Thrust pulsation of coaxial main rotor, caused by the blades relative position

Abstract

The influence of reciprocal position of the upper rotor blades in respect to the lower rotor blades is characteristic for coaxial main rotor. It is established that the initial azimuth of the blade, for example, of the upper rotor’s which does not coincide with the initial azimuth of the lower rotor blades, affects the level of vibrations caused by the rotors thrust pulsations, the level of noise, generated mainly by coaxial rotor. This paper presents numerical studies which assess the effect of the initial azimuth of the upper rotor blades ("phasing") on the helicopter coaxial rotor thrust force pulsation. The research was carried out applying the calculation method based on the nonlinear vortex theory in a non-stationary formulation. The results of the helicopter coaxial rotor with different initial azimuths of the upper rotor blade relatively to the azimuth of the lower rotor blade flow around numerical simulation are presented. The influence of the blades "phasing" on the rotor thrust coefficient change and thrust force pulsation magnitude is shown. The flow of a six-bladed coaxial main rotor (two rotors with 3 blades) was simulated in the oblique flow mode at speeds of 51.25 m/s and 71.75 m/s at the rotor angles of attack– 5⁰ and – 12⁰, respectively. The change in the coefficient of the main rotor thrust per revolution at different values of "phasing" was studied. The coaxial rotor thrust coefficient is determined by summing the lower and upper rotors thrust coefficients respectively. Thus, at some "phasing" the thrust coefficient of the lower and upper rotors increase intensifies the thrust pulsations, and at others, the peaks of the upper and lower rotors pulsations are displaced and the total coaxial rotor thrust coefficient changes per one revolution with smaller amplitude. It is established what "phasing" produce the maximum values of thrust pulsation, and at which-a minimum of thrust pulsation.