Introduction T HE inceptionof compressorinstabilityin generaloccursduring an adverse operation condition that may cause an increase in compressor blade incidence angle and, thus, higher ow de ection. Reducing the compressor mass ow at a given rotational speed past the design surge limit exhibits one of the typical scenarios that can force the compressor into unstable operation. The decreased axial velocity component caused by the mass ow reduction changes the incidenceangle leadingto an increased ow de ection that resultsin a pressureincrease.The increasedpressure,however,may cause the velocity pro le within the boundary layer to approach its separation point. In this case, the uid particleinsidethe bladesurfaceboundary layer is, roughly speaking, exposed to three different forces. The convective force tries to move the particle in the direction of the motion, whereas two coacting forces, the shear stress force and the pressure force, oppose the motion of the particle.These three forces are generally in a dynamic equilibrium condition, which can be disrupted if additional pressure buildup is triggered by an adverse off-design operating point that leads to increased ow de ection. This situationleads to a major boundary-layer ow separationalong the blade surface, hub, and casing, which indicates the inception of compressor instability. The phenomenon of the ow separation (stall) and its controlhas been well known in external aerodynamics for more than half a century, where Prandtl, Schlichting,1 and Betz2 suggestedseveraldifferentcontrolscenarios.Stabilityenhancement using casing treatment was suggested by several researchers and summarized by Greitzer. The method discussed in this Note deals with the aerodynamic control and prevention of axial compressor instability and surge. It is based on dynamically adjusting the stator blades to the compressor interstage unsteady aerodynamics. Although it is concentrated on the aerodynamiccontroland preventionof stall, the method does not deal with control speci c procedures and algorithms. However, it can be combined with any multivariable control system to provide an active feedback control to prevent inception of compressor instability during a dynamic operation.