Re: prop size?


Jay <scheevel@...>
 

Ron Weiss wrote:



"...because all the info I can get is that some reduction units don't

like 2 bladed wood props(harmonic issues). One guy used a Prince and

liked the performance, but, there was this vibration?...."



It sounds like the problem may not be the prop by itself. I would look
instead at the prop/reduction drive combination. The two components together
may produce the harmonic vibration that is noted. Let me explain:



If the reduction drive assembly is not stiff enough, then it also can
contribute to the harmonics of the system. The prop itself is not designed
to be completely stiff (nor should it be), but props are designed and
balanced so as not to destructively resonate on their own.



However, when a prop is bolted to another assembly that is not stiff enough,
the elastic/inertial response of the combination could produce unanticipated
harmonic oscillations.



In the traditional aircraft installation, the prop is bolted to the
crankshaft (prop-flange), which is VERY stiff and is also dampened by the
engine mass. In this case, the harmonic modes of the prop/engine combination
are outside the range of any prop oscillation modes, so significant harmonic
oscillations are effectively damped. This may not be the case when the prop
is connected to a reduction drive where the reduction drive is less stiff
than the standard crankshaft.



This is probably part of the reason the FAA certifies prop-engine
combinations instead of just the prop.



Something to consider..



Jay Scheevel - Tri-Q, still building

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