A340 Yaw Damper

[olirindis]

Greetings,

What does the A340 do about controlling minor excursions in yaw throughout its flight envelope as it doesn't appear to have any obvious Yaw Dampers like I've seen on other Airliners?

Regards.....

[FCeng84]

A340 Yaw Damping part of FBW Augmentation

The A340 fly-by-wire augmented control laws include damping of the yaw axis. There is not separate equipment for this purpose.

[Dani]

I don't know where you have the idea that yaw dampers on A340's have any other function or way of working than any other airplane.

btw it's wrong to believe that yaw damping is there to reduce "excursions in yaw". Yaw dampers are - in any jet with swept wings - there to reduce the risk of dutch roll.

Dani

[FCeng84]

Yaw damper is a function, not a piece of equipment

Dani - I agree with you completely. Also note that where older airplanes tended to have specific equipment called yaw dampers as that was its sole task, more highly augmented airplanes have combined the task of damping Dutch Roll with other control law functionality. In that case, the equipment providing the full range of augmentation is probably not called a yaw damper.

[olirindis]

Many thanks......got the gen I wanted!!

[grounded27]

If you would watch a modern rudder (controlled by a computer actuated by the rudder actuator) you would see a flutter or rapid constant reaction to dampen yaw unlike the old slow input that would result in safe flight but would still be more evident along the yaw axis.

[FCeng84]

Grounded27 - Please explain?!

Grounded27 - I have read your entry several times and cannot make heads nor tails out of it. Please give specifics including examples of the airplanes you refer to. Jet commercial transports with swept wings have exhibited low inherent Dutch Roll damping at least as far back as the Boeing 707. Yaw dampers have been implemented to automatically control the rudder to provide the needed damping. Yaw damping represents one of the oldest and most successful examples of aviation control augmention.

Your reference to safety (and your infrence that older models are safer) is very concerning - please explain.

[Jason Burry]

I believe Grounded27 means to say that in older jet airliners, the yaw damper response was relatively slow, and some degree of yaw divergence often existed, but well within safe limits.

In more modern aircraft with more advanced yaw damping systems, the response is much more rapid, and less yaw divergence is tolerated by the system. The rudder makes relatively quick and numerous corrections, resulting in noticably less yawing in flight.

The upswing is that yaw oscillations are nearly unnoticeable in modern airliners, while in an older machine it could be quite noticeable...

J

[FCeng84]

Dutch Roll and Yaw Damping

Dutch Roll mode frequency is typically less than 0.5 Hz. As a result, damping of the Dutch Roll mode provide on commercial transport aircraft by yaw dampers does not require particularly high bandwidth control. Early analog electronics of the first yaw damper modules provided quite adequate improvements in Dutch Roll damping. More recent designs have included more sophisticated control algorithms that provide higher fidelity turn coordination and higher bandwidth sideslip angle control, but the basic requirement to improve Dutch Roll mode damping is equally met by early model yaw dampers and the latest full authority directional augmentation control laws.

Taking another look at grounded27's entry, use of the word "flutter" jumps out at me. "Flutter" is a term reserved to describe unintended oscillation of either a control surface or airplane structure associated with lightly damped or unstable modes. To say that an airplane exhibits flutter is to suggest that it has a serious design or manufactoring flaw that would not be certifiable. To say that a control system is high bandwidth and results in rapid control surface motions is one thing. To characterize those motions as flutter is a completely different issue. Be careful with your choice of words.

[grounded27]

Taking another look at grounded27's entry, use of the word "flutter" jumps out at me. "Flutter" is a term reserved to describe unintended oscillation of either a control surface or airplane structure associated with lightly damped or unstable modes. To say that an airplane exhibits flutter is to suggest that it has a serious design or manufacturing flaw that would not be certifiable. To say that a control system is high bandwidth and results in rapid control surface motions is one thing. To characterize those motions as flutter is a completely different issue. Be careful with your choice of words.

None the less you understood my intent and YES how other to describe the rapid movement of a flight control in one word...... May be high frequency of rudder control surface movement to dampen unwanted yaw effect that may otherwise result in dutch roll and also lower the amount of vomit bags used in the aft section of large aircraft.

Just to add that on many aircraft types they take the 400Hz electrical system and increase the frequency used to read the position sensors on the actuators for cleaner operation..

And yes visually it appears to flutter to the layman's eye!!!!!!It appears to be quite insane to the point. But there is genius to this insanity. You sir should chose your post's carefully!

I took the liberty of spell checking your words in the quote for you...

[glad rag]

...... As a result, damping of the Dutch Roll mode provide on commercial transport aircraft by yaw dampers does not require particularly high bandwidth control.

Explain your terminology please, is this ANOTHER bastardised meaning of the term bandwidth??

[FCeng84]

Bandwidth explained

By bandwidth I am referring to the frequency range of the control surface commands. Because the Dutch Roll frequency is less than 0.5 Hz, there is no need for control surface motions at frequencies higher than about 1 Hz to provide increased Dutch Roll damping.

It is still unclear to me what examples grounded27 is pointing to when stating: "If you would watch a modern rudder (controlled by a computer actuated by the rudder actuator) you would see a flutter or rapid constant reaction to dampen yaw". Specifics on airplane type and phase of flight would shed more light on this topic.

[grounded27]

FCeng84

I should say if you could watch any modern jet with FCC's that control the rudder electrically especially during cruise at altitude. The rudder is in constant rapid motion to keep the aircraft in stable flight allowing for a more comfortable, clean flight not just to prevent dutch roll. The FCC's calculate and command at an electrical frequency of commonly 1600Hz. A good example of why could be explained by anyone who has had the pleasure of sitting in the back of a 747 classic.

[FCeng84]

Rudder quiet during cruise in calm air

Rudder position during cruise in calm air is steady (centered at zero except when rudder trim is inserted to balance a directional asymmetry such as an engine out). The rudder will only move in response to disturbance inputs or in conjunction with pilot or autopilot commands to maneuver laterally. In addition to Dutch Roll damping at approximately 0.25Hz, some modern commercial transports include rudder control to damp structural modes with frequencies up to about 3.5Hz in order to provide ride quality improvement, but neither of these functions moves the rudder in the absence of atmospheric disturbance or maneuver command input. Control surface oscillations without disturbance inputs would indicate limit cycling that is not permitted by design requirements.

I am not aware of any commercial flight control systems that have control surface command processing loops that run at frequencies higher than 100 Hz. Alternating current electrical power may is available with higher frequencies, but the fastest control surface command update loops run with frame times of 10ms or longer.