Raptor Pitching at High Speed

[chaos]

You could try flying it without the canopy to see if the aerodynamics of it might be contributing to a downward pitching moment at high speed. A solid horizontal stabilizer should also help prevent this.

The ratio of blade speed to forward airspeed is so high for a model heli that a "retreating blade stall" is pretty unlikely, but if it did happen the nose would pitch up. The maximum ("red line") speed for a full size heli is determined by this speed. It's true as well for the models, but I don't think any of the models are able to go fast enough to encounter this speed. Anyway, your nose down problem is something else, and I'm guessing it relates to the canopy. If that's the case, the only fix I can think of at the moment would be a horizontal stabilizer with more area.

You could do a ground check, setting the collective and cyclic sticks in the position where you notice this nose down pitching event, and while holding that setting check to see if anything in the head is binding or sloppy... apply a little twisting force to the blade holders at different points around the circle to make sure the servos are holding... flybar teeters freely, paddles tight...

Interesting problem. Hope you post the solution if you find it.