Seibel S-4 / YH-24 Sky Hawk
Charles Seibel began development on the S-4 after forming the Seibel Helicopter Company with funding from local Kansas oil investors. The S-4 was a continuation of his work on his previous design, the Seibel S-3, which he flew as a demonstrator for his design concepts; primarily a new design for a two-bladed rotor system and a simplified transmission. These features would also be incorporated into the S-4 design.
The S-4 frame was a welded steel-tube box frame, with two decks. A lower deck supported the control panel, pilot's seat, wheeled, tricycle landing gear, and a small passenger/cargo area accessible from the rear, and an upper deck carried the engine, the fuel and oil tanks, and supported the transmission and rotor assembly. A tapered, monocoque, alloy tail boom with a two-bladed antitorque tail rotor was attached at the rear of the upper deck.
In January 1949, the S-4 lifted off the ground for the first time, piloted by Johnny Gibbs. In March 1950, certification tests were completed and on 23 April 1950, the S-4 received civil certification by the CAA.
Gov. Frank Carlson and Charles Seibel at the CAA Certification Ceremony
Both the U.S. Army and U.S. Air Force showed interest in the S-4. In early 1951, the U.S. Army ordered two examples for operational and engineering evaluation in the observation, utility, and aeromedical evacuation roles. The Army designated the S-4 as the YH-24 Sky Hawk. The first Sky Hawk, serial number 51-5112, was delivered to Fort Bragg, North Carolina in April 1951; the second YH-24, serial number 51-5113, was delivered to Wright Field.
Despite the simplicity of the S-4, the Army determined that it did not provide a sufficient payload capability and the aircraft were dropped from the inventory and returned to Seibel in 1952. Only the two were built.
A larger engine, the Lycoming O-290B with 125 hp, would be installed in the aircraftand side-by-side seats, making it the S-4A.
Based on feedback from the Army during the evaluation, Seibel, shortened the fuselage of the second YH-24 (51-5113) and widened the cockpit for a co-pilot's seat next to the pilot's seat. Seibel also replaced that aircraft's original wheeled, tricycle undercarriage with landing skids. This aircraft would become the S-4B. The S-4B would serve as the basis for the design of the Cessna CH-1 Skyhook, the only helicopter Cessna ever produced.
Original design, certified by the CAA in 1950.
Featured an upgraded, 125 hp Lycoming O-290B engine.
Modified airframe based on Army recommendations during YH-24 evaluation. Two-seat cockpit and skid landing gear.
Engine: 1 × Lycoming O-290-D, 125 hp (93 kw)
Rotor diameter: 29 ft 11⁄2 in (8.88 m)
Disc area: 666 sq ft (61.9 m2)
Length: 27 ft 10 in (8.48 m)
Height: 10 ft (3.05 m)
Empty weight: 960 lb (436 kg)
Max. takeoff weight: 1,540 lb (700 kg)
Maximum speed: 65 mph (105 km/h)
Cruise speed: 58 mph (93 km/h)
Range: 98 mi (85 nmi, 158 km)
Service ceiling: 4,300 ft (1,310 m)
Rate of climb: 700 ft/min (3.6 m/s)