|Sud-Ouest SO.1310 "Farfadet"|
This is the first French convertible. It can take off vertically, hover in the air, land vertically by means of its rotor, and fly forward at a speed greater than that of pure helicopters by means of a fixed wing and an airscrew fully independent of the rotor.
The change from one state to the other is accomplished without any change at all in the external appearance.
The SO.1310 is provided with two independent power units. In the rear is an Arrius II 360hp Turbomeca unit, which supplies the jet rotor with compressed air at take-off. The all-metal three-bladed rotor is identical with the one used in the SO.1120 "Ariel III", but with a larger diameter.
In the forepart there is a Turbomeca Artouste II turbo-prop engine with a take-off power of 360hp, operating a variable-pitch airscrew. During forward flight, the rotor originally used for take-off continues to auto-rotate, but with low lift, and the greater part of the lift is transferred to the fixed wing. Mounted over the fixed wing is the pilot's cabin, fitted with dual control, and in this cabin there is room for either three passengers, a freight load or two stretchers stacked on top of each other.
The "Farfadet" first flew in May 1953. Its maiden flight, lasting about twenty minutes, was made as a true helicopter, the following one, on the same day, with the rotor and the airscrew respectively powered by their individual engines.
P.Lambermont "Helicopters and Autogyros of the World", 1958
Interest in the Unloaded Rotor-type Convertiplane VTOL was in place in France during the 1950s and resulted in the construction and testing of the SO.1310 model.
The design of the craft was interesting in that with the exception of the overhead rotor, the plane looked very much like a standard small private plane. It carried a pair of low-mounted stubby wings and a conventional-appearing tail assembly. The plane sat on a tricycle landing gear with the nose tilted slightly upward.
The powerplant for the craft was a Turbomeca Artouste II turbine which was used to power the propeller located on the normal front of the aircraft. The SO.1310 is probably the only Convertiplane-Style VTOL which has its forward-pushing - pulling in this application - in that forward position, most being in the rear of the aircraft.
For the overhead rotor, though, there is a completely separate system, a 360 horsepower Arrius II gas generator which fed compressed air for the small combustion chambers at the tips of the rotor, thus providing the propulsive force for the rotor. The overhead rotor is extremely large, and from the photos of the plane that are available, just misses the vertical stabilizer of the tail assembly.
The program had serious goals for the development of the concept with the single prototype being constructed in 1954. An unknown number of flight trials would follow, but that would be the end for the promising program, the exact reason for its demise being unknown.
S.Markman & B.Holder "Straight Up: A History of Vertical Flight", 2000
All-metal three-seat compound helicopter with small wings, fixed tricycle u/c and four-blade main rotor driven by a 360shp Turbomeca Arrius II turboshaft and forward propulsion from a 360shp Turbomeca Artouste II. Single Prot. F-WBGD FF 8 May 1953.
R.Simpson "Airlife's Helicopter and Rotorcraft", 1998