Ceiling Effect on the Aerodynamic Characteristics of Multi-Rotor Systems and Its Modeling

The ceiling effect, a phenomenon in which rotor thrust increases near a ceiling wall, holds significant potential for applications in energy-efficient drones and aerial manipulation. However, previous studies have not reached a consensus on how the changes in thrust characteristics of multi-rotor configurations differ from those of a single rotor. Furthermore, an analytical model capable of consistently describing the aerodynamics from free space to the vicinity of a ceiling has not yet been sufficiently established. In this study, we experimentally investigated the coupled effects of the ceiling effect and rotor-to-rotor interference by measuring the aerodynamic characteristics in a twin-rotor configuration while varying the distance from the ceiling wall and the inter-rotor spacing. Consequently, we revealed that the ceiling effect in a twin-rotor configuration exhibits five principal characteristics: (C1) a thrust increase exceeding that of a single rotor, which amplifies as the rotors approach each other; (C2) a thrust reduction in free space due to rotor proximity; (C3) a thrust increase as the rotors approach the ceiling wall; (C4) an expansion of the distance at which the ceiling effect emerges as the inter-rotor spacing decreases; and (C5) an asymmetric thrust distribution resulting from a thrust reduction on the adjacent side. Furthermore, we developed three progressively refined analytical models and compared them with the experimental results: Model 1, based on the induced upwash from the adjacent rotor; Model 2, which extends Model 1 by incorporating the downwash from wake vortices; and Model 3, which further refines Model 2 by accounting for the mutual interference between wake vortices. The results confirm that Model 3, which accounts for the mutual interference between wake vortices, reproduces the overall trends of the five characteristics described above and shows good agreement with the experimental results for the ceiling effect involving rotor-to-rotor interference.