Effects of rotor-airframe interaction on the aeromechanics and wake of a quadcopter in forward flight

Caprace, Denis-Gabriel [UCL] Ning, Andrew [Brigham Young University] Chatelain, Philippe [UCL] Winckelmans, Grégoire [UCL]

From small drones to large Urban Air Mobility vehicles, the market of vertical take-off and landing (VTOL) aircraft is currently booming. Modern VTOL designs feature a variety of configurations involving rotors, lifting surfaces and bluff bodies. The resulting aerodynamics are highly impacted by the interactions between those components and their wakes. This has consequences on the aircraft performance and on the downstream wake. Studying the effects of those interactions through CFD can inform the development of cheaper numerical models. In this work, we focus on the interaction between rotors and bluff bodies based on the example of a generic quadcopter in forward flight. Using the Vortex Particle–Mesh method, a CFD method well-suited for wake studies, we analyze the influence of the rotor-airframe interaction on the loads and on the development of the aircraft near wake. Our results show that the presence of the airframe induces a change in average thrust of maximum 2.4% on the rear rotors. Correspondingly, the contribution of the rotors to the overall aircraft lift and drag changes by less than 2%, suggesting that the airframe can be ignored if one seeks a simple model of the rotor forces. On the other hand, the airframe experiences peak loads due to its proximity with the blades. Thus, the rotor-airframe interaction must be accounted for in the prediction of airframe loads. Finally, comparing simulations with and without the airframe, we find that the trailing wake vortices converge to circulations less than 0.2% different. However, the vortex spacing decreases when the airframe is accounted for.