Propeller theory
 

Why is it that the propellers on piston engined aircraft are in fine pitch for start up and shut down, while turboprops are feathered?

Al

Only a few turboprops have the propellor feathered during start.
PT-6's mostly.
Others (AeroProducts/Hamilton Standard on the Electra, Garrett TPE331's, RR Dart (superb, by the way) engines, have the prop near the ground fine pitch setting, as an aid to starting.
The devil is in the details.

411A is right, and Here's a discussion of some of the details. :cool:

PT6, PW100, CT7, CT64 turboprops are all twin-spool and thus normally feathered when shut down.

Also, most single-spool (or "direct drive") installations require some sort of pitch lock to keep the blades in fine pitch when you shut down. Not sure about others but the Garrett TPE331 simply has centrifugal pins that engage as the prop spins down on shutdown.
I am sure the RR Dart has some complex sequence of microswitches and pins or something!

don't think the PT6 is a twin spool. it is a single spool with a free turbine.

the free turbine is what drives the gearbox and prop which are not mechanically connected to the spool, hence the term free turbine.

because the gearbox and prop are not connected to the spool, when you turn over a PT6 to start it, the starter generator only has to turn the spool which is also known as the gas generator, and not the spool, gearbox and prop so the prop can remain feathered without putting any strain on the starter generator.

StraightLevel

PT6 IS twin spool, however the two are not coaxial as are most twin spools.

The T58 (helo engine and its R-R Gnome twin) are likewise.
:ok:

the pt6-112a is certainly not twin spool, it's a single spool with a free turbine to turn the prop, but i do know that there are many other different pt6's, of which i know nothing about.

StraightLevel

Terminology
 

I think everyone is getting mixed up on terminology here .......but a free turbine configuration IS a twin spool...they are one and the same.

I think some confusion has arisen due to the somewhat unusual arrangement of the 2 spools in the PT6. But, just because they are not coaxial doesn't change the fact that there are still two non-mechanically connected shafts (spools)....a core(or gas generator) and a power turbine.

we were taught for our atpl's that the definition of a spool is a compressor, a shaft, and a turbine combined. anything less is not a spool.

will need to check my notes when i get a chance.

straightlevel

have just noticed that you guys are all from america.

perhaps the faa definition of a spool is different to jaa?

straightlevel

SL's reference is a rather narrow definition - valid for turbojets & turbofans.
But in the rest of the world a free turbine engine (i.e. separate power turbine, with only aerodynamic coupling to the core) is called twin spool, whether coaxial shafting or not.
(Were I of a combative bent, I'd point out that a propeller is really an unducted single-stage compressor rotor, and thus the free turbine+shaft+gearbox+prop of a PT6 constitute a "spool" per his definition.
But I won't press that point... := )