On Stability Conditions for Airfoil Equilibrium Position in Stream

The Liapunov’s stability of the equilibrium position of the heavy high-drag airfoil in a stream of fluid or gas is investigated. For a system with three degrees of freedom, cases of both ideal elastic and viscous-elastic links are considered. Necessary and sufficient conditions (criteria) of asymptotic stability are derived for the airfoil equilibrium position in the stream. In particular cases, earlier known conditions are obtained including the instability condition by Glauert-Den-Hartog.