Spool up time isn’t an issue in a prop
Sorry if I’m being difficult, but wouldn’t a turboprop be just as slow to spool up as a jet? You were meaning a piston-powered prop, weren’t you?
Sorry if I’m being difficult, but wouldn’t a turboprop be just as slow to spool up as a jet?
That depends on the powerplants. Single shaft turboprops (like the Garretts that power the Metroliner that I flew in a previous life) are almost as fast to spool up as a piston engine. Dual shaft units (like the ubiquituous PT6) are somewhat slower, but still faster than turbofans.
Good question.
I’m no expert, but I think turboprops work differently from turbojets.
With a jet, the fan blades have a fixed pitch. So in order to provide more power, you have to increase the RPMs. This takes time. Turboprops have variable pitch blades, AFAIK governed by more-or-less the same type of governor as you’ll find on Constant Speed propellers bolted to piston engines. This means that during normal flight operations the RPMs are constantly high. To increase power you don’t need to “spool up” the engine, you just need to dump more fuel in the combustion chamber. This will lead to higher torque, which initially leads to a slightly higher RPM, and that will be compensated by a courser angle on the prop blades. A much faster operation than spooling up the complete rotating assembly of a jet engine.
Having said that, I have no practical experience with jets or turboprops so I’m going from extrapolating theory only. Also the exact configuration of the engine (like what next mentioned) will have a profound effect.
