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Robert,
Unless you have some kind of catastrophic failure which causes the engine
to seize, the prop will not stop rotating even at speeds far below best glide
speed. I have attempted, and failed, to stop the prop in flight with the mixture
in cut off. There should never be a need to “crank” the engine in flight to get
a re-start in a IV, nor would there be a lack of oil pressure (unless the
failure was oil related in which case you’re probably dealing with a seizure
anyway).
Your comment about the testing being done with the engine running is, of
course, correct. There is a difference between gliding with a simulated failure
(engine at idle) and a truly dead engine...but that difference is fairly small.
I have tested this in a 320 as well a IV and found only 1-200 fpm difference.
Even with the engine actually failed the IV is an excellent glider when in clean
configuration and the prop control full aft. The MT prop will govern to a lower
RPM than a Hartzell so the MT equipped craft will do a bit better in the glide,
all else being equal. Even with the Hartzell prop a IV is capable of producing a
glide ratio that is close to double what you would have in a Cessna 182, for
example. A full feathering prop will increase the glide ratio slightly, but not
significantly, at the cost of additional weight, complexity and cost.
It is clearly an individual decision whether to equip your plane with a
feathering prop or not. Some of us have chosen the feathering prop, most of us
have gone with the non-feathering. I would suggest that proper training in
emergency procedures such as engine failure would increase your survival chances
in an engine out situation far, far more than a feathering prop without such
training.
Bill Harrelson
N5ZQ 320 1,950 hrs
N6ZQ IV under construction
Sent: Sunday, June 19, 2011 11:30 AM
Subject: [LML] Re: And how about a full feathering
prop?
Hi Robert
Thanks for the explanation, and sure as hell I am going to try to see the
glide ratio as it has been debated many times by NON lancair owners I
know.
Now for your explanation, I am thinking you are doing your test with a
running engine, but that is not when we are needing a full feather prop, right?
We need it when the mill dies on us, so no oil pressure so therefore no way to
feather in any kind of form, right?
(now I have been told by a Aerostar owner that they then just crank the
engine to get some low oil pressure and this way they feather, I am not sure if
this would work as I do not have this experience at all)
But I do know that when the engine quiets, we try to restart it first by
switching the fuel selector, crank until we figured out that it won't start and
then……well then it would be handy to have the full feather prop, right? (or do I
see this wrong and if so please tell me, I am here to learn :)
A glide ratio of 18:1 is fantastic, this is actually the same as the
Evolution, and if I am not mistaken, with the same glide speed :)
For
what I have gathered so far the lancair 4p has a glide ratio similar like a
piano LOL In other words if the engine quits then open the door, throw out your
keys because there is where you land LOL
Well,
this might not be true, or is it?
So how
about the full feathering prop ? Does anyone have experience with the before and
after installation? How much they cost and
data?
And
I needed to respond to this…
My
IV-P has a glide ratio (at 120 KIAS) of between 5:1 and 20:1, depending on
configuration. The difference is truly amazing/impressive/sobering… depending on
your situation and mind set. With the gear and flaps down and the RPM at max
(prop all the way in), the glide ratio approximates that of a brick…not really,
but you’re coming down so fast relative to forward progress that it’s difficult
to plan and execute a planned touchdown (at least for me). With the gear and
flaps up, AND THE PROP CONTROL ALL THE WAY OUT, my glide ratio at gross weight
is approximately 18:1, increasing to a bit better than 20:1 at 800# under gross.
At this glide ratio, the performance and “sight picture” approximates that of
some gliders. It’s easy to plan and execute an approach and landing to a
pre-planned spot…ideally a runway. I have practiced this many times from various
altitudes/distances from the airport to a full stop on the runway, using the
gear to increase my descent rate, and then the flaps and/or the prop control to
adjust the touchdown point. Using the prop control is ideal because the drag
effect/descent rate increase almost immediately reversible by pushing it back
in. Not so with the flaps. One
feature of my Chelton EFIS is a wind-adjusted glide range predictor. I have
tested this many times starting at altitudes up to 17,500’ and (set at 18:1) gives me a very accurate
and immediate sight picture of airfields within glide range. If it’s within the
green circle, I can turn toward an airfield and execute a safe landing there. My
MT 4-blade is not full feathering, but allows the engine to run at approximately
700 RPM when at idle and the RPM set to min. I believe this is pretty close to 0
thrust, as the engine idles at approximately 600 RPM on the ground.
So,
in summary, the throw out the keys and follow them down scenario can be executed
if you really want to go down, but a planned glide at 120 KIAS (very close to
best glide for my airplane) in clean configuration yields approximately the
performance of a 2-place sail plane—and you have the ability to transition
easily between these limits as needed. Try it; you’ll be impressed.
Bob
PS:
I considered and rejected a full feathering prop for what I have. Wouldn’t trade for any amount of
money…
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