Piasecki H‑21
in
US Air Force

US Air Force procurement began with 18 of the YH-21 Work Horse in 1952, which was the first tandem rotor helicopter used by the USAF. 33 H-21As followed and it distinguished itself in the SAR role, where it performed well in the Arctic environment. 168 H-21Bs were delivered. These were powered by the improved 1425hp Curtis-Wright R-1820-107 piston Cyclone. Most had been retired by 1966 but a few lingered on in service for almost 10 more years.