Why no CAS turboprops?

(Peter Stickney) wrote:


As I remember it, Gord was an FE on airplanes with the Allison T56.
That's a bit different - there's only one shaft, and the output to the
gearbox, compressor and turbine are all rigidly connected.
In that case, the whole engine turns at some serious RPM.
(Something like 13,000). A neat deal with teh T56 is that in flight,
it's basically a constant speed engine - the propeller pitch changes
to keep the RPM constant, while the torque varies, and the fuel
control varies the temperature to produce the desired torque.


Yes...basically this is correct. The T-56 (as used) is a great
engine prop combination. As Peter says the engine/prop runs at
100% RPM all the time in flight. Basically you give it more or
less fuel to control the power. More fuel equals more temp which
(tries to) equal more rpm which equals more pitch which equals
more go. It's called a 'narrow band engine'.

Now the Napier Eland' which the T-56 replaced on Canada's Convair
440 (making it a 580) was a convoluted sob and was fully
understood by damned few aircrew (not to mention techs). I've won
quite a few beers with that engine. Get this now...the power
lever controlled (among other things) the prop pitch control (NOT
a CSU) setting which selected the prop to a COARSER pitch, so, as
power was added the prop would actually turn faster because of
the added torque but in spite of the coarser pitch. This devilish
arrangement was a bitch to keep in trim and required almost
constant tweaking.
--

-Gord.