excitation, as opposed to the tuned elastomeric absorber
(misnamed DAMPER) found on automotive applications, which absorbs a
particular FREQUENCY of excitation. The clearance between the suspending pin
and the bushing in the crank blade determines the order of vibration to which
a particular counterweight is tuned. Many big flat-6 engines have
conterweights tuned to 5th and 8th orders. (The GTSIO-520 has the pendulous
system as well as an elastomeric absorber tuned to a particularly troublesome
FREQUENCY.)
DAMPING enters the discussion with respect to the excitation (and potential
failure) of the prop blades. Composite and wood props have a certain degree
of internal damping characteristics, which tends to make them more forgiving
of the torsional excitation imposed on them by a piston engine. Metal blades
have virtually no internal damping (with respect to vibration, they behave
much like pure springs). Therefore, metal blades are far less tolerant of
vibration inputs which can excite them at or near to their resonant frequency
(ies).
For more info on the vibration characteristics (and sensitivities) of
propellers on piston engines, visit:
http://www.epi-eng.com/PropVib.htm
To Eric: thanks for taking the time to explain Vs = sqrt(KG/RT). Lots of
good info in that posting and subsequent. While on the subject of Lear 20/30,
the story about "coffin corner" might be interesting to those
potentially-high-flyers.
Jack Kane
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