Slope Wing yaw wobble

[PDB]

First time slope soaring with a roughly put together foamboard wing based on the Flite Test Versa wing. During slow flight it had a yaw wobble, any ideas on what is causing this? 

[Graham Bowers]

50 minutes ago, PDB said:

First time slope soaring with a roughly put together foamboard wing based on the Flite Test Versa wing. During slow flight it had a yaw wobble, any ideas on what is causing this?

Only a guess on my part, however I'd look to the wing tip vertical stabilisers.

My logic is only that in the absence of better ideas, it's usually best to do what's been proven to work.

I'd be making something that resembles closely the Versa and also the traditional Windrider Bee series of models.

I have a "Bee" that was sold in the UK as an Irvine e-Xite and it flies brilliantly. This thread shows the sort of Bee I mean.

https://www.rcgroups.com/forums/showthread.php?2492998-WindRider-Bee-DS-45-5oz-Fiberglass-bagged-elevons

[Don Fry]

It’s lacking vertical stabiliser area. If it had enough, you won’t see the damping, as it swings. 

[Don Fry]

Just now, Don Fry said:

It’s lacking vertical stabiliser area. If it had enough, you won’t see the damping, as it swings. 

Pretty common in that sort of machine, the vertical stabilisers on the tips are at a mechanical disadvantage to damp the occultation, compared to a rudder well behind the wing.

comes with the geometry. 

[PDB]

Thank you both, I’ll experiment with the size of the vertical stabilisers 👍

[J D 8]

I flew a Zaggy wing on the slope that had the same symptoms as described. This was because I forgot to fit  the tip fins before the first flight. Never did fit them, the wobble would quickly settle out and I could tell which zaggy was mine.

[2.4g Shaun]

As a suggestion have a look at the Wildthing design, originally by SAS  now sold by Balsa Cabin. That has a single fin and fly's a treat.... No wobble. 

 

I would loose the tip fins, and replace them with a central one. But they also did one that looked similar  to your model (Venom I believe) that did have tip fins .

Edited October 7, 2023 by 2.4g Shaun
Typo

[Don Fry]

As JD8 did, I wouldn’t worry..ain’t going to break it. Some full size aircraft do it.

[Frank Skilbeck]

Tip fins too small and.not rearward enough, they need to extend to at least the rear of the.elevons and not extend forward of the leading edge.

[MattyB]

On 07/10/2023 at 23:33, 2.4g Shaun said:

As a suggestion have a look at the Wildthing design, originally by SAS  now sold by Balsa Cabin. That has a single fin and fly's a treat.... No wobble. 

 

I would loose the tip fins, and replace them with a central one. But they also did one that looked similar  to your model (Venom I believe) that did have tip fins .

 

The SAS Wildthing is not a true chevron - it has a lot less sweep, so it can utilise a central fin. A chevron (note - a chevron is not the same as a plank flying wing, which has very different aerodynamics) has a fair amount of sweep that is stabilised with washout, so is much better with tip fins. The problem here is their size and shape, not their overall location.

 

@PDB, If you want to learn more about optimising a chevron wing, search for threads by the Predator on RCGroups (example thread) - he was the originator of a series of mods for the model that over here we knew as the Irvine X-it, or the Windrider Bee elsewhere in the world. He was also one of the first people to expereiment with laminating film as a covering for foam models.

 

Edited October 9, 2023 by MattyB

[Keith Billinge]

I fly a lot of different wings and they all suffer from the yaw wobble to differing extents.

The secrets are:

 

Fin area has to be reasonably large.

Any fin area in front of the CG is wasted, so the fin is best to be all behind the CG......The further behind the CG the better

I'd unstick those ones you have and move them back

 

 

Cheers

KB

Edited October 9, 2023 by Keith Billinge

[PDB]

20 hours ago, MattyB said:

 

He was also one of the first people to expereiment with laminating film as a covering for foam models.

 

 

👍

 

Thank you for all of the responses which have been very helpful, putting new end plates on and will chuck it off a hill later.🙂

[PDB]

 

Larger more rearward correx end plates.

[PDB]

The new wing end plates fixed the problem, the wing flew very smooth with no yaw wobble and the kites liked it.

 

Thanks all 👍

Edited October 10, 2023 by PDB

[Robin Colbourne]

Rollin Klingberg did some research on tip fins.  His conclusion was that they should be inboard from the tip.  I think this is the video in which he talks about fin position:  Rollin Klingberg on flying wing fins
 

Edited March 10 by Robin Colbourne