Yaw Damper v Rudder Trim

[Hansard]

Probably a naive question which will reveal my ignorance........if autopilot is engaged, should the yaw damper be disengaged before adjusting rudder trim? The aircraft in question is a rurboprop with manual rudder trim. One Training Captain says yes, another says no, hence my question.

[BelArgUSA]

No - keep the yaw damper operating - if a "series yaw damper"...
If a "parallel yaw damper" - disconnect - adjust trim - re-engage yaw damper.
xxx
:)
Happy contrails

[411A]

BelArgUSA gives precise advice....and is precisely correct.
Having said this, many pilots today wouldn't likely know about a full time parallel yaw damper....unless they flew an old jet transport type.

[Fullblast]

BelArgUSA & 411A

Are you sure? B737 NG has a parallel yaw damper (no pedal movement results from yaw damper operation) , but you can adjust the rudder trim without disengage autopilot.

The Rudder System (http://www.b737.org.uk/theruddersystem.htm)


FB

[hetfield]

On series Y/D rudder pedals don't move.

[411A]

On series Y/D rudder pedals don't move.

Yup, spot on.
Now, some aircraft have both series and parallel yaw damping.
One in particular is the Lockheed L1011.
Series yaw damping normally, however, when performing an automatic approach/land maneuver (autoland), and after approach/land track is established, the rudder yaw damping shifts to parallel operation, to aid with pilot feedback for the runway alignment and rollout function.

A superb arrangement, one that keeps the pilots in the loop at all times.:ok:

[Fullblast]

A bit confusing;

From B737NG FCOM:


Yaw damper operation does not result in rudder pedal
movement. Only main yaw damper inputs are shown on the yaw damper indicator.
The pilot can override either main or standby yaw damper inputs using either the
rudder pedals or trim inputs.


No pedal movement, but you can trim with autopilot and yaw damper engaged.

FB

[Denti]

Indeed, the posted excerpt is correct for the NG, however if you fly an automatic approach the rudder servo works parallel during approach, rollout and go-around until another roll mode is selected. That makes it easy to exactly follow through and know what the autopilot is doing at any given time, same as the 1011 apparently, although the servo is not a yaw damper but just a rudder servo.

[Loose rivets]

When commuting between the US and UK, I occasionally find myself in the rear of a 777. It is always a bad experience.

The yaw damping...at least, I assume that's what's causing it, wags the tail in the lightest of chop. On one occasion, my coffee was slopping 3/4" up the sides of my cup, for hours, for no apparent reason. I've almost reached the stage that if I can't travel near the front, I don't travel.

What the hell sort of aircraft/aircraft operation, is that? The unnecessary stresses on the airframe must sum over the years and be very detrimental.

[NSEU]

The yaw damping...at least, I assume that's what's causing it, wags the tail in the lightest of chop. On one occasion, my coffee was slopping 3/4" up the sides of my cup, for hours, for no apparent reason.

Yaw damping is helping prevent yaw. I'm sure your tea would be in your lap (or the lap of the person sitting next to you) if it wasn't for this system ;)

Modern yaw damping systems even help counteract the twisting/bending of the fuselage due to crosswinds.

But, having said that, I'd imagine the system has some kind of reaction time. It wouldn't be able to eliminate all the yaw.

Rgds.
NSEU

[Loose rivets]

Modern yaw damping systems even help counteract the twisting/bending of the fuselage due to crosswinds.


Given how sophisticated the software controlling that aircraft is, I'm surprised that it can't take the last bit of 'tail wag' out of the equation.

Maybe your comment above says more than it seems. If the aircraft is so bendy that you can't stop the entire length wagging at the same time, I for one would like to see it made out of cast iron, but the BAC 1-11 was probably the last aircraft to be made thus. The damper on that was there because of the dangerous - to the newly trained on type anyway - levels of yaw/roll couple. On the 777 all this should be history. There should be no inherent wag, just the natural and often quite pleasant movements that we have all accepted for so long.

It could be the very fact that there is not this freedom of movement: computers doing such a good job of 'accurizing' the flightpath of the wings, that it occasionally leaves a residual wave that travels down the fuselage. My feeling is that something fundamentally wrong and I would bet on something like this going on.

You infer that it would be much worse without the damper. That gives me little comfort. Looking at the five or six rows of people in front of me all be doing some sort of bizarre Mexican wave, caused by the lightest of chop...and I mean light, how could this happen in a well deigned airframe?

[NSEU]

Indeed, the posted excerpt is correct for the NG, however if you fly an automatic approach the rudder servo works parallel during approach, rollout and go-around until another roll mode is selected.

Are you saying, during automatic approach, trim works in parallel... or yaw damper... or both?

I was unable to find any link between rudder trim (or YD) and the A/P in my training notes.

Thanks.
Cheers.
NSEU

[Denti]

No, i was saying that during automatic approaches the rudder servo (not the yaw damper and not the trim) works parallel. That is of course only valid for NGs that have an CAT IIIb capable autoflight system which includes automatic rollout and single engine CAT IIIa automatic approaches and go-around. For that to work it has a rudder servo and a rudder autopilot channel. Many if not most companies don't want the extra training, certifying and maintenance cost of that system and therefore never order it although it is available for at least 5 years.

And that kind of operation seems to be somewhat similar to 411s description of how his beloved L1011 works.

[NSEU]

i was saying that during automatic approaches the rudder servo (not the yaw damper and not the trim)

Thanks for the clarification , Denti :)

"If the aircraft is so bendy that you can't stop the entire length wagging at the same time, I for one would like to see it made out of cast iron,.."

The BAC111 would probably do the same thing if it was 4 times as long. I'm curious to know if you were flying a stretched 777.

I guess you have to have some tradeoffs for weight, but on the other hand, flex can help to cushion the ride (and if you compare an aircraft with a willow tree, flexing can sometimes be better than sudden failure). :}

Cheers.
NSEU

[skiingman]

My feeling is that something fundamentally wrong and I would bet on something like this going on.

You may find this article worth reading:

Boeing 787 Dreamliner Rolls Out Smoother Ride with Gust Suppression - 2007-06-04 00:00:00 - Design News (http://www.designnews.com/article/439-Boeing_787_Dreamliner_Rolls_Out_Smoother_Ride_with_Gust_Supp ression.php)

The information these (pre-787) dampers work on is from sensors of the inertial response. There has to be movement to damp. The movement is complex, and the airplane is large and flexible, and there are only a few control surfaces to effect damping with. No chance of a perfect result, at least not for all observers, some a hundred feet from the CG. Even if you act before the plane starts to move, as the 787 intends to, you still can't quiet all those motions. This is why the article mentions a focus on modes that annoy or cause illness, as opposed to those most of us find pleasant. I believe this focus isn't new, and though you may find the 777 folks didn't get it quite right, I do believe it goes beyond "yaw damping" into "ride control".

Going back a whole step, it might be said that the reason that these unpleasant motions exist is that stability carries an aerodynamic penalty, and this incentivizes engineers to replace it with sensors and dampers and all sorts of annoying math. See, for example:

Method and apparatus for reducing ... - Google Patents (http://www.google.com/patents?vid=USPAT5669582)

Unfortunately I've never had the privilege of riding in the back (or anywhere) in something like a stretched DC-8....did the early airliners have a more pleasant ride?