Experimental and numerical examination of a helicopter hovering in ground effect

This study collects experimental data of a helicopter hovering in ground effect and intends to prove the capability to compute the flow field around the measured helicopter rotor with the use of the flow solver FLOWer. Firstly, the wake trajectory of a Hughes 300C helicopter hovering in ground effect was obtained by introducing fog in the rotor disk, while visualizing the blade tip vortices with a laser light sheet and a high speed camera. Secondly, the isolated rotor of the Hughes 300C was modeled numerically and computed with the block-structured second-order flow solver FLOWer from DLR, in order to provide a computational validation for full size rotors in ground effect hover. Blade dynamics were neglected and the rotor was trimmed to the experimental thrust. It was found that the flow solver is capable of simulating the flow around a full size helicopter rotor during hover in ground effect. The wake trajectories are convected downwards slightly faster than in the experiment. The radial wake path fits well with the experimental one until high wake ages, where the numerically as well as experimentally reproduced trajectory becomes unstable. The development of the thrust value was analyzed with a Fourier transform. Disturbances due to the ground can be detected especially at higher harmonics of the blade passing frequency. For comparison, the rotor was also examined numerically out of ground effect. As expected, the oscillations are more periodic than close to the ground. An analysis of the performance data led to a benefit of 21 % due to the ground effect.