Originally Posted by arismount
The turboprops you see with condition levers and throttles only have geared engines, i.e., mechanical connection between the engine (gas producer) and the prop drive shaft. These are "constant speed engines," i.e., the condition lever sets them at idle, and then further up, into a "flight" RPM that's unchanging.
Power is controlled by the throttles, or power levers as they're more properly called. Advance the power lever, the prop takes a bigger bite of air, and generates more thrust. This tends to slow the prop, the fuel control senses this and adds more fuel, maintaining RPM. This happens so fast you can't see any change. In a sense, the prop is governing the engine through the fuel control.
That's called a "beta" control mode - the power lever sets the prop pitch and thus the load torque directly, and the FCU governor responds with the appropriate fuel flow.
On P&W powered King Airs (all of them except the B100), you have a free-power turbine, i.e., no mechanical connection between the gas producer section of the engine and the separate power turbine. The Condition lever(s) serve only to introduce or shut off fuel to the engine, OR to change the idle speed. The prop levers set the prop governor to maintain prop RPM, and the power levers control power, as above, with the exception that when you push them up, the N1 (gas producer) speed increases...and vice-versa.
Don't know the PT6 specifically, but the CT7 turboprop is also free-turbine, but has only two levers--a condition and power lever. The Condition lever sets the prop governor speed, as well as fuel shutoff. Power Lever sets the gas generator speed, effectively setting torque, and the prop governor responds to load up the prop blades. Thus the pilot can set max prop speed for TO, and slow it down for a quieter cruise. (IIRC, the condition lever also is used to select feathering)
One can always tell the difference between the two engine types by the prop blade angle when shut down. The single-shaft engine always has the blades at flat pitch (minimum drag) so it's easier to start. A free-turbine engine has the blades feathered when shut down (at least on types I'm familiar with), because the starter motor isn't directly coupled to the prop, and it doesn't care if the free turbine comes up to speed real quickly.
