In this paper, a predictor-corrector with 2nd-order backward difference (PC2B) time accurate free wake algorithm with explicit modification is formulated to investigate the rotor unsteady aerodynamic characteristics. As the rotor free wake equation possesses the characteristics of non-linearity, its solutions are susceptible to numerical instability. The modified equation of a different format is derived to investigate the accuracy and the numerical stability of the modified PC2B algorithm, and the stability and calculation efficiency of the algorithm are verified by a numerical experiment Based on the algorithm, the rotor aerodynamic characteristics are first calculated and the validation of the rotor free wake model is performed with the experimental data for hover and ramp up in collective pitch. Secondly, the rotor unsteady aerodynamic characteristics are investigated during the collective pitch maneuver phase. Results demonstrate that the fluctuation of the rotor unsteady aerodynamic characteristics during the collective pitch maneuver phase stems from the transient variation of the rotor free wake geometry.%给出了一种显式修正的二阶向后差分格式(PC2B)的时间精确自由尾迹算法,用于分析旋翼非定常气动特性.由于旋翼自由尾迹控制方程具有非线性特征,求解过程中易产生数值不稳定性问题,因此本文推导了修正PC2B算法的修正方程,分析了该算法的求解精度和数值稳定性,并通过数值试验对其稳定性和计算效率加以验证及分析.基于该算法,首先对悬停和总距突增时旋翼非定常气动特性进行了计算,通过与试验结果的对比验证了模型的有效性；其次研究了总距操纵时旋翼非定常气动特性的变化规律.研究结果表明,旋翼自由尾迹几何形状的瞬态变化是造成总距操纵条件下旋翼非定常气动特性波动的主要原因.
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