User:Born2flie/Seibel S-4

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The Seibel S-4 was a two-bladed, single-engine helicopter built by Seibel Helicopter. Designed by Charles Seibel, the S-4 was evaluated by the United States Army under the designation YH-24 Sky Hawk, but would be rejected for service. The S-4B would serve as the basis for the design of the Cessna CH-1 Skyhoook, the only helicopter Cessna ever produced.

Development
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.

In January 1949, the S-4 lifted off the ground for the first time, piloted by Johnny Gibbs. In March 1950, certification tests where completed and on 23 April 1950, the S-4 received civil certification by the CAA. A larger engine, the Lycoming O-290B with 125 hp, would be installed in the aircraft, making it the S-4A.

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, tricyle 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.

Operational history
Both the United States Army and the United States Air Force showed interest in the S-4. In 1951, the Army, ordered two helicopters for evaluation as a possible replacement for the OH-13 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. 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 pilots seat. Seibel also replaced that aircraft's original wheeled, tricyle undercarriage with landing skids. This aircraft would become the S-4B. 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 back to Seibel in 1952.

In 1952, Seibel Helicopter was acquired through a stock transfer by Cessna Aircraft. The S-4B was used to familiarize Cessna engineers with helicopter design and to train test pilots, and then both S-4 helicopters were dismantled.

Variants

 * S-4 :Original design, certified by the CAA in 1950.
 * S-4A :featured an upgraded, 125 hp Lycoming O-290B engine.
 * S-4B :Modified airframe based on Army recommendations during YH-24 evaluation. Two-seat cockpit and skid landing gear.