Comparative Study on Transonic Flutter Characteristics of Isogai Airfoil in Heavy Gas Medium

In high-speed wind tunnel flutter tests, the adoption of heavy gases such as R134a as the working medium serves as a key technical approach to address the "mass similarity" challenge in dynamic simulation of scaled flutter models. However, differences in thermodynamic properties between heavy gases and air significantly alter the aerodynamic characteristics of the model. Under transonic flow conditions, such aerodynamic differences further affect flutter characteristics and modify the distribution of the flutter boundary. Taking the Isogai benchmark airfoil as the research object, this paper constructs the transonic unsteady Aerodynamic Influence Coefficient (AIC) matrix based on the describing function method, conducts a systematic comparative analysis of steady/unsteady aerodynamic characteristics and flutter characteristics between R134a and air, and reveals the action mechanism and influence law of medium property variations. The results show that the lift curve slope, aerodynamic center position and unsteady aerodynamic influence coefficients are basically consistent between the two media at most Mach numbers, but the differences become prominent after entering the transonic regime (Ma > 0.85). Specifically, the higher lift curve slope in R134a and the altered shock oscillation behavior cause noticeable deviations in aerodynamic center position and phase characteristics of unsteady aerodynamic forces, ultimately leading to observable differences in flutter characteristics within the transonic range. This work clarifies the influence mechanism of medium effects on transonic unsteady aerodynamic characteristics, and provides important theoretical support for the application and data interpretation of heavy-gas wind tunnel flutter test technology.