Re: What is "synchronizing" propellers and why was it done?

Date:         21 Dec 97 02:32:38 
From:         "John E. Kuslich" <johnk@crak.com>
Organization: CRAK Software
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Daniel P. B. Smith wrote:
> I have read in books that four-engined propeller aircraft had special
> mechanisms for synchronizing the propellers ...
...
> Was this just done for acoustic comfort and aesthetics?  That it, were
> propellers synchronized just because the steady sound was pleasant and
> reassuring?  Or were there aeronautical reasons, and, if so, what were
> they?  Did a plane fly differently, consume less fuel, etc. if the
> propellers were synchronized?  Why?

Unsynchronized propellers provide a much more intense vibration in the low
frequency part of the spectrum than do synchronized propellers.  This is
because beat notes form at the sum and difference frequencies of the propeller
rotations.  Not only is this vibration extremely annoying and causes fatigue,
but it is much more stressfull on structural components which may have
resonances at these frequencies and endanger the structure if allowed to
continue.

JK

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John E. Kuslich, Head Geek At CRAK Software
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