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av8rps(at)tznet.com
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 Posted: Wed May 30, 2007 8:15 am Post subject: Prop Windmilling drag question |
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Here's a picture with notes from way back in about 1987 when I was flying
the prototype Avid/kitfox and practicing dead stick (which Dean Wilson had
suggested due to the unreliable nature of the Cuyuna two stroke engine).
All in all I logged 3.8 hours of dead stick time in that airplane, mostly
because it was so much fun to do. And some of that time included when it
was on floats. Fwiw - I always landed dead stick. Not once did I do an
airstart. Landing was the most important part to know in my opinion, and
also the most fun. Plus, the 1st time I tried to airstart (of course it was
on the 1st flight I shut it down), the engine wouldn't start by tugging on
the recoil, and I couldn't get enough airspeed to get the prop to turn fast
enough to help my now tired arm. Why wouldn't it start? Well, I never
thought about how cold an engine can get in a very short time of gliding
around at 60 mph up in very cold air. I later learned I needed to choke it,
but being low enough after wasting a bunch of time tugging on that recoil, I
decided it was better to fly the airplane making certain I was totally
prepared to make a good landing at the airport than it was to waste any more
time (altitude!) trying to get it going. So lesson number one I would say
when you shut off an engine in flight....be mentally prepared to land, and
have a landing site directly below you. There is no guarantee your engine
will restart!
There is no question the airplane will glide further with the engine off as
long as the prop doesn't windmill. Even with an idling engine (still
producing at least some thrust), the windmilling prop creates more drag than
if the prop is stopped. The comments made comparing our props to a
helicopter are hardly applicable. When a helicopter blade stops, that would
the equivalent to cutting off all of your wing. Of course that would create
less drag, but also less lift! So I don't think we should enter helicopter
theory in this conversation....
As far as flying an airplane with the engine totally off, I personally think
it is a really good thing to know should it ever happen unexpectedly (Has
happened 5 different times in my flying career - fwiw, two strokes as well
as a Lycoming). However, I also agree it can be risky business unless you
are very proficient with your airplane. Our Avids and Kitfoxes are such
easy to fly airplanes that teaching it is pretty easy. I had a friend of
mine doing dead stick landings in his Avid on floats in less than 5 hours of
ever having flown an airplane. Two years later when his engine failed on
takeoff (when on wheels) I think knowing how the airplane handled without
power may have saved his life possibly. His brain didn't turn to mush when
everything went quiet and smooth, which I personally believe is common when
the reality of engine failure happens to even the best of pilots. Seconds,
even fractions of a second can really count then. Knowing how an airplane
handles, sounds, and feels with the engine off is very advantageous in that
situation.
However, I will admit I have never shut off the engine in my Lake
amphibian - just too scary. Power off and dirty, that airplane has
something like a 4000 fpm sink rate along with a 64 degree best glide angle.
Somewhat equivalent to landing the space shuttle (when they say it glides
like a manhole cover it is an understatement). But my Kitfox is nothing
more than good fun in the same situation!
Just my two cents worth based on my experiences...
Paul Seehafer
Model IV - 1200
912ul on Amphibs
IVO prop
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Fox5flyer
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| Posted: Wed May 30, 2007 8:54 am Post subject: Prop Windmilling drag question |
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Everything Paul says here I agree with. However, I think what is being
forgotten in the thread is I believe that the original poster was talking
about how the airplane would act with an engine using a freewheeling clutch,
like those now supplied aftermarket on the 912x and stock on the NSI
Subarus. The NSI uses a large Sprag clutch (to dampen tortional vibrations)
that allows the prop turn one way with engine power, yet will spin backward
with no effort and I suspect the one for the 912 is similar. Shut off the
engine and the prop will spin at whatever speed the airplane is flying,
depending on the pitch setting of the prop with piston compression not being
a factor. What this will do is cause the prop to act as a large (6 foot)
disk in front of the airplane creating incredible drag, much more than a
typical prop that is wheeling slowly against compression. There's no way to
stop it from spinning, regardless of speed. I've never tried a dead stick
with mine, but when at the right place and time, I intend to. Going to full
course pitch should help some, but I'll find out. I'm sure that with an
engine failure, the spot picked to land had better be very close.
Fortunately I have the old Wurtsmith AFB 12,500 foot runway a short distance
away to try it. I'll definitely start at altitude though, just in case.
Has anyone else here using a clutch ever tried it?
Deke Morisse
N148DM
S5/Soob/CAP
NE Michigan
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Wed May 30, 2007 5:34 pm Post subject: Prop Windmilling drag question |
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Paul:
Couldn't help but to notice you didn't say anything about the ballooning
effect of quickly cutting the power on the Lake.... Especially during climb
out!
Very happy to say I personally haven't experienced it...and don't want to!
Noel
[quote] --
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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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