Rocking

[ceeb]

Hi All,

I fly quite a lot as PAX on A320 & 321 something I have noticed a lot is when well established in the cruise quite often i notice a very gentle side to side rocking of the aircraft and was just wondering what it is. I was thinking maybe ap corrections?, or it could be the wine I consume :rolleyes:

Any answers appreciated!

[ZOOKER]

ceeb,
it's called 'Dutch Roll'. I can't remember the physics/aerodynamics off-hand, but check out Google.
Many years ago I did a British Airways cockpit trip from Copenhagen to Manchester and the BAC1-11 500, (G-AVMJ, I think), displayed this phenomena superbly all the way across the North Sea.
The flight-deck crew were fascinated by it, and wondered whether any of the folks in the cabin would actually notice it.

[ceeb]

Thanks Zooker, just googled it and it explains it!

Dutch roll is a combination of yawing and rolling motion that is characteristic of all swept wing aircraft. Disturbances about the yaw axis of an airplane will result in disturbances about the roll axis. This is caused by the dihedral effect of the wings (roll due to yaw). On a swept wing airplane, this effect is due to two causes. First, when a wing with positive dihedral (tips higher than roots) is yawed the forward wing is at a higher angle of attack than the trailing wing. This results in a differential lift situation which causes a rolling moment. If the wing is also swept, the forward wing will also cause more rolling moment than the other wing due to the greater moment arm of its center of lift as well as the increased lift. This rolling produces sideslip due to roll only. This sideslip on a wing with dihedral causes a rolling moment tending to lift the down wing. The net result is a pendulum-like motion similar to the rolling motion of a Dutch ice speed-skater on the frozen canals of Holland, hence the name "Dutch Roll". Loss of directional control can occur if divergent Dutch roll is not properly damped. This is one of the reasons for the development of yaw dampers on modern day jets.

[ZOOKER]

ceeb,
that's the fella. :ok:
You beat me to it!
Now, it's time listen to some music by the Dutch band 'Focus' to celebrate your newly-acquired knowledge.
'Moving Waves', 'Focus 3' and the 'Hamburger Concerto' are all great albums to start with!

[Wizzaird]

Hi Ceeb, I fly the A320 and suspect what you are noticing is simply the ailerons responding to minor turbulence. Even in fairly smooth air the autopilot will correct any deviations in roll and this might be noticed in the cabin as a rocking sensation. At altitude the air is pretty thin so it doesn't take much disturbance to cause a roll of some sort. Full blown Dutch roll should not really occur because the flight control computers are designed to prevent it happening (by moving the rudder automatically before you would even notice it)
Hope that makes sense!

[ceeb]

Thanks Wizzaird,

Yes it always happens well established into the cruise, I do LGW-FAO a lot and it always seems to start over the Bay of Biscay. Quite a nice sensation really especially after a drop or two of red wine :O

[Cymmon]

I am so glad someone has brought this up, I have noticed it a number of times on the Monarch A320/A321 a/c flying back from Lanzarote.
Wasn´t sure if it was the a/c or the wine that gave that sensation, now I know so more wine for me next time!!!:ok:

[Agaricus bisporus]

For a start it definately isn't Dutch Roll in straight and level flight, this is more a swooping oscillation and quite distinctive. I doubt it is ever experienced in airliners these days.

Ailerons? That is not side to side but oscillations in roll.

A regular gentle side to side motion on a 2 -3 second cycle (a bit like a driver who can't steer straight and keeps correcting a bit left and right on a motorway) is the yaw damper - which is there to minimise Dutch Roll, but some designs are better than others, and some are worn or not correctly adjusted and get into a cycle that gently wags the tail. I understand that certain Airbi are particularly prone to this.

Well, that's my guess anyway.

[Wizzaird]

Ailerons? That is not side to side but oscillations in roll.


Side to side rocking in my book is called oscillations in roll. It is countered by moving the ailerons

Most likely the effect decribed by the OP is a combination of disturbances in roll and yaw caused by external conditions which the AP/YD will do its best to counter but it can only react after the event!

[Scarbagjack]

This is interesting.
I do a quite a bit of flying ( as a passenger ) and I have also noticed this feeling of gentle side to side rocking like the tail is wagging.
To try and clarify what I mean by side to side rocking I would say that it is not like the aircraft wings are dipping but that the rudder was being applied without any roll of the aircraft around the central axis but just that soft push on the shoulder from one side to another whilst staying perfectly vertical. ( best I can do in layman speak!:O)
I have only noticed it on the A320 and in calm cruising conditions. I would asume that if this sensation was occuring in choppy conditions, it would be hard to distinguish from the bumps.
I have never noticed this happening on any Boeing aircraft.
I am sorry I have no explanation or even theory for this, but the sensation is most definitely real.

[grizzled]

It's actually designed into ABs. Once an aircraft has been in cruise for more than 2 hours ""Papa Floor Protection" kicks in. The gently induced motion lulls the flight crew into a semi-comatose state so they won't do anything to interfere with the aircraft's systems whilst enroute.

Five minutes prior to "Top of Descent" you'll notice a short sharp bit of light turbulence. This is the auto alert system gently waking the crew so they can begin the (minimal) interaction required in order to get you all safely on the ground.

As you can all verify, it works well!

;)

[Rollingthunder]

http://www.aviationexplorer.com/Blue%20Angels%20Navy%20F-18%20Aircraft%20Team/Blue%20Angels%20Navy%20F-18%20Jet%20Aircraft%201%20(13).jpg

[Agaricus bisporus]

Wizz - maybe in your book, but not in the rest of aviation's.

As I said above, side to side - lateral - "tail wagging" oscillation (as so well described by scarbagjack) is in YAW, which is controlled by the RUDDER, and modified/damped by the YAW DAMPER, which is what has caused this effect we're discussing. The damper gets into a closed feedback loop such that it makes a tiny correction one way, shortly after senses the small deviation in yaw it has caused, and corrects it. and so on. and on.and on. and on. And you feel a gentle wagging - but probably only if you sit towards the back of the aircraft. Over the wings it will not be noticeable.


When it happens in a Boeing the crew at the rear of the aircraft usually report it and the yaw damper gets a bit of TLC to cure it. Airbi seem to do it far more.

[L4key]

Ah but is it the Airbus wagging the tail...? :}

[crippen]

Noticed it on the 380. A lady put her compact mirror on the food tray,and the sunshine thro the window was drawing little circles on the ceiling. About 2 second cycles.:8

[ThreadBaron]

The gently induced motion lulls the flight crew into a semi-comatose state

It is no coincidence that 'yaw' is the root of the word 'yawn'.

[Juliet Sierra Papa]

At altitude the air is pretty thin so it doesn't take much disturbance to cause a roll of some sort.

Not quite, the thinner the air, the less the resistance.