Interactional aerodynamic modeling and analysis of multi-rotor aircraft using computational fluid dynamics

Abstract

Over the past decade, advancements in battery, electronics, and motor technology have led to the rapid popularization of electric vertical takeoff and landing (eVTOL) aircraft which use multiple rotors for thrust generation and control. The distributed-electric propulsion architecture allows for flexible placement of multiple lifting and propulsive rotors, leading to a variety of potential advanced configurations. However, when rotors operate in close proximity aerodynamic interactions can lead to degraded (or enhanced) performance. The current batteries powering most eVTOL aircraft exhibit low energy density relative to the hydrocarbon fuels used by conventional helicopters. Under this limitation, it is especially important to maximize the aerodynamic efficiency of eVTOL aircraft in order to realize meaningful payload capacity, endurance and range. In this body of work, computational fluid dynamics (CFD) is used to simulate the aerodynamics of multiple rotors (and wings) operating in close proximity. Simulations of numerous multi-rotor/rotor-wing systems are compared to those of isolated rotors and isolated wings operating under the same conditions in order to extract the interactional aerodynamic effects. Through these simulations, the physical mechanisms driving the interactions are established and potentially advantageous designs/configurations are identified. In particular, the aerodynamic interactions between two in-line rotors operating in edgewise flight is established and an associated aft-rotor performance penalty is identified. This analysis is extended to systems with varying vertical and longitudinal hub-hub separation where the aft rotor thrust and torque at each separation distance is evaluated. The interactions of in-line rotors are also investigated for when they are canted (and each rotor's rotation axis is tilted). Laterally canted and longitudinally canted rotors are considered and the most aerodynamically efficient configuration is identified. While much of the existing literature on rotor-rotor interactions pertains to forward flight, close-proximity rotors are also expected to interact when operating close to the ground. Pairs of side-by-side rotors are simulated in ground effect (IGE) at two ground heights and two hub-hub separation distances and their thrust production is compared to that of an isolated rotor out of ground effect (OGE). Canted side-by-side rotors are also simulated IGE, and their thrust production is compared to uncanted side-by-side rotors at the same height above the ground and hub-hub spacing. Beyond rotor-rotor interactions, the influence of a rotor operating below and behind a wing is also considered. This rotor placement is found to improve system performance by enhancing wing lift through rotor induced suction. Wing lift enhancement is investigated at a range of wing incidence angles, rotor disk loadings, flight speeds and rotor positions and the most advantageous configurations/conditions are identified. Rotor-wing interactions are further investigated for a full vehicle by simulating a quad rotor tail sitter (QRTS) using CFD. Rotor-wing and wing-rotor aerodynamic interactions are identified and compared for three rotor mounting positions. These results are used to inform CFD-CSD coupled simulations of the QRTS which show reduced power over the QBiT even with interactional aerodynamic penalties.