Turbomeca Astafan

Turbomeca at Bordes is Europe's largest maker of small and medium-sized turboshaft engines. It was logical for it to move into the small-fan-jet field, albeit belatedly, with small fans using existing turbines as core engines. The first adaptation, christened Astafan, was based on the 592 s.h.p., 441kW Astazou IIIN and started at 1,3701b, 6kN thrust, first running in 1969. Since then development has been through Astafans II, III and IV giving respectively 1,5651b, 7kN, 1,7651b, 8kN and 2,4141b, llkN. The IV is scheduled for certification in 1973.
It will be recalled that the Astazou is a simple singleshaft turboshaft with two axial and one centrifugal compressor stages and three compressor-cum-power-extractor stages. Turbine-entry temperatures are not divulged but the rather high s.f.c. of the IV, 0-791b/lb/hr, suggests temperatures below 1,000°C.
Turbomeca's own v.p. fan is now used, mounted on the Astazou's output shaft with modified reduction gearing. The tendency has been for the bypass ratio to increase, from 7-35 with the I to 7-6 with the IV. Fan gear ratios have been adjusted accordingly from 0 • 2357 to 0 • 2297.
Adoption of a v.p. fan and bypass ratio of 7-6:1 is a natural development. One sees the Astafan as almost a propeller engine with a small-diameter, high-density propeller which can work efficiently at supersonic speeds, as a result of turbine technology. There are a lot of advantages to this arrangement. The main one is the ability to improve take-off performance and reduce noise level at the same time. Dowty has demonstrated an almost 50 per cent noise reduction when its v.p. fan is moved from cruise to take-off pitch. It also offers the possibility of reverse thrust as a free bonus. Both Turbomeca and Dowty—it is anyone's guess as to who thought of it first—have suggested a helicopter installation where the fan is set at zero pitch for take-off and spare power is used to drive the rotor.
Once airborne, power to the rotor is tailed off and forward thrust is supplied by putting the fan into coarse pitch.
Lift would be provided by small fixed wings. The transition period should be interesting.