Originally Posted by
Shaun
Well, they still have the same blades on the inside, but they also have propeller blades on the outside. The propeller blades on the outside are usually driven by the air being moved by the jet engine. Sort of like an automatic transmision without fluid. Others are driven off of the shaft of the turbine through a gear reduction gear box.
That is kind of right but doesn't really explain it clearly. There are 2 types of turbo props, direct drive and free-spinning. A direct drive propellor is driven directly from the compressor shaft through a reduction gear assembly. A free-spinning is not directly connected to the engine. A free turbine power section is added to the engine after the turbine section. The exhaust gases from the engine drive the free power turbine . The power turbine is connected to the propellor gearbox by a shaft. A PT-6 is a free spinning turbine, you can spin the prop without spinning the engine, with a direct drive, when you spin the prop, you are actually spinning the engine itself. You can tell which type of engine is the aircraft just by looking at it. A free-spinning prop, when shut down, is in the feathered position, a direct drive engine it is the low pitch high rpm position, or flat looking if you stand infront of the aircraft and notice alot of blade is exposed when shut down. But as said before, they are all turbines, they just function slightly differently.