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Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 1:11 am
by TripleDelta
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?

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 1:31 am
by Klaus
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...

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 2:02 am
by TripleDelta
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?

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 2:09 am
by dl757md
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

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 3:02 am
by FredT
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

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 4:40 am
by timz
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?

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 4:49 am
by dl757md
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

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 5:36 am
by FredT
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

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 9:47 am
by Starlionblue
Here's a nice drawing that illustrates the whole thing:

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

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 10:33 am
by SlamClick
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.

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 3:48 pm
by TripleDelta
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?

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Sun Aug 15, 2004 4:08 pm
by L-188
Ever blow on a pinwheel?

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

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Mon Aug 16, 2004 12:54 am
by PaveLowDriver
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!

RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Mon Aug 16, 2004 2:22 am
by SlamClick
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.



RE: Free-turbine Turboprop - What Makes It Turn?

Posted: Mon Aug 16, 2004 3:47 am
by FredT
A figure I remember is that for the C130, about 7% of the thrust comes from the exhaus gases.

Regards,
Fred