Fuel flow variation with change in ambient air temp

[Virtus]

Situation: You have air at +20 degrees C entering your turboprop engine and with the current settings you are producing 75% torque - fuel flow is 500 lb/hr.

If you change the ambient air temp to -30 degrees C and adjust the power lever to maintain 75% torque, will the fuel flow be 500 lb/hr?

The engine is direct-drive and the RPM is at 100% and does not get adjusted.

I'd also be interested in what happens in a free-turbine or turbojet/fan where the spool speed is allowed to change.

Cheers!

[Brian Abraham]

Some figures taken from a PT-6 powered airframe

ISA-30 384pph
ISA-20 399
ISA-10 404
ISA 409
ISA+10 413
ISA+20 418
ISA+30 423
ISA+37 439

With increasing temp the N1 spool will need to increase RPM

[Virtus]

With increasing temp the N1 spool will need to increase RPM

Makes sense as it would need more air (in volume) to compensate for lack of density.

I'm assuming that's for the same torque setting?

Do you know what happens for a direct-drive engine like a TPE331 where the N1 speed doesn't change?

[Brian Abraham]

I'm assuming that's for the same torque setting?Yes, and same altitude. Figures for a TPE331 installation

-15 376
-5 377
5 380
15 384
25 389
35 395
45 402