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747-8 'Active' Control Laws  
User currently offlinePart147 From Ireland, joined Dec 2008, 510 posts, RR: 0
Posted (3 years 11 months 4 weeks 1 day 7 hours ago) and read 7414 times:

I've decided not to mess up the 747-8 Flight Tracking and Production thread by posting this new thread...

I'm intrigued by this comment ...

Quoting 747classic (Reply 8):
The outboard wing oscillation problem will be solved by new software, counteracting the impending oscillation with new control laws for the outboard ailerons. Further tests of the new software are proceeding.

As I understand it, we now have a new aircraft with an inherent flight control issue that now requires a software 'fix'.

Will Boeing actually fix the problem in the future or will they just rely on the 'fix' to give the illusion that it HAS been fixed??? Especially since the issue occurs at certain speeds with the flaps and gear extended - possibly making emergency operations/landings that bit more challenging???

In my mind, I see the 747-8's control system falling into the realm of military-type fly-by-wire systems which are necessary to maintain control of an 'inherently' - 'slightly' - unstable civil aircraft.


It's better to ask a stupid question during training, rather than make a REALLY stupid mistake later on!
11 replies: All unread, jump to last
 
User currently online747classic From Netherlands, joined Aug 2009, 2179 posts, RR: 14
Reply 1, posted (3 years 11 months 4 weeks 1 day 6 hours ago) and read 7280 times:

IMO this intriguing question could be better transferred to the Tech/Oper forum, but here we go :

Quoting Part147 (Thread starter):
As I understand it, we now have a new aircraft with an inherent flight control issue that now requires a software 'fix'.



We have an aircraft that has an oscillation issue at a certain part in the flight envelope (mid weight near cruise speed).

Quoting Part147 (Thread starter):
Will Boeing actually fix the problem in the future or will they just rely on the 'fix' to give the illusion that it HAS been fixed???



We don't know what Boeing is planning. Boeing can decide that the software fix is temporarily and only used to get the first aircraft certified and delivered. But It could also be the final fix.

Quoting Part147 (Thread starter):
Especially since the issue occurs at certain speeds with the flaps and gear extended - possibly making emergency operations/landings that bit more challenging???



That's not correct. This issue is only valid at a certain (cruise) speed, weight and altitude combination, with flaps up and gear up.
The other "full flap flutter issue" is solved by modification of the wing gear doors.

Quoting Part147 (Thread starter):
In my mind, I see the 747-8's control system falling into the realm of military-type fly-by-wire systems which are necessary to maintain control of an 'inherently' - 'slightly' - unstable civil aircraft.



On earlier 747 variants the outboard ailerons are locked in the neutral position above certain air speeds (747-400) or with flaps selected up (747 classic). On the 747-8 the outboard ailerons (and spoilers) are "fly by wire" only to obtain equal low speed handling qualities as the 747-400 (common type rating). Also this system is used to give the outboard ailerons a "flap function" to augment the lift with flaps selected (shifting out of the neutral position).
Boeing now used this "fly by wire" system to a give a little bit outboard aileron deflection to eliminate the impending oscillation. on certain speed and weight combinations, calculated by the new added software. The only draw back could be a very slight increase in drag while flying in the affected part of the flight envelope.
IMO you can see it as a kind of yaw damper, available on several aircraft since the 707 and not as an 'inherently' - 'slightly' - unstable civil aircraft.

[Edited 2010-10-30 02:01:16]


Operating a twin over the ocean, you're always one engine failure from a total emergency.
User currently offlineAA737-823 From United States of America, joined Mar 2000, 5873 posts, RR: 11
Reply 2, posted (3 years 11 months 4 weeks 1 day 4 hours ago) and read 7124 times:

Quoting Part147 (Thread starter):
In my mind, I see the 747-8's control system falling into the realm of military-type fly-by-wire systems which are necessary to maintain control of an 'inherently' - 'slightly' - unstable civil aircraft.

The same could be said for Mach Trim, which eliminates the effect of Mach Tuck, in many commercial airliners (737, 757, etc).
Granted, it isn't done with fancy electronics, but it's still an automated system which preserves safety in the face of the laws of physics.
And, Mach Tuck happens EVERY TIME you exceed a certain speed, whereas this 747-8 issue only happens a certain weight+speed+altitude combinations.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 3, posted (3 years 11 months 3 weeks 6 days 18 hours ago) and read 4590 times:

Quoting Part147 (Thread starter):
Will Boeing actually fix the problem in the future or will they just rely on the 'fix' to give the illusion that it HAS been fixed???

If the problem goes away it has been fixed...you may not like the particular method Boeing used to fix it, but fixed is fixed. It's like calling a yaw damper the "illusion that it HAS been fixed" for aircraft which Dutch roll tendencies (which is basically all swept wing jets).

Quoting Part147 (Thread starter):
Especially since the issue occurs at certain speeds with the flaps and gear extended - possibly making emergency operations/landings that bit more challenging???

In addition to the above (this particular issues doesn't seem to be the landing config issue), as long as the aileron system is working you don't have a problem, and even if it quits working you have an oscillation. That has long-term fatigue implications, but isn't a reason you couldn't land the airplane (or would even really notice).

Quoting Part147 (Thread starter):
In my mind, I see the 747-8's control system falling into the realm of military-type fly-by-wire systems which are necessary to maintain control of an 'inherently' - 'slightly' - unstable civil aircraft.

It's still stable...oscillation puts too many cycles on the structure, so you've got a fatigue issue, but it's not like the aircraft handling is changing by addition of modal suppression.

This is no more military-type control than load alleviation in the flight controls (which both Airbus and Boeing use).

Tom.


User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 4, posted (3 years 11 months 3 weeks 5 days 23 hours ago) and read 4397 times:

Quoting tdscanuck (Reply 3):
than load alleviation in the flight controls (which both Airbus and Boeing use).

And, which Lockheed developed and used first, on civil jet transport aircraft.
Specifically...the L1011-500 model.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 5, posted (3 years 11 months 3 weeks 5 days 14 hours ago) and read 4226 times:

Part147, I think you're overestimating the seriousness of the "situation". These "fixes" are not a new thing. They've been used for decades. Yaw dampers have been around for at least fifty years (right?). As mentioned by 411A, dynamic load alleviation was used on the L1011 back in the 80s. The 340NG models use the elevators to relieve bending loads on the fuselage.


Here's the crucial point:

Quoting tdscanuck (Reply 3):
In addition to the above (this particular issues doesn't seem to be the landing config issue), as long as the aileron system is working you don't have a problem, and even if it quits working you have an oscillation. That has long-term fatigue implications, but isn't a reason you couldn't land the airplane (or would even really notice).

It's not like the aircraft will crash or be harder to control. You're alleviating fatigue.

Perhaps an imperfect comparison would be balance rods in automotive engines. Will the engine work without them? Yes, but you'll get more vibration, meaning less passenger comfort and more fatigue. But it won't be dangerous.



"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 6, posted (3 years 11 months 3 weeks 5 days 14 hours ago) and read 4226 times:

Quoting 411A (Reply 4):
And, which Lockheed developed and used first, on civil jet transport aircraft.
Specifically...the L1011-500 model.

I'm becoming convinced that the answer to any question of the form "When was X first used on commercial aircraft?" is either the Lockheed L1011 or Concorde.

Tom.


User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 7, posted (3 years 11 months 3 weeks 5 days 12 hours ago) and read 4183 times:

Quoting tdscanuck (Reply 6):
I'm becoming convinced that the answer to any question of the form "When was X first used on commercial aircraft?" is either the Lockheed...

Don't know anything about the Concorde, however...I was having a discussion with two now retired Boeing design engineers about a year ago, and when the subject of the Lockheed L1011 came up, their comments were the following...

"When we got a first careful look at the L1011 design, and especially the autoflight systems used therein, it was a HUGE wake-up call to those of us at Boeing at the time, because...Lockheed was waaay ahead in that department."

Lockeed developed two distinct items that were only used on their L1011 design...Direct Lift Control and...the all-flying stabilizer for pitch control.
In addition, Lockheed was the first to use hydraulic system fusing directly from the factory, not added later.


User currently offlinePGNCS From United States of America, joined Apr 2007, 2833 posts, RR: 45
Reply 8, posted (3 years 11 months 3 weeks 1 day 10 hours ago) and read 3737 times:

Quoting tdscanuck (Reply 6):
I'm becoming convinced that the answer to any question of the form "When was X first used on commercial aircraft?" is either the Lockheed L1011 or Concorde.

Tom.

  That's not far off, Tom! I know nothing of Concorde, but can say that the L-1011 was the most amazing leap of technology over it's peers I ever saw, and is still my all-time favorite aircraft to fly; it still has some absolutely fantastic things otherwise unavailable. What a great machine!

Quoting 411A (Reply 7):
Don't know anything about the Concorde, however...I was having a discussion with two now retired Boeing design engineers about a year ago, and when the subject of the Lockheed L1011 came up, their comments were the following...

"When we got a first careful look at the L1011 design, and especially the autoflight systems used therein, it was a HUGE wake-up call to those of us at Boeing at the time, because...Lockheed was waaay ahead in that department."

Boy were they right!


User currently offlineMax Q From United States of America, joined May 2001, 4655 posts, RR: 19
Reply 9, posted (3 years 11 months 3 weeks 1 day 10 hours ago) and read 3728 times:

I have always been a big L1011 fan and rate it as one of the finest widebodies ever made.



But Lockeed did not make it simple. The all flying tail is an example, magnificant engineering to be sure and a safety bonus undoubtedly to have that exceptional pitch authority.




You don't see it incorporated on any other modern jet transports though. It's all about economics and the extra weight and complexity involved just don't pay.



I think there are limited examples of DLC out there if i'm not mistaken though. I believe this is the mod for some Aircraft operating out of LCY to allow them to fly a steeper glideslope ?



The best contribution to safety is a competent Pilot.
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (3 years 11 months 3 weeks 1 day 9 hours ago) and read 3697 times:

Quoting Max Q (Reply 9):
I think there are limited examples of DLC out there if i'm not mistaken though.

C-17 definitely has it...hardly commercial jet transport territory, but at least the technology lives on.

Quoting Max Q (Reply 9):
I believe this is the mod for some Aircraft operating out of LCY to allow them to fly a steeper glideslope ?

I think the LCY-type version is sort of a bastard step-child of DLC...not nearly as elegant, but the same aerodynamics at work in a cruder fashion.

Tom.


User currently offlineXaraB From Norway, joined Aug 2007, 210 posts, RR: 0
Reply 11, posted (3 years 11 months 2 weeks 6 days 23 hours ago) and read 3552 times:

Quoting tdscanuck (Reply 6):
I'm becoming convinced that the answer to any question of the form "When was X first used on commercial aircraft?" is either the Lockheed L1011 or Concorde.

And the occasional VC10...



An open mind is not an empty one
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