timz
Posts: 6163
Joined: Fri Sep 17, 1999 7:43 am

Feathered Or Flat-pitch Props?

Tue Jan 04, 2000 7:46 am

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
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Feathered Or Flat-pitch Props?

Tue Jan 04, 2000 10:27 am

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.

Hope that helps. There's more if you want!

Best Regards,

Buff
 
Buzz
Posts: 694
Joined: Sun Nov 21, 1999 11:44 pm

RE: Feathered Or Flat-pitch Props?

Wed Jan 05, 2000 2:53 am

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
Posts: 6163
Joined: Fri Sep 17, 1999 7:43 am

RE: Feathered Or Flat-pitch Props?

Sat Jan 08, 2000 4:21 am

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?
 
Guest

RE: Feathered Or Flat-pitch Props?

Sat Jan 08, 2000 8:11 am

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
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Feathered Or Flat-pitch Props?

Sat Jan 08, 2000 9:42 am

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.

Good series of questions!

Best Regards,

Buff

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos