Is the CV-22 / MV-22 Osprey classified as a helicopter, plane or both?
If the President had to hitch a ride on one, would it become Airforce One (CV-22) or Marine One (MV-22)?
Under the FAA, a CV/MV-22 Osprey is considered to be a powered lift aircraft. Under 14 CFR §1.1:
Powered-lift means a heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low speed flight that depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on nonrotating airfoil(s) for lift during horizontal flight.
That designation would depend on the branch of service operating the aircraft. A USAF CV-22 carrying the President would have the call sign Air Force One during the operation. A USMC MV-22 carrying the President would have the call sign Marine One.