The approaches of compressible dynamic stall control using real-time airfoil adaptation and slatted airfoils are compared. Each method attempts to solve the unsteady e ow separation and the underlying causes differently. The approaches lead to unexpected results: For the slatted airfoil, dynamic stall alleviation on the main airfoil with a fully stalled slat occurred, and for the shape adapting airfoil, leading-edge attached e ow with trailing-edge separation was obtained. In both cases, no dynamic stall vortex was present. As can be expected, the control effectiveness of each method varies over the full cycle and depends on the Mach number due to the new factors introduced by the use of the methods. These issues are addressed. conditions. At low Reynolds numbers and Mach numbers, it arises from a large leading-edge adverse pressure gradient. At slightly highervaluesofthesequantities,whenthelocale owbecomessuper- sonic, the mechanism changes to that due to an interaction between the local supersonic e ow and the laminar separation bubble, at very low pressure gradients. Tripping the airfoil causes dynamic stall again from the strong leading-edge adverse pressure gradient. At higher Mach numbers, shock-induced separation initiates dynamic stall. A rotor blade encounters a variety of freestream conditions de- pending on the local e ow conditions. Hence, it may see dynamic stall arisingfromany of these causes. In general,athigher Reynolds numbers, two main mechanisms, the adverse leading-edge pressure gradient and the shock-induced separation, can be expected to pre- vail. It is imperative that any dynamic stall e ow control schemes that are developed be robust and effective against all dynamic stall onset mechanisms over the full range of e ow conditions observed in the rotor e ight envelope. Specie c to dynamic stall control is that the unsteady vorticity e uxincreasesdramaticallywhenanairfoilisrapidlypitchedpastthe staticstallangle.Thisisparticularlyinterestingbecauseoneeffectof unsteadinessistoreducetheequivalentangleofattackfortheairfoil, which is responsible for the delay of stallmeasured.Eventually,this unsteady vorticity abruptly coalesces into the dynamic stall vortex.