09/30/2009. Remarks by Johan Visschedijk: "Charles M. Seibel formed Seibel Helicopter Company Inc. at Wichita, Kansas, USA in 1948. Registered N5152, the two-seat S-4 flew in January 1949, powered by an 108 hp Lycoming O-235-C1 engine. It was followed by the S-4A (N5153 c/n 2), that was similar, but powered by an 125 hp Lycoming O-290-B engine.

Subsequently the S-4 was also fitted with an 125 hp Lycoming O-290-B engine, and both N5152 and N5153 were acquired in 1951 by the US Army under the designation YH-24 Sky Hawk, and given the s/n 51-5112 and
51-5113. They were returned to Wichita in late 1952, by that time Seibel had become a division of Cessna.

A side-by-side trainer was also produced in 1951, designated S-4B, and registered N5154 (c/n 4)."