Why does a turbo prop loose torque while climbing?

Question

2) During a normal climb, if the Power Lever positions are not changed, the torque produced by the Pratt and Whitney PT6 engine will ____decrease__________________________ (increase, decrease, remain the same)as the airplane climbs. (Assume standard atmospheric conditions)(Explain your answer)(10 points)

This is my final exam question and I have searched the manual to no avail. Please help me.

Answer 1

pressure decrease->drag decrease->torque decrease

i suppose the prop blades remain constant pitch.

Answer 2

because the air density changes with altitude.. you have to enrich the mixture you should have a egt guate and lean and enrich according to that .. but me if this happined change the prop angle. and also enrich the mixture. ok

Answer 3

Turbines produce less power with increasing altitude. Less power, less torque. Simple.

Answer 4

And how exactly do you enrich the mixture on a turboprop?

Answer 5

The power lever is one input to how much fuel is metered into the combustion chamber.  It's not a direct input, though.  It's only telling the Fuel Control Unit what you're requesting out of the engine.

Bleed air is tapped from the compressor section and fed to the Fuel Control Unit to aid in properly metering the fuel.  For any given power lever setting, the higher you go, the less pressure is coming off of the compressor.  The FCU will compensate for the reduced pressure by metering less fuel to the engine.  Less air + less fuel = less torque.

If the FCU didn't take a reading of how much air is available from the compressor section and allowed the pilot to directly control the fuel via the power lever, there would be a lot of torched PT6s out there.

Answer 6

A turboprop engine is no different than any other, it will always produce less power up high due to the decrease in air density. It is suffering just like a piston engine at altitude cause it has to compress its own combustion air too but there is less up high to compress. If you do not move the power lever your airspeed and climb rate will decrease to to the point where you are not climbing anymore and you will be going slow

Answer 7

Good grief Charlie Brown, without going in to great detail about turbine engines on turbo propeller driven aircraft the torque will stay the same.  The propellers are turned at 100% torque all the time through the gearbox.  The pitch can be changed from high to low, but he torque will remain the same.  To maintain constant torque the fuel control unit (FCU) will add more fuel as required do to propeller pitch change.  This is all linked from the governors to the FCU or ECU depending on the PT-6 you are taking about.

Torque is a measure of load and is properly expressed in pound-inches (lb.-in.) or pound-feet (lb.-ft.) and should not be confused with work, which is expressed in inch-pounds (in.-lbs.) or foot-pounds (ft.-lbs.).

The typical turboprop engine can be broken down into assemblies as follows:

(1) The power section assembly, which contains the usual major components of gas turbine engines (compressor, combustion chamber, turbine, and exhaust sections).

(2) The reduction gear or gearbox assembly whìch contains those sections peculiar to turboprop configurations.

(3) The torquemeter assembly, which transmits the torque from the engine to the gearbox of the reduction section.

(4) The accessory drive housing assembly.

A gas turbine engine that delivers power through a shaft to operate something other than a propeller is referred to as a turboshaft engine.  Turboshaft engines are similar to turboprop engines.  The power takeoff may be coupled directly to the engine turbine, or the shaft may be driven by a turbine of its own (free turbine) located in the exhaust stream.  The free turbine rotates independently.  This principle is used extensively in current production turboshaft engines.