An advanced Tilt-Wing concept evolved as a joint NASA/Vertol program in the late 1950s that never advanced beyond the wind tunnel stage. A pure Tilt-Wing concept, the craft featured six-propellers driven by a thousand horsepower variable-frequency electric motor. Additional lift was also acquired by double-slotted flaps that covered about 60% of the chord. The extremely small-in-diameter propeller mounts gave the plane a completely different look.