I was testing a B70 for another guy and found a dynamic function I have never encountered before.  This thing starts a violent bouncing pitch oscilation at 5knots on the runway and is almost uncontrollable. The real plane would have self-destructed. Once it is off the ground, it continues the pitch problem up to around 20000 feet, where it starts to settle down. From 40000 to 100000 feet it is totally docile and flies like a dream at mach4.7. This is a totally new problem to me and seems like an irritation between the sim and the .air file. Yet, it sits real nice on the runway.  Is there anyone that has some insight to this wierd behavior??  The plane, I mean.

Thanks for the reply. The model is AF5 based and is quite simple but accurate. It does everything so well I hate to quit. I'll keep beating on it for a while.