The Seibel S-4 was a two-bladed, single-engine

helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...

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 Skyhook, 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 were completed and on 23 April 1950, the S-4 received civil certification by the

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. A larger engine, the Lycoming O-290B with 125 hp, would be installed in the aircraft, making it the S-4A. Cessseibelh54elo-43-

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 In aviation, the first officer (FO), also called co-pilot, is the pilot who is second-in-command of the aircraft to the captain, who is the legal commander. In the event of incapacitation of the captain, the first officer will assume command o ...

'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.

Design

The S-4 frame was a

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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 Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator * Helicopter rotor, the rotary wing(s) of a rotorcraft ...

assembly. A tapered,

monocoque Monocoque ( ), also called structural skin, is a structural system in which loads are supported by an object's external skin, in a manner similar to an egg shell. The word ''monocoque'' is a French term for "single shell". First used for boats, ...

, alloy tail boom with a two-bladed

antitorque A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...

tail rotor The tail rotor is a smaller rotor mounted vertically or near-vertically at the tail of a traditional single-rotor helicopter, where it rotates to generate a propeller-like horizontal thrust in the same direction as the main rotor's rotation. The ...

was attached at the rear of the upper deck.

Operational history

Both the

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and

U.S. Air Force The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Sign ...

showed interest in the S-4. In early 1951, the U.S. Army ordered two examples for operational and engineering evaluation in the

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, 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

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in April 1951; the second YH-24, serial number 51-5113, was delivered to

Wright Field Wilbur Wright Field was a military installation and an airfield used as a World War I pilot, mechanic, and armorer training facility and, under different designations, conducted United States Army Air Corps and Air Forces flight testing. Loca ...

. 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.

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.

Specifications (YH-24)

References

External links

Global Security
{{USAF helicopters 1940s United States helicopters S-4 Single-engined piston helicopters Aircraft first flown in 1949