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Aviation History
1978
1978 - 2750.PDF
1680 FRANCE'S AEROSPACE INDUSTRY IGHT international, 4 November 1978 ,,n®OM Astafan: power for the Fouga 90 HUGH FIELD reports from Pau DEVELOPMENT of the Fouga 90 for the French Air Force offers the first serious hope for the future of the Turbomeca Astafan, an unusually small engine which first ran nine years ago. Astafan is very much the personal project of "Le Patron"-—M Szydlowski, co-founder in 1938, and now president, of Turbomeca. Astafan is a direct development of the Astazou turbo- props, which power the Jetstream and Pucara and the gas-generator Astazous of the Alouette, Gazelle and Dauphin. This unique range of small turbines achieves fuel economy by reversing the usual cycle of controlling power alterations. The engines run at constant r.p.m. Change of power is first signalled by a demand for a change of propeller pitch. A fuel governor then assesses the increase or decrease of load and adjusts fuel flow to compensate accordingly. Turbomeca is alone in using this fuel-control technique because it has stayed with the single-shaft engine. Com petitors' turboprops rely on at least two shafts and obtain rapid power response from the use of a free power tur bine. If the single-shaft unit is to vary its r.p.m. the inertia of the rotating mass must be overcome fast enough for an acceptable response. Turbomeca's success lies in the development of a fuel-control unit capable of giving responsive control while allowing the gas generator to rotate at constant r.p.m. Having achieved such control, by direct adjustment of propeller pitch, Turbomeca was able to apply the technique to fan power. There are two versions of the Astafan. Basic develop ment from the Astazou XVI has led to the Astafan IIG with 1,5581b (707kg) thrust, adopted for the Fouga 90. Use of the Astazou XX core, with compressor augmented by an additional axial stage, leads to the Astafan IV with 2,1001b (951kg) thrust. Turbomeca has two Turbo Commanders among its flight-test fleet, and I was given a brief demonstration of the engine in flight during a recent visit to Pau. Engine hours were at a premium, ruling out flying of the higher- powered aircraft. I was, however, able to sample the engine at 1,7001b (770kg) (series IIB4) rating in F-BSTM. This aircraft was originally converted by Miles Aircraft, and since first flying with Astafans during 1971 has accumulated about l,450hr. The outer casing of the 6-5:1 ratio fan is supported from the centre body by three struts, through which compressor bleed air is supplied to the intake lip for anti-icing. The struts have no aerodynamic function, and the fan meets the air directly without any straightening stators. A double row of stators straightens the flow behind the fan. The engine installation in the Commander underlines the unit's small size. Overall length is 78in (2m) and the diameter over the fan is 2ft 3in (70cm). There is no obvious test equipment, and the aircraft has a normal category certificate of airworthiness. (Early photographs show it carrying the experimental label F-WSTM.) The throttle quadrant has a familiar Astazou look about it, carrying r.p.m. levers with thumb wheels for fine adjust ment. My pilot, M Vaast, says that these have been eliminated on the Astafan IV. Fan blades are fully fine for starting, and the automatic cycle runs the engines up to 70 per cent r.p.m. in about 80 seconds. A criticism of the Astazou was its slow starting procedure. It appears that the Astafan has not yet reaped full benefit of the development put into the turboprop. The test engines have electrical high-pressure fuel cocks which open automatically during the starting cycle, and the combined starter/generator reverses its function as the r.p.m. rise through 50 per cent. Vaast uses 95 per cent for ground manoeuvring to reduce noise. This setting is obtained by winding up the thumb wheels. Fan noise is similar to that of the turboprop, but the installation can be made soundproof so that the noise intensity is no longer obtrusive. We line up for take-off on runway 31 and inch the r.p.m. to 100 per cent. Two red lights on the glareshield are extinguished, confirming that the engine configuration is now correct for take-off. The nature of the power control is such that the levers can be slammed open if desired. The engine note changes as the fans start to claw for air, and the noise is left behind almost immediately as the aircraft accelerates. Initial acceleration is slow. The Commander seems to use quite a lot of runway on this hot afternoon, but the sensation is deceptive for it is not long before Vaast warns against exceeding Vxo once we level off. Turbomeca has always been a little idiosyncratic about its instrumentation. It is no surprise, therefore, to find that instead of engine-pressure ratio gauges indicating the airflow it is shown by pressure-increment gauges. They show the SP in millibars, so a gauge reading of 120 mb is virtually the same as 112 EPB. Since the r.p.m. are constant, and therefore of no value in setting power, the pilot must use fuel flow or pressure increment as his datum. The former is recom mended, and I find that setting a total flow of 3601it/hr
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