<hi style=afpcap>Piasecki designed the first operational tandem rotor helicopter, with the Navy receiving the HRP-2 initially, followed the Air Force who ordered 18 YH-21 Workhorses for service evaluation. These were followed by 32 H-21As for the Air Rescue Service principally in the Alaskan region. The H-21B was the next version which was obtained for service with Troop Carrier Command to perform airlift of assault troops. 163 H-21Bs were ordered between 1951 and 1957. The Army received the H-21C named the Shawnee with 334 being purchased from 1951 until 1956. In 1962 the designation was modified with the addition of the C prefix to signify the type as a cargo helicopter, while the Rescue SH-21Bs became the HH-21B. </hi>
