Turbomeca Astafan
| Astafan | |
|---|---|
| Type | Variable-pitch turbofan |
| National origin | France |
| Manufacturer | Turbomeca |
| First run | 1969 |
| Developed from | Turbomeca Astazou |
The Turbomeca Astafan is a single-spool, variable-pitch turbofan engine developed from the Turbomeca Astazou. Despite successful flight-testing, an efficient, quiet and clean design (compared to turbojets and conventional turbofans) and some commercial interest, the Astafan never entered series production. The engines were only flown on the Fouga 90 prototype and Turbomeca's two test aircraft.
The engine combines an Astazou-derived centrifugal compressor with up to three axial compressor stages (depending on the version of the engine), driven by a common driveshaft. The fan section is composed of a unique variable pitch fan and fixed stator blades mounted at the front of the engine. Blade pitch is varied by a hydraulic piston mounted in the fan hub, while the fan itself is powered via a reduction gear from the main driveshaft.
The engine is designed to operate at a constant speed, no matter the thrust output—a characteristic permitted by the Astafan's use of precise fuel metering and fan blade pitch adjustment. To control the engine, the pilot first sets the speed lever, which adjusts fuel flow in proportion to the selected engine speed. Then the pilot moves the thrust lever (analogous to a throttle), actuating the fan pitch piston by way of a mechanical linkage and hydraulic valve, and varying the blade pitch. In response, the speed governor holds engine speed steady by adjusting fuel delivery to the combustor. The control system also monitors turbine temperature and adjusts blade pitch (and therefore engine load) to maintain it within the preferred range of 350 °C to 450 °C. Additionally, the system automatically protects against excessive fuel delivery or unusual changes in blade pitch. Using the thrust lever, pilots can start an Astafan in fine pitch (for minimal resistive torque), select takeoff (coarse) pitch, feather the fan, or even reverse the fan pitch in flight (with the fan providing reverse thrust in lieu of conventional reversers).
Turbomeca owned two Rockwell Turbo Commander aircraft, and arranged for Miles Aircraft to modify each of them to accept two podded Astafan engines inboard of the nacelles. The nacelles were stripped of their turboshaft engines, and converted to carry extra fuel. One of the Turbo Commanders, a model 680V-TU, originally bore the experimental registry F-WSTM, and was later redesignated F-BSTM. The other Turbo Commander, F-BXAS, was either a 690A or B model.
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