The Sikorsky YH-19 was the first American practical single-rotor utility helicopter. The first in Sikorsky's successful S-55 series, it solved the center-of-gravity problems of earlier models by shifting the engine to the front and the passenger compartment to beneath the rotor hub. Other innovations included offset-flapping hinges and hydraulically boosted irreversible controls. Designed for the Air Force for arctic rescue, the S-55 served all U.S. military branches throughout the 1950s, especially during the Korean War and in most major military conflicts of the early Cold War.