<p>This study meticulously examines the aerodynamic performance of a heavy-lift electric vertical take-off and landing (eVTOL) aircraft with coaxial rotors. Focus is placed on the impact of rotor spacing and the windmilling state of the lower rotor. Analysis involves both experimental assessments and numerical simulations. The optimal spacing for aerodynamic performance in the coaxial rotor system is found to be no less than <i>Z/D</i> = 0.145, resulting in nearly 88% of the ideal coaxial thrust. At this spacing, the lower rotor experiences a 16.8% thrust decrease and a 27.5% torque increase, while the upper rotor remains relatively unchanged. Aerodynamic interference boosts electrical power consumption by 21% and reduces motor efficiency by 5%. In windmilling, at <i>Z/D</i> = 0.175, the lower rotor rotates at 22% of the speed of the upper rotor. Compared to normal operation of coaxial rotors, the upper rotor’s thrust decreases during windmilling, while the lower rotor generates drag. These combined effects result in a 7.62% reduction in total thrust of the coaxial system during windmilling. To aid in visually exploring coaxial rotor aerodynamics, a 3D model of the rotor was accurately recreated using 3D scanning technology. The experimental findings presented in this paper offer valuable insights for the design of other large-scale coaxial eVTOL systems.</p>