Turbo prop question

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

"Frank Olson" wrote in message
news:0ShQj.90501$rd2.26964@pd7urf3no...
Bertie the Bunyip wrote:
Chris W wrote in news:74gQj.67968$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?


I didn't catch the name of the plane. but as I remember, there were 4
seats in the passenger area, and it was pretty roomy. The only other
distinctive thing I can remember is it had a fairly large cargo door
with a power assist to close it because it was so far over head when
it
was open.




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.

Envious people - generally stupid.

wrote in news:e508d435-db20-4b21-9c9b-
:

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

Matt W. Barrow wrote:
"Frank Olson" wrote in message
news:0ShQj.90501$rd2.26964@pd7urf3no...
Bertie the Bunyip wrote:
Chris W wrote in news:74gQj.67968$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?


I didn't catch the name of the plane. but as I remember, there were 4
seats in the passenger area, and it was pretty roomy. The only other
distinctive thing I can remember is it had a fairly large cargo door
with a power assist to close it because it was so far over head when
it
was open.



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.

Envious people - generally stupid.




Touché! :-)

"Bertie the Bunyip" wrote in message
.. .
wrote in news:e508d435-db20-4b21-9c9b-
:

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.

TonyP.

"muff528" wrote in
news:krtRj.968$lc6.775@trnddc04:


"Bertie the Bunyip" wrote in message
.. .
wrote in news:e508d435-db20-4b21-9c9b-
:

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

"Bertie the Bunyip" wrote in message
...
"muff528" wrote in
news:krtRj.968$lc6.775@trnddc04:


"Bertie the Bunyip" wrote in message
.. .
wrote in news:e508d435-db20-4b21-9c9b-
:

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.

TP

On Apr 28, 7:24 pm, "muff528" wrote:
"Bertie the Bunyip" wrote in .130...



"muff528" wrote in
news:krtRj.968$lc6.775@trnddc04:

"Bertie the Bunyip" wrote in message
...
wrote in news:e508d435-db20-4b21-9c9b-
:

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.

TP

Propeller speeds are controlled by the governor, which controls
propeller blade pitch. Adjusting pitch up or down loads or unloads the
prop, which will change its RPM. The governor is controlled by the
propeller RPM control on the panel. Many twins have either a
synchronizer that will adjust one governor a bit to keep the RPM the
same as the other propeller, or a synchrophaser that does the same
thing plus adjusts the rpm the tiniest bit to minimize the cabin
noise. The pilot can adjust the synchrophaser to get the prop blades
passing the fuselage at the same time or alternately or whatever makes
the least noise. The synchro will then hold everything right there.
If an airplane doesn't have either of those and the RPM is
obviously off, the pilot is either ignoring it or doesn't notice. He
can adjust one prop lever to stop it.
An excerpt from Hartzell's material at
http://www.hartzellprop.com/engineer...tant_speed.htm

"On twin-engine aircraft, whether they are reciprocating or
turbine-powered, the propeller governor or synchronizer may be
supplemented with an electronic synchrophaser. One purpose of the
synchrophaser is to match the two propellers to the same RPM. However,
the more difficult and relevant task of the synchrophaser is to match
not only the RPM, but also the phase position between the two
propellers. In other words, when a blade on one propeller passes the
fuselage, a blade on the other propeller is operating at the same
speed and is at a specific and predefined relative position. In doing
so, airframe vibration and cabin noise can be substantially reduced. "

Dan

"muff528" wrote in
news:SMuRj.25645$TS5.1624@trnddc08:


"Bertie the Bunyip" wrote in message
...
"muff528" wrote in
news:krtRj.968$lc6.775@trnddc04:


"Bertie the Bunyip" wrote in message
.. .
wrote in news:e508d435-db20-4b21-9c9b-
:

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

wrote in message
...
On Apr 28, 7:24 pm, "muff528" wrote:
"Bertie the Bunyip" wrote in
.130...



"muff528" wrote in
news:krtRj.968$lc6.775@trnddc04:

"Bertie the Bunyip" wrote in message
...
wrote in news:e508d435-db20-4b21-9c9b-
:

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.

TP

Propeller speeds are controlled by the governor, which controls
propeller blade pitch. Adjusting pitch up or down loads or unloads the
prop, which will change its RPM. The governor is controlled by the
propeller RPM control on the panel. Many twins have either a
synchronizer that will adjust one governor a bit to keep the RPM the
same as the other propeller, or a synchrophaser that does the same
thing plus adjusts the rpm the tiniest bit to minimize the cabin
noise. The pilot can adjust the synchrophaser to get the prop blades
passing the fuselage at the same time or alternately or whatever makes
the least noise. The synchro will then hold everything right there.
If an airplane doesn't have either of those and the RPM is
obviously off, the pilot is either ignoring it or doesn't notice. He
can adjust one prop lever to stop it.
An excerpt from Hartzell's material at
http://www.hartzellprop.com/engineer...tant_speed.htm

"On twin-engine aircraft, whether they are reciprocating or
turbine-powered, the propeller governor or synchronizer may be
supplemented with an electronic synchrophaser. One purpose of the
synchrophaser is to match the two propellers to the same RPM. However,
the more difficult and relevant task of the synchrophaser is to match
not only the RPM, but also the phase position between the two
propellers. In other words, when a blade on one propeller passes the
fuselage, a blade on the other propeller is operating at the same
speed and is at a specific and predefined relative position. In doing
so, airframe vibration and cabin noise can be substantially reduced. "

Dan

Thanks Dan. I was referring to the few seconds after the engines are
throttled back on jump run......during that interval when the engines are
revving down. This is when the sound is most obvious from the ground. Do the
governors or synchronizers work (or are they supposed to work) during that
brief deceleration window?
This is when the differences between the PT6's and the Garretts are most
noticeable, with the Pratts sounding "mushier". Now another ? ......Is
synchronization of twin Garretts (or other directly coupled engine/prop)
done primarily by propeller pitch adjustments without regard, within reason,
to engine RPM?

TonyP.