Posted for Gary Casey <glcasey@adelphia.net>:

 Rick gave a most excellent description of prop controls that answered  some
of the questions I have had forever (why is that twin sitting  there without
the props feathered?).  But your question was about  adding an accumulator to
hold a conventional non-feathering prop in  the high pitch position after an
engine failure.  As was said there  are always tradeoffs.  First, the oil is
fed to the prop through the  front main bearing, using the bearing itself as a
(leaky) seal.  the  accumulator would have to be very large, I would think, to
be able to  hold oil pressure on the prop for any length of time and you need
it  to work for maybe 15 minutes.  How do the accumulators on a  feathering
prop work then?  They are only there to unfeather the prop  and to do that
only have to supply a single dollup of oil, after  which the pins in the prop
stop it from going back to the feathered  position.  I guess the lesson for
twin drivers is that you only get  one chance to unfeather the prop - after
you do that and can't get  the engine to restart you can't change your mind
and feather it  again.  When would you want to go to high pitch when there is
no  engine oil pressure?   I think rarely, as most engine failures result  in
an engine that still spins and maintains oil pressure.  The engine  would have
to stop turning, but then I'm not sure there is a big drag  difference between
high and low pitch positions - both are stalled  and represent flat plate
drag.  If the engine loses oil pressure, but  keeps on turning it won't be
doing that for very long.  I thought  about all the tradeoffs and elected to
keep it simple.  Stayed with a  conventional prop.
 
 Gary Casey
 On Oct 27, 2006, at 3:01 AM, Lancair Mailing List wrote: