The TechOne FPV900 is very-fast on 4S but the bottom line is that this wing has too much power for the stiffness of the elevons. It flies very nicely at 1/2 to 3/4 throttle but at full power on a 4S pack, the elevons are too flexible and that causes it to tuck (dive) when the airspeed becomes too great.
I will be stiffening the elevons and trying again in a few days so stay tuned.
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