While I have not experienced instability on the three airplanes I have used flaperons on, I was disappointed in how the plane flew with them "on". The planes seemed to lose aileron effectiveness at low speed- needing almost full stick to make corrections when the flaperons were fully deployed. I used them on a 40 size corsair to help steepen the glide path for landing, and it did help the approach, but the plane felt very unresponsive in roll. Maybe more aileron throw is needed, will have to try it next time.