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Neutral Spiral Stability/Overbanking Tendencies  
User currently offlinenovember678x From United States of America, joined Sep 2013, 3 posts, RR: 0
Posted (1 year 1 month 1 week 2 days 2 hours ago) and read 2610 times:

Quick question I may:

Do passenger jets have neutral spiral stability and overbanking tendencies? In other words, do they hold any bank angle with the yoke neutral (non fly-by-wire)? I assume FBW aircraft would behave the same without protection.

Thank you!

12 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 1, posted (1 year 1 month 1 week 2 days 2 hours ago) and read 2593 times:

From memory, swept wing airliners tend to have weaker directional than lateral stability, and are thus prone to dutch roll, which is why they have yaw dampers. This is pretty much the opposite to being prone to spiral divergence, which requires stronger directional than lateral stability.

I don't know if they would hold the bank with a neutral yoke though.

[Edited 2013-09-11 22:41:04]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently onlineBMI727 From United States of America, joined Feb 2009, 15779 posts, RR: 27
Reply 2, posted (1 year 1 month 1 week 2 days 1 hour ago) and read 2580 times:

Quoting november678x (Thread starter):
Do passenger jets have neutral spiral stability

I want to say that most aircraft suffer from spiral divergence, but I use the word "suffer" loosely since it typically develops slowly and will only be unsafe if the pilot doesn't realize what is going on.



Why do Aerospace Engineering students have to turn things in on time?
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 3, posted (1 year 1 month 1 week 1 day 20 hours ago) and read 2492 times:

Quoting BMI727 (Reply 2):
Quoting november678x (Thread starter):
Do passenger jets have neutral spiral stability

I want to say that most aircraft suffer from spiral divergence, but I use the word "suffer" loosely since it typically develops slowly and will only be unsafe if the pilot doesn't realize what is going on.

At how much bank though? If you stay under 25 degrees, will the aircraft tend to stay at that bank angle (or decrease it) or will it tend to diverge?



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineroseflyer From United States of America, joined Feb 2004, 9666 posts, RR: 52
Reply 4, posted (1 year 1 month 1 week 1 day 16 hours ago) and read 2428 times:

EASA has the clearest definitions for what type of stability are required. It's a bit easier to read than the corresponding FAA requirements:

Go to page 1-B-16

http://www.easa.europa.eu/agency-mea...-002-RM/decision_ED_2003_02_RM.pdf
CS 25.171 General
The aeroplane must be longitudinally, directionally
and laterally stable in accordance with the provisions
of CS 25.173 to 25.177. In addition, suitable stability
and control feel (static stability) is required in any
condition normally encountered in service, if flight
tests show it is necessary for safe operation.



If you have never designed an airplane part before, let the real designers do the work!
User currently offlineLH707330 From United States of America, joined Jun 2012, 799 posts, RR: 0
Reply 5, posted (1 year 1 month 1 week 1 day 15 hours ago) and read 2421 times:

Quoting november678x (Thread starter):
Do passenger jets have neutral spiral stability and overbanking tendencies? In other words, do they hold any bank angle with the yoke neutral (non fly-by-wire)? I assume FBW aircraft would behave the same without protection.

Airliners, including FBW ones, are designed to have positive stability, such that they can recover from any minor bump without pilot input, though as starlionblue mentioned, some suffer from dutch roll under certain regimes, and hence have dampers. Most low-wing airliners have a few degrees of dihedral, which provides for roll stability.


User currently offlineroseflyer From United States of America, joined Feb 2004, 9666 posts, RR: 52
Reply 6, posted (1 year 1 month 1 week 1 day 14 hours ago) and read 2393 times:

Quoting LH707330 (Reply 5):

Airliners, including FBW ones, are designed to have positive stability, such that they can recover from any minor bump without pilot input, though as starlionblue mentioned, some suffer from dutch roll under certain regimes, and hence have dampers. Most low-wing airliners have a few degrees of dihedral, which provides for roll stability.
The static lateral stability (as shown by the
tendency to raise the low wing in a sideslip with the
aileron controls free) for any landing gear and wingflap
position and symmetric power condition, may
not be negative at any airspeed


Yes stability is required in the FARs, however stability augmentation systems may be used. The Yaw Damper is an example. Some other airplanes use fly by wire control systems to augment stability. As there is push for more and more efficiency, inherent static stability is slowly being augmented by stability control systems. There are strict requirements for these, and usually they are only required in specific thrust/airspeed/flap configurations.



If you have never designed an airplane part before, let the real designers do the work!
User currently offlinenovember678x From United States of America, joined Sep 2013, 3 posts, RR: 0
Reply 7, posted (1 year 1 month 1 week 1 day 13 hours ago) and read 2377 times:

Just to make sure I understand; if you make a 10, 30, or 60 degree bank on a boeing jet and nuetralize the ailerons then it returns to wings level?

Thank you


User currently offlineroseflyer From United States of America, joined Feb 2004, 9666 posts, RR: 52
Reply 8, posted (1 year 1 month 1 week 1 day 13 hours ago) and read 2365 times:

Quoting november678x (Reply 7):
Just to make sure I understand; if you make a 10, 30, or 60 degree bank on a boeing jet and nuetralize the ailerons then it returns to wings level?

Not necessarily. It depends on how the airplane is trimmed. For an explanation of trim, CG and stability, you can read below:

http://www.boeing.com/commercial/aer...zine/aero_02/textonly/fo01txt.html



If you have never designed an airplane part before, let the real designers do the work!
User currently offlinePihero From France, joined Jan 2005, 4590 posts, RR: 77
Reply 9, posted (1 year 1 month 1 week 1 day 11 hours ago) and read 2325 times:
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Quoting roseflyer (Reply 4):
EASA has the clearest definitions for what type of stability are required.
Quoting roseflyer (Reply 4):
The aeroplane must be longitudinally, directionally and laterally stable in accordance with the provisions of CS 25.173 to 25.177.

Yes, ...But the said provisions are very very important :

"(1) From 1·13 VSR1
to VMO/MMO..
(2) From VMO/MMO to VFC/MFC, unless
the divergence is –
(i) Gradual;
(ii) Easily recognisable by the pilot;
and
(iii) Easily controllable by the pilot"


Quite a few aircraft display that sort of - acceptable - behaviour ; The worst is almost certainly the 743 : At 30° of bank, you'd have to input nearly 90° opposite ailerons and rudder.

Another airplane, not certified then to modern western standards was downright dangerous : The Tupolev 134, In my opinion the scariest kite I've ever flown.



Contrail designer
User currently offlinemusang From United Kingdom, joined Apr 2001, 865 posts, RR: 7
Reply 10, posted (1 year 1 month 1 week 1 day 10 hours ago) and read 2311 times:

"Just to make sure I understand; if you make a 10, 30, or 60 degree bank on a boeing jet and nuetralize the ailerons then it returns to wings level?"

No. I've only tried this on a 737 up to 30 degree, but when trimmed in pitch, yoke neutral, it stays banked. Very stable.

On a test flight in a 146 we did a 60 degree bank through more than 90 degrees of heading. Again, pitch trimmed, ailerons neutral, it felt like it was on rails and showed no tendency to increase or decrease the bank angle.

Regards - musang


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 11, posted (1 year 1 month 1 week 1 day 7 hours ago) and read 2257 times:

Quoting Starlionblue (Reply 1):
From memory, swept wing airliners tend to have weaker directional than lateral stability, and are thus prone to dutch roll, which is why they have yaw dampers. This is pretty much the opposite to being prone to spiral divergence, which requires stronger directional than lateral stability.

I don't know if they would hold the bank with a neutral yoke though.

Yaw dampers artificially give airliners dutch roll stability. Without that some are actually unstable in dutch roll (the 727 was one under certain conditions). As you say, natural dutch roll stability is tends to mean to spiral instability. So as strong dutch roll stability can be achieved artificially with a Y/D this means natural directional stability does not have to be strong so something like neutral spiral stability can be achieved at the same time.

As a sim engineer, dutch roll and spiral stability are simulator qualification tests we regularly have to run so I'm familiar with how these characteristics are exhibited. I've never flown a real airliner, but the full flight sims I've worked with all pretty much stay at the bank angle they were at wheel release, initially at least. Over a period of time they will slowly either return towards wings level (stable) or increase bank angle (divergent). If they didn't stay roughly at the same bank like that, you would always have to hold some wheel angle to maintain bank in a turn, which you don't need to do.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlinePihero From France, joined Jan 2005, 4590 posts, RR: 77
Reply 12, posted (1 year 1 month 1 week 20 hours ago) and read 2172 times:
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I think we have to be more precise in this thread.
1/- We are talking about *dynamic* stability, i.e an aircraft out of straight and level flight for some disturbance, be it outside influences or pilot-induced.
2/- We are as a matter of fact talking about two doifferent things affecting our aircraft behaviour :
a/- lateral stability, i.e skid or slide
b/- spiral stability, i.e bank angle
The effects of a given - low - bank angle on a swept-wing airliner cocern both stability criteria above.
a/- the lower wing has an apparent greater AoA than the outside wing, therefore tending to cancel the slide, hence the bank : it is the *dihedral effect of a swept wing*.
b/- If pilot induced, i.e with a reason for the pilot to get into a turn - and the use of the rudder to keep the ball centered -, the outside wing will be apparently faster than the inside wing, hence gets an increased lift which tends to increase the bank angle. The way to control this is, of course, input outside aileron to somehow equalize the lift on both wings.
Of course, this effect increases with the wing-sweep value, that's why it was so prominent on the 707 and the 747 1, 2 and 3 as they had a greater angle of sweep than modern aircraft.

This feature is the reason why for certification, we have the proviso that stability must be achieved...
"(1) unless the divergence is –
(i) Gradual,
(ii) Easily recognisable by the pilot; and
(iii) Easily controllable by the pilot"

Quoting musang (Reply 10):
On a test flight in a 146 we did a 60 degree bank through more than 90 degrees of heading. Again, pitch trimmed, ailerons neutral, it felt like it was on rails and showed no tendency to increase or decrease the bank angle.

Impressive, but of little use to everyday's flying : You wouldn't trim for banks above 30° and the exit of the turn could be very intertesting   

Rezgarding the yaw damper... it's usefulness depends a lot on airplane configuration (clean or with flaps and gear...) The 737 for instance finds it useful in landing configuration.



Contrail designer
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