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Yaw Damper

Mon Sep 01, 2003 11:45 pm

Can somebody explain me the function/use of the Yaw Damper please?

Many thanks

RE: Yaw Damper

Tue Sep 02, 2003 12:29 am

Dear Luxair:
The yaw damper is a stability component which prevents Dutch roll, the yaw damper is generally part of the autopilot, and acts upon the rudder to prevent the aircraft from yawing left/right (maintain its stability on the yaw axis)...
In a jet aircraft with swept wings, when the aircraft yaws, the wing so moving somewhat "forward" becomes subject to extra lift, raising that wing, and the consequence is extra drag... this then pulls that wing back, and the aircraft opposite wing then moves forward, gets subject to extra lift and extra drag... some old jets had very nasty Dutch roll behavior, to the extreme of complete loss of controls and stability...
Some airplanes are severely limited (altitude and/or speed) when the yaw damper(s) are inoperative, such as the 727... some are not limited and easy to control, or not even prone to Dutch roll, like the 747...
How do you prevent Dutch roll, is by using large tail fin surfaces, which improve yaw stability, also longer fuselages achieve that task... The stretched DC8 versions were much more stable than the short ones... The short 747 SP required a taller tail fin which assisted to maintain yaw stability...
A 720 (short 707) once had a Dutch roll incident in which the increasing violence of the unstable aircraft, eventually tore one engine nacelle off its wing...
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RE: Yaw Damper

Tue Sep 02, 2003 12:35 am

Swept winged aircraft have a tendency to "Dutch Roll". The "Dutch Roll" is a yawing motion coupled with a rolling motion. It can be initiated by the deflection of the ailerons. The aileron that is "up" in a roll will cause more drag than the "down" aileron on the opposite wing. The distance from the aileron to the airplane's center of gravity will then cause a moment force that will yaw the airplane in the direction of the "up" aileron.
In certain conditions, the "Dutch Roll" will increase the magnitude the yaw and roll to unsettling levels.
Looking forward from the cockpit, the nose of the airplane will appear to be moving in a circular motion with the airplane rolling left and right and yawing left and right at the same time.
The yaw damper senses this tendency to yaw and automatically deflects the rudder to counter the yaw. During normal flight operations on swept wing aircraft, the rudder pedals are rarely used in turns, as the yaw damper will assist in maintaining a coordinated turn without initiating a "Dutch Roll".
In the early days of the swept winged jet airliners, flight simulators were not sophisticated enough to provide training for recovery from a "Dutch Roll" where the yaw damper is inoperative. So, on a training flight, the yaw damper would be turned off, the "Dutch Roll" initiated, and a recovery practiced. If the "Dutch Roll" reached large amplitudes, the recovery could generate high enough loads on the pylons that result in shedding the pylon and engine.
A few airplanes lost one pylon and made a successful, but there was a 707-220 on a Boeing test flight where both outboard pylons left the airplane. In this case the crew crash landed on a river bed, the flight crew were lost and the flight test engineers survived because they went to the rear of the cabin. There was a Convair 880-22M that lost all 4 pylons during a training flight and all were lost.
Flight simulators, today, can be used to train for "Dutch Roll" recovery under much safer conditions.
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RE: Yaw Damper

Tue Sep 02, 2003 1:16 am

The yaw damper is a computer/gyro that strategically moves the rudder or trim tab to prevent Dutch Roll. On the Fokker 100, for example, the yaw damper has eight degrees of authority in moving the rudder to stabilize the aircraft. Hope this helps!
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B-52s don't take off. They scare the ground away.
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RE: Yaw Damper

Tue Sep 02, 2003 4:41 am

Yeaphhh, thanks a lot to all of you for your very precies answers!!!

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RE: Yaw Damper

Tue Sep 02, 2003 8:29 am

A couple of clarifications due to the VAST number of aircraft out there...

First, the yaw damper system is most often a stand-alone system that does not communicate with the autopilot and can be operated by itself. The yaw-damper engage switch is however most often right on the autopilot control panel because many autopilot systems require the yaw damper system to be operational whenever the autopilot is engaged.

Secondly, yaw damper systems typically use two sources of information: Bank angle and yaw acceleration. Many yaw damper computers have the accelerometer built right in.

The bank angle signal causes a direct rudder deflection proportional to the bank angle. The acceleromter causes a direct rudder deflection proportional to the yaw rate-of-change. In other words, if the aircraft was in a steady 5 degree per second yaw with no bank (not likely) the yaw damper would not cause a rudder deflection.

Even with a yaw damper system, the F28 fish-tails through the sky. If you sit in the back and watch closely, you're feel the aircraft slightly roll left and right continuously. The Dutch seem to be experts at "Dutch roll"...

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RE: Yaw Damper

Tue Sep 02, 2003 2:08 pm

In some aircraft, the B757 for one, the Yaw Damper aids in turn coordination.

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