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Free-turbine Turboprop - What Makes It Turn?  
User currently offlineTripleDelta From Croatia, joined Jul 2004, 1124 posts, RR: 7
Posted (10 years 4 months 2 weeks 4 days 1 hour ago) and read 17791 times:
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In a free-turbine turboprop, the compressor turbine and the rear stage turbine shafts are not linked, right? If so, what makes the compressor turbines turn once the engine is started?


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14 replies: All unread, jump to last
 
User currently offlineKlaus From Germany, joined Jul 2001, 21525 posts, RR: 53
Reply 1, posted (10 years 4 months 2 weeks 4 days 1 hour ago) and read 17766 times:

Isn´t it always the "highest pressure" compressor/turbine shaft that´s driven by the starter in a multi-shaft configuration? The other shaft(s) would then "come around" when the high-pressure core has lit up and is producing enough thrust to drive the other turbines...

User currently offlineTripleDelta From Croatia, joined Jul 2004, 1124 posts, RR: 7
Reply 2, posted (10 years 4 months 2 weeks 4 days ago) and read 17737 times:
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That's what I thought as well, if I understood you right. The starter operates the compressor turbines, while the rear stage (the free turbine) ones are operated by gases produced in the combustion chambers. But does that mean that the starter (or an equivalent motor) turns the compressor turbines throughout the whole flight?


No plane, no gain.
User currently offlineDl757md From United States of America, joined May 2004, 1562 posts, RR: 16
Reply 3, posted (10 years 4 months 2 weeks 4 days ago) and read 17731 times:

Unlike multi shaft turbine engines which have concentric shafts, freeturbine turbo props have their shafts in a tandem arrangement. The power section contains a compressor section and turbine section which are on a common shaft. The load section, which has a turbine that drives the propeller, is on a separate unconnected shaft that is in tandem with and downstream of the power section. The advantage of the setup is that in the case of a prop strike the power section of the engine is protected from physical damage.

Dl757md



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User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (10 years 4 months 2 weeks 3 days 23 hours ago) and read 17718 times:

In a free-turbine turboshaft engine, you have a multi-turbine arrangement just as in a multi-spool turbojet/fan engine. First a turbine which drives the compressor, then a power turbine which drives the shaft. As the power turbine is not linked to a compressor, that engine is a free-turbine engine.

The advantages are in efficiency. You can have a turbine which is ideally suited for the purpose of driving the compressor, and another turbine which is ideally suited for driving the power shaft.

Further, you don't have to turn the power shaft with the starter when starting which is a significant advantage. The starter has enough to do turning the gas generator without having to turn the prop as well. Hence, a free-turbine turboprop aircraft will typically have it's propellers in feather when on the ground while a single-turbine turboprop will have locks preventing the propellers from feathering on shutdown, as in reciprocal engine aircraft.

Regards,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineTimz From United States of America, joined Sep 1999, 6903 posts, RR: 7
Reply 5, posted (10 years 4 months 2 weeks 3 days 21 hours ago) and read 17691 times:

Some free turbines use concentric shafts, don't they? If the gases flow front to back, and the prop's at the front, wouldn't it have to?

User currently offlineDl757md From United States of America, joined May 2004, 1562 posts, RR: 16
Reply 6, posted (10 years 4 months 2 weeks 3 days 21 hours ago) and read 17688 times:

Some free turbines use concentric shafts, don't they? If the gases flow front to back, and the prop's at the front, wouldn't it have to?

Actually the intake is aft and the flow of air through the engine is back to front with exhaust exiting the front. That is why you see the large curved, usually polished exhaust pipes near the front of the engine. The compressor is actually aft, the 1st turbine is in the middle, and the power turbine is at the front of the engine.

Dl757md



757 Most beautiful airliner in the sky!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 7, posted (10 years 4 months 2 weeks 3 days 20 hours ago) and read 17671 times:

In most free-turbine engines the air comes in through the front and exits at the rear, as per usual. And yes, that means concentric shafts... unless you want to take out the power at the rear of the engine, of course.

The PT-6 is of course immensely popular in many applications and has reverse-flow, with the power turbine up front. I've also seen APU:s with that configuration. Still, I'd say that reverse flow is the exception. Turning the air around isn't an issue and keeping exhaust and intake air separated is a lot easier if the air only goes front to back...

Regards,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently onlineStarlionblue From Greenland, joined Feb 2004, 17191 posts, RR: 66
Reply 8, posted (10 years 4 months 2 weeks 3 days 16 hours ago) and read 17623 times:

Here's a nice drawing that illustrates the whole thing:

http://travel.howstuffworks.com/turbine6.htm



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User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 9, posted (10 years 4 months 2 weeks 3 days 16 hours ago) and read 17612 times:

If I am reading the thread-starter question correctly, the replies have sort of danced around the answer.

"what makes the compressor turbines turn once the engine is started?"

Answer: Airflow, or exhaust gas flow, just like any other turbine engine arrangement. The expanding exhaust gases flowing downstream through the engine are turned by stators to impinge at the optimum angle on the turbine blades. The turbine blades are little airfoils and they start turning in response to this flow. Basically, they windmill. In chord section they also resemble the sails of a fore-and-aft rigged sailboat as viewed from above.

The electric starter on a PT-6 (for example) just spins the N2 shaft up to a speed where ignition can begin and the process can spin itself up to speed. After starter cutout, it is all gas flow that does it.

The design does have some advantages for turboprop/turboshaft applications.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineTripleDelta From Croatia, joined Jul 2004, 1124 posts, RR: 7
Reply 10, posted (10 years 4 months 2 weeks 3 days 10 hours ago) and read 17579 times:
AIRLINERS.NET CREW
PHOTO SCREENER

The electric starter on a PT-6 (for example) just spins the N2 shaft up to a speed where ignition can begin and the process can spin itself up to speed. After starter cutout, it is all gas flow that does it.

Hmm...well that makes sense once you think about it  Smile/happy/getting dizzy. It did seem strange (after a while) that the starter would be able to operate the compressor at the speed it usually does operate...the starter would then probably be bigger than the engine itself...
But if the gases produced in the combustion chambers are that powerful, couldn't they be used to give a percentage of thrust as well (if ducted properly and if they don't loose much of their energy spinning all those turbines)? Is that how those "propjet" engines (like on the C425) work?



No plane, no gain.
User currently offlineL-188 From United States of America, joined Jul 1999, 29840 posts, RR: 58
Reply 11, posted (10 years 4 months 2 weeks 3 days 10 hours ago) and read 17578 times:

Ever blow on a pinwheel?

Same idea, except in this case the "breath" comes from the hot section.



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User currently offlinePaveLowDriver From , joined Dec 1969, posts, RR:
Reply 12, posted (10 years 4 months 2 weeks 3 days 1 hour ago) and read 17532 times:

TripleDelta,

In a free turbine configuration, although the compressor (N1 or Ng) turbine is not directly linked to the power turbine (N2 or Nf) the turbine rotors are still directly adjacent to each other in their respective engine chamber.

After the fuel air mixture is ignited in the combustion section, the expanding gas is routed over first over the N1 (compressor) turbines which share a common shaft with the engine's compressor section. After passing over the N1 turbines, the exhaust gas then passes over the N2 (power) turbine which then is linked to some sort of gearbox (generally being backshafted through the middle of the existing N1 shaft) before being used to drive a propeller/rotor/transmission, etc.

Klaus is correct....the engine starter drives the compressor section until its RPM is high enough to keep the engine self sustaining. With a free-turbine set-up, it's also possible to start the engine without having the N2 turbines turn. This is what allows a number of helicopters to do a "locked start" utilizing the rotor brake. In this case, the N1 is section is up and running (albeit at a low RPM), while the Nf turbine remains stationary.

The short answer to the thread starter question is that in the "suck-squeeze-bang-blow" model of jet engine operation, both the N1 and N2 turbines are in the "blow" section. So even though the starter has dropped out, once sufficient combustion is occurring the N1 turbines are still turning.

My experience has been mostly the PT6, T53, and T64....so if I'm off-base, I'm all ears.

Hope that helps.

Cheers,

-Charlie
PAVE LOW LEADS!


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 13, posted (10 years 4 months 2 weeks 3 days ago) and read 17516 times:

You are right, TriDelt the exhaust gases do produce thrust. In some installations it is an amount that is part of the aircraft performance. What do these three aircraft have in common?

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Answer is, more or less the same engine.

BTW a lot of smaller turbine aircraft use a "starter-generator" which is a single appliance that functions as a starter motor during that phase, then switches to become the primary electrical generator during the rest of the operation.





Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 14, posted (10 years 4 months 2 weeks 2 days 22 hours ago) and read 17490 times:

A figure I remember is that for the C130, about 7% of the thrust comes from the exhaus gases.

Regards,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
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