william
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Airbus 320 Curious Flight Profile

Fri Nov 07, 2008 6:44 pm

Hi folks, I was recently on a couple A320s and A319s and at cruise I noticed the aircraft does a slight rocking motion. Why is that? Is it a characteristic of the Autopilot? Sit in the last row with seat of the pants you can notice it. And seeing the shadow move to and fro o n the wing confirms I was not crazy.
 
PGNCS
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RE: Airbus 320 Curious Flight Profile

Fri Nov 07, 2008 7:50 pm

It's not unusual and is actually commonly more pronounced on some other aircraft, notably the 757 (especially the 753).
 
KELPkid
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RE: Airbus 320 Curious Flight Profile

Fri Nov 07, 2008 8:00 pm



Quoting PGNCS (Reply 1):
It's not unusual and is actually commonly more pronounced on some other aircraft, notably the 757 (especially the 753).

The most pronounced yaw "wandering" I have ever experienced was towards the back of an SQ 747-400, on takeoff from SFO for HKG, my wife and I were in row 57...and I think PhilSquares assured me (via a tech/ops posting I later made) that the aircraft was most likely either at gross weight for that takeoff or very close to it.
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
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Starlionblue
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RE: Airbus 320 Curious Flight Profile

Fri Nov 07, 2008 10:47 pm

The MD-8x is quite "good" at porpoising. I asked some pilots about it and they said sometimes the auto-trim has a hard time finding "center" so keeps compensating back and forth.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
william
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RE: Airbus 320 Curious Flight Profile

Fri Nov 07, 2008 11:05 pm

It was a gentle side to side rolling motion. Is the autopilot computer having a hard time trying to find level is it built into the programing because its easier to allow a little rocking instead of even keel flying?
 
WPIAeroGuy
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RE: Airbus 320 Curious Flight Profile

Fri Nov 07, 2008 11:22 pm



Quoting William (Reply 4):
It was a gentle side to side rolling motion. Is the autopilot computer having a hard time trying to find level is it built into the programing because its easier to allow a little rocking instead of even keel flying?

Essentially. Absolute center is never known, you're always going to be off by a little bit. The goal is to get to a value very close to center as smoothly as possible, which usually involves some overshoot and correction as the aircraft constantly oscillates about what it thinks center is. On longer planes this wobbling is pronouned the farther you are from the axis of rotation. Also, there are constant disturbances which add to the complexity of finding center.
-WPIAeroGuy
 
PhilSquares
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RE: Airbus 320 Curious Flight Profile

Fri Nov 07, 2008 11:24 pm



Quoting William (Reply 4):
It was a gentle side to side rolling motion. Is the autopilot computer having a hard time trying to find level is it built into the programing because its easier to allow a little rocking instead of even keel flying?

What you are feeling is the yaw damper doing it's job. The yaw damper counteracts "dutch roll" which is a characteristic of swept wing aircraft. The yaw damper also puts slight rudder inputs in to ensure coordinated turns are made. You tend to feel it more as you reach the ends of the longitudinal axis, ie sit towards the back of the aircraft.

While it is part of the autoflight system, it's a function that's constantly on. As has been pointed out, the longer the fuselage the more noticable it is, so aircraft like the 757 that are fairly long tend to exhibit the yaw damper function a little more than a 737 would.
Fly fast, live slow
 
PGNCS
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RE: Airbus 320 Curious Flight Profile

Sat Nov 08, 2008 12:31 am



Quoting Starlionblue (Reply 3):
The MD-8x is quite "good" at porpoising. I asked some pilots about it and they said sometimes the auto-trim has a hard time finding "center" so keeps compensating back and forth.

You are talking about pitch, and the MD-80 isn't normally bad, and is CERTAINLY better than the classic 737's at finding the trimmed condition. The MD-80 stab is trimmed by the alternate trim motor which trims much more slowly than we can when hand flying. Occasionally on level off (especially after an IAS descent) the MD-80 will work for a bit to get it right, but rarely does it require disconnecting the autopilot and trimming it by hand, though some guys do that at top of climb, mainly as a technique for adjusting roll and yaw trim. As an aside, some MD-90's are quite well-known for porpoising. They have a different (and faster acting) autopilot trim system.

The original question here is unrelated to this, as it was about yaw displacement. That is a normal experience, and is more commonly experienced in "long" aircraft; the 757 (and especially 757-300) is especially notorious. It is caused by the yaw damper and is perfectly normal, although some people find it disconcerting or even mildly nauseating.
 
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Starlionblue
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RE: Airbus 320 Curious Flight Profile

Sat Nov 08, 2008 12:50 am

Thanks for info PGNCS. And yes I know t was somewhat unrelated. I just felt like talking a bit about other oscillations.  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
AA737-823
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RE: Airbus 320 Curious Flight Profile

Sat Nov 08, 2008 11:27 am



Quoting William (Thread starter):
Sit in the last row with seat of the pants you can notice it.

When in the last row, I ALWAYS sit with the seat of my pants.
And if I didn't, I'd fully expect to be noticed as well!



I've actually never noticed this phenomenon on A32X series aircraft, but I will admit that the tail is relatively far from the wing, at least visually, so it stands to reason that it would perform similarly to a "longer" plane. That is, a 73G seems to have its wings near the center of the longitudinal axis, whereas the A320 wings seem a bit far forward, making the tail section further away from the CW.
 
PGNCS
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RE: Airbus 320 Curious Flight Profile

Sat Nov 08, 2008 10:05 pm



Quoting Starlionblue (Reply 8):
Thanks for info PGNCS. And yes I know t was somewhat unrelated. I just felt like talking a bit about other oscillations.

No problem; oscillations can be fun!  Wink
 
william
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RE: Airbus 320 Curious Flight Profile

Sun Nov 09, 2008 1:40 am

Many posts here state it was the yaw damper working. I thought the yaw damper was on the rudder controlling lateral motions? What I experienced was a gentle rocking or rolling motion.

Thank you about the point that there is no true level. I imagine there are numerous forces acting upon the aircraft at any given time during a flight.
 
PGNCS
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RE: Airbus 320 Curious Flight Profile

Sun Nov 09, 2008 1:44 am



Quoting William (Reply 11):
Many posts here state it was the yaw damper working. I thought the yaw damper was on the rudder controlling lateral motions? What I experienced was a gentle rocking or rolling motion.

It was the yaw damper working. Understand that displacements in yaw and roll interact in swept wing aircraft.
 
william
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RE: Airbus 320 Curious Flight Profile

Sun Nov 09, 2008 3:09 am



Quoting PGNCS (Reply 12):

Thank you for explaining the yaw damper,that makes sense,thank you.
 
PGNCS
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RE: Airbus 320 Curious Flight Profile

Sun Nov 09, 2008 5:22 am



Quoting William (Reply 13):
Quoting PGNCS (Reply 12):


Thank you for explaining the yaw damper,that makes sense,thank you.

If you are interested here's a modestly interesting bit on yaw dampers specific to the 727:

http://www.boeing-727.com/Data/systems/infoyawdampers.html

I know you were more interested in the Airbus, but this gives a generalized idea of what they do. To address your questions about Dutch Roll, you might want to refer to this page:

http://www.av8n.com/how/htm/equilib.html

Some aircraft require a yaw damper(s) to be operational and/or require special training for extricating yourself from a Dutch Roll situation (e.g. the B-727, which can be vicious). Hopefully this answers your questions!  Smile

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