Abstract
The surface pressure change during a pitching motion of an airfoil was measured, and was analyzed to obtain a precursor of separation. A pitching airfoil was adopted to produce a predictable separation, because it is necessary to know a precise time to start separation to distinguish the precursor from other signals. The sinusoidal wave amplified exponentially in time was found to appear just before separation. This fluctuation is regarded as an instability wave of boundary layer based on the velocity profile with an inflection point.
