"muff528" wrote in
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"Bertie the Bunyip" wrote in message
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"muff528" wrote in
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"Bertie the Bunyip" wrote in message
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"muff528" wrote in
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"Bertie the Bunyip" wrote in message
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wrote in
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:
On Apr 25, 4:37 am, Bertie the Bunyip wrote:
wrote in news:de5298d4-778b-475f-9321-
:
On 25 Apr, 11:53, Frank Olson
wrote:
Bertie the Bunyip wrote:
Chris W wrote in
news:74gQj.67968$y05.19485 @newsfe22.lga:
I was channel surfing the other day and caught the tail
end of
some
show
about rich people and their planes. Anyway the pilot of
this
one
single
engine turbo prop said there was no mechanical connection
between
the
prop and engine. How can that be?
It's a free turbine. Most modern turboprops use that
system.
There's a
seperate turbine that runs the prop geabox. He's
technically
incorrect
since that turbine is part of the engine...
Bertie
Rich people. More money than brains.- Hide quoted text -
I'm not in disagreement with Bertie, wouldn't dare, but it
"could be" electric drive, hydraulic drive.
Nah, not on a turbine.
These are used in other vehicles. Most cars in the US
use an automatic transmission that includes a
torque convertor for example.
I am sure that it is in this case a free turbine - gas drive
- that connects to a propellor - gas-gas drive.
http://www.gasgas.com/index.shtml
Yeah, almost all modern turboprops use this system. Some of the
older ones, like the RR Dart or the Allisons were gear driven
off the main turbine, but this is all but abandoned these days.
In fact, even
"jets"
are really just ducted fan turboprops with the fan driven by a
turbine
in the exhaust.
Bertie
The Garrett TPE 331 is still a fixed turboprop. Lots of
those
flying, especially on some upscale kitplanes. Isn't the
Swearingen Merlin powered by those, too?
http://www.aircraftenginedesign.com/pictures/TPE331.gif
Dan
Yeah, Garret are one of the holdouts. They're on alot of
airplanes. the Metro, the turbine commanders. I think there's a
Walter that's in th esame class being used on quite a few
airplanes as well. The Pratts are all free turbines, AFAIK. The
older ones are centrifugal engines with a revese flow, that is,
the intake is on the back and they exhaust in front, which is why
the PT6 's have those stovepipes just behind the prop, and the
newer 120 and newers are axial with the air coming in the front
and the fire going out the back. The 120 has a prop brake with
which you can stop the prop while the engine is running ( on the
ground) and use the engine for an APU.
Bertie
Amazing how after a while you can tell the difference between the
Pratts and the Garretts from the ground while the plane is at
13,500 ft above you. Many times we had a Twin Otter with Pratts
and a Casa 212 with Garretts flying on the same day and you could
tell which was on jump run just by the sound. I just assumed it
was because of the different methods of coupling the props to the
powerplant.
!!! I don't think I could except by seeing the airplane. Yeah, you
get tuned in to any engine you're familiar with, though, don't you?
Bertie
Yeah, I think the fact that they were flying within a few minutes of
each other all day makes the comparison easier. The main difference
seems to me to be the "loudness" of the beat freq. generated by
slightly "out-of-sync" props, especially when the "cut" occurs just
before exit time. The Garretts sounded "gutsier" for lack of a
better term and the beat frequency amplitude was much greater than
the PT6's. Even during the climbout to jump altitude the Garretts
seemed to sound more authoritative. But that brings up another
question that I have wondered about. For a twin, for example, does
the force of the atmosphere pushing against the props have an
"equalizing" effect on the propeller speeds of turbine-coupled props
on slightly out-of-sync engines? I would intuitively think that such
a moderating effect would account for differences in beat sounds
between them and direct-coupled twins which were a few rpm's out.
Well, I've only flwon one type with the turbine directly geared to
the prop and it was a single lever operation which meant you were
altering the power as well as the prop pitch whne you mover the
lever. it had no autosynch so you did it the old fashioned way and it
was out of synch a good bit of the time. I can't remember hat we had
in the King Airs I flew, but the PW120 had an autosynch so that might
account for it.
Bertie
Thanks, I never thought of propellers as such complex devices. Dan's
link to Hartzell was pretty informative.
On the fixed shaft turboprops they're extremely complex. All sorts of
gadgets on them to keep them under control. on the old Allisons the most
common reason for shutdown by far was a prop problem of one type or
another. but giving your observation about the Garret vs PW it's
probably due to the Garret fineing off as the throttles close as opposed
to the Pratts where they do, but not so much...
Bertie