TwistStang yaws

[Paul Taylor]

So it was not real windy today and I put up a few flights with my F-Twister. It yaws and bobbles.
I noticed this two weekends ago but thought it was due to the wind. But today proved the wind was not the  problem because it was a very lite breeze. Not the 20+ mph winds we had two weekends ago.

I am not sure what is going on but my plane seem to "YAW" back and forth while flying, and it looks to me that the outboard wing bobbles up when it comes across the wind. What would cause this problem? Ryan looked at it and said the outboard tip was low. So we pulled some of the tip weight out of the wing. It still did it some but not as much.

Where do I start?

Could it be a simple tweeking of the flaps?
Add a trim tab?
Take out more tip weight?

What say ye?

[Clint Ormosen]

Paul, we're sure the wing is dead straight (not warped), right? Also the flaps are even? I'll assume that they are for this post.
Is the outboard wing low flying upright and inverted? If not, then it's not the tip weight. Tweak a flap and put the weight back in the tip. Even add a little to help the bobble.
Yaw problems are usually misalignment problems at wing/fuse joint or wing to stab alignment (as viewed from above the model). Measure the distance from flap hingeline to elev hingeline and make sure the wing is square in the fuse. Also check for excess engine or rudder offset.

[phil c]

Yawing(seeing the inboard tip swinging fore and aft) comes from some imbalance between the design(wing offset mainly), the rudder, engine offset, CG, leadout position, and prop precession(nose turns in on sharp outsides).  Most people these days start with everything set as neutral as possible and go from there, as needed.  For example, the rudder and fin may look straight, but since the plane is flying around a circle it actually has a bit of an angle of attack, or maybe it is just slightly warped or crooked, so you may need to adjust the rudder neutral to correct.  Put the leadouts in the calculated position(one of the Line programs will help) and make sure you have located the CG accurately(within 1/8 in.).  It doesn't have to be where the designer suggests, but you have to know so you can set the leadouts.  When the plane yaws that may also cause some rolling.

Bobbles, or rolling movements come from lateral imbalances.  Warps, twisted flaps, uneven airfoils along the wing, tip weight, and any feedback from the yaws.  The Twister wing is pretty square and also more subject to gusts than a tapered wing.  Even a light breeze, if it is gusty, will make the wing wobble.  The profile fuselage can also cause turbulence over one wing or the other, depending on the wind direction, and cause a bit of a roll.  Make sure the flaps are flat and untwisted and exactly neutral with each other.  Then dewarp the wing as needed to get it to fly level and keep even tension on both inside and outside loops.  Tweaking the flaps gives you a wing with changing airfoil shapes all along it.  That will make it do funny things in some maneuvers but not others.  Once the wing is straight trim the tip weight to get just a tiny bit of roll in tight maneuvers.

When all that is done you still will probably be getting some precession effects from the prop.  A Rabe rudder is the best solution, but you can use smaller, lighter props to reduce the effects.  The difference between a wood prop and a plastic one can be huge.

[Dennis Vander Kuur]

Paul,
If your Twister has adjustable leadouts you should check to see that the slider bolt is tight. Mike's Jamison was flying "weird" and a close inspection found that the adjustable leadout slider bolt was loose. This allowed the leadouts to move around during maneuvers. Good luck on taming your Twister.
DennisV