Timz From United States of America, joined Sep 1999, 6644 posts, RR: 7 Posted (13 years 11 months 3 weeks 3 days 4 hours ago) and read 2554 times:
In the old days, when you shut down a piston or turboprop aero engine the prop would invariably (?) go into flat pitch-- to make it easier to eventually restart, I assume. This is still true of the Jetstream, but most other in-production turboprops now use free-turbine engines (PT6, PW120, CT7 etc) which feather the props on shutdown. The question: how firm is the rule that free-turbines always feather and other turbines always go to flat pitch? I think the Britannia was an exception; were/are there any others?
Buff From Australia, joined Mar 2007, 0 posts, RR: 1 Reply 1, posted (13 years 11 months 3 weeks 3 days 1 hour ago) and read 2510 times:
The BAe31/41 engines shutdown just like any other turbine engine. The propeller works the same as well - it is spring loaded to feather. Oil pressure maintains fine and reverse pitch. The reason the Garretts (Allied Signal now I think - can't remember!) appear to shut down in fine pitch is due to the pilot engaging "blade latches" during the shutdown procedure. Blade latches prevent the feathering spring from driving the prop to feather. On occasion because of distraction or rusted latches, the prop does not "latch" and feathers. If this happens, the engine cannot be started until the latches are re-engaged. The Garretts are fixed shaft engines and do not have a free turbine as do P&W turboprops. During engine start, a feathered prop on a Garrett engine will produce too much drag for the starter to rotate the engine sufficiently for a safe start. The engine WILL overtemp during the start.
Buzz From United States of America, joined Nov 1999, 697 posts, RR: 23 Reply 2, posted (13 years 11 months 3 weeks 2 days 8 hours ago) and read 2499 times:
Good answer Buff!, Buzz here. To confuse things further, maybe i'll describe the difference between Garrett engines and Pratt + Whitney PT6 Free Turbine engine.
The Garret (ok, i grew up a long time ago and don't keep up with the name changes) has a prop connected to the gearbox, connected to the engine shafts. Everything turns together.
The Pratt PT6 free turbine engine has the prop connected to a gearbox (hey, turbines work best rotating FAST!) but the gearbox has it's own set of turbine blades in the exhaust. It's "free to rotate" at whatever speed it can. All that exhaust going by sure makes a lot of horsepower.
Slightly different topic: I crew chief a DC-3 for fun. We normally shut down with the prop in high RPM (flat pitch), we came in for landing like that.... ready for a go-around. When you shove the blades to low RPM /feather it takes a few gallons of oil in the prop dome. Unfeather on one of those big Ham-Standard props dumps those few gallons of oil in the nose case of the engine. Can you say hydro-lock next start? g'day
Timz From United States of America, joined Sep 1999, 6644 posts, RR: 7 Reply 3, posted (13 years 11 months 2 weeks 6 days 7 hours ago) and read 2489 times:
Thanks, guys. I didn't know the J31/J41 engines are springloaded to feather. As usual I've got the followup question: what about older non-free-turbine engines like the Dart, Tyne, Allison 501-- and piston engines? Do they automatically feather unless the pilot latches them on shutdown? Has anyone seen a picture of an Electra or DC-6 or F27 parked with feathered props?
Buddster From , joined Dec 1969, posts, RR: Reply 4, posted (13 years 11 months 2 weeks 6 days 3 hours ago) and read 2485 times:
I would like to add to Timz's questions. Is the purpose of the spring-to-feather so in case of engine failure the feathering is done automatically? If the prop pitch is controlled with oil pressure, I would imagine that a shutdown/failed engine would not be able to feather the prop due to lack of oil pressure.
Buff From Australia, joined Mar 2007, 0 posts, RR: 1 Reply 5, posted (13 years 11 months 2 weeks 6 days 2 hours ago) and read 2487 times:
That is substantially correct. To help in the understanding, the pitch of turboprops is usually a balance of forces: Oil pressure driving the pitch to fine and further to reverse, and spring pressure driving the pitch in the reverse direction towards feather. Some turboprops have an additional isolated oil reservoir to quickly and positively feather the propeller in flight. A separate manifold in the prop dome channels this oil pressure to the feathering side of the pitch actuator, accelerating the feathering action of the spring. Otherwise the bleeding off of engine oil pressure may be so gradual the prop takes too much time to feather by itself.
If anyone has spent any amount of time around a BA341 operator's ramp (like ACA in Dulles) every now and then you will see a Jetstream with a propeller feathered. That's probably because the pilot either forgot or was late to engage the latches.
Some PT6 engines are equipped with blade latches - you will find these mostly on floatplanes like the DHC-6 and DHC-2T. Once started, the prop is in flat, i.e. zero thrust pitch. This enables starting the engine without going anywhere!
Sorry I can't remember the shutdown procedure on the HS748 - RR Dart engines. They too are a fixed shaft engine operating the prop through a RGB (reduction gear box). I believe they have automatic latches.