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Faro
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FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 10:28 am

I believe it is not the case today but maybe sometime in the future, can regulators take into account the effect of FBW flight control systems with regard to neutralising pitch/yaw oscillations in the design of new aircraft so that one can implement smaller/lighter empennage surfaces?

Faro
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tdscanuck
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 2:30 pm

Quoting faro (Thread starter):

I believe it is not the case today but maybe sometime in the future, can regulators take into account the effect of FBW flight control systems with regard to neutralising pitch/yaw oscillations in the design of new aircraft so that one can implement smaller/lighter empennage surfaces?

It is the case today, and yes they do.

Tom.
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 3:05 pm

Quoting tdscanuck (Reply 1):
It is the case today, and yes they do.

Do current production FBW airliners possess sufficient natural static and dynamic stability to be safely flown by the pilot in the unlikely event that the flight control system, while redundant, suffers a complete failure?
 
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Faro
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 3:27 pm

Quoting airbuske (Reply 2):
Quoting tdscanuck (Reply 1):
It is the case today, and yes they do.

Do current production FBW airliners possess sufficient natural static and dynamic stability to be safely flown by the pilot in the unlikely event that the flight control system, while redundant, suffers a complete failure?

Yes, my question exactly. Not an expert but I thought that today the regulator requires a minimum level of natural, non-FBW stability. If that is true it entails an empennage of a certain minimum size. How then can you reduce that via FBW?

Faro
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roseflyer
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 6:20 pm

Quoting airbuske (Reply 2):
Do current production FBW airliners possess sufficient natural static and dynamic stability to be safely flown by the pilot in the unlikely event that the flight control system, while redundant, suffers a complete failure?

The only fly by wire airplane that can be controlled with a full flight control electronic system failure is the 777. It has backup pitch via stabalizer cable, roll via spoilers cables and yaw via engines. Its crude, but can allow the pilot to aim the imminent crash at something soft or give them some time to get the primary or backup systems working again.

A complete flight control system failure is close enough to statistically impossible that airplanes can be certified. The rules are extremely strict with independent power sources, wire separation, computers built by different manufacturers using different code, etc. You have to lose engines, fail a RAT and drain the batteries to get something like that to happen.

Load alleviation using FBW controls allows for smaller control surfaces and less structure in an airplane since the plane can by limited in certain conditions whereas it cannot be done with cable controls. It is all very specific to each airplane, but things like acceleration limits can significantly reduce the stress on an airplane. An accident like the AA A300 in Queens would be far less likely to happen with a thoroughly modern FBW load alleviation system in place.
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 6:39 pm

Quoting RoseFlyer (Reply 4):
The only fly by wire airplane that can be controlled with a full flight control electronic system failure is the 777.

Incorrect. That can be achieved on all FBW aircraft, from the Airbus A320 to the latest Bombardier.
Moreover, you should get a look at the A380 flight control architecture and its unmatched - for some time - redundancy.
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Northwest727
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 7:16 pm



Quoting Pihero (Reply 5):
Incorrect. That can be achieved on all FBW aircraft, from the Airbus A320 to the latest Bombardier.
Moreover, you should get a look at the A380 flight control architecture and its unmatched - for some time - redundancy.

I thought with "full flight control electronic system failure," the only things the pilot could do in a Airbus (not including the A380) would be to control the pitch via the trim wheel, and use the rudder and differential thrust for lateral control (yaw). Wouldn't that imply no roll control? I'm no Airbus pilot though, so I do not know the procedures in place.

Edit: Incorrect quote reference

[Edited 2011-02-08 11:23:14]
 
David L
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 7:23 pm

Quoting Pihero (Reply 5):

Phew. I was beginning to think I'd imagined all the previous discussions over the past 11 years here! I seem to recall that the rudder is still available on the Airbuses, via cables and hydraulics... though it's about time I was spectacularly wrong again.
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 7:26 pm

Quoting Northwest727 (Reply 6):
Wouldn't that imply no roll control?

But wouldn't that be coupled to yaw? Not ideal, perhaps, but workable in an emergency?
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 7:26 pm

Quoting Pihero (Reply 5):

Incorrect. That can be achieved on all FBW aircraft, from the Airbus A320 to the latest Bombardier.
Moreover, you should get a look at the A380 flight control architecture and its unmatched - for some time - redundancy.

How does the A320 have roll control? I was under the impression, the 777 is the only FBW plane with cables to the wing.

I'd be really curious to learn more about the A380. Never really seen an architecture map of it.
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 7:44 pm

Quoting faro (Thread starter):
I believe it is not the case today but maybe sometime in the future, can regulators take into account the effect of FBW flight control systems with regard to neutralising pitch/yaw oscillations in the design of new aircraft so that one can implement smaller/lighter empennage surfaces?

That could be an important factor in making blended wing body aircraft more efficient. They lack the long moment arm to mount the tail on, so they will probably require larger and heavier vertical stabilizers, unless they can be certified with relaxed stability. Manufacturers could probably save a fair bit of weight if they can reduce the size of the stabilizers.
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 8:52 pm

Quoting David L (Reply 7):
Phew. I was beginning to think I'd imagined all the previous discussions over the past 11 years here! I seem to recall that the rudder is still available on the Airbuses, via cables and hydraulics... though it's about time I was spectacularly wrong again.


Why would Airbus want to have the "rudder" as the last functioning control surface? Of the "primary controls" the rudder is the least used and required control surface.
 
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Faro
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 9:15 pm

Getting a little back on topic, one simple question: assuming one had sufficient actuation authority with the FBW system out, are the A320/A330/777/etc stable in pitch and yaw or not? Are their empennages sufficiently big to naturally dampen oscillations on their own or not?

Faro
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Pihero
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 08, 2011 11:53 pm

Quoting Northwest727 (Reply 6):
I thought with "full flight control electronic system failure," the only things the pilot could do in a Airbus (not including the A380) would be to control the pitch via the trim wheel, and use the rudder and differential thrust for lateral control (yaw). Wouldn't that imply no roll control?

Roll control is achieved through the rudder pedals as * induced roll* : push the right rudder, for instance, you generate yaw to the right, then your left wing will go faster than the right one, therefore generating more lift, thence a bank to the right.
One doesn't need to use engine differential thrust for turns as the rudder is powerful enough (as a matter of fact, that situation makes one realise how powerful these control surfaces are (the rudder and the stabiliser) as you'd only need very small inputs to fly the airplane.

Quoting RoseFlyer (Reply 9):
I was under the impression, the 777 is the only FBW plane with cables to the wing.

Those cables are linked to the spoilers 4 and 11 IIRC on each wing. Other cables link the trim wheel to the stabiliser.

Quoting RoseFlyer (Reply 9):
I'd be really curious to learn more about the A380. Never really seen an architecture map of it.

See this, then A380 Flight controls
The A380 doesn't have a manual back-up but a dedicated independent system, a simpler FBW in fact called *Electrical back-up*

Quoting 474218 (Reply 11):
Why would Airbus want to have the "rudder" as the last functioning control surface? Of the "primary controls" the rudder is the least used and required control surface.

It's not *the last* control surface as the THS is also used in this back-up mode.
Remember the 747 which lost its entire fin ? It was quite unflyable. That fin, on a *normal airplane* - not a flying wing - is essential for stability..
It is interesting to note that the rudder mechanical back-up disappeared on the 340-600, for an electrical one.

Quoting David L (Reply 7):
Phew. I was beginning to think I'd imagined all the previous discussions over the past 11 years here! I seem to recall that the rudder is still available on the Airbuses, via cables and hydraulics

Long memory, David !

Quoting faro (Reply 12):
assuming one had sufficient actuation authority with the FBW system out, are the A320/A330/777/etc stable in pitch and yaw or not? Are their empennages sufficiently big to naturally dampen oscillations on their own or not?

Yes. As a matter of fact, one has too much authority if anything.
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474218
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 2:43 am

Quoting Pihero (Reply 13):
It's not *the last* control surface as the THS is also used in this back-up mode.
Remember the 747 which lost its entire fin ? It was quite unflyable. That fin, on a *normal airplane* - not a flying wing - is essential for stability..


By the "fin" I assume you mean the vertical stabilizer, the loss of which is a completely different than losing use of the rudder.

The rudder is the least required "primary flight control surface". Provided it will fair when power it is lost, safe flight is possible.

Remember the Aircoupe doesn't even have a rudder!
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 6:19 am

Quoting airbuske (Reply 2):
Do current production FBW airliners possess sufficient natural static and dynamic stability to be safely flown by the pilot in the unlikely event that the flight control system, while redundant, suffers a complete failure?

Yes. The problem in the event of complete control failure is lack of control, not loss of stability.

Quoting faro (Reply 3):
Not an expert but I thought that today the regulator requires a minimum level of natural, non-FBW stability.

They do. But it's a lot lower than the level you have with all the FBW active. The certification standard for failures is safe operation and landing...it's OK if the handling sucks or causes a high crew workload, provided that it will still get you home.

Quoting faro (Reply 3):
If that is true it entails an empennage of a certain minimum size. How then can you reduce that via FBW?

It's the difference between normal flight characteristics and characteristics with the FBW partly failed. With non-FBW, the systems are basically the same (minus stuff like yaw dampers). With FBW, you can get artificial stability through closing control loops, so you can achieve the same flight characteristics with smaller surfaces. With loss of the FBW you revert to the natural stability of the aircraft, which can be considerably worse than with the FBW on but it still stable and controllable.

Quoting faro (Reply 12):
assuming one had sufficient actuation authority with the FBW system out, are the A320/A330/777/etc stable in pitch and yaw or not

Yes.

Quoting faro (Reply 12):
Are their empennages sufficiently big to naturally dampen oscillations on their own or not?

Yes, other than phugoid, but that's usually almost undamped on all designs anyway so nobody really cares.

Quoting Pihero (Reply 13):
The A380 doesn't have a manual back-up but a dedicated independent system, a simpler FBW in fact called *Electrical back-up*

This is where it gets thorny to talk about "full flight controls failure." There are FBW systems out there, like the A380 and 787, where complete and total failure of all electrical power on all buses will kill you. But those systems have so many different levels of power and actuator redundancy that the probability of simultaneous failure of all systems is vanishingly small.

Tom.
 
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Faro
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 8:26 am

Quoting tdscanuck (Reply 15):
Quoting faro (Reply 3):
Not an expert but I thought that today the regulator requires a minimum level of natural, non-FBW stability.

They do. But it's a lot lower than the level you have with all the FBW active. The certification standard for failures is safe operation and landing...it's OK if the handling sucks or causes a high crew workload, provided that it will still get you home.

Interesting, so gusty conditions/wind shear/etc are out of the question. Interesting to note Pihero's dissenting opinion in the last comment in reply 12...

Which I guess is rather academic anyway since the probability of losing all FBW control is ridiculously remote. Can someone kindly confirm that no such total FBW control loss has occurred on any FBW type since the introduction of the A320 in the late 1980's?

Faro

[Edited 2011-02-09 00:49:48]

[Edited 2011-02-09 00:52:52]
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 9:03 am

There was an article about the different Airbuses in the french magazine Air&Cosmos, they relate one incident where a A320 pilot accidentilly switched of the FBW system during cruise. As the aircraft was naturally stable the crew flew the aircraft with the trim+rudder until the system had rebooted, no-one in the cabin felt anything. I suppose it should be true.
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 9:04 am

Quoting faro (Reply 16):

Which I guess is rather academic anyway since the probability of losing all FBW control is ridiculously remote. Can someone kindly confirm that no such total FBW control loss has occurred on any FBW type since the introduction of the A320 in the late 1980's?

I may be missing some incident but I believe such an incident has never occurred. No FBW Airbus or Boeing has ever crashed due a problem with the flight control systems.
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David L
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 10:13 am

Quoting 474218 (Reply 14):
The rudder is the least required "primary flight control surface". Provided it will fair when power it is lost, safe flight is possible.

Doesn't that assume the ailerons are functioning?
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 10:27 am

Quoting 474218 (Reply 14):
The rudder is the least required "primary flight control surface". Provided it will fair when power it is lost, safe flight is possible.

I don't know about more or less required flight control. All I know is that without rudder, your flight stability is degraded - as every bank will be in most cases induce reverse yaw and a sideslip into the turn.
On a multi engine airplane, an engine failure will very quickly make you totally and utterly change your mind about the usefulness of a powerful rudder ! Believe me, you'll need it a lot more than anything else !

Quoting 474218 (Reply 14):
Remember the Aircoupe doesn't even have a rudder!

Incorrect : It has the control surfaces ; what it lacks is the *rudder pedals*, replaced by a system linking the control wheel to the ailerons and rudders for well coordinated turns.

Quoting faro (Reply 16):
Quoting tdscanuck (Reply 15):
Quoting faro (Reply 3):
Not an expert but I thought that today the regulator requires a minimum level of natural, non-FBW stability.

They do. But it's a lot lower than the level you have with all the FBW active. The certification standard for failures is safe operation and landing...it's OK if the handling sucks or causes a high crew workload, provided that it will still get you home.

Interesting, so gusty conditions/wind shear/etc are out of the question. Interesting to note Pihero's dissenting opinion in the last comment in reply 12...

This is where the discussion gets interesting : we have two ways of defining FBW :
1/- all electrics are lost and we're left with manual back-up . I persist in saying that one has sufficient authority on the aiplane three axis (just remember that the A320 stabiliser, as one single moving pitch control is bigger than a DC-3 wing).
2/- FBW is about control laws, so if we revert to direct control, i.e. a stick movement is proportional to a flight surface deflection, therefore making the piloting of the aircraft in that situation no differet from any "classical" airplane. In this case, I personally have not seen / experienced / felt any stability problem with the flying.
Now, is the 'Bus fin / rudder smaller, relatively speaking, than the 737 ? I cannot say. Visually, I'd say they're quite equivalent. But it's just me.

[Edited 2011-02-09 02:29:49]

[Edited 2011-02-09 02:32:32]

[Edited 2011-02-09 02:34:49]
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Klaus
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 11:30 am

Quoting Pihero (Reply 13):
See this, then A380 Flight controls

Very interesting to see the stabilizer trim screws of the A380, A340 and A320 side by side for comparison...
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 11:47 am

Quoting faro (Reply 16):
Can someone kindly confirm that no such total FBW control loss has occurred on any type since the introduction of the A320 in the late 1980's?

I actually don't think that is possible, but somebody correct me if I'm wrong. Could it have been that they accidently turned of some flight control reverting the airplane back to Direct Law?
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Starlionblue
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 3:28 pm

Quoting travelavnut (Reply 22):
Quoting faro (Reply 16):
Can someone kindly confirm that no such total FBW control loss has occurred on any type since the introduction of the A320 in the late 1980's?

I actually don't think that is possible, but somebody correct me if I'm wrong. Could it have been that they accidently turned of some flight control reverting the airplane back to Direct Law?

Sorta depends how you define it. Is reversion to Direct Law a failure of the FBW system if it doesn't lead to a crash? I mean, the system has managed to activate its own fail-safe if you will.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
travelavnut
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 3:53 pm

Quoting Starlionblue (Reply 23):
Quoting travelavnut (Reply 22):
Quoting faro (Reply 16):
Can someone kindly confirm that no such total FBW control loss has occurred on any type since the introduction of the A320 in the late 1980's?

I actually don't think that is possible, but somebody correct me if I'm wrong. Could it have been that they accidently turned of some flight control reverting the airplane back to Direct Law?

Sorta depends how you define it. Is reversion to Direct Law a failure of the FBW system if it doesn't lead to a crash? I mean, the system has managed to activate its own fail-safe if you will.

Reading back I see I made a mistake and quoted the wrong post   My post should have been;

Quoting ferpe (Reply 17):
There was an article about the different Airbuses in the french magazine Air&Cosmos, they relate one incident where a A320 pilot accidentilly switched of the FBW system during cruise. As the aircraft was naturally stable the crew flew the aircraft with the trim+rudder until the system had rebooted, no-one in the cabin felt anything. I suppose it should be true.

I actually don't think that is possible, but somebody correct me if I'm wrong. Could it have been that they accidently turned of some flight control computers reverting the airplane back to Direct Law?
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Pihero
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 5:31 pm

Quoting ferpe (Reply 17):
There was an article about the different Airbuses in the french magazine Air&Cosmos, they relate one incident where a A320 pilot accidentilly switched of the FBW system during cruise.
Quoting travelavnut (Reply 24):
I actually don't think that is possible, but somebody correct me if I'm wrong.

Everything is possible but here, to have a full reversion to mechanical back-up means at least the loss of ELAC 1 and 2 and SEC 1 and 2... and these lightswitches are separateed on the overhead panel -ELAC 1 and SEC 1, on the left, ELAC 2 and SEC 2 on the right... You'd really need to want these swit-ches off to do so !

Quoting travelavnut (Reply 24):
Could it have been that they accidently turned of some flight control computers reverting the airplane back to Direct Law?

Careful, as in *Direct law*, one has pretty much a normal airplane with a stick and a rudder pedals set. It is hardly an emergency situation.
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PGNCS
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 9:31 pm

Quoting travelavnut (Reply 22):
Quoting faro (Reply 16):
Can someone kindly confirm that no such total FBW control loss has occurred on any type since the introduction of the A320 in the late 1980's?

I actually don't think that is possible, but somebody correct me if I'm wrong. Could it have been that they accidently turned of some flight control reverting the airplane back to Direct Law?

It is entirely possible if you turn off the ELACs and SECs.

Quoting Starlionblue (Reply 23):
Is reversion to Direct Law a failure of the FBW system if it doesn't lead to a crash?

No. A failure of some type has occurred (could be hydraulic, electric, or many other things) causing the FBW to work in a more basic mode, but be assured that the FBW is still working and commanding the control surfaces of the aircraft to move based on pilot inputs.

Quoting travelavnut (Reply 24):
Quoting ferpe (Reply 17):
There was an article about the different Airbuses in the french magazine Air&Cosmos, they relate one incident where a A320 pilot accidentilly switched of the FBW system during cruise. As the aircraft was naturally stable the crew flew the aircraft with the trim+rudder until the system had rebooted, no-one in the cabin felt anything. I suppose it should be true.

I actually don't think that is possible, but somebody correct me if I'm wrong. Could it have been that they accidently turned of some flight control computers reverting the airplane back to Direct Law?

Again, it's possible. You can make FBW cease to function by turning off the flight control computers. Direct Law does not work if the computers are turned off or inoperative. The airplane is still flyable as Pihero has stated, but if you turn off the ELACs and SECs your sidestick is out of business.

Quoting Pihero (Reply 25):
Quoting travelavnut (Reply 24):
Could it have been that they accidently turned of some flight control computers reverting the airplane back to Direct Law?

Careful, as in *Direct law*, one has pretty much a normal airplane with a stick and a rudder pedals set. It is hardly an emergency situation.

Correct, and well said! (Though the malfunctions that caused the reversion to Direct Law might make declaring an emergency prudent...)  
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 9:35 pm

Quoting Pihero (Reply 25):
Quoting travelavnut (Reply 24):
Could it have been that they accidentally turned of some flight control computers reverting the airplane back to Direct Law?

Careful, as in *Direct law*, one has pretty much a normal airplane with a stick and a rudder pedals set. It is hardly an emergency situation.

I know, but I thought maybe the article got it wrong and mistook going to Direct Law for actually turning off the complete control system. It´s clear that in this case it was a aviation/space magazine, but even aviation journalist get it wrong sometimes   Also I thought it wasn´t possible to completly turn off the FBW system during cruise (just guessing on my part though).

Reading your answer I realize it is actually possible but the chance of doing it by accident seems quite remote to say the least.

[Edited 2011-02-09 13:37:13]
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Pihero
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 11:14 pm

Quoting travelavnut (Reply 27):
I realize it is actually possible but the chance of doing it by accident seems quite remote to say the least.

Nothing better than a picture. See the overhead panel
View Large View Medium
Click here for bigger photo!

Photo © Robert Domandl


the (very) small flight control panels are
- on the left, below the ADIRS panel. We have here ELAC 1 / SEC 1 / FAC1
-on the right, underneath the *Power-up sequence* placard : ELAC 2 / SEC 2 / SEC 3 / FAC 2.
One really has to deliberately press on these switchlights to cancel them.
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Starlionblue
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RE: FBW Flight Controls To Reduce Empennage Size?

Wed Feb 09, 2011 11:43 pm

Quoting PGNCS (Reply 26):
Quoting Starlionblue (Reply 23):
Is reversion to Direct Law a failure of the FBW system if it doesn't lead to a crash?

No. A failure of some type has occurred (could be hydraulic, electric, or many other things) causing the FBW to work in a more basic mode, but be assured that the FBW is still working and commanding the control surfaces of the aircraft to move based on pilot inputs.

I was sort of being rhetorical.  
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tdscanuck
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RE: FBW Flight Controls To Reduce Empennage Size?

Sat Feb 12, 2011 6:18 am

Quoting faro (Reply 16):
Interesting, so gusty conditions/wind shear/etc are out of the question.

Not at all...it's just more work. Going to direct mode is, at worst, doing direct position-to-position coupling between stick/yoke and control. The airplane is still statically and dynamically stable in all axes (not counting pitch phugoid), so it's completely flyable. It's just going to take a lot more constant corrections by the flight crew since they have to cover all all the artificial stability that was lost when the primary flight control laws dropped out.

Quoting faro (Reply 16):
Interesting to note Pihero's dissenting opinion in the last comment in reply 12...

I don't think he's dissenting...he was talking about surface authority (which is typically *larger* in direct mode), I was talking about natural damping (which is typically *smaller* in direct mode).

Tom.
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Sat Feb 12, 2011 2:28 pm

Quoting BMI727 (Reply 10):

That could be an important factor in making blended wing body aircraft more efficient.

I do not think that a BWB could be safely flown without FBW; aerodynamically it will be very similar to the flying wing. Northrup's first flying wing designs proved to be dangerously unstable and pretty much unrecoverable in a stall, which is primarily why they were not adopted at the time. It was FBW that made the B2 possible, and I expect that it will be equally necessary for any civilian adaptation.
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tdscanuck
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RE: FBW Flight Controls To Reduce Empennage Size?

Sat Feb 12, 2011 7:44 pm

Quoting SEPilot (Reply 31):

I do not think that a BWB could be safely flown without FBW; aerodynamically it will be very similar to the flying wing.

A flying wing, and BWB, can be perfectly safely flown without FBW.

Quoting SEPilot (Reply 31):
Northrup's first flying wing designs proved to be dangerously unstable and pretty much unrecoverable in a stall

It can be unstable/unrecoverable once you actually stall it...there's no circumstance in which a commercial airliner is supposed to actually stall (other than testing) and several ways of guarding against it. A simple alpha-limiter (which can be implemented in FBW but doesn't have to be) is all it would take.

Quoting SEPilot (Reply 31):
It was FBW that made the B2 possible, and I expect that it will be equally necessary for any civilian adaptation.

B2 had the *huge* additional restriction of having no vertical control surfaces. That's what drove them to requiring FBW, it's got essentially zero native yaw stability. Even the Northrop flying wing, although it had no explicit vertical fin, had reasonable vertical surface area from the engine supports.

I haven't seen a credible commercial BWB design yet that doesn't have some combination of winglets and engine pylons that make it very different from a B2 in terms of stability. A BWB can be stable in all axes up to stall, and probably stable post stall with 50+ extra years of aerodynamics under our belt. Although pretty much everyone is going full FBW now, so it's probably a moot point.

Tom.
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Sat Feb 12, 2011 8:21 pm

Quoting tdscanuck (Reply 32):

A flying wing, and BWB, can be perfectly safely flown without FBW.

I thought I had read that the flying wing was unstable, and that was the primary reason for its failure. But I will defer to your much greater knowledge of the subject.
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RE: FBW Flight Controls To Reduce Empennage Size?

Sat Feb 12, 2011 11:04 pm

Quoting SEPilot (Reply 31):
I do not think that a BWB could be safely flown without FBW; aerodynamically it will be very similar to the flying wing.

A BWB can be done without FBW. But, it would be better with it, since the required size of the vertical surfaces will be reduced, saving weight.
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RE: FBW Flight Controls To Reduce Empennage Size?

Sun Feb 13, 2011 2:58 am

Quoting SEPilot (Reply 33):
I thought I had read that the flying wing was unstable, and that was the primary reason for its failure. But I will defer to your much greater knowledge of the subject.

I think it was. But it was also built 60 years ago. As Tom says aerodynamics have made a couple of steps forward since then.
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RE: FBW Flight Controls To Reduce Empennage Size?

Sun Feb 13, 2011 9:58 am

Quoting RoseFlyer (Reply 9):

How does the A320 have roll control? I was under the impression, the 777 is the only FBW plane with cables to the wing.

You can roll an aircraft with the rudder. Looks ugly, feels ugly, but works.
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RE: FBW Flight Controls To Reduce Empennage Size?

Sun Feb 13, 2011 10:17 am

Quoting faro (Thread starter):
I believe it is not the case today but maybe sometime in the future, can regulators take into account the effect of FBW flight control systems with regard to neutralising pitch/yaw oscillations in the design of new aircraft so that one can implement smaller/lighter empennage surfaces?

Tails are usually not sized by stability requirements. If it was for stability in cruise, the empenage could be mush smaller.

The vertical tail is usually sized by engine failure at take-off and the requirement to keep the aircraft on the runway at limited speed (VMCG - V Minimum Control Ground). Larger vertical tail of course helps lateral stability in flight, but modern (or not so modern, been around for nearly 50 years now) yaw dampers can compensate. Without yaw dampers swept wing aircraft would fly funny and the supply of sick bags needs to be increased.

Same for the horizontal tail. An A380 is not naturally stable in cruise. As soon as it took off, the center of gravity is shifted and the aircraft flies with a slight negative stability margin. Of course, the tail contributes drag and weight.
The size is again determined by other criteria, which cannot be changed by fancy fly by wire. Getting the nose wheel off the runway with one engine failed at most unfavorable CG position and low speed is the key criteria.
Horizontal tail sizing is a compromise between operational flexibility (possible CG location) and weight & drag.

So, FBW (or more correct: flight control laws - you could technically have a full authority EFCS that pulls cables instead of transmitting its inputs via electronic signals) do help and have made aircraft more efficient. But stability in (normal) flight is not the sizing criteria for today's civil airliner's tail. Many requirements emerge from take-off and landing with failed engine. Think of an ugly situation like one engine go-around, fully configured, with CG at most forward position.
The disadvantage is that the tails are a bit too powerful in normal flight. That is why the control authority on the vertical tail is limited by systems. The A380 again sets a new mark by introducing the first command control system for lateral control on an airliner. It works so well that pilots might miss an engine failure in cruise. To prevent that a system is in place that yaws the aircraft into the failed engine, so the pilot can correct. Without that the aircraft would simply keep the sideslip at zero degrees.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Sun Feb 13, 2011 8:06 pm

Quoting SchorschNG (Reply 37):
Same for the horizontal tail. An A380 is not naturally stable in cruise. As soon as it took off, the center of gravity is shifted and the aircraft flies with a slight negative stability margin.

Sorry, I find that hard to believe, if you mean negative stability margin = reduced static stability I can be with you but if you mean if we cut the inner loop of the FBW the A380 would diverge abruptly in the lateral plane I just can't believe you, for 2 reasons:

1. There is not point in having negative margin, the stabilizer would have to counter the divergence of the A/C all the time, this would cause trim drag and thus negate the idea of reduced stability margin. At an extreeme neutral margin would be more sensible as it minimizes trim drag.

2. I don't think you can certify a negative margin civil airliner, especially as the FBW is based on that from the A320 and A330/340 which both are naturally stable. That FBW architecture is not disigned to be 100% fail-safe like the military ones.

Please give proof of you claim.
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RE: FBW Flight Controls To Reduce Empennage Size?

Sun Feb 13, 2011 9:18 pm

Quoting ferpe (Reply 38):
2. I don't think you can certify a negative margin civil airliner, especially as the FBW is based on that from the A320 and A330/340 which both are naturally stable. That FBW architecture is not disigned to be 100% fail-safe like the military ones.

Nothing is 100% failsafe – military fighter planes just happen to have ejection seats.
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Mon Feb 14, 2011 10:54 am

Quoting ferpe (Reply 38):
Please give proof of you claim.

How do you think this "proof" should look? You are free to believe whatever you want, and if you consider my statement wrong I honestly don't care.

I got the statement at a lecture from a guy working at Airbus. He was involved in the flight control system design of the A380 and lectured to this topic. You'll find the text at:
http://www.fzt.haw-hamburg.de/pers/S...007_09_27_A380_Flight_Controls.pdf
You'll find nothing about negative stability margin there, it was an oral statement.

The pitch stability of an aircraft doesn't just turn into disaster from one point to the other. Initially the aircraft will only experience a slow and slightly increasing up and down movement (called Phugoid). A sudden and severe pitch happens later.
A continued flight at strong negative stability is only useful for some special purposes, like being more maneuverable or having better characteristics in supersonic regime.

Actually, the lecturer answered to a question and said exactly that an increased "instability" over that the A380 already has does not make sense.
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RE: FBW Flight Controls To Reduce Empennage Size?

Mon Feb 14, 2011 1:14 pm

Quoting Pihero (Reply 28):
Quoting travelavnut (Reply 27):
I realize it is actually possible but the chance of doing it by accident seems quite remote to say the least.

Nothing better than a picture. See the overhead panel
Quoting Pihero (Reply 28):
Quoting travelavnut (Reply 27):
I realize it is actually possible but the chance of doing it by accident seems quite remote to say the least.

Nothing better than a picture. See the overhead panel



Thanks a lot Pihero! You must be quite physically challenged to hit all those buttons by accident.
Live From Amsterdam!
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Mon Feb 14, 2011 6:04 pm

Quoting SEPilot (Reply 33):
I thought I had read that the flying wing was unstable, and that was the primary reason for its failure

The flying wing is unstable in certain flight regimes, notably post-stall. This is not unique to the flying wing...there are flight regimes outside the normal operating envelope that are unstable for almost all aircraft. The only thing that made the flying wing unique is that post-stall is an easier flight condition to get into than some of the other unstable spots you can put an aircraft. Stall is also one of the few that is explicitly called out in the FARs as something you have to be able to recover from.

Quoting SchorschNG (Reply 40):
How do you think this "proof" should look?

An AMOC statement for how they got it certified with negative stability on an axis would be a good start. There's no way EASA/FAA didn't require extra paperwork to cover that.

Tom.
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Mon Feb 14, 2011 8:08 pm

Quoting SchorschNG (Reply 40):
Actually, the lecturer answered to a question and said exactly that an increased "instability" over that the A380 already has does not make sense.

I looked into the A380 document and found what can be expected, the A380 is a normal stable aircraft in all flight regimes inside it's envelope. What the guy actually said does not contravene that, you misunderstood what he said. He said that a further reduction in the stability margin on the aircraft would not improve the performance.

Most likely it could but then they would have had to design the aircraft backup flight modes differently, right now it is a reduced control bandwidth system with emergency electrical command in all axis, yaw and pitch damping is still provided. But such a system can never be accepted by EASA/FAA as a backup system for an unstable aircraft.

Lets also relativate the "unstable" FBW myth a bit for military aircrafts ( I was involvd in one of the systems flying today), a military combat aircraft has a very wide range of loads to be carried fore and aft of the arodynamic lift center (which wary it's position a lot within the flight enevelope, from subsonic to supersonic etc), thus sometimes the aircraft is quite stable and sometimes slightly unstable. Thus the fulltime FBW enable you to design an aircraft that tries to fly with the smallest possible stability margin in all relevant parts of the flight envelope, you want to minimize trim drag.
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RE: FBW Flight Controls To Reduce Empennage Size?

Mon Feb 14, 2011 8:20 pm

Re the A320 crew that switched of the FBW I checked the article again, it says the system has been deliberately switched of during certification and during a commercial US flight. I find that hard to believe, especially if it was a no ferry flight.

I would think such a crew would be fired directly should this be verified and the black box would carry the evidence. I can believe it was carried out as part of a test flight after repairing the system in some way but then the manufacturer must have approved of it being done. Your thoughts?
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RE: FBW Flight Controls To Reduce Empennage Size?

Mon Feb 14, 2011 8:34 pm

Quoting ferpe (Reply 43):
I looked into the A380 document and found what can be expected, the A380 is a normal stable aircraft in all flight regimes inside it's envelope. What the guy actually said does not contravene that, you misunderstood what he said. He said that a further reduction in the stability margin on the aircraft would not improve the performance.

He actually said that the A380 in cruise is "slightly unstable". But being unstable can mean only a negative damping on the rather sluggish Phugoid motion, which can easily be controlled by the pilot, despite higher workload.
But basically we are both saying the same thing: Anything close to neutral stability will deliver best results, when again the tail is actually sized by pitching the aircraft at take-off.
The total gain of optimized CG is apparently limited, Boeing does not install such system at all.

The tail is a rather inefficient lifting surface, so producing lift (negative as well as positive) is never desirable. So something close to neutral stability is probably the best compromise, also because it probably can still be certified if a low failure probability can be proven.
The A380 for example has a separate FBW back-up system, probably also augmented by simple pitch stability systems.

Flying wings can actually be very stable in pitch, but tend to be bitches in lateral motion. That killed the YB-49, and the B-2 uses a full authority flight control system. Again, having the pilots sitting on ejections seats eases some requirements.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
ferpe
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RE: FBW Flight Controls To Reduce Empennage Size?

Mon Feb 14, 2011 9:36 pm

All stable aircraft which does not have a pitch damping loop has a pughoid motion, the frequency and amplitude is dependant on the stability margin. The pughoid is very easy to control for the pilot. The likewise normally prevailing spiral instability causes a pilot more work on an undamped aircraft. To control the spiral instability you need a wing leveler. The pronounced dutch roll is the nasties as it can make airline passangers sea-sic and is quite tricky to dampen for a pilot, that is why the first damper you add on a normal aircraft is the yaw damper to control the ducth roll.
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tdscanuck
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 15, 2011 5:31 am

Quoting SchorschNG (Reply 45):
He actually said that the A380 in cruise is "slightly unstable". But being unstable can mean only a negative damping on the rather sluggish Phugoid motion, which can easily be controlled by the pilot, despite higher workload.

I think this might be a translation problem, but you can't have negative damping...damping can never get lower than zero.

You can have negative stability margin, but that terminology usually isn't applied to phugoid motion because the phugoid has essentially zero damping on most designs...it's either meta-stable or stable depending on which definition you use. But it's got such a long period that most pilots will damp it out without even realizing it.

Quoting SchorschNG (Reply 45):
The total gain of optimized CG is apparently limited, Boeing does not install such system at all.

The performance gain is real, but you have to trade that against weight (extra and/or larger pumps and tubes) and a bunch of extra failure modes that then have to be analyzed, designed, and certified.

Tom.
 
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 15, 2011 6:08 am

Quoting SchorschNG (Reply 37):
The size is again determined by other criteria, which cannot be changed by fancy fly by wire

I concur, flight control system architecture should not affect actual control surface size, regardless of damping method.
Not quite sure where this thread is going with regard to A380 vs B2 pitch stability, but for the flight control system design I've been involved with thus far, surface sizing is left up to the aerodynamics folks, surface size and deflection then dictates the force required from the PCU which then in turn gives a hydraulic pressure and flow rate requirement.

Now, as for whoever writes the code for the FBW electronic yaw damper, that's another story...
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Faro
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RE: FBW Flight Controls To Reduce Empennage Size?

Tue Feb 15, 2011 10:38 am

Quoting SchorschNG (Reply 45):
He actually said that the A380 in cruise is "slightly unstable". But being unstable can mean only a negative damping on the rather sluggish Phugoid motion, which can easily be controlled by the pilot, despite higher workload.

Just to clarify, by slightly unstable, does that mean that a nose-up pitch divergence due to a gust will have a tendency to become slightly more nose-up in normal/direct law? I find that amazing if it were indeed the case.

Also, if this 'slight instability' is only in the cruise does it really matter? Can you still get meaningful gusts at +35,000 feet outside of cumulonimbus activity?

Faro

[Edited 2011-02-15 03:37:21]
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