Helicopter Yaw Rate Damping w/ SCAS (SAS)

[LongbowInstructor]

Does anyone have any information dealing with yaw rate damping on the ground? This would be to prevent "Dutch Roll" movement of the tail? Anything would be helpful....

Thankshttp://www.army-technology.com/projects/apache/apache1.html

[Shawn Coyle]

I'm not sure what it is you want. On the ground, there should be no yaw rate to be damped.
Many helicopters with AFCS incorporate a weight on wheels sensor to change gains when on the ground, just to further complicate matters.
Dutch roll is normally referred to as an aerodynamic phenomena where roll and yaw movements couple together - on the ground neither of these should be present.
If you are talking about some tail boom movement on the ground that is a different story, and I don't think that a yaw damper would help here, as it would only sense yaw rates of the fuselage.
Need more details about the problem.

[Gregg]

If you have a ground tailboom mode, you can use a stability system to damp it out. However, you add a series of complications which include needing a very reliable weight on gear switch that turns these gains on for ground operations and off for flight operations.
It would probably be more efficient to cure the tailboom mode by changing the frequency response of the tailboom by other methods.

[Wunper]

If it’s the first torsional and lateral bending mode of the fuselage you are referring to there is normally a lot of work done here to prevent this occurring,

With the Navy Lynx that I was involved with a few years ago , the tail wag was due to culmulative mechanical hysteresis in the yaw control run and yaw servo jack mount security (lack of it), it would kick in quite literally above about 100kts, we had a handful of “tank slappers” to use motorcycle parlance in the fleet before an oversight on an inspection periodicity on the yaw control run was unearthed. It could minimised on those Lynx by knocking off the No 1 Hydraulic system which provided the Yaw control run power.

I am not sure which aircraft you are referring to unless your name holds the clue but I understand a lot of mods have been done to play around with the torsional stiffness of the tailboom below the 1P frequency and nifty work with elastomeric mounts on the vertical tail to tailboom joint.

[Flytest]

The Sea King AFCS incorporated a yaw rate damper, but don't recall it being any use in the scenario you refer to.

Wunper

Would your experiences then explain why I spent half of my time on MARTSU up the back end of Lynx fixing cracks???;)

Our response to the problems was to beef up the airframe at the upper longerons and transportation joint, including surrounding frames, thus achieving the effect of preventing cracks in those locations and initiating new ones elsewhere :rolleyes:

FT

[Shawn Coyle]

I spoke to very knolwedgeable Apache folks, and the problem may be due to the weight on wheels switch sensing 'airborne' and turning on the yaw SAS.
A bit more info about what the problem is would be helpful.