Vibration Characteristics Of Blades Research Articles

Sensitivity of Rotor Blade Vibration Characteristics to Torsional Oscillations

A theoretical investigation of dynamic response characteristics of helicopter rotor blades in forward flight was carried out with special emphasis on the torsional degrees-of-freedom. The finite element method was applied in the formulation of the coupled equations of motion for flapwise bending and torsion for blades with noncolinear elastic, mass, and aerodynamic axes. The sensitivity of blade vibration characteristics with respect to several basic structural, geometric, and aerodynamic properties as well as flight conditions was evaluated. Numerical results for sample blades were plotted to show the variation of the coupling between bending and torisonal components of the response.

Vibration Characteristics of Pre-twisted Blades of Asymmetrical Aerofoil Cross-Section

SummaryThe natural frequencies and mode shapes of vibration of cantilever aerofoil cross-section blades of pre-twist angle in the range 0 to 90 degrees are obtained. The beams are 152·4 mm long and the width / thickness ratio is such that they may be regarded as slender. Theoretical frequency ratios and mode shapes of vibration, neglecting shear and rotary inertia effects, are obtained for two sets of beams, one with clockwise pre-twist relative to the root cross-section and the other with anti-clockwise pre-twist. The effect of variation in the value of the centre-of-flexure coordinates upon the natural frequency ratios and mode shapes of vibration is investigated. The theoretical results are compared to corresponding experimental results.

Vibration Characteristics of Low Aspect Ratio Compressor Blades

This paper presents the results of a study of the vibration characteristics of low aspect ratio compressor blades. The treatment is based on thin shell theory and the Rayleigh-Ritz method is used to obtain the eigenvectors and eigenvalues. The object is to elucidate those characteristics which are inaccessible using beam theory. Results are presented which show the variation of the natural frequencies and mode shapes with angle of twist, aspect ratio, and angle of inclination of the base of the blade. A three-dimensional plot of the bending mode frequencies versus aspect ratio and twist angle is presented. Although the surfaces describing the variation of frequencies for specific modes do not intersect, there is a point of contact. This contact point is significant in the transition of mode shapes along the frequency surfaces. It is demonstrated that the “stiff-direction” or “in-plane” vibration of the untwisted plate evolves into coupled bending modes as the twist angle increases from zero and that the character of these modes changes in the vicinity of the contact point.

Vibration Characteristics of Straight Blades of Asymmetrical Aerofoil Cross-Section

SummaryTheoretical and experimental natural frequencies and modal shapes up to the fifth mode of vibration are given for a straight blade of asymmetrical aerofoil cross-section. The theoretical procedure consists essentially of transforming the differential equations of motion into a set of simultaneous first-order equations and solving them by a step-by-step finite difference procedure. The natural frequency values are compared with results obtained by an analytical solution and with standard solutions for certain special cases. Good agreement is shown to exist between the theoretical results for the various methods presented. The equations of motion are dependent upon the coordinates of the axis of the centre of flexure of the beam relative to the centroidal axis. The effect of variations of the centre of flexure coordinates upon the frequencies and modal shapes is shown for a limited range of coordinate values. Comparison is made between the theoretical natural frequencies and modal shapes and corresponding results obtained by experiment.

Mechanical evaluation of gas-turbine blades in their actual centrifugal field

Blade vibration and fatigue have been a major problem in the development of gas turbines. Earlier test techniques developed for determining blade vibration and fatigue characteristics were satisfactory for stationary blades, but did not provide the centrifugal field needed for rotor-blade tests. This paper describes a test technique and facility on which blades are tested in their actual centrifugal environment.

Turbines

Vibration characteristics of pre-twisted composite blades are analysed using a three-noded triangular cylindrical shell element. The specific example of glass fibre reinforced plastic material is analysed with its material damping. The effect of different parameters such as pre-twist, fibre orientation, skew angle, taper ratio and aspect ratio on natural frequency and system loss factor is investigated. It is found that the maximum fundamental frequency and maximum fundamental loss factor occur at different twist angles and at different fibre orientations. The influence of taper ratio and aspect ratio on 0° and 90° fibre orientations is reported. The effect of skew angle on natural frequency and on system loss factors is seen to be low.