Still hanging out at the Pro Pilot Board?
No, I let the membership lapse, though I might go back some day. Long story...
Just to include some content related to the thread subject:
In one of my previous lives I worked on TPE 331 engines for a time. Normally the 331, being a direct drive single spool turboprop, must always maintain a positive torque in flight in order to maintain control of the engine. Too much power output at idle and the airplane has difficulty slowing down, not enough and the negative torque sensing system dumps prop hub pressure to increase blade angle thus eliminating negative torque. (prop driving the engine) As I recall, power management theory for 331s is a pretty sizable chapter in the line maintenance training manual so I'll try to avoid going into excessive detail here.
When in alpha mode, the engine RPM is controlled by the prop governor (blade pitch) and the torque output is controlled by the power lever. (fuel flow) When in beta mode the blade angle is controlled by the position of the power lever through the prop pitch controller and beta tube while the RPM is controlled by the engine fuel controller.
Proper adjustment is accomplished by setting the flight idle blade angles and flight idle fuel flows to M/M specs, which involves adjusting the beta tube depth to set blade angle and a fuel control adjustment screw to establish the idle fuel flow. (power) The small amount of tolerance in these settings is usually utilized to get both engines on a twin to make equal thrust at the power lever idle position and avoid yawing moments when both power levers are brought to idle for landing. The transition between alpha and beta modes normally takes place during the landing roll as the prop "falls off the governor" with decreasing airspeed.
This small amount of beta tube and fuel flow adjustability could also be utilized to somewhat reduce aircraft floating tendency caused by excess idle power output. This is a particular problem on certain airframe types with low wing loading. On single engined agricultural application airframes like the Thrush and Weatherly some operators were (allegedly) modifying the prop pitch controller by grinding their own PPC cams such that beta mode could be achieved in flight. Normally this is only possible by bringing the power lever "over the hump" in flight, a practice generally frowned upon with ample reason. This "modification" (unapproved) enabled them to land the airplanes in a reasonable distance without risking an inadvertent entry into reverse blade angle range.
With the PT-6A being a free turbine engine the methodology of the engine/prop rigging are different but the result similar.
westhawk