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Feathering props

Old 22nd Feb 2017, 00:56
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Feathering props

Hi all.

While reading about the recent Melbourne Kingair accident, I've been seeing 'feathering' mentioned a lot, so trying to understand what it is (Non-pilot, so excuse my poor lingo here).

I've been googling, and it seems it's a way for the pilot to a) reduce drag on a failed engine and b) protect the engine from windmilling (Possibly causing further damage). And what happens is the blades are angled so that they cut cleanly into the airflow (So, leading edge of the prop into the wind?). And that helps cause wind resistance in the spinning direction, as the blade is flat (Like a cricket bat? Unless you're playing a cut shot.. )

And if all is OK, (i.e. You have time), you would then attempt to restart it.

Would you un-feather .. (de-feather... not sure what the term is) it to start it, as now it's in a resistance mode (Trying to stop it from spinning). Or is the engine powerful enough not to care about that. I'd have guessed that running a prop when it's feathered would strain the engine, and be pretty useless for providing thrust? So, would the engine be started while feathered.. and then ... de-feathered completely to run normal again?

And then auto-feather - does that mean the engine is feathered automatically in the event of a failure? If so, why would you ever disable auto-feather?
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Old 22nd Feb 2017, 02:39
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Yes you are correct about the reasons for feathering. Primarily it is a means to reduce drag, in terms of restarting mid air it depends on the aircraft type. For my type when a relight is attempted the propeller is moved out of the feather position by the unfeather pump. The windmill effect you speak of actually is used to provide initial acceleration (usually done on the ground with the starter generator). This is only ever done at altitude where the effects of the windmill are safe. In terms of auto feather this is engaged on takeoff and landing but disengaged in the cruise.
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Old 22nd Feb 2017, 02:41
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If it's a recip engine, unfeathering the prop usually creates enough torque to restart the engine, provided it is not badly damaged. Think of a rolling start with a clutch and manual transmission in a car or truck.

Single shaft turbines can also be restarted this way, because th prop torque is directly coupled to the compressor rotor. With a two- or three-shaft machine, no such luck, although the engine starter may be used.
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Old 23rd Feb 2017, 01:59
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Single shaft turbines can also be restarted this way, because th prop torque is directly coupled to the compressor rotor. With a two- or three-shaft machine, no such luck, although the engine starter may be used.
The Pratt & Whitney PT6A-41/42 turbine installed in a King Air is a free turbine, not a fixed or single shaft turbine. The propeller and power turbine are not mechanically connected to the Compressor Turbine and Compressor.
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Old 23rd Feb 2017, 02:02
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you would have to think long and hard before restarting a turbine. Turbines are extremely reliable and for one to shut down or fail there must be a very good reason for it.
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Old 23rd Feb 2017, 02:16
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Thanks pinkpanther1 - When you mention safe conditions for the windmilling, is that because of the drag it causes - resulting in a lot of yaw towards the dead engine?
Also, why would you ever want auto-feather disengaged? Is there a time that, if the engine fails, you wouldn't want it to feather?
Thanks for explaining.

Thanks barit1 - the 'push starting a car' is a good way of explaining it for us earth-bound people. Makes sense.
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Old 23rd Feb 2017, 03:53
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... is that because of the drag it causes - resulting in a lot of yaw towards the dead engine?
Yes, the drag is so significant on all such twin engine types that the aircraft cannot maintain its altitude. All propeller powered aircraft (with 2 or more engines) have the ability to feather the props of failed engines for this reason.

Yes, there is yaw towards the dead engine.

... why would you ever want auto-feather disengaged?
Things get very interesting when you try to answer this question. From what I have read in this thread, it appears this operator had a policy of taking off with autofeather (AFX) disengaged - for the reason that other King Airs in their fleet were not AFX equipped and they wanted the takeoff procedure to be standard across the fleet. There are sound safety arguments behind this approach.

But, on the other hand, it can be said that to intentionally disable a major safety feature such as AFX is foolhardy! Takeoff is the phase of flight for which the AFX is primarily intended.

Is there a time that, if the engine fails, you wouldn't want it to feather?
Yes, after takeoff, when the aircraft has climbed to a comfortable altitude (say 5000-10,000 feet). In this case, with the possibility of being able to restart the engine, it is more simple to do it with the propeller windmilling, rather than feathered.
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Old 23rd Feb 2017, 04:19
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you would have to think long and hard before restarting a turbine. Turbines are extremely reliable and for one to shut down or fail there must be a very good reason for it.
Gets interesting with a fixed shaft engine such as a Garrett? Engine components start disembarking at a rapid pace!
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Old 23rd Feb 2017, 04:37
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it can be said that to intentionally disable a major safety feature such as AFX is foolhardy!
On the King Air with the Raisbeck mods it is mandatory to have serviceable auto feather and to use it.
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Old 23rd Feb 2017, 11:08
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On the King Air with the Raisbeck mods it is mandatory to have serviceable auto feather and to use it.
That is interesting as VH-ZCR appears to have a Raisbeck conversion:

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Old 23rd Feb 2017, 11:39
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FGD
From what I have read in this thread, it appears this operator had a policy of taking off with autofeather (AFX) disengaged - for the reason that other King Airs in their fleet were not AFX equipped and they wanted the takeoff procedure to be standard across the fleet. There are sound safety arguments behind this approach.
I think that comment from the EN thread (is that the thread you were referencing?) was in relation to the AAV crash, not the EN crash, but please correct me if I'm wrong.


Megan,
Please let me just clarify your statement, you can have "Raisbeck mods" without requiring the AFX on the B200. James developed more than just props on the B200!

A more correct statement would be B200s fitted with Hartzell-Raytheon or Hartzell-Raisbeck 4 blade props require auto feather.
A B200 fitted with McCauley 4-blade props does not.

Last edited by Car RAMROD; 23rd Feb 2017 at 11:59.
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Old 23rd Feb 2017, 12:36
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FDG135 - technically there are twin-engine aircraft where the propellers can't be feathered. An example:

https://en.wikipedia.org/wiki/Champion_Lancer


Cralis - with regards to reasons for having it disengaged, I can see two (in addition to the one posted above).

First, it doesn't always work. If you're expecting it to feather automatically, and it doesn't, then in a situation that's already extremely busy you're going to have a plane that's not behaving how you expect (because the propeller isn't feathered) and another action (manually feathering it) to deal with. Better to just have it disabled, so that you fully expect that it'll need to be handled manually.

Second, I know some pilots are wary of automatic systems. Better, perhaps, to have a plane that's behaving exactly how you expect than to have one that's trying to make things better for you and just adding to the confusion. We saw this sort of thing in the Asiana 777 crash; the pilot assumed that the automatic systems were taking care of airspeed control, and didn't realise that they weren't until it was too late to recover.
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Old 23rd Feb 2017, 13:06
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On the King Air with the Raisbeck mods it is mandatory to have serviceable auto feather and to use it.
So, what law would you be breaking if you chose not to arm the AFX prior to taking off in your 4 blader King Air?
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Old 23rd Feb 2017, 13:18
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138- Pilot to comply with requirements of aircraft's flight manual etc


The AFX requirement is stated in the Limitations section.


I would also suspect that if you ignored it, killed people etc, you'd probably be up for negligence, misconduct etc.

There's a reason why the equipment is certified with that requirement. Do some people really think they know better?
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Old 23rd Feb 2017, 13:20
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Thanks for the clarification Ramrod, but since we are talking auto feather I took the assumption folks would read "Raisbeck mods" as referring to engine/props. My error. The Raisbeck mentions -52 engines and Quiet Turbofan Props, whereas ZCR had -42, so not applicable? Is it a prop thing, engine thing, or combination of both? And for what reason to make auto feather mandatory over and above the stock standard machine?
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Old 23rd Feb 2017, 13:20
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... technically there are twin-engine aircraft where the propellers can't be feathered. An example:
Slatye,
Ha! I just knew somebody would come up with some ultra obscure twin that had fixed pitch props - and that it would be British!

Well, in this case this type is American. A quick look over that Wikipedia page brought a few laughs - thanks! Here are a few quotes from that page:

The Lancer's performance when flying on a single engine is notably poor; in a column for AOPA Pilot, author Barry Schiff summarized the airplane's single-engine performance by writing that "...it doesn’t have any".
The engine nacelle placement hampers visibility,[3][4][6] particularly for the rear-seat pilot,[4] and for both pilots during banked turns.[3] Schiff compares the engine nacelles to "...horse blinders that [result] in disorienting tunnel vision".[4]
The close proximity of the engines and propellers to the front-seat pilot's head create elevated noise levels described as "remarkable"[3] or even "paralyzing".[6]
Its lackluster performance is generally attributed to abundant form drag from the wing and tailplane struts and unusually large strut-braced fixed main landing gear legs, which are about 3 inches (76 mm) in diameter and 5 feet (1.5 m) long.[2][3][4]
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Old 23rd Feb 2017, 13:26
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The Raisbeck mentions -52 engines and Quiet Turbofan Props, whereas ZCR had -42, so not applicable.
Still applicable, Megan - applies in all cases where the prop has four blades. That is my understanding.
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Old 23rd Feb 2017, 13:29
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No probs Megan. I took it as the props but thought I'd get a little pedantic
You can get Raisbeck props on -42 too

If I remember correctly, the D4N-3A designator is the Hartzell-Raisbeck, and the E4N-3G is Hartzell-Raytheon.
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Old 23rd Feb 2017, 14:07
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Nothing to do with feathering props but in line with the philosophy of training in case of a safety system ( auto feathering props ) not working . Emirates used to do all it's single engine work on the B777 in the sim with the TAC failed . The TAC is a computer that senses thrust diffencial between engines and automatically adds rudder to assist the pilot with control.
After a few real life engine failures where the TAC worked as designed and the pilots massively over controlled the aircraft . This training philosophy was revoked. ( till the next flight training manager anyway )
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Old 23rd Feb 2017, 23:54
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Thanks, Slatye - that makes a hell of a lot of sense, as to why you wouldn't/shouldn't leave it engaged all the time.

I use X-Plane, a flight sim (I know - pa! Not the real thing! But it's the best I can do for now...). And on the KingAir, I've always thought this thing (circled in red) had something to do with feathering. I'm now guessing I'm wrong, but might go a bit crazy when you feather an engine?

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