Turbo Prop operation: Fixed shaft and FPT Alpha and Beta modes
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Turbo Prop operation: Fixed shaft and FPT Alpha and Beta modes
I have a question regarding free power turbine and fixed shaft turbine power lever controls in both alpha and beta modes.
I am most familiar with the PT6 engine in the B200 and B350 aircraft. Am I correct to say that the power lever will control fuel flow in the alpha range and in the taxi (beta) range it will change propellor pitch to control power when taxiing?
If we now compare this idea to a fixed shaft turbine engine such as a Garrett, one would select the flight range (98%ish turbine speed range) on the conditioner lever and power lever would simply control propellor pitch. Does this also apply to the beta (72%ish) ground range or does fuel control now come into the game?
On top of that, if someone could please give me a simple explanation as to why EGT rises in a fixed shaft turbine with an increase in power (propellor pitch only), I would be very appreciative.
I hope my understanding so far is accurate enough to start of the discussion. Thank you
I am most familiar with the PT6 engine in the B200 and B350 aircraft. Am I correct to say that the power lever will control fuel flow in the alpha range and in the taxi (beta) range it will change propellor pitch to control power when taxiing?
If we now compare this idea to a fixed shaft turbine engine such as a Garrett, one would select the flight range (98%ish turbine speed range) on the conditioner lever and power lever would simply control propellor pitch. Does this also apply to the beta (72%ish) ground range or does fuel control now come into the game?
On top of that, if someone could please give me a simple explanation as to why EGT rises in a fixed shaft turbine with an increase in power (propellor pitch only), I would be very appreciative.
I hope my understanding so far is accurate enough to start of the discussion. Thank you
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hey !
basicly yes, but there are two abbrevations for what "beta" is. the one call beta everything below flight idle , the other call beta the range between flight and ground idle and pulling the power lever further back they call it reverse. in the range between flight and ground idle you are correct, the turbine speed and output is fixed and the props are controlled directly via the power lever. reverse is again governor controlled on the pt6 and the turbine will spool up when you pull the power lever to full reverse.
thats not correct. on the garret the condition levers selects a speed range and the power lever the output - the prop pitch is just a resulting factor from the governor. when you push the power forward more fuel is introduced and the turbine tries to spin faster. the governor reacts and sets a higher prop pitch to absorb this more power and keep turbine speed the same . the higher pitch produces more thrust and the plane goes faster.
its basicly the same on the pt6 with the main difference the pt6 has not a mechanical coupling between the power turbine and the core engine ( gas generator) so just prop speed is kept constant via the governor and the core turbine speed varies due to power changes -on the garrett everything what spins inside is kept contant since all has a common mechanical connection.
an increase in power needs more fuel to be burned so the answer should be obvious. i think you mean the situation of keeping climb power on a climb out until the engine temps out , right?
hope this helps !
Am I correct to say that the power lever will control fuel flow in the alpha range and in the taxi (beta) range it will change propellor pitch to control power when taxiing?
If we now compare this idea to a fixed shaft turbine engine such as a Garrett, one would select the flight range (98%ish turbine speed range) on the conditioner lever and power lever would simply control propellor pitch.
its basicly the same on the pt6 with the main difference the pt6 has not a mechanical coupling between the power turbine and the core engine ( gas generator) so just prop speed is kept constant via the governor and the core turbine speed varies due to power changes -on the garrett everything what spins inside is kept contant since all has a common mechanical connection.
On top of that, if someone could please give me a simple explanation as to why EGT rises in a fixed shaft turbine with an increase in power (propellor pitch only), I would be very appreciative.
hope this helps !
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aerobat,
Thank you for the detailed explanation. My mis understanding of the EGT rise was due to my mis understanding of the governor/turbine relationship.
Thank you for the detailed explanation. My mis understanding of the EGT rise was due to my mis understanding of the governor/turbine relationship.
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On the Garrett beta ranges from full reverse to a little above idle "providing limited thrust for taxi" so beta extends into positive thrust also?
In the manufacturers operating manual, it states:
"the forward limit of the beta range is set by the FLIGHT IDLE mechanics", any idea on how this works? I presume it has something to do with airspeed?
After all, I think flight idle and ground idle are the same detent, so technically you could put the engines into beta in flight if it were not for these FLIGHT IDLE mechanics.
In the manufacturers operating manual, it states:
"the forward limit of the beta range is set by the FLIGHT IDLE mechanics", any idea on how this works? I presume it has something to do with airspeed?
After all, I think flight idle and ground idle are the same detent, so technically you could put the engines into beta in flight if it were not for these FLIGHT IDLE mechanics.
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Garret beta in a 331 powered Merlin
Flight idle has is a mechanical stop - pull the lever past the stop and your in reverse. There was no squat switch lockout on my Merlin. In fact on a short field I would get it clear the stop during the flair, then lay on the reverse when down, great fun 90 kts to zero in about 600 feet. Make the mid field turn off on a 3500 ft runway. When having this kind of fun, keep in mind the nose wheel steering is off and you must steer by modulating reverse. Doing both reverse and the brakes at the same can result in blown tire as it easy to use the brake when you need more reverse on that side. What worked best for me in the Merlin was to use only one form of direction control at a time.
On 331s Beta mode is characterized by power lever controlling blade angle directly and RPM being controlled by the fuel governor. Alpha mode is characterized by blade angle being controlled the prop governor and fuel flow by the power lever.
In Beta mode the 331 controls prop pitch directly by movement of the power lever. The Prop Pitch Control pulls the beta tube aft when the power lever is pushed forward. The prop pitch follows the movement of the beta tube. Screwing the beta tube further in or out is how a mechanic sets the flight idle blade angles. In the beta mode, engine RPM is controlled and held constant by the fuel governor. As blade angle is increased by forward power lever movement (causing aft beta tube movement), the governor adds fuel to maintain RPM. A rise in indicated torque, fuel flow and EGT will be noted when the power lever is moved forward. (if the prop has been bumped OFF the start locks!*) Pulling the power lever back pushes the beta tube forward and the prop pitch follows the beta tube movement. A reduction in indicated torque, fuel flow and EGT will be noted until the prop reaches reverse pitch. This should approximately match the pitch angle of the blades when they are on the start locks. As the blades reach a reverse angle, the torque, fuel flow and EGT indications will increase as reverse blade angle increases.
When applying takeoff power, the beta tube will reach a position where the prop pitch stops increasing and the mechanical linkage from the power lever to the fuel governor demands increased fuel scheduling. This position is a function of the "cut", or shape of the PPC cam, the adjustment of the beta tube when the flight idle blade angles were last set and the rigging of the linkage rods connecting the power lever to the fuel governor. At this point the RPM will increase into the governing range of the propeller governor. Any further forward movement of the power lever now directly controls fuel flow and prop blade angle is now controlled by the prop governor to maintain RPM. This is the transition from Beta to Alpha mode. Exactly where the transition takes place is by design, but also dependent upon the accuracy of the rigging process as performed by the mechanic.
On landing the 331 will "fall" from Alpha to Beta mode as the airspeed reduces (and prop drag increases) in the ground roll if the pilot leaves the power lever in the idle position. However most pilots will pull the power levers "over the hump" on most landings, which pushes the beta tubes forward, manually transitioning the engines/props into Beta mode.
* If a prop is still on the start locks, forward P/L movement will not cause blade angle increase and torque, fuel flow and EGT will not increase. The fuel governor will maintain RPM but the engine will not increase it's power output. A pilot/mechanic I was once acquainted with crashed a Metro into a hangar because he attempted to take off with one prop on the start lock. (upon power application for takeoff, please check the engine instruments?)
In Beta mode the 331 controls prop pitch directly by movement of the power lever. The Prop Pitch Control pulls the beta tube aft when the power lever is pushed forward. The prop pitch follows the movement of the beta tube. Screwing the beta tube further in or out is how a mechanic sets the flight idle blade angles. In the beta mode, engine RPM is controlled and held constant by the fuel governor. As blade angle is increased by forward power lever movement (causing aft beta tube movement), the governor adds fuel to maintain RPM. A rise in indicated torque, fuel flow and EGT will be noted when the power lever is moved forward. (if the prop has been bumped OFF the start locks!*) Pulling the power lever back pushes the beta tube forward and the prop pitch follows the beta tube movement. A reduction in indicated torque, fuel flow and EGT will be noted until the prop reaches reverse pitch. This should approximately match the pitch angle of the blades when they are on the start locks. As the blades reach a reverse angle, the torque, fuel flow and EGT indications will increase as reverse blade angle increases.
When applying takeoff power, the beta tube will reach a position where the prop pitch stops increasing and the mechanical linkage from the power lever to the fuel governor demands increased fuel scheduling. This position is a function of the "cut", or shape of the PPC cam, the adjustment of the beta tube when the flight idle blade angles were last set and the rigging of the linkage rods connecting the power lever to the fuel governor. At this point the RPM will increase into the governing range of the propeller governor. Any further forward movement of the power lever now directly controls fuel flow and prop blade angle is now controlled by the prop governor to maintain RPM. This is the transition from Beta to Alpha mode. Exactly where the transition takes place is by design, but also dependent upon the accuracy of the rigging process as performed by the mechanic.
On landing the 331 will "fall" from Alpha to Beta mode as the airspeed reduces (and prop drag increases) in the ground roll if the pilot leaves the power lever in the idle position. However most pilots will pull the power levers "over the hump" on most landings, which pushes the beta tubes forward, manually transitioning the engines/props into Beta mode.
* If a prop is still on the start locks, forward P/L movement will not cause blade angle increase and torque, fuel flow and EGT will not increase. The fuel governor will maintain RPM but the engine will not increase it's power output. A pilot/mechanic I was once acquainted with crashed a Metro into a hangar because he attempted to take off with one prop on the start lock. (upon power application for takeoff, please check the engine instruments?)
As far as the tpe 331, i always explained it that in alpha mode you had the underspeed governor" min selected speed add fuel", overspeed governor" adjust prop to keep speed under" manual fuel valve "select ff and the overspeed adjusts prop" and fuel bypass"srl tq/temp control". In reverse the beta cam sets prop angle and the underspeed governor ramps up fuel to stay onspeed.
And there is a 104% governor too
Probably oversimplified, but if you cant explain simply......
And there is a 104% governor too
Probably oversimplified, but if you cant explain simply......
Last edited by rigpiggy; 11th Aug 2017 at 21:09. Reason: Addition
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I have a question regarding free power turbine and fixed shaft turbine power lever controls in both alpha and beta modes.
I am most familiar with the PT6 engine in the B200 and B350 aircraft. Am I correct to say that the power lever will control fuel flow in the alpha range and in the taxi (beta) range it will change propellor pitch to control power when taxiing?
I am most familiar with the PT6 engine in the B200 and B350 aircraft. Am I correct to say that the power lever will control fuel flow in the alpha range and in the taxi (beta) range it will change propellor pitch to control power when taxiing?
If I recall correctly, the correct definition of B range is whenever the prop RPM is being controlled by the power lever. This happens in flight too, not just in ground, as you will be in B range in every approach and landing.
On FPT, in Beta the power lever will indeed control prop RPM until reaching the "coarse pitch limit" from the governor, then the prop governor will pick it up (thus entering Alpha range) until reaching full fine pitch. Power lever will also control fuel flow in B too.
In Alpha range the governor, via the prop levers, will control prop RPM and the power levers will control torque (and also fuel flow)
It's been some years since I parted ways with the PT6, so feel free to correct me
