I really don't know enough about wing tips to understand how they help the plane except as a drag points and reduce drag from vortex. On this craft they are used for yaw control, but I wonder if I bring the tips rear-ward more, would, that help the yaw stability?
First thing to remember is that as a fighter the plane was designed to travel quite fast, rudder drag can oppose such a requirement and therefore the rudders are a little small to keep scale and remain truly effective.
Second thing with wing tip rudders is that the closer they are to the CG, (in the direction of flight), the less effective they will be and the more effected by turbulence or crosswind gusts. In other words more twitchy!
Thirdly, Wing tip rudder incidences MUST be parallel to the direction of flight. If the LE of the rudder is "Toed out" slightly the yaw stability is lost totally and actually the craft will definitely oscillate in yaw as the craft is flying in level flight.
Last thing to consider is mass distribution. On the fighter the mass was more concentrated around the fuselage and so the ease of pivoting the craft in yaw did not require huge drag rudders but with the FB the mass is more distributed. (the wings weigh a greater percentage of the weight than the original craft). This "Spread out weight" makes the rudders even less effective in their original form.
Things to try! I will ignore mass distribution as this requires a redesign and is therefore not going to happen.
1. Fly faster. Bigger motor faster prop or the like.
2. Increase the rudder/fin area to get scale responses and not scale look. An old cheat is to use a clear rigid plastic fin extension that is invisible in flight but gives the increased effectiveness. This can also include a fuselage mounted fin or spin also of clear plastic to give a central fixed fin behind the CG point.
3. Add a degree or two of dihedral to the wings.
4. Angle wing tip rudders slightly inwards at their leading edge (1 or 2 degrees only). Whilst this adds to overall drag it has a great effect on yaw stability in that each wing tip rudder has a drag amount and as a wing advances or the plane yaws the advancing rudder has an increase in drag and the retreating one has a marked decrease in drag. the changes in drag oppose the yaw movement and the craft will re-balance due to the drag differential.
I will ponder on the issue further but my main recommendation at this time are to increase the drag rudder area slightly, (5 to 10 percent only and angle the rudders in slightly and equally on each wing tip.
I hope this helps.
Have fun!
