Flying Wing 'Wobble'

[Extreme Bob]

I think this is probably the right forum for this question.

It concerns something I've noticed with a few flying wings I've built, namely that sometimes while flying they seem to develop a side-to-side (yaw) wobble while flying. It's hard to describe but it's as if someone was waggling the rudder stick, even though there is no rudder on these wings....

Does anyone have an explanation for this phenomenom? I've tried checking that the winglets are parralel to the direction of flight and as best as I can tell, they are...

Thanks,

Mark.

[Ollie]

If there is a slight roll associated with the wobble then it is a lack of yaw stability. It can be eliminated by increasing the vertical tail area. Tape a piece of card stock to each fin to temporarily increase its area. Reestablish the previous CG and test fly to see if that corrects the wobble.

[Sparky Paul]

I've looked at that "head-shaking" on my ZAGIs.. it's a vertical area thing, as Ollie mentions.
Add some fin area -below- the wing.
I tried everything from no vertical at all to the added area. Adding the area helps.

[Extreme Bob]

Thanks guys. :)

[Buster Brown]

I have a book that describes the Horten brother's wings, both the history and design evolutions. They called such side-to-side oscillations "dutch walking", and attributed it to a lack of vertical stabilizer. That seems to be the conscensus of other articles I've read that describes the problem, and a possible solution to it.
That's all I really know about it at the moment.

[JMP_blackfoot]

It concerns something I've noticed with a few flying wings I've built, namely that sometimes while flying they seem to develop a side-to-side (yaw) wobble while flying.

Having flown a number of Horten wings, we found that this Dutch roll may happen when the C.G. is slightly too far back, and that is happens when flying fast at low angle of attack.
The effective dihedral of a swept wing is dependent on the angle of attack. Look at your wing from the front, and lift its nose , you will see that the apparent dihedral becomes negative.
Dutch roll occurs when there is too much dihedral for the vertical fin area, or too little fin for the dihedral.