Slips in turns and landing with winglets

Martin I fully understand this discusion, having some time wheeling around an extended wing open cirrus but I am afraid Bruce there isnt quite at cloud base yet.

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.

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

The statement attributed to Dr Maughmer that winglets don't stall is baloney. They do stall and it is easy and interesting to observe by simply tufting them and watching as you increase yaw or slip in a turn.

I found on the Duo Discus XL the inboard winglet stalls at a yaw string angle of about 15-20 degrees (not sure how that corrolates to actual aircraft yaw angle). But the amount of slip required to cause the yarn tufts to blow backward was a little more than a pilot would naturally use. So Schempp has chosen a mounting angle that, while not optimum in perfectly straight flight, is tolerant of pilot "abuse." A stalled winglet is effectively a mini speed brake. Not welcome.

KS

On Monday, February 8, 2016 at 12:55:19 PM UTC-5, kirk.stant wrote:
On Sunday, February 7, 2016 at 6:11:14 PM UTC-6, Martin Gregorie wrote:

Doesn't this follow from the fact that the yaw string is a fair distance
in front of the wing?

Think about it: if the tipwise axis of the wing is exactly aligned with
the radius of the turn, which it should be for maximum climb efficiency,
the yaw string should also be at right angles to the turn radius but,
being a good 2m/6ft or thereabouts in front of the wing, the turn radius
it is on is angled forward of the turn radius the wing is on, which
therefore makes it point out slightly out. This looks like a slight slip
to the pilot.

Draw this situation on a piece of paper, but exaggerate the situation by
drawing the circle diameter and about twice the wingspan and you'll see
what I described.

If you redraw the diagram with correctly scaled turn radius and wingspan,
the angular difference between the wing radius line and the yaw string
radius line will be very small, but this needs to be adjusted because the
curved shape of the canopy will amplify the angle of yaw string in a
slip. Now Dick Johnson's advice makes perfect sense.

Practically, if the yaw string is central or pointing slightly out in a
thermal you're doing it right. If its pointing a long way out or to the
inside of the turn you're sideslipping while turning and this is hurting
your climb rate by generating excessive drag.

Martin, I believe this is only part of the answer. You are correct that a perfectly coordinated turn (slip ball in the center) should show some slip if the yaw string is forward of the CG (and conversely, a centered forward mounted yaw string indicates a skidding turn - beware in the pattern!) but there is also the overbanking effect of long wings seeing different local airflow velocity in a turn; this can be countered by holding aileron out of the turn (inefficient and draggy) or by adding a little bit of slip via top rudder. This slightly increases the angle of attack of the lower wing and decreases the angle of attack of the top wing and voila, no overbanking, ailerons are flush and efficient, and you gleefully outclimb the newb with the perfectly aligned yaw string. It's really noticeable in my LS6 (probably due to the dihedral) - it settles into a nice slipping climb with the yaw string off about 10 degrees to the outside, no aileron deflection, and just a touch of top rudder. Extra bonus - you get some lift off the fuselage! (See "String Theory").

Kirk
66

John Coutts, world champion, stated this as well. I had the pleasure of two days dual instruction with him in Minden. His demonstration of this effect convinced me of the benifit of a little top rudder, increasing the angle of attack of the inner wing, while thermaling.

On Sunday, February 7, 2016 at 5:09:42 PM UTC-5, wrote:
I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?

Sorry for the questions, but would appreciate any help in this.
Karl is more forthright, than I was going to be!
When I installed homebuilt winglets ( Maugner airfoil) on my 20E,
I set them at the recommended -ve angle to the horizontal axis
of 3 deg from the zero lift setting. Upon tufting these winglets,
it was quickly apparent that keeping them both unstalled was
almost impossible ( Note , the flow at the wingtip is not
along the axis) I added 2 deg extra negative AoI and the problem
became manageable, but I still now pay a lot more attention to
slip ( or not).

Slipping turns ( without winglets) are useful.
Years ago, flying a rented Libelle std I found climbing against the
Std Cirrus discouraging. After ballasting to aft CG and circling
with a fair amount of slip, I could climb with the best Cirrus
drivers. But they left me on the glide!

John firth; an old no longer bold pilot.