The experimental and numerical computational investigation of co-axial rotor performance has been increased over the past decade in order to understand complex interactions in co-axial rotor flows to improve design of unmanned-aerial vehicles. Nevertheless, the issues related rotor aerodynamic performance, wake interactions, etc. are not well understood. In the current work, aerodynamic interactions in co-axial rotor have been investigated with both experimental and numerical methods in hover flight by varying tip diameters, rpm, axial distance, etc. In order to calculate the co-axial thrust efficiency, in-house test bench has been created. On the numerical side, the three-dimensional unsteady Navier-Stokes equation is solved using a pressure-based, segregated, compressible and time-accurate solver of OpenFOAM. A sliding mesh interface procedure is utilised to link rotating regions and SST k - omega model is employed for the turbulence modelling. The computational results indicate relatively good agreement with in-house experimental data.