Airfoil stall plays a central role in the design of safe and efficient lifting surfaces. We typically distinguish between static and dynamic stall based on the unsteady rate of change of an airfoil’s angle of attack. Despite the somewhat misleading denotation, the force and flow development of an airfoil undergoing static stall are highly unsteady and the boundary with dynamic stall is not clearly defined. We experimentally investigate the forces acting on a two-dimensional airfoil that is subjected to two manoeuvres leading to static stall: a slow continuous increase in angle of attack with a reduced pitch rate of 1.3 $\times$ 10⁻⁴ and a step-wise increase in angle of attack from 14.2°to 14.8°within 0.04 convective times. We systematically quantify the stall reaction delay, or the timespan between the moment the blade exceeds its critical static stall angle and the onset of stall, for many repetitions of these two manoeuvres. The onset of flow stall is marked by the distinct drop in the lift coefficient. The reaction delay for the slow continuous ramp-up manoeuvre is not influenced by the blade kinematics and its occurrence histogram is normally distributed around 32 convective times. The static reaction delay is compared with dynamic stall delays for dynamic ramp-up motions with reduced pitch rates ranging from 9 $\times$ 10⁻⁴ to 0.14 and for dynamic sinusoidal pitching motions of different airfoils at higher Reynolds numbers up to 1 $\times$ 10⁶. The stall delays for all conditions follow the same power law decrease from 32 convective times for the most steady case down to an asymptotic value of 3 convective times for reduced pitch rates above 0.04. Static stall is not phenomenologically different than dynamic stall and is merely a typical case of stall for low pitch rates where the onset of flow separation is not promoted by the blade kinematics. Based on our results, we suggest that conventional measurements of the static stall angle and the static load curves should be conducted using a continuous and uniform ramp-up motion at a reduced frequency around 1 $\times$ 10⁻⁴.

翼型失速在安全高效的升力面设计中起着核心作用。我们通常根据翼型迎角的不稳定变化率来区分静态和动态失速。尽管有些误导性的表示，经历静态失速的机翼的力和流动发展是高度不稳定的，并且动态失速的边界没有明确定义。我们通过实验研究了作用在二维机翼上的力，该机翼经过两次机动导致静态失速：攻角缓慢连续增加，俯仰率降低至 1.3$\times$10 ^-4和在 0.04 次对流时间内攻角从 14.2° 逐步增加到 14.8°。对于这两个操作的多次重复，我们系统地量化了失速反应延迟，或叶片超过其临界静态失速角的时刻与失速开始之间的时间跨度。流动失速的开始以升力系数的明显下降为标志。缓慢连续斜坡上升机动的反应延迟不受叶片运动学的影响，其发生直方图正态分布在 32 次对流时间附近。静态反应延迟与动态斜升运动的动态失速延迟进行比较，降低的俯仰率范围为 9$\times$10 ^-4到 0.14 以及在高达 1 的更高雷诺数下不同翼型的动态正弦俯仰运动$\times$10 ⁶ . 所有条件下的失速延迟都遵循相同的幂律，从最稳定情况下的 32 次对流次数下降到 0.04 以上的降低俯仰率时的 3 次对流次数渐近值。静态失速在现象学上与动态失速没有区别，只是低桨距速率下失速的典型情况，其中叶片运动学不促进流动分离的开始。根据我们的结果，我们建议静态失速角和静态负载曲线的常规测量应使用连续且均匀的斜升运动进行，频率降低约 1$\times$10 ^-4。