Arplast will very happily say their propellers flex in result to shock
loadings. ( act as a spring if you like ) I would disagree very much though
that a two stroke with reduction and shock absorber impart the same loading
as a direct drive installation. A two stroke with shock absorber and large
reduction is the most easy to accomidate. A direct drive four stroke is
much harder on a propeller. Slow reving direct drive four strokes are the
worst of all. The easiest way to imagine the shock loading is to count the
firing per revolution of the propller. The fewest revolutions must be using
a bigger load to drive the propeller until the next stroke. Resonance
effects can dramatically increase stresses maybe particularly on composite
propellers.
Please do not use any Arplast propellers for installations for which they
were not intended the elacticity of the propeller is generaly designed to
suit the installation for which they were intended.
Jerry
Flying Hi - Wish I was
jerry@flyinghi.demon.co.uk
----------
From: Duncan MacFadyean <Halcrow_MRO@compuserve.com>
Subject: BMW aircraft conversion designer -Reply
Date: 30 June 1998 13:47
Message text written by MARK CLARK
>I believe they are havingTORSIONAL RESONANCE problems.<
Correct that to HAVE HAD.
> the
propeller is a form of spring,<
Try getting the manufacturer of a popular experimental propeller to admit
to this!
>This is due to the torque reversals that occur at resonance point. Any
engine that produces 1 power stroke per revolution is bad news when
it comes to torsional vibration<
Actually the propeller sees little difference btween a geared two cylinder
engine and a direct drive four cylinder of the same power; six or more
cylinders is another story.
>It would take a very complex analysis to determine the
magnitude of this problem.<
An engine propeller combination is actually quite easy to analyse with the
modern availability of PC`s and appropriate software. The difficult bit is
trying to get the propeller maker to nandover data he doesn`t have! Added
to which.........
>This is best analysed empirically on an engine
test stand;<
Which is partly where 102 running hours of testing and development work
went.
See also www.sierra.net/skyranch/article2.htm and
www.prime-mover.org/engines/torsional/sport-av92.html ; the latter confirms
the solution we have taken as being the most appropriate. Compare that with
the alternative engines on the market, some of which have NO torsional
damping device.
For more info., pick up a brochure at Cranfield ( Sportair stand 82 & 83)
Duncan McFadyean
----------
|