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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 N5152 c/n 1 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 and returned to Cessna in 1952; the second YH-24, serial number 51-5113, was delivered to Wright Field, modified to S-4B.YH-24 51-5113 with a 165hp Franklin 6A4-165-B3, was modified as a side-by-side trainer for Army testing at Fort Sill OK. Both ships were eventually scrapped by Cessna.

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 1950 S-4A N5153 c/n 2. Flight tests by CAA's Hal Hermes.
Seibel 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.
S-4 Skyhawk
Engine: 108hp Lycoming O-235-C1
Rotor: 29"2"
Length: 27'10"
Useful load: 580 lb
Max speed: 65 mph
Cruise: 58 mph
Range: 100 mi
Ceiling: 4300'.
Seats: 2 tandem
Engine: 125hp Lycoming O-290-B
Length: 24'6"
Seats: 2-3
Engine: 1 × Lycoming O-290-D, 125 hp (93 kw)
Rotor diameter: 29 ft 112 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)
Crew: 1-2
S-4B / YH-24
Engine: 165hp Franklin 6A4-165-B3
Seats: side-by-side



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