Prediction of Rotor Instability at High Forward Speeds. Volume V. Flapping and Flap-Lag Instability

摘要

The volume deals with an investigation of the unstable flapping or flatwise bending tendency which arises during operation at high forward speeds and reduced rotational speeds. The principal parameters influencing flapping or flatwise bending are enumerated and discussed. The simplified analysis used a single degree of freedom, rigid flapping for an articulated blade or the first flatwise bending mode for a nonarticulated blade. This analysis was used to calculate response to an instantaneous gust, and practical operating boundaries were defined in terms of maximum allowable flapping or bending excursions. The simple analysis was used to investigate a range of various parameters. The simple analysis was supplemented with similar calculations with the extended normal mode transient analysis. These results showed that the simpler analysis predicted all trends correctly and provided results which were somewhat conservative with respect to those of the normal mode transient analysis. The existence of a flap-lag instability was investigated with the normal mode transient analysis, which included blade stall, compressibility, and reverse-flow effects. Three advance ratio conditions were chosen, with the rotor operating in high-speed flight under stalled conditions. While large control pulses were found to produce blade excursions which were unacceptably large from a practical standpoint, no tendency of the rotor to become catastrophically unstable was noted. (Author)