Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Pros And Cons Of Speed Vs Vector Stabilty  
User currently offlineChamonix From France, joined Mar 2011, 316 posts, RR: 1
Posted (2 years 10 months 1 week 2 days 10 hours ago) and read 3369 times:
Support Airliners.net - become a First Class Member!

Tom tdscanuck mentioned this:
"Airbus does primarily flight path vector stability (reduced speed stability) while Boeing does a blend that favours speed stability over flight path vector stability."
What are the pros and cons of speed vs vector stabilty?
Many thanks!

15 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (2 years 10 months 1 week 2 days 6 hours ago) and read 3317 times:

Quoting Chamonix (Thread starter):
What are the pros and cons of speed vs vector stabilty?

Vector stability is closer to the ideal of what the pilot really wants to do...control where the airplane is going. However, you can't do it without heavily augmented flight controls (basically, FBW).

As a result of the above, most planes have natural speed stability. This is what pilots are used to during training so, by the time they get up to sophisticated aircraft that don't necessarily have to exhibit speed stability, they're accustomed to aircraft with speed stability. Artificial speed stability in FBW makes the plane's behavior familiar and intuitive.

Neither is wrong, they're just different.

Tom.


User currently offlinercair1 From United States of America, joined Oct 2009, 1308 posts, RR: 52
Reply 2, posted (2 years 10 months 1 week 17 hours ago) and read 3137 times:
AIRLINERS.NET CREW
CUSTOMER SERVICE & SUPPORT

Quoting tdscanuck (Reply 1):
Neither is wrong, they're just different.

Maybe another way to ask the question is:
- is there any credible data (not bias) that would indicate one or the other has led to more/less incidents?
Might be an interesting research project.



rcair1
User currently offlineChamonix From France, joined Mar 2011, 316 posts, RR: 1
Reply 3, posted (2 years 10 months 1 week 14 hours ago) and read 3099 times:
Support Airliners.net - become a First Class Member!

Quoting rcair1 (Reply 2):
Maybe another way to ask the question is:
- is there any credible data (not bias) that would indicate one or the other has led to more/less incidents?
Might be an interesting research project.

Excellent idea!
The topic is under-exposed and under-rated yet over-riding in terms safety and technology.
Google draws a big blank.
I like to compare the two using cars as an analogy.
Vector stability makes me think of ASR and ESP control stabilty systems that that neutralise over-steer and under-steer as well adjusting the active suspension ( AutoTrim in cars).
Speed stabilty gives me the idea of raw performance and power sliding that is more reactive that produces more NVH (Noise,Vibration,Harshness) at the expense of comfort.
I remember comparing two MB 560 SELs in back-to-back testing.
One had all the wizardry,the other was plain vanilla.
The difference was scary in the wet!

[Edited 2011-09-22 11:40:22]

[Edited 2011-09-22 11:40:49]

User currently offlineStarlionblue From Greenland, joined Feb 2004, 16994 posts, RR: 67
Reply 4, posted (2 years 10 months 1 week 10 hours ago) and read 3055 times:

Quoting Chamonix (Reply 3):
The topic is under-exposed and under-rated yet over-riding in terms safety and technology.

Not really. That is, both philosophies will give you ridiculously safe operation, as evidenced by the negligible accident rates.

It's not like we have a statistically significant number of similar crashes to draw conclusions from. The only conclusion is that either way commercial aviation is very safe.

Quoting Chamonix (Reply 3):
Vector stability makes me think of ASR and ESP control stabilty systems that that neutralise over-steer and under-steer as well adjusting the active suspension ( AutoTrim in cars).
Speed stabilty gives me the idea of raw performance and power sliding that is more reactive that produces more NVH (Noise,Vibration,Harshness) at the expense of comfort.
I remember comparing two MB 560 SELs in back-to-back testing.

Apples and oranges. In cars all that stability stuff is used at the edge of the envelope, a place where airliners don't venture unless something has gone really wrong.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (2 years 10 months 6 days 19 hours ago) and read 2990 times:

Quoting rcair1 (Reply 2):
is there any credible data (not bias) that would indicate one or the other has led to more/less incidents?

Not really. "Vector stability" is most often implemented in a pitch control law called "C*", which goes back at least to the early 70's. It's got a long record in military aircraft where, to my knowledge, it's never been indicted in an incident.

Quoting Chamonix (Reply 3):
Google draws a big blank.

You need to know what to search for...try "C* flight control law" as a starting search string.

There's a good overview paper of this particular control law in civil aircraft out of Cranfield University:
https://dspace.lib.cranfield.ac.uk/handle/1826/186

Quoting Chamonix (Reply 3):
Vector stability makes me think of ASR and ESP control stabilty systems that that neutralise over-steer and under-steer as well adjusting the active suspension ( AutoTrim in cars).
Speed stabilty gives me the idea of raw performance and power sliding that is more reactive that produces more NVH (Noise,Vibration,Harshness) at the expense of comfort.

ASR/ESP are much more like envelope protection than they'e like basic axis flight control laws.

Tom.


User currently offlineRaginMav From United States of America, joined May 2004, 376 posts, RR: 1
Reply 6, posted (2 years 10 months 6 days 14 hours ago) and read 2921 times:

Well I guess I'm the one that has to say it:

The Flight Path Vector stability of modern Airbus aircraft contributed to the crash of AF447. If the wheel/stick/whatever got 'heavy' in the PF's hands, perhaps there wouldn't have been "aft inputs" on the wheel/stick/whatever all the way to impact.

Flame away.


User currently offlinerwessel From United States of America, joined Jan 2007, 2311 posts, RR: 2
Reply 7, posted (2 years 10 months 6 days 13 hours ago) and read 2911 times:
Support Airliners.net - become a First Class Member!

Quoting RaginMav (Reply 6):
The Flight Path Vector stability of modern Airbus aircraft contributed to the crash of AF447. If the wheel/stick/whatever got 'heavy' in the PF's hands, perhaps there wouldn't have been "aft inputs" on the wheel/stick/whatever all the way to impact.

Stick forces are normally* quite light in a stall - minimal aerodynamic load, after all.

*Assuming a non-accelerated stall


User currently offlineRaginMav From United States of America, joined May 2004, 376 posts, RR: 1
Reply 8, posted (2 years 10 months 6 days 13 hours ago) and read 2909 times:

Quoting rwessel (Reply 7):
Stick forces are normally* quite light in a stall - minimal aerodynamic load, after all.

Are you referring to an Airbus aircraft? And what are those forces like in the moments leading up to the stall?

[Edited 2011-09-23 12:46:07]

User currently offlineStarlionblue From Greenland, joined Feb 2004, 16994 posts, RR: 67
Reply 9, posted (2 years 10 months 6 days 9 hours ago) and read 2859 times:

Quoting RaginMav (Reply 8):
Quoting rwessel (Reply 7):
Stick forces are normally* quite light in a stall - minimal aerodynamic load, after all.

Are you referring to an Airbus aircraft? And what are those forces like in the moments leading up to the stall?

I think the point rwessel was making was that if you have feedback to the controls, it's not like a stall will give you high stick forces.

On an Airbus, it is just spring loaded anyway right?

Quoting RaginMav (Reply 6):
The Flight Path Vector stability of modern Airbus aircraft contributed to the crash of AF447. If the wheel/stick/whatever got 'heavy' in the PF's hands, perhaps there wouldn't have been "aft inputs" on the wheel/stick/whatever all the way to impact.

First off, the investigation is not complete. Having said that, Airbus pilots should be trained and comfortable in the characteristics of their aircraft. If FPV stability contributed, it is because they made several errors. FPV stability as a concept does not seem flawed to me based on AF447.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (2 years 10 months 6 days 8 hours ago) and read 2848 times:

Quoting RaginMav (Reply 6):
The Flight Path Vector stability of modern Airbus aircraft contributed to the crash of AF447.

Not in the way you're describing it.

Quoting RaginMav (Reply 6):
If the wheel/stick/whatever got 'heavy' in the PF's hands, perhaps there wouldn't have been "aft inputs" on the wheel/stick/whatever all the way to impact.

The PF was pulling full aft stick...that takes a lot of force. He had to know what he was doing (although he may not have known why). Exactly the same thing would have happened in a non-C* airplane if you held full aft stick/yoke.

Tom.


User currently offlinerwessel From United States of America, joined Jan 2007, 2311 posts, RR: 2
Reply 11, posted (2 years 10 months 6 days 8 hours ago) and read 2846 times:
Support Airliners.net - become a First Class Member!

Quoting RaginMav (Reply 8):
Are you referring to an Airbus aircraft? And what are those forces like in the moments leading up to the stall?

I meant in general, particularly in aircraft that actually have some force feedback from the control surfaces to the stick, since that's what would be the "natural" feel.

In most aircraft, a low-speed stall will have relatively low stick forces leading up to the stall, but will usually require a fair bit of stick deflection. That's not universally true, in some aircraft, if you're trimmed for a fairly high speed, you need some force to pull the stick back far enough to stall, but it's not usually all that much, certainly a lot less than the opposite (the amount of push you need to fly at high speed when trimmed for low speed).

In the AF447 case, and assuming they were flying an aircraft with conventional controls (the A330 provides no feedback from the control surfaces, and handles trim automatically), they would not have been trimmed for a particularly high speed anyway. Their *true* airspeed and Mach number were high, but their *indicated* airspeed, which is what mostly affects your control loads (and trim), was not.

All that's relative of course - if you're flying a B-29 (a 737 sized aircraft with *unpowered* controls), the stick forces are high no matter what you're doing.

And aircraft with a stick pusher will artificially increase the stick load as you approach the stall. An interesting question is if a conventional stick pusher would have helped – basically those depend on the same alpha vanes that the stall warning runs on. That went off for something like 50s before the AF447 before the airspeed dropped below the cutoff limit. And if they were willing to ignore that, would they not have ignored the pusher as well?


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 12, posted (2 years 10 months 6 days 5 hours ago) and read 2825 times:

Quoting rwessel (Reply 11):
I meant in general, particularly in aircraft that actually have some force feedback from the control surfaces to the stick, since that's what would be the "natural" feel.

I'm not aware of any modern widebody with force feedback from the surfaces to the stick...they all use irreversible hydraulic controls, or electric actuators (which are inherently irreversible). Any feel you get on a widebody is going to be artificial, just like an A330.

Tom.


User currently offlineRaginMav From United States of America, joined May 2004, 376 posts, RR: 1
Reply 13, posted (2 years 10 months 5 days 17 hours ago) and read 2753 times:

Quoting Starlionblue (Reply 9):
Airbus pilots should be trained and comfortable in the characteristics of their aircraft. If FPV stability contributed, it is because they made several errors. FPV stability as a concept does not seem flawed to me based on AF447.

I agree that FPV stability is not inherently wrong, but it is unquestionably different. I stand by my statement that it contributed to the crash. rwessel makes this statement:

Quoting rwessel (Reply 11):
(the A330 provides no feedback from the control surfaces, and handles trim automatically)

and that is exactly my point. I am not a fan of aerodynamic cues and trim movements being masked. The Falcon 7x, for all it's splendor, is guilty of the same thing.

Quoting rwessel (Reply 11):
Their *true* airspeed and Mach number were high, but their *indicated* airspeed, which is what mostly affects your control loads (and trim), was not.

Good point. It's not like the aircraft was trimmed for 300+ KIAS and then that speed got cut in half during their climb. Nonetheless, masked trim movement and FPV stability led to a situation where the airspeed indicator was, obviously, by far the easiest way to see their low speed state. And of course the investigation thus far has indicated unreliable airspeed information.

This brings up a slight variation of my theme: no direct indication of AoA available on the flight deck. Come on, governing authorities of the world, make this a requirement!!! I don't know who led the charge for the US Navy to adopt the AoA indicator the way they have, but I would imagine he would roll over in his grave if he knew modern airliners are not equipped with such a simple system.

Sorry for slightly highjacking the thread, so back on topic for me.

FPV stability is an intriguing concept. I haven't had the fortune of doing this yet, but some day i look forward to putting the HUD's FPVon the touchdown zone of a runway, and-voila!-stabilized approach, at least in a flight path sense. If a pilot is astute enough to look, he or she could see how many degrees away from optimum glide path they are, and use that info to make more timely, and better informed corrections, or go-around decisions. That certainly is an advancement in safety.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 14, posted (2 years 10 months 5 days 17 hours ago) and read 2747 times:

Quoting RaginMav (Reply 13):
Quoting rwessel (Reply 11):
(the A330 provides no feedback from the control surfaces, and handles trim automatically)

and that is exactly my point.

There may be something to trim masking, but no modern widebody airliner provides aerodynamic feedback from the control surfaces regardless of who made it.

Quoting RaginMav (Reply 13):
This brings up a slight variation of my theme: no direct indication of AoA available on the flight deck.

It's in two places...the PLI (pitch limit indicator) and the barberpole on the speedtape. These are actually *more* useful AoA measures since AoA, as an absolute value, doesn't tell you much. It's only when coupled with airspeed and configuration that you get what you really want (how close to stalling am I?)...which is exactly what the PLI (how much more pitch do I have available) and speed tape (how much slower can I get) tell you.

Quoting RaginMav (Reply 13):
I don't know who led the charge for the US Navy to adopt the AoA indicator the way they have, but I would imagine he would roll over in his grave if he knew modern airliners are not equipped with such a simple system.

They are...most airlines just don't take the option.

Quoting RaginMav (Reply 13):
I haven't had the fortune of doing this yet, but some day i look forward to putting the HUD's FPVon the touchdown zone of a runway, and-voila!-stabilized approach, at least in a flight path sense.

It's awesome.

Tom.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16994 posts, RR: 67
Reply 15, posted (2 years 10 months 5 days 8 hours ago) and read 2690 times:

Quoting RaginMav (Reply 13):
I agree that FPV stability is not inherently wrong, but it is unquestionably different. I stand by my statement that it contributed to the crash

Different, sure. But it is hardly new and groundbreaking anymore. It's been around for over a quarter century in service. Any pilot who flies a FBW Airbus should be well aware of the idiosyncrasies of his aircraft.



"There are no stupid questions, but there are a lot of inquisitive idiots."
Top Of Page
Forum Index

Reply To This Topic Pros And Cons Of Speed Vs Vector Stabilty
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
Pros And Cons Of Piloting A Giant Jet? posted Sat Feb 13 2010 05:42:39 by 747400sp
Pros And Cons Of 737NG Overwing Exit Design posted Thu May 21 2009 16:55:34 by BuyantUkhaa
Pros And Cons Of "Plugged" Windows posted Mon Oct 24 2005 21:41:45 by AT
Cost Of F/J Seat Vs Y Class Seat (Y = 100) posted Thu Jul 21 2011 03:11:53 by faro
Size Of Wing And Handling Of Turbulence posted Wed Jun 29 2011 18:57:25 by jfrworld
Ground Speed And True Air Speed Differences posted Sun Jun 12 2011 06:56:48 by Quokka
L-1011 Speed Vs. Drag posted Mon Apr 25 2011 15:03:20 by PGNCS
Architecture And Functionality Of Control Towers? posted Sun Apr 24 2011 19:56:18 by web500sjc
Efficiency Of A330 Vs. DC-10 posted Mon Dec 13 2010 10:51:37 by tsugambler
Nose Shape Of 787 Vs. Traditional Shape posted Thu Aug 5 2010 12:18:03 by tsugambler

Sponsor Message:
Printer friendly format