PT6A questions on Starship
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Ok so the prop lever only controls RPM, the governor automatically changes the blade angle to maintain this RPM, we have no direct control over the blade angle.
When engine power (torque) is increased the prop wants to spin faster, the governor says no I want 1600 RPM only so to do this I will increase the blade angle to maintain this RPM, the increased blade angle is what generates our thrust.
So basically the governor and the blade angle changing is where the magic happens?
@Captain-Bakon Well, you do have direct control over the blade angle - in the forward-Beta range....
But the simulation in MSFS is far too rudimentary for that - so don't fret
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Ok so what and how do I use Beta?
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Alpha angle on prop blade is related to air flow, just like alpha on a wing. As stated above, adjusting alpha will change torque/hp - result should be change in RPM, but governor catches this and keep RPM steady. Governor is unable to catch quick changes, and you will experience surge in RPM.
- Why it is difficult to taxi in alpha. The lag feels like being towing by a rubber band.
Beta is a fixed blade angle (or wing installation), as you find on all simple aircraft and r/c models.
Locked in beta, you change power by change RPM, instant response. (and instant death when airborne)
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Have only experience with PW125 (Fokker 50) , there is a throttle setting near beta, causing harmonize vibration in propeller, you can hear and feel it. Of course you must avoid it, and quickly transit through it . Maybe red band RPM in Starship is similar? I do not know.
- Why it is difficult to taxi in alpha. The lag feels like being towing by a rubber band.
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So basically Beta temporarily changes it to a fixed-pitch prop?
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You guys have been a great help, now my confusion is gone! thank you
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Ok this is my final understanding:
In a turbo prop;
N1 spool creates exhaust gas which spins the other power turbine N2 via windmill effect, power turbine connected to gearbox to reduce 50,000 RPM to 1700 RPM, gearbox turns prop shaft which is directly connected to prop itself.
Throttle controls N1 gas generator by modulating fuel flowProp control is for RPM, governors will maintain this RPM AT ALL COSTS!!!!!!!!!!!!! So if we increase torque the governers will say NO, they will maintain RPM at all costs, as torque is increased or decreased the governors will adjust blade angle to maintain RPM, the blade angle change is what produces our thrust! not the prop RPM!
Torque is primary parameter in PT6A-67A for Beechcraft Starship 2000 (BE-2000)
Change torque via gas generator via throttle levers, blade angles change and THIS is what produces thrust, RPM is always held constant by the governors and the RPM is ONLY changed by the prop controls, prop control ONLY changes target RPM nothing else, we have no direct control over blade angle in alpha range (forward thrust range)