Revolutionary Vertical Lift Technology (RVLT) Project Technical Highlights https://www.nasa.gov/directorates/armd/aavp/armd-aavp-rvlt/revolutionary-vertical-lift-technology-rvlt-project-technical-highlights/
Revolutionary Vertical Lift Technology (RVLT) Project Technical Highlights
One model used blade pitch for control and the other model used rotor speed for thrust