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.

On Thu, 08 Jan 2004 05:14:07 GMT, Rick wrote:

wrote:

What aircraft would use that setup Rick? I can't imagine how you
could drive a prop directly from a turbine engine with no
reduction gearing? (or did I misread you?)

I think you misread. A turboshaft engine can be used for
anything but driving a propeller directly. They are used to
power everything from generators and air compressors to
ships and water pumps. Some of them, like the GE LM2500
drive the output shaft directly from the power turbine at
3600 rpm so they can be direct coupled to a generator. They
do not use a gearbox.

Rick

The LM2500 definitely has a reduction gear, and is not (in the USN)
used to power a generator. In most applications two LM2500 are
connected to a common reduction gear.

Al Minyard

On Thu, 08 Jan 2004 05:26:55 GMT, "Gord Beaman" ) wrote:

Rick wrote:

wrote:

What aircraft would use that setup Rick? I can't imagine how you
could drive a prop directly from a turbine engine with no
reduction gearing? (or did I misread you?)

I think you misread. A turboshaft engine can be used for
anything but driving a propeller directly. They are used to
power everything from generators and air compressors to
ships and water pumps. Some of them, like the GE LM2500
drive the output shaft directly from the power turbine at
3600 rpm so they can be direct coupled to a generator. They
do not use a gearbox.

Rick

Ok, that makes sense as far as it goes...I'm a little surprised
at the low speed of that turbine though but then, I'm used to
aircraft turbines that rotate much faster, could be something to
do with weight requirements though. The other part of your post
seems to say that you 'can' drive a prop directly and that's what
I was querying actually. Most aircraft turbines driving props
rotate at 13,000 - 14,000 RPM and that's much too fast for props
so you need reduction gearing.


You are right, the LM2500's are geared down, they operate
MUCH faster than a ships propeller, which is what they drive.

Al Minyard

Alan Minyard wrote:

The LM2500 definitely has a reduction gear, and is not (in the USN)
used to power a generator. In most applications two LM2500 are
connected to a common reduction gear.

If by "most" you mean the numerical majority, most LM2500
installations are in power generation. A single engine
direct drives a generator.

Comparatively few of the engines are used for geared marine
propulsion systems.

Rick

Rick wrote:


If by "most" you mean the numerical majority, most LM2500 installations
are in power generation. A single engine direct drives a generator.

Comparatively few of the engines are used for geared marine propulsion
systems.


Oops, big foot in mouth, just found out there are about 300
more LM2500 engines in military use than civilian power
generation.

Don't know what percentage of the military installations are
geared propulsion though. Probably most of them.

Rick

In article , ] says...
In article , Abe
wrote:
In article ,
says...
A question-- why haven't any CAS aircrarft been designed with
turboprops or the pusher varient (I foreget what you call it)?

The Argentinian Pucara is/was a CAS aircraft with turboprops.

The Pucara is usually denoted a COIN (Counter insurgency) aircraft.
Flying one around a modern battlefield is going to be unhealthy.

Absolutely, which demonstrates nicely why turboprops aren't used in CAS
aircraft

"Abe" wrote in message
...
In article , ] says...
In article , Abe
wrote:
In article ,
says...
A question-- why haven't any CAS aircrarft been designed with
turboprops or the pusher varient (I foreget what you call it)?

The Argentinian Pucara is/was a CAS aircraft with turboprops.

The Pucara is usually denoted a COIN (Counter insurgency) aircraft.
Flying one around a modern battlefield is going to be unhealthy.

Absolutely, which demonstrates nicely why turboprops aren't used in CAS
aircraft

Gee, my text indicates there are two mission categories for air support of
ground forces--CAS and AI. No mention of "COIN". Which leads me to conclude
that COIN is not a mission per se, but more of a "condition set" for
missions--the actual missions in support of COIN operations would still be
CAS and AI.

Brooks

Rick wrote:

Rick wrote:


If by "most" you mean the numerical majority, most LM2500 installations
are in power generation. A single engine direct drives a generator.

Comparatively few of the engines are used for geared marine propulsion
systems.


Oops, big foot in mouth, just found out there are about 300
more LM2500 engines in military use than civilian power
generation.

Don't know what percentage of the military installations are
geared propulsion though. Probably most of them.

Rick

Good man...I have much admiration for a person who, when he finds
himself in error, admits it. Good on you sir.
--

-Gord.

wrote:

who, when he finds himself in error,

Thanks, but I am used to it ;-)

Rick

Rick wrote:

wrote:

who, when he finds himself in error,

Thanks, but I am used to it ;-)

Rick

Yes, I suppose we who do it a lot, deserve less and less credit
as time goes on because it becomes easy for us but now if dudley
were to ever do it then he'd deserve a purple heart, a
ticker-tape parade and a national holiday created for him. Hell,
I even saw Rasimus do it recently. Excellent.
--

-Gord.