The X-planes are a series of experimental United States airplanes and helicopters (and some rockets) used to test and evaluate new technologies and aerodynamic concepts. Most of the X-planes have been operated by the National Advisory Committee for Aeronautics (NACA) or, later, the National Aeronautics and Space Administration (NASA), often in conjunction with the United States Air Force. The majority of X-plane testing has occurred at Edwards Air Force Base.