Mystery of Yaw Damper

[Dreamlinerwannabe]

I have heard of this term for awhile and I never know how does it look like.

I then go to google, made me even more confused.

yaw damper - Google Search

When I input Yaw Damper on google search for images,

I see whole light aircraft, whole big jet, instrument panel....

So my question is, how does yaw damper look like? Which picture is true?

Thanks!

[de facto]

Hi there,

First picture is a B737 yaw damper.

[Rick777]

They are probably all true. The yaw damper is a piece of equipment that lives in the tail section of a lot of aircraft and may be mechanical or hydraulic operated. All the pilot will ever see of it is the switch to turn it on, and some planes don't have a switch.

[717tech]

Some aircraft claim to have one (Dash-8 200/300) but I don't think it actually does a thing!

[LimaFoxTango]

Some aircraft claim to have one (Dash-8 200/300) but I don't think it actually does a thing!

Au contraire, try flying with it off and you'll see.

[aterpster]

727 had two of them. Turn them both off at high altitude and you could very well roll over on your back.

[Rick777]

The KC135 didn't have one and neither did some 707s. They had a rudder axis on the auto pilot which usually worked fine, but had to be turned off for take off and landing. Always funny to watch new guys Dutch rolling down final.

[fantom]

Goodness me! Dutch roll/ yaw damper. Do pilots know anything these days?

[aterpster]

Rick777:

My company's early 707s had a part-time yaw damper that had to be off for takeoff and landing. Perhaps mid-way, or earlier, the deliveries changed to a full-time "series" yaw damper. At that point in production the anti-skid brakes changed from those awful thumping (can't recall the name) anti-skid to full modulating anti-skid.

[tdracer]

Large, low wing commercial aircraft tend to have nasty Dutch Roll characteristics (high wing aircraft are generally much better). While there are some design characteristics that can reduce Dutch Roll, for the most part they tend to make other aspects of aircraft handling/control worse.....

The exception is the yaw damper - a properly functioning yaw damper makes Dutch Roll a non-issue. A yaw damper is pretty much what it sounds like - an automated system that moves the rudder automatically to address oscillatory yaw behavior.

I've always heard the 707 Dutch Roll behavior was really bad (with regard to what aterpster wrote - I hadn't heard the 727 was as bad or worse than the 707 - not disagreeing, just relating Boeing tribal lore).

The last fatal Boeing Commercial flight test crash was a 707 back in 1959. Boeing had introduced a new 707 yaw damper, and the customer pilot was doing his best to test it out by making large control inputs to induce Dutch Roll. One of the inputs was so severe that the subsequent recovery resulted in engines 1, 2, and 4 departing the aircraft . The loss of three engines and associated damage was such that they couldn't make it back to an airport and had to set it down in a field north of Seattle. Four of the eight occupants didn't survive. The story as I heard it was, the pilots and flight engineer stayed in the flight deck, while the other occupants braced for the crash in the back - those in the flight deck perished while those in the back survived.

ASN Aircraft accident Boeing 707-227 N7071 Arlington, WA

[TLB]

Yaw Damper

Had one of those on the old CF-101 Voodoo. Part of the Automatic Flight Control System, it had a separate ON/OFF switch and performed two functions:

1. if you manually applied a bunch of rudder, say to the left, and then let go & centralized the rudder, the aircraft would yaw back the other way (to the right) and after one cycle the yaw damper would in fact dampen out all the yaw instead of yawing back & forth until the aircraft stability would eventually zero out the yaw;

2. more importantly, it would automatically counter adverse yaw effects. In other words, if you want to turn left, you input left aileron and the aircraft starts to roll to the left, but it will also yaw in the opposite direction (yaw right in this case). The yaw damper would recognize this and automatically input left rudder to counter the right yaw. You could keep your feet off the rudders and roll left or right and the yaw damper would always keep the ‘ball in the centre’.

[aterpster]

tdracer:

I've always heard the 707 Dutch Roll behavior was really bad (with regard to what aterpster wrote - I hadn't heard the 727 was as bad or worse than the 707 - not disagreeing, just relating Boeing tribal lore).

The fact Boing had to split the rudder on the 727 so it could have two yaw dampers should tell you Boeing fact from Boeing lore.

[grounded27]

Yaw Damp in modern aircraft is a calculation in the FCC that constantly corrects rudder position, if you saw the rudder in flight it would look like it was fluttering constantly. Even in the 727 yaw damp is more or less a function of the auto pilot. If I remember older systems were less rate/gyro driven but simply tightened up rudder authority with airspeed hence the above statement about switching off on the 707 for t/o and landing.

[aterpster]

grounded:

Yaw Damp in modern aircraft is a calculation in the FCC that constantly corrects rudder position, if you saw the rudder in flight it would look like it was fluttering constantly. Even in the 727 yaw damp is more or less a function of the auto pilot.

The 727 didn't have a FCC, rather a lousy two-axis Sperry auto-pilot. The two yaw dampers were independent of that lousy auto-pilot in all respects.

[Rick777]

I never flew a 727 with yaw damper off, but the sim was a lot harder to recover than the 707. Or maybe it was too long flying new airplanes. I went from A320 to left seat 727. Just wanted to say I flew it.

[Piper19]

To the original message; none of the google results show a yaw damper. The first picture tells you about the 737 yaw damper, but shows one of the PCUs I think. The yaw damper is in that same area. It depends from aircraft to aircraft how big it is, but it's basically not 1 item. Older aircraft tend to have more mechanical linkages whereas modern aircraft calculate the amount of rudder deflection in computers. The yaw damper is closely connected mechanically to pilots pedals, rudder ratio changer, autopilot actuators and the trim/neutral offset cam. Rods and springs make sure that e.g. the rudder pedals do not move when yaw damper makes inputs etc...
I've uploaded a technical drawing for you
Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting

[doubleu-anker]

Yes, of course, the yaw damper controlled rudder input. If you lost the Y/D the book said attempt to control the Dutch roll with aileron/spoiler input. However if you knew when to manually apply rudder input, to control the D/R, it was far more effective than fighting it with ailerons/spoilers. Had to "know what you were doing" though, or didn't even think about it. With A/P engaged the clue for rudder input, was in the control wheel displacement. Taking lag into consideration, the rudder input was applied when the control wheel was in the exact opposite position (on the A/C I used to operate) as the handling pilot would initially expect.

[stilton]

I don't agree that Boeing had a split Rudder on the B727 so they could have two yaw dampers (or dampeners as some of our southern friends called them)


The 757 /767 has two yaw dampers but does not have a split rudder.


I think it was more for redundancy of having two separate powered rudders on the 727.


I do agree, after seven years of flying that lovely Aircraft that you don't want to go above 25000 feet without at least one Yaw Damper and without any it had very unforgiving dutch roll.

[J.L.Seagull]

To the original poster... and to those looking for a chunk of metal.. i.e. an actual component called the "Yaw Damper"... IT DOESN'T EXIST! (At least not on the A330, A340, B777 and all modern aircraft.)

As grounded27 mentioned, Yaw Damping on FBW aircraft is a software function of the of the PFCS. Using inertial references, the primary flight control computers calculate an opposing input and feed it to the rudder servos.

A330s and A340s have a BYDU (back-up yaw damper). If the PRIMs fail, hydraulic fluid (B&Y) enters the BYDU, drives a generator to supply electricity for a small computational unit and a set of gyros. Signal output then commands a hydraulic actuator inside the BYDU to give inputs to the rudder servos input system, thus driving the rudder to provide a yaw damping function.

[Leftofcentre2009]

The pictures of the actual mechanical Yaw Dampers that you see in your search results are off railway vehicles.

The mechanics of it are similar to a motor car shock absorber in that it is a cylinder and piston filled with oil. It is connected to the rail vehicle chassis/body at one end and the wheel bogie at the other end and prevents the bogie from "shimmying" at high speed.

In the UK, rail regulations stipulate that trains must have them fitted if they travel faster than 75mph.

Now back to aviation . . .