Crossunder

24th May 2019, 11:18

Given: A PW121-powered turboprop will use the assumed temperature method for take-off.

Unlike a jet engine that - in this case, will operate at a lower N1, the TP pilots must use a reduced torque setting, e.g. 80% instead of 97,5% TQ.

Since the actual OAT is lower than the assumed OAT, the air density is higher than assumed, and the jet engine will thus produce more thrust (for a given N1) than the take-off calculations are based on. A bonus.

But what about the turboprop? RPM is constant at 1200, and 80% torque should be 80% torque, no matter what the OAT and air density. Sure, the required fuel flow might differ, but will the engines give more thrust at an OAT of 15 degrees C, that at 49C?

The only interesting (to me) variable is blade angle, and perhaps the propeller blades will be at a less efficient angle at lower air densities, in order to absorb the power from the turbine without increasing propeller RPM?

So - for a jet engine we can safely conclude that for a given N1 it will produce more thrust at 15 degrees C than at 49C. But what about the turboprop? Will there be any additional thrust due to the difference between assumed and actual OAT, when using assumed (flex) temperature?

Was that question clear as mud?

Unlike a jet engine that - in this case, will operate at a lower N1, the TP pilots must use a reduced torque setting, e.g. 80% instead of 97,5% TQ.

Since the actual OAT is lower than the assumed OAT, the air density is higher than assumed, and the jet engine will thus produce more thrust (for a given N1) than the take-off calculations are based on. A bonus.

But what about the turboprop? RPM is constant at 1200, and 80% torque should be 80% torque, no matter what the OAT and air density. Sure, the required fuel flow might differ, but will the engines give more thrust at an OAT of 15 degrees C, that at 49C?

The only interesting (to me) variable is blade angle, and perhaps the propeller blades will be at a less efficient angle at lower air densities, in order to absorb the power from the turbine without increasing propeller RPM?

So - for a jet engine we can safely conclude that for a given N1 it will produce more thrust at 15 degrees C than at 49C. But what about the turboprop? Will there be any additional thrust due to the difference between assumed and actual OAT, when using assumed (flex) temperature?

Was that question clear as mud?