<hi style=afpcap>The helicopter was in its infancy during the early '40s, with the types being allocated in the series R for rotary wing from 1941 when the first example was ordered, until 1947 when the H prefix was adopted. The Sikorsky Aircraft Division was at the forefront of helicopter design the R-4 being its first product. This was developed into the R-6 retaining the rotor and transmission system of the R-4, but with a refined fuselage. The XR-6 prototype was followed by five XR-6A development versions, with production of almost 200 R-6A. In addition Nash-Kelvinator built 26 similar YR-6As. </hi>
