Analysis of the features of the main rotor with negative offset of the flapping hinge
In this paper a new concept of a high-speed main rotor of a single-rotor helicopter is considered. A new concept of a high-speed main rotor is to use the design rotor hub, which allowing to obtain a natural frequency of flap motion blade lower than the rotational speed of the rotor. Reduction of the natural frequency, as well as its increase (screws with positive flapping-hinge offset or rotors with elastic fastening of blades) leads to a phase shift of the flapping motion of the blades. The phase shift of the blades flapping motion leads to an increase the angles of attack on the advancing side of rotor and to a decrease in the retreating side, and thereby detracts from the appearance of a stall at high flight speeds. The analysis of the effect of its design features on aerodynamic characteristics is carried out. Presented are the equations of the flapping motion of main rotor blades with negative offset of flapping hinges, as well as of the moment on the rotor hub. The paper also considers the method for calculating the coefficients of the first harmonic of the blade flapping motion and the expressions for calculating angles of attack in blade sections. The analysis of the operation of rotors with both negative and positive flapping-hinge offset is carried out, an estimation of the influence of some rotor parameters on angles of attack of blade sections is obtained. Also presented are the results of calculating such rotors that illustrate the possibility of increasing angles of attack on advancing blades and reducing them on retreating blades by means of the swash plate deflection.