You are correct. With the looong wing you need an appropriate amount of verticle surface for the lateral stability. I was not getting enough from the one centered on the fuse. The plane would wag (put your arms straight out like a wing and rotate your torso back & forth = wag in my vocabulary) from time to time and with a streamer on the wing tip it became uncontrolable. Adding the tip verts made that all go away as well as adding to the overall stability and flyability of the bird.

rrh