Estimation and reduction of helicopter hub vibratory forces due to blade-to-blade dissimilarity in composite rotor system

Abstract

In the conventional helicopters, the main rotor provides necessary lift capability to meet various mission requirements. However, the main rotor system is the principal source of distractive vibrations, which restricts helicopters from achieving higher speed, maneuverability, agility, and crew effectiveness. These vibrations can be divided into two categories. The first category of vibrations is inherent due to the asymmetric nature of a rotor in forward flight and are present even in the case of a balanced rotor (tracked rotor) system. The second category of vibrations is due to the blade-to-blade dissimilarity which results from manufacturing uncertainties, highly vibratory operating conditions and environmental effects. High levels of vibration and noise are key issues of concern for helicopter industries. Further, the extent severity of these vibrations increases due adding of few more harmonics vibrations coming from blade-to-blade dissimilarity. This dissimilarity may be result of various uncertainties, blade degradation due to highly vibratory operating conditions and environmental effects. Therefore, it is important to estimate the extent of extra vibratory loads coming from this dissimilarity. Most of the efforts until now are focused on minimization of hub vibratory loads by assuming the balanced rotor system whereas the dissimilarity is minimized separately using “track and balance” system. This study is focused on the estimation of hub vibratory loads due to dissimilarity and its elimination using active twist control.