november678x
Topic Author
Posts: 3
Joined: Tue Sep 10, 2013 1:49 am

Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 5:02 am

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!
 
User avatar
Starlionblue
Posts: 17208
Joined: Fri Feb 27, 2004 9:54 pm

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 5:38 am

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." - John Ringo
 
BMI727
Posts: 11172
Joined: Mon Feb 02, 2009 9:29 pm

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 6:15 am

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 avatar
Starlionblue
Posts: 17208
Joined: Fri Feb 27, 2004 9:54 pm

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 10:56 am

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." - John Ringo
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 3:22 pm

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!
 
LH707330
Posts: 1555
Joined: Fri Jun 15, 2012 11:27 pm

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 4:03 pm

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.
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 4:55 pm

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!
 
november678x
Topic Author
Posts: 3
Joined: Tue Sep 10, 2013 1:49 am

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 5:52 pm

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
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 6:25 pm

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!
 
Pihero
Posts: 4232
Joined: Mon Jan 31, 2005 5:11 am

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 8:26 pm

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
 
musang
Posts: 788
Joined: Sun Apr 08, 2001 4:11 am

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 8:59 pm

"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 avatar
Jetlagged
Posts: 2562
Joined: Sun Jan 23, 2005 3:00 pm

RE: Neutral Spiral Stability/Overbanking Tendencies

Thu Sep 12, 2013 11:54 pm

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.
 
Pihero
Posts: 4232
Joined: Mon Jan 31, 2005 5:11 am

RE: Neutral Spiral Stability/Overbanking Tendencies

Fri Sep 13, 2013 10:51 am

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

Who is online

Users browsing this forum: No registered users and 14 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos