From: Epijk@aol.com
Message-ID: <97.240d1a0c.29b65932@aol.com>
Date: Tue, 5 Mar 2002 12:24:02 EST
Subject: Re: Low MMOI Props
To: lancair.list@olsusa.com
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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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