This paper examines the aerodynamic drag force acting on a tilt-rotor UAV that has three mini fans and a main rotor with a tilt mechanism. The mini fans are embedded in the nose and the left and right wings. The main rotor is located near the center of the vehicle, and its front half is surrounded by the trailing edge of the nose in rotary-wing mode. The downward airflow from the fans and the main rotor generates an aerodynamic drag force called momentum drag, which is linearly proportional to the airspeed of UAV. To verify the existence of momentum drag, parameter identification of drag coefficients is performed from experimental data where the UAV flies forward in rotary-wing mode. The drag force is also investigated using computational fluid dynamics simulations. These experimental and numerical results are consistent with theoretical results based on momentum theory.

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