YH-24 in flight
Role light helicopter
Manufacturer Seibel Helicopter
Designer Charles Seibel
First flight January 1949
Primary user United States Army
Number built 2
Developed into Cessna CH-1

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 Skyhook, the only helicopter Cessna ever produced.


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

Gov. Frank Carlson and Charles Seibel at the CAA Certification Ceremony

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-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.

Operational history

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.


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.

Specifications (YH-24)

Orthographic projection of the Seibel S-4.
Orthographic projection of the Seibel S-4.

Data from U.S. Army Aircraft Since 1947[1]

General characteristics



  1. ^ Harding 1990, p.224
  2. ^ Bridgman 1951, p. 288c.

Further reading