PT6A questions on Starship
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The large black exhaust under the prop is the oil cooler outlet. The NACA intake on the bottom is the oil cooler inlet. The rectangular ports on the sides are the inertial separator bypass. Great questions!
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Ok so PLEASE correct me if I'm wrong:
Starship PT6A-67A is configured like a jet, intake at front, exhaust at back!
COMPRESSOR/COMBUSTION/TURBINE (FORMS GAS GENERATOR SPOOL N1) => DRIVES POWER SPOOL (N2) VIA WINDMILL EFFECT FROM EXHAUST GAS => POWER SHAFT DRIVES GEARBOX, WE HAVE GEARBOX TO REDUCE RPM FROM 50,000 TO 1600 OR 1700 => GEARBOX DRIVES PROPELLER SHAFT
So we have three shafts
- Gas generator shaft (N1) consists of compressors and centrifugal turbine, makes EXHAUST gas,
- Power shaft (N2) NOT CONNECTED to N1 shaft is ONLY spun by EXHAUST gas generated by N1, Power shaft POWERS the GEARBOX?
- Gearbox shaft REDUCES RPM from 40,000 to 50,000 to about 100 - 1700 RPM so the props don't spin too fast causing extremely loud noise and I believe if the props spin too fast they wont produce as much thrust
So in summary:
GAS GENERATOR N1 SPOOL ==> POWER SPOOL N2 ==>GEARBOX ==> PROPELLER
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Seems to be correct.
N1 turbine drives the compressor - necessary waste of energy
Then there is a so called free turbine. Designed to take out as much energy from gas as possible, using several stages.. Exhaust should be only a cold fart. This drives the propeller via gearbox. Problem with high prop RPM - tip blade must not reach speed of sound. -
Seems to be correct.
N1 turbine drives the compressor - necessary waste of energy
Then there is a so called free turbine. Designed to take out as much energy from gas as possible, using several stages.. Exhaust should be only a cold fart. This drives the propeller via gearbox. Problem with high prop RPM - tip blade must not reach speed of sound.@eker What if it goes supersonic? deafness and no thrust?
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So how does changing the blade angle (which the prop levers do via the governors) change RPM?
How is more thrust produced if the propeller RPM is the same but torque increases?
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So how does changing the blade angle (which the prop levers do via the governors) change RPM?
How is more thrust produced if the propeller RPM is the same but torque increases?
You're looking at it with a bit of a flawed understanding of how constant speed props work. Think of it like this. The prop controls don't "adjust the blade angle." The prop controls tell the prop governors what RPM they need to govern the propellers to. So while the pitch of the blades change when you adjust that governed RPM, the pitch of the blades ALSO change any time you move the power levers when the props are in the governing range.
So basically, when you increase power, you speed up the engine. This would also speed up the prop, if the governor didn't change the blade angle to prevent that from happening.
Take Starship for example. When you have the props set at 1600RPM in flight, the governors are changing the pitch of the blades any time you add or remove power just to keep the props spinning at 1600RPM.
More thrust is produced with higher torque at the same RPM because the prop takes a bigger "bite" out of the air. It's not about how fast its turning, its about how hard its working to move air.
Hope that makes sense!
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Is that why the redline for the props is around 1700 RPM? They sure like to overrun that for a few moments on takeoff.
@jmarkows The props running past the redline is realistic behavior for a constant speed prop, but is indicative of poor technique on the pilots part. When that happens, the power is being increased so quickly that the prop governors can't react fast enough to govern the props to their maximum RPM.
The result is the "surge" that you refer to. If it happens on takeoff, its because you added takeoff power too fast. I treat Starship the same way I treat turbine engines in real life. Slowlyyy bring the power in so the RPM doesn't bounce off the governor, then once they're at 1700RPM, a nice easy push up to around 96% torque. That last nice easy push part takes me a good 6-7 seconds.
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You're looking at it with a bit of a flawed understanding of how constant speed props work. Think of it like this. The prop controls don't "adjust the blade angle." The prop controls tell the prop governors what RPM they need to govern the propellers to. So while the pitch of the blades change when you adjust that governed RPM, the pitch of the blades ALSO change any time you move the power levers when the props are in the governing range.
So basically, when you increase power, you speed up the engine. This would also speed up the prop, if the governor didn't change the blade angle to prevent that from happening.
Take Starship for example. When you have the props set at 1600RPM in flight, the governors are changing the pitch of the blades any time you add or remove power just to keep the props spinning at 1600RPM.
More thrust is produced with higher torque at the same RPM because the prop takes a bigger "bite" out of the air. It's not about how fast its turning, its about how hard its working to move air.
Hope that makes sense!
@SinkRate Are you a RW Turbo pilot? Do you think you can go in depth and explain exactly how and why the throttle, prop, and fuel condition levers work? It would help me a lot! thanks
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@jmarkows The props running past the redline is realistic behavior for a constant speed prop, but is indicative of poor technique on the pilots part. When that happens, the power is being increased so quickly that the prop governors can't react fast enough to govern the props to their maximum RPM.
The result is the "surge" that you refer to. If it happens on takeoff, its because you added takeoff power too fast. I treat Starship the same way I treat turbine engines in real life. Slowlyyy bring the power in so the RPM doesn't bounce off the governor, then once they're at 1700RPM, a nice easy push up to around 96% torque. That last nice easy push part takes me a good 6-7 seconds.
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@SinkRate Are you a RW Turbo pilot? Do you think you can go in depth and explain exactly how and why the throttle, prop, and fuel condition levers work? It would help me a lot! thanks
@Captain-Bakon I can do my best. Currently I'm flying a large-cabin business jet, but I also own a recip and have plenty of experience in turboprops. Both the PT6 and the TPE-331.
Essentials:
The power levers are connected to the fuel control unit, and essentially regulate fuel flow to the engine. This allows the pilot to control turbine RPM, or N1, as its represented to you on the EICAS in Starship.
See my first post for the prop controls.
The condition levers, depending on which type of turboprop we're talking about, also interact with the FCU. They're used to command an idle speed, as well as the first line of defense when it comes to shutting off fuel to the engine.
We could get quite a bit more involved, but these are the basics.
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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?
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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)
