RussianJet
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Please Explain Prop-Feathering

Thu Jun 27, 2013 4:09 pm

As the title says.

I understand that it's the changing of the angle of the prop blades to reduce drag, particularly in an engine-out scenario. But, how physically does it work, what are the mechanisms involved? Are there any large-ish props that don't have the capability? What other situations is it of use in aside from engine failure?

Grateful in anticipation of your replies.

RJ
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Woodreau
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RE: Please Explain Prop-Feathering

Thu Jun 27, 2013 4:35 pm

Generically, to control the pitch angle of the propeller blades, engine oil pressure is usually used. During an engine failure, the oil pressure goes away, and springs push the propeller blades into the feathered position. Very generic explanation.

Some aircraft have autofeather systems, so that if you don't feather the props manually, it feathers the props for you during an engine failure.
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Starlionblue
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RE: Please Explain Prop-Feathering

Thu Jun 27, 2013 5:38 pm

If you want to know the nitty gritty details about prop governors, McCauley's ancient but still excellent booklet with "Professor Kliptip" explains the whole thing very well. http://www.mccauley.textron.com/von_klip_tip_cs_propeller.pdf .

Some more details.
- In single engine planes with a variable pitch (AKA "constant speed") prop such as the Cessna 172RG or the Piper Arrow, if you lose oil pressure in the governor, the prop automatically goes to full forward, that is fine pitch. It's your only engine, so if something goes wrong with the prop governor you want the prop to be able to handle any torque you throw at it.
- In multiple engine planes with a variable pitch (AKA "constant speed") prop such as anything from the Piper Apache to large turboprops like the Dash-8, if you lose oil pressure in the governor, the prop automatically goes to full back, that is beyond coarse pitch to feather. This is because on a multi-engine prop you want the failed prop to feather and not induce any adverse yaw.

Quoting RussianJet (Thread starter):
What other situations is it of use in aside from engine failure?

I can't think of any but given the characteristics of props in an engine failure scenario, auto-feather is very important on many models. A failed engine with a prop full forward is the equivalent of having the same size solid disc as the propeller disc hanging out there on the wing producing drag and thus adverse yaw. This can quickly become quite dangerous.

Having flown an old light twin with a critical engine and no auto-feather (the Pa-23 Apache) I can tell you that if an engine fails (as so often seems to happen during flight training) the plane is downright nasty until you feather the prop. You are pushing so hard and so long on the rudder pedal that your leg starts aching while you go through the engine failure checklist. If you mismanage this situation you will plummet to your death. No kidding. Light twins are unforgiving machines when an engine fails. Auto-feather would have been nice at this point!
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Francoflier
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RE: Please Explain Prop-Feathering

Thu Jun 27, 2013 5:50 pm

Quoting RussianJet (Thread starter):
What other situations is it of use in aside from engine failure?

Free turbine engined aircraft use it on the ground. It allows to keep the engine turning at idle while producing zero thrust. Useful during ground maneuvering and quick turnarounds.

Props are also feathered before shutdown as otherwise the prop would keep spinning forever. (again, free-turbine engines)

[Edited 2013-06-27 10:52:49]
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SAAFNAV
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RE: Please Explain Prop-Feathering

Thu Jun 27, 2013 9:07 pm

With regards to how:

Normally there is a piston in the barrel of the prop, actuated by the engine oil pressure. This moves a cam forwards and backwards, which in turns imparts rotary movement on bezel gears to change the angle.

Here's 2 pictures of a C-130 prop schematic:



On larger aircraft, like a C-130 or P-3, you need oil pressure to feather the engine. But, since you have A/C busses to power, the Aux Feather Motors, it's not a problem. If all the oil leaks out, you are stuffed. A windmilling prop creates 6x the drag of a feathered one.

On smaller props, commonly found on PT'6 etc, the prop is feathered before you start.
During start, oil pressure builds up and rotates the blade angle to the idle position. An 'auto-feather' design is achieved by massive springs, as well as asymmetric counterweights. Due to centrifugal force, they will swing outward due to lack of a counter-pressure from the oil system.

This picture is from an An-2's prop.



Hope this helps.
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timz
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RE: Please Explain Prop-Feathering

Thu Jun 27, 2013 9:20 pm

Free turbine engines usually feather automatically when they're shut down-- but the Britannia didn't, did it? Why not?

But if it's not a free turbine, like the TPE331, the blades are in flat pitch when the engine shuts down-- the pilot has to latch them somehow into flat pitch before shutdown?
 
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Francoflier
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 1:34 am

Whether the prop feathers when the engine shuts down is not related to whether the engine is a free turbine or not.

Older designs tended to use oil pressure to push the prop to high pitch angles. Feathering them involved pumping oil into the hub, which meant using residual oil pressure or, when it failed, an auxiliary oil pump. Like SAAFNAV mentioned, if you had an oil leak, you were in trouble.

Nowadays, props tend to have springs and counterweights to allow them to go high pitch on their own when the oil pressure drops. That way it provides a built in autofeather system, although it is often complemented with an automatic system or auxiliary pumps or valves as well.

Quoting SAAFNAV (Reply 4):
On smaller props, commonly found on PT'6 etc, the prop is feathered before you start.
During start, oil pressure builds up and rotates the blade angle to the idle position.

Actually, on most PT-6 powered aircraft, the prop is kept feathered all the way through the start, until power is needed to move.
It is also manually feathered before shutdown. If the engine is shut with the prop in fine pitch on the ground, it will keep spinning for literally minutes as, without aerodynamic load on them, they will move very slowly into high pitch.
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RussianJet
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 1:46 am

Thanks very much for the highly informative posts chaps - much appreciated.
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SAAFNAV
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 5:01 am

Quoting francoflier (Reply 6):

Actually, on most PT-6 powered aircraft, the prop is kept feathered all the way through the start, until power is needed to move.
It is also manually feathered before shutdown. If the engine is shut with the prop in fine pitch on the ground, it will keep spinning for literally minutes as, without aerodynamic load on them, they will move very slowly into high pitch.

Oh ok, I didn't know that, I thought you just cut the fuel.

The C-130 prop was best described to us as 'suitable system of linkages and gears, with smoke, mirrors, magic and witchcraft.
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DashTrash
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 12:50 pm

Quoting Starlionblue (Reply 2):
large turboprops like the Dash-8, if you lose oil pressure in the governor, the prop automatically goes to full back, that is beyond coarse pitch to feather.

The blades on the PW120 powered Dash 8s are driven both directions by oil pressure. If you lose oil pressure, the feather pump (not the real name for it, can't remember and it's probably an accumulator anyway) will drive the blades into feather via the aux feather switch. You can also unfeather it that way. If you lose oil quantity, you're screwed. Prop will not feather because there are no springs in the system, just counterweights and oil.

I would hazard a guess that the Brasilia is the same way. Same engine and prop combination.

In training the drilled it into our heads not to question an oil pressure warning light (>45psi). If it comes on, you shut the engine down.
 
SAAFNAV
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 3:48 pm

Quoting RussianJet (Thread starter):
What other situations is it of use in aside from engine failure?

Cruise engine shutdown to extend endurance.. Mostly a military/coast guard procedure, no pax liner would do that.
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RussianJet
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 3:58 pm

Quoting SAAFNAV (Reply 10):
Cruise engine shutdown to extend endurance.. Mostly a military/coast guard procedure, no pax liner would do that.

That sounds odd. Even if a prop is feathered, surely it still creates a reasonable amount of drag? Would it really extend endurance by that much?
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DashTrash
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 4:45 pm

Quoting RussianJet (Reply 11):
That sounds odd. Even if a prop is feathered, surely it still creates a reasonable amount of drag? Would it really extend endurance by that much?

I can't give you a number, but a V1 cut in a Dash 8 and unfeathered prop requires full rudder deflection to keep it somewhat straight. Feather the prop and it's around half. Huge difference in drag between a windmilling prop and a feathered one.
 
timz
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 5:22 pm

Quoting francoflier (Reply 6):
Whether the prop feathers when the engine shuts down is not related to whether the engine is a free turbine or not.

Do any non-free-turbine aircraft leave the props feathered when shut down?
 
DashTrash
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 6:01 pm

Quoting timz (Reply 13):
Do any non-free-turbine aircraft leave the props feathered when shut down?

I'm not sure it's possible to start one if the prop is feathered. Too much drag on the shaft from spinning feathered blades and will cause a hot start.
 
SAAFNAV
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RE: Please Explain Prop-Feathering

Fri Jun 28, 2013 6:40 pm

Quoting RussianJet (Reply 11):

That sounds odd. Even if a prop is feathered, surely it still creates a reasonable amount of drag? Would it really extend endurance by that much?

Well, you shut down one engine to save on fuel. The only way to shut it down is by feathering it. That would be the only other reason to feather a prop besides a malfuntion.

Quoting DashTrash (Reply 14):

I'm not sure it's possible to start one if the prop is feathered. Too much drag on the shaft from spinning feathered blades and will cause a hot start.

True. On the T56, even with the blades fully fine, the compressor section has got bleed air valves to unload the compressor during start.
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Francoflier
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RE: Please Explain Prop-Feathering

Wed Jul 03, 2013 8:10 pm

Quoting timz (Reply 13):
Do any non-free-turbine aircraft leave the props feathered when shut down?

During normal shutdown on the ground, no. It's not necessary and you would overtemp the engine when starting.
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