Compressor Map - Fan Map

Fan Map

As the second illustration shows, a low pressure ratio fan (such as that used on a high bypass ratio turbofan) has a range of working lines. At high flight speeds, the ram pressure ratio factors up the cold nozzle pressure ratio, causing the nozzle to choke. Above the choking condition, the working lines tend to coalesce into a unique steep straight line. When the nozzle unchokes, the working line starts to become more curved, reflecting the curvature of the nozzle characteristic. With falling flight Mach number, the cold nozzle pressure ratio decreases. Initially this has no effect upon the position of the working line, apart from the curved (unchoked) tail, which becomes longer. Eventually, the cold nozzle will become unchoked at lower flight Mach numbers, even at full throttle. The working lines will now become curved, gradually migrating towards surge as flight Mach number decreases. The lowest surge margin working line occurs at static conditions.

Owing to the nature of the constraints involved, the fan working lines of a mixed turbofan are somewhat steeper than those of the equivalent unmixed engine.

The fan map shown is for the bypass (i.e. outer) section of the unit. The corresponding inner section map typically has longer, flatter, speed lines.

Military turbofans tend to have a much higher design fan pressure ratio than civil engines. Consequently the final (mixed) nozzle is choked at all flight speeds, over most of the throttle range. However, at low throttle settings the nozzle will unchoke, causing the lower end of the working lines to have a short curved tail, particularly at low flight speeds.

However, ultra-high bypass ratio turbofans have a very low design fan pressure ratio (e.g. 1.2, on the bypass section). Consequently, even at cruise flight speeds, the cold (or mixed final) propelling nozzle is unchoked. The fan working lines become more spread-out with flight Mach number, because they move bodily up the map, towards the top right hand corner. As a result, the static working line can be well into surge.

One solution is to have a variable area cold (or mixed) nozzle. Increasing the nozzle area at low flight speeds brings the fan working line away from surge.

An alternative solution is to fit a variable pitch fan. Scheduling the pitch of the fan blades has no impact upon the position of the fan working lines, but can be used to move the surge line upwards, to improve fan surge margin.