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There are at least other ways to force the prop to
coarse pitch in the event of oil pressure loss: counterweights and a pressure
accumulator. And the accumulator is obviously time-limited; once the
pressure is used up the prop would return to fine pitch. Hartzell used an
accumulator with their early 'Claw' model props, but changed to counterweights
after some overspeed incidents.
That said, your basic point is correct: any additional
mechanism to put the blades into coarse pitch results in extra
weight.
-- DJ
I spent some time with Harry Griswold of
AeroComposites at Lakeland this year discussing prop pitch with engine failure.
It turns out that the answer may not have to do with philosophy of handling
emergencies but with mechanical complexity (cost) and weight.
If you spin a prop without a governor, the
mechanical forces will drive it to flat pitch. That means that there must be a
force applied to drive the prop back to coarse pitch. This is typically done by
using oil pressure to drive pistons that move the prop. Of course, when the
engine stops, the oil pressure goes away, and the prop spins to flat pitch if it
is windmilling.
The only way to reverse this behavior is to have a
massive spring in the hub to force the prop to coarse pitch at engine out. Then
the pistons use engine oil to drive the prop to fine pitch. This spring
increases complexity (cost) and weight.
Ted Noel
L-IV-P Eagle
at airport, starting ground checks
N540TF
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