Variable pitch fan

The Variable Pitch Fan is a mechanism used to vary the fan pitch similar to that of a variable-pitch propeller. One example is the Turbomeca Astafan turbofan engine. Another is the proposed Rolls-Royce Trent Ultrafan. Variable Pitch Fans are also used in some other applications, such as industrial.

In a conventional turbofan, usually a single shaft (the "low-pressure" or LP shaft) connects the fan, the intermediate-pressure compressor and the low-pressure turbine, whilst a second concentric shaft (the "high-pressure" or HP shaft) connects the high-pressure compressor and high-pressure turbine. However, in the Rolls-Royce RB211 and Trent engine series, the intermediate-pressure compressor is not connected to the LP shaft but is mounted on a concentric IP shaft, which is driven by the intermediate-pressure (IP) turbine

Each compression component (e.g. fan) has a compressor map which (usually) has been generated by comprehensive testing of the unit on a compressor rig. Engineers can superimpose operating lines on these compressor maps to show the locus of operation of the unit in terms of pressure ratio, corrected flow and efficiency. Speed and surge margin can also be inferred.

On a modern civil turbofan engine, the fan is a single stage device, usually developing a pressure ratio of less than 1.9:1 at maximum corrected speed.

At cruise flight speed (e.g. Mach 0.8), the ram rise in the intake supplements the fan pressure ratio to choke the secondary (i.e. cold) nozzle, so that the cold air in the bypass stream reaches sonic velocity (i.e. Mach 1.0) at the nozzle throat. As the engine is throttled back the reducing cold nozzle pressure ratio is sufficient to still choke the nozzle resulting in a straight, positive slope, operating line on the fan map. At lower engine thrust the cold nozzle may well unchoke causing the operating line to start curving away from the straight choked line. At a lower cruise flight speed (e.g. Mach 0.75) the cold nozzle may well be still choked at full throttle, but will unchoke early as the is engine throttled back.

However, the situation changes at low flight speeds. The ram rise in the intake is negligible, so the cold nozzle pressure ratio is not even as high as the fan pressure ratio because of the bypass duct pressure loss. Consequently, the cold nozzle is likely to be unchoked, resulting in a highly curved operating line, significantly closer to the surge line than the high speed cruise operating line, resulting in a lower fan surge margin particularly at low thrust settings.

At intermediate flight speeds the fan operating lines will be curved and lie somewhere between the low speed and cruise speed operating lines. If a curved low flight speed operating line is extrapolated to higher flow, it tends to coalesce with the straight high flight speed operating line, also extrapolated.

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Wikipedia