Analysis of helicopter's rotor airloads in transition flight regime

A computational model is developed to predict helicopter's rotor blade airloadings and examine the high vibratory airloadings acting on the rotor hub in the transition flight regime. Experimental results and theories from several references are applied to model the blade structural dynamics in bending, the strength and core size of the blade tip vortex, and to calculate the rotor free wake geometry and blade sectional airloading. This rotor wake model features the modeling of close blade-vortex interaction (BVI) and includes the vortex deformation and a maximum limit on vortex-induced lift coefficient of a blade section. It is found that with this modeling of close BVI, the computational singularity and instability found in previous rotor wake models can be prevented, and the computed results can predict the high airloading variations caused by close BVIs. Five flight conditions of a H-34 helicopter in and near the transition flight regime are chosen as test cases. The agreement of the computed blade sectional airloadings with the measured data is good. From the study of the relation between the computed blade airloading distribution and wake geometry, the influence of the close rotor vortex wake on the blade airloading during transition flight is better understood. It is found that at low forward speeds the close blade tip vortices induce high spanwise pressure gradients and high sectional airloadings at the blade tip portions for most of azimuth angles. The close rotor wake geometry changes with the forward speed. This change causes the different areas around the rotor disk to have different vortex-induced airloading changes with the forward speed. These different airloading changes around the rotor disk cause high vibratory airloadings acting on the rotor hub during transition flight.