ngr
Posts: 172
Joined: Fri Apr 13, 2001 6:38 am

"Free" Turboprop

Wed Dec 03, 2008 1:54 pm

Hello all,

The search was not working correctly when I posted this, so sorry if this topic has been discussed before.

I have been told that modern turboprop engines have a freely-rotating propeller...that is the turbine section is allowed to startup and reach optimal speeds before the propeller begins to rotate. Some specific examples I recall were the C-130J and the ATR-72 series, among others.

I have been under the impression that the shafts were always directly connected by some gearing ratio, but I am interested in learning about this other type of turboprop. How do the gearboxes work in the "free" turboprop, and how do they keep engine parts from grinding or stripping during the "connection" process?

Thanks!
 
cptspeaking
Posts: 567
Joined: Mon Apr 24, 2006 2:49 pm

RE: "Free" Turboprop

Wed Dec 03, 2008 2:04 pm

The Pratt & Whitney PT6 is a great example of a free spooling turboprop. It is widely considered the most reliable engine in the world also.

Basically, the turbine section's exhaust drives an impeller which spins the propeller. There is a reduction gearbox involved also between the turbine and propeller shaft. The concept of a free turbine means that in theory, you can stand next to the plane and hold the prop while the engine is starting, and (up until a certain point), it won't move until you let go.

Don't have time to get into many details now, but this is why you hear King Airs start well before the prop spins up...most Kng Airs (excluding the 100 series) have PT6 engines.

http://en.wikipedia.org/wiki/Pt6
...and don't call me Shirley!!
 
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Jetlagged
Posts: 2562
Joined: Sun Jan 23, 2005 3:00 pm

RE: "Free" Turboprop

Wed Dec 03, 2008 3:02 pm



Quoting Ngr (Thread starter):
I have been under the impression that the shafts were always directly connected by some gearing ratio, but I am interested in learning about this other type of turboprop. How do the gearboxes work in the "free" turboprop, and how do they keep engine parts from grinding or stripping during the "connection" process?

A free turbine means the propeller has it's own turbine which is driven by the exhaust energy from the gas turbine part of the engine. Reduction gearing is used to reduce this power turbine's RPM to a speed suitable for a propeller.

Other turboprops have the propeller directly connected to the gas turbine spool through reduction gearing. For turboprops with two spools it would be the LP spool it would be geared to.

There aren't any clutches or gear engagement involved. The spools on any turboprop (free or fixed turbine) are only connected aerodynamically, there is no mechanical connection. On the ATR42/72 there is a propeller brake which prevents the propeller and it's turbine from turning while the gas generator runs. This "hotel mode" allows the gas generator to be used like an APU while on ground.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
vc10
Posts: 1342
Joined: Thu Nov 15, 2001 4:13 am

RE: "Free" Turboprop

Wed Dec 03, 2008 3:22 pm

Another example of a free turbine turbo prop is the Bristol Proteus engine, which was developed in the early 1950s and produced about 4000 HP.

The danger of a free turbine was if the free turbine shaft broke then the turbine would be
off loaded and go rapidly into overspeed. However should the compressor shaft fracture then h it's turbine would be offloaded and run to overpeed, but in this case the compressor will slow down so the energy would be removed from it's turbine.

To prevent the free turbine overspeeding and coming apart an overspeed device was fitted to the engine which should sense an approach to an overspeed and would close the fuel High Pressure cock and so remove the energy

The free turbine shaft run inside the main engine's shaft and drove the prop via a reduction gear
The Proteus had a number of teething troubles, which delayed it's introduction into service, but it did become a very good engine, though too late to become a huge success

http://en.wikipedia.org/wiki/Bristol_Proteus

http://en.wikipedia.org/wiki/Bristol_Britannia

Quoting CptSpeaking (Reply 1):
The concept of a free turbine means that in theory, you can stand next to the plane and hold the prop while the engine is starting, and (up until a certain point), it won't move until you let go.

On the Britannia this was not just theory but practice, as you had to release the prop brake prior to starting engines and should you have a tail wind then the prop and it's turbine would start to rotate the wrong way which would result in a slow and hot start. To prevent this a person would hold the prop during the intial start and only release it when he felt it trying to turn in the correct direction.

I think "health and Safety " department would have anight mare at this today

Littlevc10

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