An experimental and a theoretical investigation of rotor pitch damping using a model rotor

Sotiriou, Constantinos Panteli (1990) An experimental and a theoretical investigation of rotor pitch damping using a model rotor. University of Southampton, Doctoral Thesis.

Abstract

An experimental investigation was conducted to study rotor pitch damping in hover, axial flight (climb speed to tip speed ratios: 0 - 0.549), and low speed forward flight (advance ratios: 0.05 - 0.088), for a variation in rotor thrust coefficient 0.001 - 0.013, and for the range of flapping frequency ratios squared 1.0 - 1.11. The tests were conducted at the following oscillation ratio frequencies, (frequency of oscillation divided by rotor angular speed), 0.186 for hover and axial flight and 0.1068 for low speed forward flight and in hover for the case of flapping frequency equal to unity. The nature of the damping of the rig was investigated since it was found that frictional effects contaminated the estimates of the rotor aerodynamic damping. To do this a parameter estimation method, based on a nonlinear minimisation procedure, was developed that does not require a large amount of data to identify the viscous and frictional damping coefficients from the free response of the rotor. A theoretical model for low frequency coupled rotor body dynamics was developed and the effects of rotor and inflow degrees of freedom on rotor damping were investigated. Results indicate that both rotor and inflow degrees of freedom are required in the mathematical description of low frequency coupled rotor-body dynamics for the adequate estimation of rotor damping. The dynamic inflow degrees of freedom were described by using momentum theory and by assuming a nonuniform distribution for the steady induced velocity given by a Glauert wake. A blade flexibility study was conducted to define the flapping frequency for blade-linear spring configurations producing both time invariant and time dependent boundary conditions at the spring location. Comparison of theoretical damping estimates with the experimental data in hover and axial flight show that at oscillation ratio frequency 0.186, the no-inflow model is in reasonable agreement with the experimental data and that the inclusion of dynamic inflow theory produces no improvement in the experimental data correlation. In hover and in low speed forward flight at oscillation frequency ratio 0.1068, the inclusion of dynamic inflow theory provides an improvement in the experimental data correlation indicating that for these conditions inflow degrees of freedom are also required for the adequate prediction of rotor pitch damping.

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