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Old Jun 23, 2011, 10:48 PM
treefinder
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Noting the comments on tufting the wings (on one of the threads) and tried to find the vid I did last year on OSG vs Gentle Lady, but guess I ditched it when I cleaned up my hard drive. So. trying again: This is first vid, almost dusk, in between the storms and wind not to my liking for getting decent video. I used sewing thread for the tufts, based on last year's experience with yarn (the yarn never moved no matter what I did - too thick, i guess). If weather improves (sounds now like Saturday) I'll get this out again and also the conventional foil and get some comparison videos.

The video is with a KFM2 wing on the OneSheetGlider (60"span). The threads generally pointed aft at rest before flight. In Flight it looks like most are curled into the pocket behind the step except for a couple longer ones that generally stay straight. I launched her without resetting the elevator trim for climb, so the first part of the climb was attempting to prevent loops, and I edited it out. vid starts after I reset trim and was in a pretty steady climb.

I think I'm going to reset the locations for near LE, just ahead of the step and at 75% of chord instead of what's shown: near LE, and just at step. Any other thoughts?

Untitled (2 min 53 sec)
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Old Jun 24, 2011, 06:42 AM
Jack
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Interesting stuff, Springer.

I guess it is the turbulence from the nose and fuselage and near the center of the wing that makes that first thread want to curl nearer to the camera a lot.

Is that a one layer of FFF high step? About 1/4"?

Jack
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Old Jun 24, 2011, 07:59 AM
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Step is about 3/16, it's dollar store foam. I'm not sure whether it's because the closer thread is affected by fuse, etc or just longer and getting in the major flow over the wing . I'm moving them around for next vid after the rain quits (and do the other wings).
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Old Jun 24, 2011, 08:29 AM
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I'm not sure what can be read into the behavior of the tufts other than the zone downstream of the step seems to be quite turbulent judging by the dancing about of the longer 'tuft'. The shorter ones may have 'stuck' in position because i note that they stayed curled even after the plane landed.

I'll see if i can do something similar with a wing of an 'un-stepped' airfoils to see if the flow is noticeably more stable.

Steve
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Old Jun 24, 2011, 08:35 AM
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Springer that was interesting. I looked at the last few seconds of flight over and over again. Notice how the long thread moves toward the fuse, and the one tucked in untucks for a few seconds? The plane is effectively falling into the turn at one point, and the air flows from the wing tip toward the root. You can also see the tucked yarn untuck just before touchdown. Looks like the "trapped vortex" got untrapped then. Just my interpretation.

The yarn needs to be soft and reasonably short to be effective. I got some "baby yarn) from Walmart. It is thinner and much softer than regular yarn. I'll be using it as soon as I can sort out the power problem on my test plane.

Place the yarn in rows that overlap where the yarn is taped down. That will give you flexible yarn across the area measured. I'd put the yarn spaced 1/2" along the wing for a foot or so, and down the chord from leading edge to trailing edge. That gives good flow visualization. Then when you do test flights do some power off and (if you can) power on stalls. We need to see the flow at high alpha approaching and into stall. Seeing it coming out of the stall will also be informative.

Roger
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Old Jun 24, 2011, 10:37 AM
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springer, great vid and good first go. Adding to maguro points, I might say to try 100% cotton thread and either roll it / rub it out to soften it up as much as possible. To me, it looked like the thread was a bit stiff? Other than that, good show! We got a bunch of rain down here too. I wonder if Steve (JPF) will stick some string on his Elf for the test? If I get a break in the rain, I can give it a try with my MiMi as well for a comparison, (AG03 airfoil), if JPF does not get to it first. Been wanting to put my cam on it anyway. Hope the rain breaks for you soon too...

Mark
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Old Jun 24, 2011, 11:09 AM
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Well, we got a break in weather (of sorts) up here, and I got out again. Unfortunatly before I read all your posts, so haven't added any of the suggestions. (next time!) I moved the thread so it formed 3 sequential tufts along the chord, and was also able to get the conventional wing set up the same way and flown. The Picture shows how they lay on the wings at rest.

In the flight vids, I launched into the wind and climbed to altitude at about 15deg or so; did a turn to right, turn to left; cut power and adjusted trims to glide; did a couple cross wind runs and retrimmed to come in for landing. Wind was a bit higher on the second flight (conventional) and the landing wasn't where I wanted it. You can also see a stall right at the end, that made me realize I should have done some nose up into stalls and some dives during the flight (d'oh!) next time....

The thing that surprised me most on KFM was that the threads turned to the center of the plane, rather than the tip at the step. we've all been thinking that the vortex is spilling off the wing tips. But even the tuft at the tip is turning inward most of the time. Seems like the longer threads get caught in the "bubble" but then sometimes escape depending on speed, angle, etc. while the shorter ones just stay in the bubble.
OSG KFM2 airflow (3 min 12 sec)

Conventional was pretty boring, flow just over wing, although I might have seen some separation at the first two threads from the camera, they seemed to lay pretty flat most of the time, but occasionally would lift or relax.
OSG conventional airfoil airflow (2 min 36 sec)

A few more bits of data for rumination.....

For next time, I'll try unwrapping the thread to be more "tufty" and perhaps shorten each, add more to get a better "surface" flow visualization. I was concerned about the stiffness, but figured it would be less stiff than the yarn I used last summer. Seems like when flow is moving over the wing that they do in fact lay down and align with flow, though. I'm a bit concerned about putting too much on the wing surfaces, as the chord is only 5", and I don't want to have the presence of the tufts dictate the flow.
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Old Jun 24, 2011, 11:27 AM
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Roger: on the second video (which I guess is still processing as I write), watch the tuft at the wing tip step. It starts flowing straight back, but relatively quickly during the climb curls inward and stays like that most of the flight. And, of course, all the other ones turn inward, or flow straight, none turn outward even in the turbulent portions of the flight or during inside or outside turns. not what I would expect.

As you noted in first vid, the threads get caught in the bubble/vortex, whatever, and even though they don't straighten fully, they do relax when the plane lands (as do the threads on conventional foil), so it seems that they are really getting caught in something other than just random turbulence.
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Old Jun 24, 2011, 12:47 PM
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Very interesting, the flow seems to be inward. Wonder what wing tips would do to that flow? Great work springer! Keep it up. Have you thought about putting a few pins in the wing and having a few threads elevated a bit from the surface to see what the air just above the wing is doing?

Mark
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Old Jun 24, 2011, 05:51 PM
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It's been a tough week. The car needed an expensive brake job, the house air conditioner failed and that repair bill was as bad as the brake job. I thought I had good news to report. My new speed controller came in the mail, and I wired it up just after paying the air conditioner repair man. Bad move. The billowing blue smoke the instant I connected the battery, clued me in to the fact that I wired the ESC battery connector in reverse, and didn't bother to check it before I plugged it in. I just put the order in for yet another new speed controller. Geesh.Roger
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Old Jun 24, 2011, 06:01 PM
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Sorry about ranting guys. Springer thanks for the additional videos. Did you notice on the second video when you pitch up to land how the tufts rise up off the wing. That's flow separation at a relatively low angle of attack. Very interesting.

Roger
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Old Jun 24, 2011, 10:00 PM
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Roger: Really sorry to hear about the magic smoke escape! (Hate it when that happens!). I assume you are talking about the conventional wing with the pitchup? Yeah, I noticed it and that was what made me think "I should have stalled both of them" (d'oh!) Hopefully tomorrow will be a bit calmer and nice and sunny and I'll get some more sessions. I think I'll do the UC wing as well, although I'd expect the flow to be pretty much the same as "conventional" wing.
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Old Jun 25, 2011, 03:41 AM
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super work, what you might like to do is once you have got the tuffting to where you want it run some tests with a removable wingtip plate to see if adding a tip plate makes any diff at all.

it would seem that the untipped wingtip is allowing air from the very edge of the wingtip, to roll round the step of the kf step and then poss roll all the way along the inside back edge of the kf step?

of course this is just a possiblity but the addition of a tip to eliminate the possiblity of tip vorticies having any effect on the performance of the rest of the step, would be invaluable information, and one other tuffting test to tick off the list.

RCT
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Old Jun 25, 2011, 03:50 PM
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Following many years experience of wind tunnel results at various wing chords and speeds,, can state that flow OVER a wing tends to move inwards,(Due to lower pressure, which associated with lift) and outwards UNDER the wing, the inward and ourward flows being strongest at the tips unless there are modifiers like end plates, tip tanks etc; these two opposing flows result in the rotating tip vortex; for example, looking at the starboard wing tip from behind, the vortex will be anti-clockwise.The longer the wing relative to the tip chord, the smaller the vortex as a percentage of total flow, especially if there is wash-out,(tip at less incidence than root) usually desireable on high aspect ratio wings to avoid inward tip stalling in tight turns when the relative speeds of each tip can be considerable, (Turning in a thermal for instance.) The differential between top and bottom flows is increased for swept wings so full size aircraft frequently have chordwise "fences" to mitigate the effect and increase lift at slower speeds/higher angles of attack.
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Old Jun 25, 2011, 06:51 PM
Dreamer
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Joined Sep 2008
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hi guys,

I did a cfd project on a KF wing, but the model i used was a very badly CADed, but we got some pretty interesting results.

the problem was to figure out the curvature of the leading edge. through some iterations i found that some leading edge shapes added to the drag of the wing. again the meshing around the step is where all the magic happens, so I have to do some iterations on this mesh.

We did an infinite wing assumption, with this we got a max Cl of about 0.75. I don't know how accurate it is. Now that there are no deadlines to actually do the CFD, i'm thinking of redoing the analysis.

I'm hoping i can get some basic CL, curves, and L/D characteristic of KF airfoils as well as some moment coefficients...

ill post our report and some pics soon its really cool to see the vortexes, especially during higher AoA
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