Question for all you turbo-prop guys.

captainphil

captainphil

Well-Known Member
What is the difference between a direct drive turboprop and a free-turbine turbo prop? Is it just the way the air is entered into the engine or does it have to do with the fact that one automatically feathers with the fly-weights when the engine is shut down while the direct drive keeps a flat pitch upon shut down? They all look the same to me :confused:
 
The question is kinda-sorta OK.

You have Direct Drive and Free Power turbines, or regular flow (I forget the technical term) and reverse flow engines.

Direct Drive means the gas generator is hooked mechanically to the gearbox. You can usually tell this as these engines are parked in a flat pitch (Garrett, Allison 5000 and Rolls-Royce Darts being examples)

Free Power Turbines have no mechanical connection between the gas generator and the gearbox, using a turbine wheel(s) to transmit power from the gas generator to the gearbox. These engines typically shutdown in feather. (PT6s, GEs on the SAAB and Pratts on the Dash)

The Direct Drive engines typically have mechanical locks holding the props in flat pitch when they're shut down. If the blades aren't flat, then there is too much drag created for the starter to start the engine.

Free-power turbines don't have that issue, as the prop doesn't have to turn to start the engine.

Regular flow means air goes in the front of the engine and flows to the back, and it's aligned with the airplane. Basically, the air goes straight through the engine (Garrett)

Reverse-flow means the air enters the engine after a 180 degree turn around, flowing from the tail of the airplane to the nose. (PT6)

Does that work?
 
Polar742 said:
The question is kinda-sorta OK.

You have Direct Drive and Free Power turbines, or regular flow (I forget the technical term) and reverse flow engines.

Direct Drive means the gas generator is hooked mechanically to the gearbox. You can usually tell this as these engines are parked in a flat pitch (Garrett, Allison 5000 and Rolls-Royce Darts being examples)

Free Power Turbines have no mechanical connection between the gas generator and the gearbox, using a turbine wheel(s) to transmit power from the gas generator to the gearbox. These engines typically shutdown in feather. (PT6s, GEs on the SAAB and Pratts on the Dash)

The Direct Drive engines typically have mechanical locks holding the props in flat pitch when they're shut down. If the blades aren't flat, then there is too much drag created for the starter to start the engine.

Free-power turbines don't have that issue, as the prop doesn't have to turn to start the engine.

Regular flow means air goes in the front of the engine and flows to the back, and it's aligned with the airplane. Basically, the air goes straight through the engine (Garrett)

Reverse-flow means the air enters the engine after a 180 degree turn around, flowing from the tail of the airplane to the nose. (PT6)

Does that work?
Click to expand...

Just one addition- "free power" is typically referred to as "free turbine." Instead of a direct drive, "fluidic coupling" is used.... basically meaning the air is used as the coupling between the 2 turbine spools. It's not all that different from a turbofan engine, except in the case of a turboprop you have all the crazy gear boxes and fly weights and such for the prop.

FYI- when the oil pressure is dumped from a prop without the locking mechanism (be it a turboprop or regular piston prop), the prop will always feather. The free turbine props don't require that locking mechanism simply due to the design.
 
All you need to know is that Direct Drive is Better because A) It's much more complex...extra mechanical bits whirring around at insane speeds and B) It's inevitably much louder due to the fact that the engine must run at a significantly higher N1 on the ground. Complexity+Noise=Win
 
Boris Badenov said:
All you need to know is that Direct Drive is Better because A) It's much more complex...extra mechanical bits whirring around at insane speeds and B) It's inevitably much louder due to the fact that the engine must run at a significantly higher N1 on the ground. Complexity+Noise=Win
Click to expand...


Games, Set, Match... I've flown even louder turboprops than your old ride!
 
Boris Badenov said:
All you need to know is that Direct Drive is Better because A) It's much more complex...extra mechanical bits whirring around at insane speeds and B) It's inevitably much louder due to the fact that the engine must run at a significantly higher N1 on the ground. Complexity+Noise=Win
Click to expand...

Nice! :laff:

Phil
When you get a chance find a copy of the "Turbine Pilots Flight Manual." Great book for making the piston to turboprop/jet transition. (Or any complex system for that matter.) Somewhere around $40 new, and I'm sure you can find used on Amazon. It goes into all that stuff in detail, but in terms that are easy to understand.
 
Seggy said:
Turbo Prop Guys all have bigger balls than Polar742....
Click to expand...
Yeah but Polar has the hat.....
DSCN1346.jpg
 
Lee D said:
Nice! :laff:

Phil
When you get a chance find a copy of the "Turbine Pilots Flight Manual." Great book for making the piston to turboprop/jet transition. (Or any complex system for that matter.) Somewhere around $40 new, and I'm sure you can find used on Amazon. It goes into all that stuff in detail, but in terms that are easy to understand.
Click to expand...

Just bought it from school store :)
 
Boris Badenov said:
All you need to know is that Direct Drive is Better because A) It's much more complex...extra mechanical bits whirring around at insane speeds and B) It's inevitably much louder due to the fact that the engine must run at a significantly higher N1 on the ground. Complexity+Noise=Win
Click to expand...

That's why I loved the Chinook helicopter. Two Allison turbines pushing 3 drive shafts through 5 separate transmissions to spin over 2 thousand pounds of rotor blades. Complex and very loud. But not a whiny Garrett loud - a deep manly loud. :)
 
Ian J said:
That's why I loved the Chinook helicopter. Two Allison turbines pushing 3 drive shafts through 5 separate transmissions to spin over 2 thousand pounds of rotor blades. Complex and very loud. But not a whiny Garrett loud - a deep manly loud. :)
Click to expand...

All true, but I think there's a requirement somewhere in the Man Book that you have to be able to go faster than a Caravan...
 
Seggy said:
Turbo Prop Guys all have bigger balls than Polar742....
Click to expand...

Hmmm....So my time flying Rolls-Royce Darts doesn't count?

I guarantee my time behind those (and PT6s.....and Garret TPE-331s) will check your little bit of PT6 time any day.

That motor would either start or catch on fire, since I manually controlled the start process.

None of this "move the condition lever" and abort for hot/hung/no light pansy stuff of the PT6 days.

None of this "Select, Start, Guard" kiddie campus autostart on a Garrett.

Push the button. Condition Lever to Fuel on. when it hangs, add some gas, but not too much or you'll smoke it.

Set "Trim of the Day" for a max power takeoff, for every take off.

And a scream that would make a Garrett retreat.....

Silly wookie.
 
captainphil said:
What is the difference between a direct drive turboprop and a free-turbine turbo prop? Is it just the way the air is entered into the engine or does it have to do with the fact that one automatically feathers with the fly-weights when the engine is shut down while the direct drive keeps a flat pitch upon shut down? They all look the same to me :confused:
Click to expand...

Boris Badenov said:
All true, but I think there's a requirement somewhere in the Man Book that you have to be able to go faster than a Caravan...
Click to expand...

Yes, but the Hook can sling 3 loaded Caravans and still fly. The reverse cannot be said.

Don't dis Big Windy.
 
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