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Posted for "rtitsworth" <rtitsworth@mindspring.com>:
Kevin,
Nice write-up from McCauley (below, along with my original off-list reply),
although the McCauley blurb is also a bit confusing in that:
1. McCauley calls a normal (non-feathering) prop a "constant speed prop"
(which is the commonly used term), but a feathering prop/governor is
technically also a constant speed device.
2. McCauley inter-mixes the feathering and non-feathering pictures (which
keeps the reader guessing until you figure out that feathering is shown with
blue oil and constant speed with red oil. Then, on the first governor
picture they switch to yellow oil (go figure).
I now understand that you're looking for: A normal (non-feathering) prop
with an accumulator that is rigged to engage (open) at the low RPM prop
control setting (high oil pressure) such that it would force the prop to a
high pitch setting (low rpm) even if the oil pump/governor was not working
(supplying oil pressure), such as in an engine-out (or oil loss) scenario.
I'm not a prop expert, but I haven't heard of this arrangement/setup.
However, given all the various configurations that have been attempted
throughout aviation history, I would be surprised if someone hadn't tried it
(or something like it) sometime/somewhere before. I'm assuming this would
be a normal (non-feathering) prop with a custom governor/accumulator
arrangement (i.e. nothing special with the prop, just the
governor/accumulator). Would be interested to see/hear what others know.
One concern might be how much oil is "leaked" back out of the prop/governor
oil channel in the crank (and into the oil sump) at the front crank bearing.
I believe a small amount of leakage there is typical and normally not an
issue as the governor just supplies more oil/pressure and the "leakage" is
all internal to the engine (back to the sump). In an engine out (or governor
failure) scenario, this "leakage" would cause the prop/accumulator to loose
pressure (overtime) and thus fall back to the low pitch (high drag) setting
anyway (eventually). This is not an issue with the accumulator setup on a
feathering prop, since the accumulator is only used to get the prop out of
feather for restarting, which is a short term event.
Note: That depending on how the accumulator system was implemented, it would
still not be fully fail safe (seems nothing is), since an oil loss failure
in the governor and/or the prop hub would simply exhaust/leak the high
pressure oil from the accumulator without applying the pressure to the prop
and thus the prop would return to the low pitch (high rpm, high drag)
condition. This isn't all bad in that it is no worse than not having the
accumulator in the first place and a governor failure on take off (the most
critical failure in a feathering prop) would result in a low pitch setting
(desirable) like it normally does with a non-feathering prop.
I would be curious as to whether it would be possible to "make" a
counterweighted Hartzel by starting with a feathering prop, setting the
stops to prevent full feather and removing the unnecessary accumulator and
ground lock-out pins. This of course might be considered a non-certified
engine/prop combination and thus require 40 vs 25 hours phase 1 testing.
I've also heard of electric governor/prop pitch setups (rather than oil),
but know little (nothing) about them. Seems if you have enough battery
juice, an electric system could position/maintain the prop in either setting
regardless of the engine/oil system health. However, the question then
becomes "if I have an electrical system failure - which way do I want the
prop to fall/move to". Seems the answer would be fine pitch (flat) to cover
a sudden electrical/governor failure at takeoff. An in-flight
electrical/governor failure (with the engine still working) is not much of
an emergency per say even if the prop drops to fine pitch automatically
(just pull some throttle out to prevent an overspeed). Perhaps the
electrical systems work more like a servo and don't move freely in either
direction - which would seem fine too (in most scenarios).
Rick
ES/TSIO550 - building (with MT counterweighted prop)
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