The H-19/HRS-1 was the first practical single-rotor utility helicopter and the first in Sikorsky's successful S-55 series. The centre-of-gravity problems of earlier models were solved 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. This example is now on display at the Steven F. Udvar-Hazy Center of the Smithsonian NASM. Ex US Marines 127799.