N650PF was a G-II modified for the joint NASA and Lockheed Propfan Test Assessment program. The aircraft was fitted with a turboprop engine driving a "propfan" on the left wing only. A thick shield was applied to the side of the fuselage nearest the fan blades for protection. Also of interest are the long instrumentation boom, the small counterweight on the left wing, and the much larger counterweight on the right wing. The PTA program ran for about a year.