Two articles that cover this in depth are: 1. Revised Fold-Away Rotor Aircraft Concepts Emerge From Special Operations X-Plane Program. December 2024: https://www.twz.com/air/revised-fold-away-rotor-aircraft-con...
2. Bell’s Plan To Finally Realize A Rotorcraft That Flies Like A Jet But Hovers Like A Helicopter. September 2021: https://www.twz.com/41997/bells-plan-to-finally-realize-a-ro...
The second article covers decades of prior wind tunnel testing on the folding rotor concept.