High Alpha Wing Rock

[Gene Bond]

How to avoid it, How to cure it, What causes it????

Today, with the cheap, light power systems we have available, we can fly our toy planes way beyond what is possible or practical with full-scale, on-board pilots. No way they could survive the kind of stuff we put our models through :)

3D flying is mostly way beyond realistic and scale, and therefore we break new ground and find new brick walls to break through. We solve problems that can't even exist in real planes!

So, to the point, Some planes have bad wing rock in high alpha, and some don't. I've searched a ton of threads to try to figure out the cause and cure, with not much success.

Knowing there's some incredibly smart guys hanging out in this forum, I figured I'd try to tap the talent.

I know that by reflexing ailerons, it reduces the effect.

It doesn't seem that cg has a great effect, in my testing.

I'm not sure if vertical surfaces on the wings (fences, tip plates, or whatever) really cure it, but have seemed to help on one model.

Other ideas, suggestions, or known cures?

[JetPlaneFlyer]

I've no experience of 3D type flying but I'd guess that the rocking may be due to tip stalling.. Maybe slats or washout could help, but at the expence of inverted performance. Parallel chord wings should be less prone than tapered planform... maybe :rolleyes:

Steve

[nmasters]

It's an odd post-stall vortex shedding mode. The rocking frequency matches the Strouhal number (http://en.wikipedia.org/wiki/Strouhal_number) with the wing span as the reference length. The NASA deep stall maneuvering study (http://www.dfrc.nasa.gov/gallery/photo/Schweizer-1-36/HTML/ECN-26847.html) of the early 1980s observed this on some of the flights but the S number isn't mentioned in the report.

[Mike the Snake]

In the electric 3D section here, "Tribute 3D FX" thread", around page 10 I think, there's some GREAT vids of one modeler who's added SFG's and a high-wing above the canopy to a Tribute 3D FX, which has very noticeable wing-rock in stock form.

The flights in the vids are Very impressive, and the work done by edfrules (I think that's his call name here) has completely eliminated any hint of wing rock.

[Gene Bond]

Thanks, Mike, I'll check it out.

[Zoom]

I think nmasters is on the right track. I agree it's an alternating vortex shedding phenomenon, not unlike Karman vortices (http://en.wikipedia.org/wiki/Von_K%C3%A1rm%C3%A1n_vortex_street). In a nutshell, Karman vortices can be observed peeling off one side of a bluff body, and then the other, in a periodic manner at very low Reynolds numbers. Whenever you shed a vortex, you will get a pressure disturbance on that side of the wing. So when this mode is active, you get the right wing tip dropping then the left tip, then the right tip, etc. I have not concluded what it is that makes this condition better or worse, but you can imagine that it is somehow related to the plan form of the wing, and span wise 3D geometry (fences, struts, ribs under heat shrink covering, dihedral, etc).

Let me also add that I believe even small changes in a wing shape can effect this a lot. I have a friend who made two fan fold Edge 540s. They were both cut from the same plans. The construction was similar but not exactly the same, and equipment setup was similar, but again not exactly the same. One had a bad case of wing rock and the other had virtually none. Looking at the two, side by side, from an aerodynamic frame of mind, I could not see why one behaved so much differently than the other.

Good question Gene, and one that I have pondered for quite some time. Look forward to more feedback from other modelers.

- Dave -

[macboffin]

Two cures ; reflex ailerons, or plenty of washout to reduce tip stalling,and try small end plates angled outwards about 45 degrees, height about one fifth of wing chord.

[Gene Bond]

Thanks, guys.

I sorta guessed I had figured why and how, but the reinforcement helped me decide to try some fences. Purely TLAR, On my AcroMaster (45"ws, 13.5"root chord, 9"tip chord) I made some fences which are 1" ahead of the LE, 2" tall and below (wraparound) that extend to the hinge line of the ailerons. I taped them on at ~85% span (TLAR, again), and they work.

I'm going to try 2 more things, making them taller, and trying them just on the bottom. Inverted doesn't seem to rock, probably due to the taller canopy.

My rough (grade school mechanics, as it was referred to in another thread) rule of thumb is that any vertical surface ahead of the cg reduces the vertical stab+rudder effective area by about .25x ... ie: 1sqin ahead takes away 1/4sqin at the rear.

I think I'm on the right track, as the plane has some much more wicked rudder snaps, now (not always good) !!!

[RCAV8R13]

Gene,
My Mini Vertigo Pro Light (5.25oz) has never rocked once. I think it is the super light wing loading.
RCA

[Joel K. Scholz]

I work for Jakks Pacific and worked on a project that you may know as the XPV. I initially tried the design with a symmetrical foil, but there was noticeable wing rocking in high alpha modes. I went to essentially a flat bottom foil and essentially the problem went away, or at least it was greatly reduced

[Joel K. Scholz]

The red and black model has the flat foil.

[msnyder]

Man, those look wicked! Esp. the red and black one.

[podavis]

Inverted doesn't seem to rock, probably due to the taller canopy.

It could be that having the cg high versus lows changes the effect of lateral cg offset from positive feedback (reinforcing) to negative feedback (damping). The mechanism would be the lateral motion of the cg during rocking causing a moment in sympathy or against the moment caused by the vortex shedding. I'm speculating about this.