Aerodynamic effects of rotor-rotor interaction on a tandem rotor in ground effect*

In this study, we investigate the effect of rotor-rotor interaction on the thrust generation and the wake structure of a tandem rotor operating near the ground. For this purpose, the force and velocity field measurements are performed with varying the normalized rotor separation distance (0.25 ≤ s/R ≤ 2.45, where s is the distance between the adjacent rotor tips and R is the radius of the rotor) and the normalized ground height (0.50 ≤ h/R ≤ 2.00, where h is the distance between the rotor tip-path plane and the ground). The interaction between the rotors causes the thrust generated by a tandem rotor to be less than that generated by a single rotor at the same h/R. The thrust loss is more pronounced at lower s/R as the tandem rotor approaches the ground. The measured velocity field shows that the thrust loss is due to the reinjestion of tip vortices rising above the rotor tip-path plane by the fountain flow formed between the rotors. As the tip vortices are reinjested into the rotor disk, the turbulence intensity of the rotor inflow is increased compared to a single rotor, which in turn reduces the thrust generated. The flow characteristics described above appear in the form of a vortex ring between the two rotors, which will be discussed in detail in the presentation.