I've been trying to make an intermeshing rotor heli, it works fine on the ground, but as soon as I turn up the collective, the blades collide with something. [I'm not going to pretend to know what they collide with, even in slow motion they're impossible to follow.]

Any ideas on how to fix this? It's just a proof-of-concept right now, but if I can figure it out, I'll probably make something like the Kaman K-MAX.