Time‐Frequency Analysis of Experimental and Analytical Rotor Thrust during a Rotor Speed Change

Abstract

A study was conducted examining a small-scale, two‐bladed rotor undergoing a change in rotor speed. The data acquired consists of time histories of nonstationary signals; therefore, a Stockwell transform was employed to analyze the data in lieu of traditional Fourier transform-based methods. The analysis included extraction of time‐varying frequency content of the rotor thrust and hub motion, instantaneous rotor speed, damping ratios, and instantaneous phase difference between thrust and hub motion. This work is divided into two parts. First, an analytical study was conducted to understand how rotor transient response, determined using a time‐marching solution, is affected by the inclusion of a simplified elastic support structure model, and is used to demonstrate the utility of the Stockwell transform. The second part of the study compared the results from an analytical model of the NASA Multirotor Test Bed (MTB) undergoing a rotor speed change to wind tunnel data. The current analytical model of the MTB, including an elastic support structure, properly predicted the measured trends in the frequency content of the thrust; however, it underpredicted the amplitude of these unsteady loads.