8. Mastering the Fan Curve: Understanding Stall, Surge, ac axail and Efficiency Understanding the fan performance curve is crucial for selecting and operating an axial fan efficiently, as it graphically illustrates the relationship between airflow (volume), static pressure, ac fans and power consumption[1]. The curve shows that as the airflow rate decreases (due to increasing system resistance), the static pressure generated by the fan increases up to a maximum point, after which it drops rapidly[1]. This region of rapid pressure decrease is highly unstable and is characterized by two critical aerodynamic phenomena: stall and surge[1][16]. Stall occurs when the fan's angle of attack—the angle at which the blade meets the incoming air—becomes too high, causing the air to separate from the blade's surface[16].