Since the inside of the propeller is moving slower than the outside, the blade is progressively more swept toward the outside, leading to a curved shape similar to a scimitar - a practice that was first used as far back as 1909, in the Chauvière make of two-bladed wood propeller used on the Blériot XI.
•
General Electric's GE36 Unducted Fan was a variation on the original propfan concept, and appears similar to a pusher configuration piston engine.
Progress D-27, a propfan engine that the Ivchenko Progress company developed in the 1980s for commercial and military transport aircraft