jpilotj

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

jpilotj

aerobat77

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 !

jpilotj

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.