Free-running turbines / turboprop...

[cagarini]

Not 747-related, but I always had a catch for turboprops, so...

the videos bellow include a very good explanation of why turbines cancel torque so effectively:

https://youtu.be/hATrgAsgevY?t=139

The full video here:

https://www.youtube.com/watch?v=hATrgAsgevY

In particular operations ( startup / shtudown ) of a PT-6 in a C-90 here, in another video:

https://www.youtube.com/watch?v=J7Rs6sCHN4s

[Will]

I flew the British Aerospace J32 with Garrett TPE331 engines, and the de Havilland Twin Otter, with PW PT6 engines. The TPE331s were direct-drive through a gear box, which meant that the propeller would turn if any part of the turbine was rotating.

In contrast, the PT-6 was free-turbine, so you could do fun tricks, like hold the propeller still while the engine started. The turbine would spool up to normal speeds, but as long as you had a firm grip on the propeller, it wouldn't rotate. Then as soon as you let go, it would instantly spool up to about 2000 RPM (and slice your hand off unless you were careful).

Some airplanes used free-turbine engines as generators. With a "prop brake" that was activated from the cockpit, you could start the engine and use the turbine as a generator and to supply hydraulic pressure and bleed air (like an APU), while the prop brake kept the propeller from rotating while passengers and baggage were loaded. Then before taxi, you'd release the prop brake and it would spool right up.

I never had as much confidence in an engine as I did in the PT6.

[cagarini]

The Jetstream 32 Wow ! I really loved that commuter.

Back in my youth, way away from flight simulation, the days I built aircraft kits,  there was an AIRFIX model for the Jetstream 31 :-), and in January 2018 I had the chance to fly in one from LPPS to LPMA :-) !!!! LOVED IT !!!!

One question, if you still recall from your RL experience, and a long time debate among flight simulator users / developers that comes up everytime I point out that MSFS ( all variants until the recent one ) and even X-plane incorrectly model free-running turbines like the PT-6:

- In the PT-6, assuming you're flying straight and level, fixed throttle and condition, varying prop RPM shouldn't affect FF even for wide ranges of variation, right ?

The ELITE Kingair B-200 is the only I found correctly modelling it, or, to put it more correctly, the way I believe is correct, since Fuel Flow doesn't vary with prop lever inputs under the conditions described above...

[Will]

I may not be very useful here, because my time on the PT6 was almost 30 years ago...

But yeah, RPM and fuel flow are not linked, except at very low power settings. Turboprops don't use RPM as a primary marker of power. RPM was was more of a marker of how much stress the engine was under. For a takeoff or in single engine operations, you wanted maximum output, so you'd use maximum RPM. But in normal flight, you'd want to relax the engine a bit, so you'd dial back the RPM so the propeller wasn't turning as fast, but that is independent of how much torque the engine is providing. For a fixed RPM, the engine could create a wide range of torque. Just as an example, the propeller would be set at a constant cruise RPM of 1500, and the torque would vary between almost nothing (idle descent) to almost max (normal cruise).

The power levers set a commanded torque, which directly impacted fuel flow. More torque, more power, hence more fuel. But all of that would be with constant RPM. (Commanding more torque led to increased propeller blade angle, not to faster propeller RPM.)

So what happens if torque is constant and you only vary the RPM? I don't see that this would have an effect on fuel flow, because again fuel is linked directly to power (torque), which is not dependent on propeller speed except in the ground idle regime.

Things were similar in the TPE331, even though it was direct-drive. We'd set 100% RPM for takeoff and then 97% for cruise, and that was basically it unless you were feathering the prop. The RPM would stay constant at 97% over a wide range of power settings (and FF). I can't say for sure that pulling the props back to 97% would result in constant FF, but it would make sense, because how much fuel goes into the engines was controlled by the power levers, not the propeller levers.

[cagarini]

Thx Will !

That makes sense, even for a "non-free" / "direct-drive" turboprop like the one in the Jetstream, Garret-equipped.

[Will]

You're welcome! Thank you for letting me walk down memory lane for a bit. :-)