First flight of a gas turbine powered helicopter (Sud-Ouest SO.1120 Ariel III)
Wednesday, April 18, 1951
The gas turbine (Turbomeca Artouste) powered propulsive ducts at the blade tips.
See Presented in Paris, FLIGHT, 22 June 1951 for an image of the aircraft. This reference states that the first flight occurred 19 April 1951 but the source reference for this milestone is believed to be the accurate date.
Source: AHS Calendar 2009, April