  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 1.3e-4 and a step-wise increase in angle of attack from
14.2{\deg} to 14.8{\deg} within 0.04 convective times. We systematically
quantify the stall reaction delay 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
9e-4 to 0.14 and for dynamic sinusoidal pitching motions of different airfoils
at higher Reynolds numbers up to 1e6. The stall delays for all conditions
follows the same power law decrease from 32 convective times for the most
steady case down to an asymptotic value of 3 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. 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 1e-4.
