In 1928, Tesla received his last patent, U.S. Patent 1,655,114, for a biplane capable of taking off vertically (VTOL aircraft) and then be "gradually tilted through manipulation of the elevator devices" in flight until it was flying like a conventional plane. Tesla thought the plane would sell for less than $1,000.:251 Although the aircraft was probably impractical, it may be the earliest known design for what became the tiltrotor/tilt-wing concept as well as the earliest proposal for the use of turbine engines in rotor aircraft.[improper synthesis?]
What was the patent number for Tesla's final patent? For what invention was U.S. Patent 1,655,114 granted? What was Tesla's belief as to the selling price of the biplane? What kind of engines did the biplane design have?