Are you sure on those numbers? And I am pretty sure the one touch start was only a decade or two ago. I thought Garret direct drive was only 5% to 10% more efficient depending on engine model (the more recent the PW6 engine, the closer in efficiency they are).

Tim

Steve, where does that efficiency come from, direct drive? What makes it more efficient than a Pratt. Very curious.

Pratts are all installed backwards for one thing!! The inlet air has quite a turn to make to enter the engine. This is not to be confused with a reverse flow.

Both the Pratt and garret are reverse flow meaning the compressed air makes a turn before it enters the combustion chamber. The Pratt just has that additional inlet ducting before the inlet air even hits the engine.

This also causes a big loss in efficiencies when in icing. When you need to use the inlet doors engine performance suffers. No such problem on the garrets. There is no noticeable loss of engine performance with engine heat on.

Big help when in icing and you want full power to climb out of those conditions!

The direct drive turbine is a more efficient design. All larger turbo props are designed like that as are jet engines.

Good post, yup the inertial separator robs us of power when in any water vapor/FOD type conditions.

Does the derating of some the P&W's cause some of the power loss too? Meaning bigger engine, burning more, but derated? Does the derating improve engine longevity?

Yes, down low.

Turbine engines usualy have their best BSFC at or near max power.

They make more kWH of power per kg of fuel BUT the burn rate is higher.

Note that this is not the airframe's most efficient speed.

The most efficient turbine engine is the one that's just barely big enough, run WFO.. Problem is in the real world, you need excess power, things do wear, hotter than standard day, etc. .

In the case of the PC-12, you make far less than rated power down low, but it in essence flat rates the engine to a higher altitude.

Switching gears, the big negatives of a non free turbine engine are:
Prop either has to default to flat pitch to shut down (T-56s and other big ones) or have start locks (Garretts). Start locks are a tad bit more complexity in the hub. Props that don't feather by default (T-56s) have some "evil" failure modes.

And Starting.. PT6s have relatively small batteries and start current requirements, because you are just spinning up the gas generator.. Yes, the power section is slowly rotating but it's not a huge load on the starter.

Garretts have to spin up EVERYTHING, turbines, compressor, gearbox, prop to at least self sustaining speed.

Takes a bigger starter. In the case of T56s, it's an air starter, which in most planes is powered by the APU bleed air, just like bigger jets.

Or if you fly the Hawkeye like me, we and the U-2 are the last common planes tied to an air start cart to get running.

If the Garretts were that good and efficient they would be on pressurized SETP.
I dont see any except on older out of production twins.

Pratt as FADEC on PT6s in helicopters for years.
They want too much money for it thats why we dont see it on the TBM.

I agree the garretts are better in lots of ways

Lower fuel flow
Higher TBO
Lower overhaul costs

There is a conversion from PT-6 to Garrett for the Caravan. Improves HP and efficiencies.

Unfortunately the SETP does not lend itself to a Garrett conversion easily.

There are inefficiencies introduced by the exhaust routing required for SETP installs.

The Texas Turbine conversion makes the Caravan a much better performing aircraft, especially on amphibious and float planes.

7500hr tbo is the biggest benefit.

3500hrs from P&W is not even close.

That being said do the parts have similar life limits.

Michael I am not sure on the specific parts. I have never heard a complaint one way or the other.

I had my engines rebuilt to the 7000hr TBO stds about a 1.5 yrs ago and they cost around 250k each.

As far as derating I think it prolongs an engines life and makes overhauls cheaper.

I'm sure that Garrett has the same life limits on it's internal parts as P&W. That seems a fair price to pay for the -10's. Agreed on the derating, it's what's been communicated to me.

I have an HSI coming up, which I'll document.

What is this power robbing inertial separator you speak of There is one less button or switch to worry about on the Piper factory turbines. Always on.... always inefficient, but you don't have to worry about a performance hit in icing or IMC

The M600 is very cool in a number of ways.

I know you know that!

The engine is so derated you don't need the typical inlet found on a normal PT-6 installation.

You have NACA inlet simple and effective.

Love it!

I agree the garretts are better in lots of ways

Lower fuel flow
Higher TBO
Lower overhaul costs

There is a conversion from PT-6 to Garrett for the Caravan. Improves HP and efficiencies.

Unfortunately the SETP does not lend itself to a Garrett conversion easily.

There are inefficiencies introduced by the exhaust routing required for SETP installs.