Originally Posted by
Crossunder
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?
Using your example, 80% torque will not be 80% regardless of OAT as you will reach your ITT limit sooner at 49C than at 15C. So, for eg your TQ for 49C may be say 72% (arbitrary figure) while at 15C it might be 80%. The power output of the engine will be the same because the NH (or NG) at both OAT is about the same. Just like a jet engine because that’s what it is.