Comparison of In-Flight Measured and Computed Aeroelastic Damping: Modal Identification Procedures and Modeling Approaches

ABSTRACT The Operational Modal Analysis technique is a methodology very often applied for the identification of dynamic systems when the input signal is unknown. The applied methodology is based on a technique to estimate the Frequency Response Functions and extract the modal parameters using only the structural dynamic response data, without assuming the knowledge of the excitation forces. Such approach is an adequate way for measuring the aircraft aeroelastic response due to random input, like atmospheric turbulence. The in-flight structural response has been measured by accelerometers distributed along the aircraft wings, fuselage and empennages. The Enhanced Frequency Domain Decomposition technique was chosen to identify the airframe dynamic parameters. This technique is based on the hypothesis that the system is randomly excited with a broadband spectrum with almost constant power spectral density. The system identification procedure is based on the Single Value Decomposition of the power spectral densities of system output signals, estimated by the usual Fast Fourier Transform method. This procedure has been applied to different flight conditions to evaluate the modal parameters and the aeroelastic stability trends of the airframe under investigation. The experimental results obtained by this methodology were compared with the predicted results supplied by aeroelastic numerical models in order to check the consistency of the proposed output-only methodology. The objective of this paper is to compare in-flight measured aeroelastic damping against the corresponding parameters computed from numerical aeroelastic models. Different aerodynamic modeling approaches should be investigated such as the use of source panel body models, cruciform and flat plate projection. As a result of this investigation it is expected the choice of the better aeroelastic modeling and Operational Modal Analysis techniques to be included in a standard aeroelastic certification process.

摘要：运行模态分析（Operational Modal Analysis）是一种在输入信号未知时，广泛应用于动态系统识别的方法。该方法依托一类仅利用结构动态响应数据即可估计频率响应函数（Frequency Response Functions）并提取模态参数的技术，无需掌握激励力的相关信息。此类方法适配于测量由大气湍流等随机输入引发的飞机气动弹性响应。研究团队沿飞机机翼、机身及尾翼部署的加速度计采集了飞行过程中的结构响应数据。本次研究选用增强频域分解（Enhanced Frequency Domain Decomposition）技术以识别机身动态参数，该技术的假设前提为：系统受宽带频谱激励，且功率谱密度近似恒定。系统识别流程基于对系统输出信号功率谱密度（Power Spectral Density）的奇异值分解（Single Value Decomposition），而功率谱密度通过常规快速傅里叶变换（Fast Fourier Transform）方法估算得到。该流程已应用于多种飞行工况，以评估待研究机身的模态参数及气动弹性稳定性趋势。研究将本方法获得的实验结果与气动弹性数值模型提供的预测结果进行对比，以验证所提出的仅基于输出信号的分析方法的一致性。本文的研究目标为，将飞行中测得的气动弹性阻尼与通过数值气动弹性模型计算得到的对应参数进行对比。本次研究需考察多种气动建模方法，例如源面板体模型、十字形及平板投影模型。通过本次研究，期望筛选出更优的气动弹性建模与运行模态分析技术，以纳入标准气动弹性认证流程。