This is an unwanted gyro motion which is a variation of a 2 revolution resonance. It causes a pilot-induced accident on the CL-840 prototype when increasing amplitude caused the divergent flapping motion to cut through the canopy of the aircraft. This was also caused by a plunge motion. The condition was fixed by stiffening the blades, and control system.
- proutyLockheedAH56Cheyenne1992[^1]