Slips in turns and landing with winglets

On Friday, March 11, 2016 at 6:31:54 PM UTC-8, C2 wrote:
I asked Dr.
Mark Maugham about how much slip angle the winglets on my discus would tolerate before they stall. This was his response.

"In our flight tests, it wasn't possible to stall the winglet at any sideslips we could generate. There is a misconception that a winglet behaves as a vertical tail, but in reality, it operates in a "bubble" of induced velocities generated by the wing. So, when the glider is at some yaw angle, the flowfield around the winglet does not "experience" nearly as much."

I hope this helps.

Unless I missed something here, all of you misinterpreted the argument Johnson was making. Basically, he stated (I was there when he said it) that the geometry of a coordinated sailplane in a turn placed the yaw string slightly ahead of the airflow passing over the wings. Thus the yaw string would be displaced slightly to the outside of the turn (again, assuming the sailplane is in coordinated flight). At no time did he advocate flying uncoordinated in a turn.

I worked out the geometry involved and found that this displacement would be less than the width of the yaw string. I told Dick so, but he could be pretty stubborn at times :-)

Tom

This keeps coming up and seems to be generally believed, particularly when it is accompanied by a nice diagram where invariably the scale of the glider and the circle is utterly mismatched. If you do the calculations for average circle the resultent angular dis placement is about one and a half degrees. I am sure I cannot discern that in the fluttering string. Perhaps others can

Cheers

Paul

This discussion is exemplary of why r.a.s is not a place to learn.
There is the usual low fact:fancy ratio and high fertilizer quotient, spiced with a little unwarranted personal vindictive.
The personal experience is useful, yet the only analytic comment that is sound is the quotation of Mark Maughmer. We need less "physics" and more aerodynamics to understand this phenomenon.
In addition to aileron drag, spanwise flow is importantly improved for ships without spoilers with a little slip (the proof of the pudding is in the eating). Regardless of improved climb, there's a real safety benefit: in a steep turn, in an un-spoilered ship with long wings, if the yaw string is kept centered, the stick will be hard against the top stop. Now there is only two-axis control. Thermal turbulence will readily cause an incipient spin, as I have experienced many times -- until I learned how and why to slip steep turns. Slipping the turn enough to (approximately) center the stick:
- most importantly improves control (a good thing for the spouse & kids, and the others sharing the thermal)
- decreases span-wise flow, improving the lift distribution, and
- decreases aileron drag, which probably improves turning sink rate, but more important, helps avoid stalling the inboard wing.
Ships with spoilers have improved circulation at the outboard part of the wing, decreasing the need to slip turns.
Credits: Dr. Mark Maughmer, Tilo Holinghaus