Experimental studies of non-stationary aerodynamic characteristics of a helicopter airfoil vibrating in pitch angle

Аuthors

Kirillov O. E.^*, Mirgazov R. M.^**, Golubev N. V.^*, Petruhin D. A.^*

Central Aerohydrodynamic Institute named after N.E. Zhukovsky (TsAGI), 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia

*e-mail: spintest@tsagi.ru
**e-mail: ruslan.mirgazov@tsagi

Abstract

There is presently no reliable theoretical approach capable to predict the unsteady aerodynamic forces acting on the airfoils at the helicopter blade sections operating near or beyond the dynamic stall angle of attack. However, numerous wind-tunnel tests of vibrating airfoils do provide a valuable experimental foundation for establishing phenomenological models, through which certain semi-empirical predictions can be made.

The article presents the results of experimental studies of non-stationary integral and distributed aerodynamic characteristics of the helicopter's airfoil in stationary and non-stationary modes. The stationary mode is a fixed airfoil in a uniform steady airflow. The non-stationary mode is an airfoil vibrating in the pitch angle in a uniform steady airflow. The studies were being performed in the vertical, closed-circuit TsAGI wind tunnel with an open test section at Reynolds numbers Re = 270,000 and 540,000, and at reduced frequencies from 0.06 to 0.26.

A particular feature of the study was application of two methods for aerodynamic characteristics determining, namely, a direct method of forces measuring using balance, and calculating forces by integrating the pressure distribution along the chord. The obtained results are compared to each other, and their satisfactory agreement in the stationary mode is shown.

Another feature of the study is the analysis of forces pulsations and pressures on the airfoil surface. In particular, using the wavelet analysis, a phenomenon called “frequency explosion”, specific for the dynamic stall of the flow, is demonstrated.

Keywords:

vibrations in pitch angle, dynamic stall, wind tunnel , wavelet

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