April 2019

Sud-Ouest SO.1120 Ariel III

First gas turbine-powered helicopter

First flight: April 18, 1951

SNCASO (abbreviated from Société Nationale des Constructions Aéronautiques du Sud-Ouest, or commonly, Sud-Ouest) was formed in 1936 from the merger of several French aircraft manufacturers. (SNCASO merged with SNCASE in 1957 to form Sud Aviation, which became part of Aérospatiale in 1970, and is today part of the Airbus legacy.)

The Sud-Ouest SO.1120 Ariel III tip-jet helicopter flew for the first time on April 18, 1951. It was powered by a 260 hp (195 kW) Turbomeca Artouste turbocompressor, which provided compressed air for the tip jets at the tips of its 35.4 ft (10.8 m) diameter 3-bladed rotor. The helicopter had a conventional layout; but instead of a tail rotor, it used the exhaust from the turbine as a source of high-pressure air for directional control.

The Ariel series of helicopters represented a new approach to power a rotorcraft. Up to that time, helicopters were powered by relatively heavy piston engines and associated cooling systems. Helicopter designers were desperate for ways to improve the power-to-weight ratio of their machines. Tip jets seemed to provide the answer they were looking for; but even though tip jets negated the need for heavy gearboxes and complicated anti-torque systems, it was not without compromises. The long-sought breakthrough by SNCASO creative engineers did not arrive until a turbine engine was married to a helicopter as the source of compressed air for the tip jets. As its name implies, the Ariel III was not the first Ariel in the series; and the Ariel I and Ariel II differed in design and powerplant. Only the Ariel III used a turbine engine for the compressor. The Ariel III was the first turbine engine powered helicopter to fly (albeit it was not a “direct drive” turbine-powered machine as nearly all other turbine-powered helicopters have been — see note below).

Two prototypes of the Ariel III were built, both capable of carrying 3 persons side by side. The two prototypes (registered as F-WFUY and F-WBBL) flew very successfully for quite some time. In the end, the Ariel series of tip jet helicopters’ greatest contribution was that they paved the way for SNCASO to develop and produce the most successful of the reaction rotor helicopters, the SO.1221 Djinn. The Ariel III also pointed the way for future rotorcraft designers to use turbine powerplants as the engine of choice for helicopters.

The Ariel development was greatly influenced by Theodor Laufer, who during World War II was a member of Friedrich von Doblhoff's team in Austria and played an important role in developing the WNF 342 tip jet helicopter. While Henrich Focke and his team were hired by SNCASE, Laufer was hired by SNCASO to develop the von Doblhoff concept for the Ariel I-III and follow-on helicopters until eventually the Djinn was successfully developed and fielded.

Note: The media sometimes erroneously state that the Kaman K-225 turbine-powered demonstrator was the first turbine-powered helicopter to take flight. It was, however, the first turboshaft-driven helicopter to fly; first flight was in Dec. 1951, 8 months after the Ariel III.

Main Photo: San Diego Air & Space Museum

Text: Erasmo Piñero, Jr.

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