1 Answers
A variable pitch fan is similar in concept to that of a variable-pitch propeller and involves progressively reducing the pitch of the fan on a turbofan as the engine is throttled. Although variable pitch fans are used in some industrial applications, the focus of this article is on their use in turbofan engines. No production engine uses such a feature; however, it will likely be required on at least some of the next generation of high bypass ratio turbofans.
One of the methods used to reduce Thrust-specific fuel consumption is to improve Propulsive Efficiency. This involves reducing the effective jet velocity of the engine by reducing specific thrust. This, in turn, reduces the optimum fan pressure ratio required and consequently the cold nozzle pressure ratio. At cruise flight speeds the nozzle is choked and the fan working line is fairly steep and linear. However, at low flight speeds the ram pressure rise in the air intake is so low the nozzle is well un-choked. Consequently, the fan working line is highly curved and well to the left of the cruise flight speed working line, potentially reducing the fan surge margin to a dangerous level, particularly at lower throttle settings. Readers unfamiliar with surge lines, working lines, etc. should read the Wikipedia article on Compressor map.
There are two solutions to this problem:
1] Open up the cold nozzle area at low flight speeds, which moves the fan working line away from the surge. This has little effect on the position or slope of the surge line.