Why does my wing wobble?

[mclarkson]

I have a scratch-built blue foam wing (here's the thread).

It flies great and has become one of my very favorite planes, but ...

At low speeds, it has a wobble, mainly in the yaw. At higher speeds, it's fine, but when I slow it way down it starts to wobble back and forth on me. The close I get to stall speed, the worse the wobble gets. (I'll try to provide a quick video later.)

I have fins on the wingtips. Ironically, they are the biggest fins I've got on any of my wings. Is it likely that they are still too small? Is there something else that's likely to be causing this? Any suggestions appreciated.

[JetPlaneFlyer]

Sounds like Dutch Roll: http://en.wikipedia.org/wiki/Dutch_roll

If that's what it is then cure would be to either make the fins bigger or better yet; change their shape to put their area further back and make them higher but less wide (higher aspect ratio).

[mclarkson]

It does indeed seem like Dutch Roll. Guess I'll have to try tweaking the fins a bit, then re-tweaking the CG, etc. I love this wing in general and people who fly it fast don't seem to have the problem.

[CHELLIE]

The Problem Could be Dutch Roll, But it looks to me like you have to much dihedral in the wing, and the wing is trying to correct itself all the time, remove most the Dihedral, and the problem should go away, just my 2 cents worth Take care and have fun, Chellie

[mclarkson]

Chellie! Nice to see you back!

However ... the angle of the photos is misleading; the wing has no dihedral at all.

[63 nova]

It looks like your wing doesn't have as much sweep as the wing you designed it from. I have designed a few wings and when there is not enough sweep your back end will try to wash out on you at lower speeds.

[mclarkson]

Ahhh ... I don't know. Meanbaby over an RCG sent me the wing(s) in the mail. They were cut by his friend Papo. I know that they had to cut an inch or so off the very ends of the wings to fit the box, but I did not design or cut this wing.

Not that I'm doubting you about the sweep, mind you. Just sayin'.

edit// The sweep's in the neighborhood of 35 degrees, FWIW.

[63 nova]

That should be enough sweep then my wing is just over 33 degrees my winglets are slightly cambered out from the wing tips to stop wobble.

[mclarkson]

My winglets have a very slight toe-in. I'm thinking about increasing it.

When you say camber, do you mean camber (tipped in at the top/bottom)? Or toe-in/out (tipped in at the front/back?) Is yours positive or negative?

[CHELLIE]

Originally Posted by mclarkson

Chellie! Nice to see you back!

However ... the angle of the photos is misleading; the wing has no dihedral at all.

Its Nice to be Back I have been very very busy trying to get the IRS off my Back finally got them off my back, I sold my 5th wheel truck and 30ft trailer and got those buggers off my back LOL and I have a little extra money to pay off bills with now, whew, I can breath a little better now LOL,

what are the elevons made from on your wing, if they are made from foam, they might be twisting and bending causing instability, they look to be a little to wide too, i like to make my elevons/ailerons from 3/4" to 1" balsa aileron stock, much much stronger and stiffer, and they are good for 200mph Plus Also check your lateral balance from wing tip to wing tip, might be out of balance, Hope that helps, Chellie

[mclarkson]

Chellie, the ailerons are made of 1/4" balsa covered in a skin of packing tape. I don't think they're the problem, as this shows up at very low speeds, rather than high.

[63 nova]

The winglets are tipped out I guess that would be positive camber.

[mclarkson]

Okay, thanks. My wings have very little camber if any. I'll look at adjusting that.

[JetPlaneFlyer]

Like i said in the first reply.. The fins really need to be further back and higher aspect ratio. The problem with the very low aspect ratio fins (wide chord but short in height) is that they have a very flat lift slope so need high yaw angle to generate much lift (i.e. 'sideways lift' to correct the yaw), that means they allow the plane to yaw at smaller angles with little resistance.

Solution is to make the fins a more 'upright' shape like a conventional vertical stabiliser, and locate as far back as practical. Making them out of thinner material or sanding in a basic airfoil shape could only help too, the quite thick foam slab with square edges cant be doing much for the aerodynamics.