The influence of interactional effects on quadrotor performance in forward flight was evaluated taking into account square and diamond configurations, forward and backward tilt angles and a range of hub spacings including overlapping blades. The analysis was based on the wind-tunnel measurements and simulations from five computational methods with different fidelity levels. The outcome indicates that the efficiency of a diamond configuration improves in comparison with isolated rotors for non-overlapping rotor spacings, while the interactions in square alignments are detrimental for all analysed test cases. The computational results showed good agreement with the measurement data for the forward rotor plane tilt, however for the increased rotor-wake interactions at a backward tilt angle the spread between the calculated values, especially for torque, could be observed with the general trends maintained. The study proves the diamond configuration with 1.2 D rotor spacing and tip-to-tip rotor phasing to be most favourable in analysed conditions in terms of both aerodynamic performance and acoustic signature.

A comparison between high-fidelity simulations of a half and a full model approach of the Volocopter-2X (VC2X) in cruise flight is conducted. To ensure an identical flight condition for both cases a flight mechanics trim is performed. Therefore, the computational fluid dynamic (CFD) code FLOWer is loosely coupled to the flight mechanics tool VFAST. The half model simulation setup can save up to half of the cells and therefore also computational time. However, it is only valid for a center of gravity lying on the geometric symmetry plane and a coordinated flight with a zero sideslip angle and no roll rate. The comparison between the half and full model shows no visible difference in the mean rotor revolution speed and only small difference in the pitch attitude. The Volocopter flow is characterized by a high density of small-scale vortex structures. These result, on the one hand, from the rotor wake and rotor-rotor interactions and, on the other hand, from rotor-airframe interactions. A vortex tube developes near the symmetry plane above the rotor disc of the VC2X, which is more prominent in for the half model. The differences on the level of the individual rotors is for most rotors small, only the front outer rotors show bigger deviations.

High fidelity CFD simulations of seven rotors in compact configuration at three different flight scenarios are performed. The cases are hover, as well as 50 km/h and 100 km/h forward flight. For comparison each rotor is simulated isolated with the same RPM and pitch angle as in the configuration. Additionally, a bigger isolated rotor with the same area as the complete configuration is simulated. For the configuration as well as the bigger rotor a flight mechanic trim is performed using the flight mechanic tool VFAST. The CFD simulation is performed with FLOWer. In hover, only the center rotor showed a significant Figure of Merit (FM) drop of 16% compared to the isolated rotor. The thrust of the outer rotors is increased at the tip areas facing outwards, while the tip areas towards the center and the tip areas of the center rotor showed reduced thrust compared to the isolated rotor. The wake contraction at the outer rotors is increased compared to the bigger rotor. For the 50 km/h forward flight the efficiency of the front rotors is increased (10%-17%) and the rear ones decreased (11%-16%). In this case the wake is directly convected from the front rotors into the rear rotor planes and strong vortex interactions occur. For the 100 km/h case the efficiency gain of the front rotors is reduced to 3%-11% and the decrease to 5%-9%. Due to the higher pitch the wake of the rotors flows away from the rotor plane and the rotor-rotor interactions are reduced.

High-fidelity CFD simulations of hovering flights of the multi-rotor Volocopter 2X (VC2X) aircraft with three different heights above the ground and an out of ground reference case are presented. The tool chain applied consists of the CFD solver FLOWer, which is loosely coupled to the flight mechanic tool VFAST. For all simulations an adequate representation of the flight mechanics is of crucial importance since the high number of trim degrees of freedom of the VC2X has a significant influence on the flight physics. A short introduction and validation of VFAST is carried out, which shows that VFAST already provides valuable stand-alone results for the considered flight envelope. The in ground and out of ground CFD simulations showed a highly complex flow field for the VC2X. With increasing ground proximity the number of vortex structures induced by the ground increases and the rotor wake is characterized by strong fluctuations. Basic rotorcraft in ground effect phenomena, like a high-pressure area below the rotor plane and aircraft power reduction are observed for low flight heights. The decreased power requirement is mainly traced back to a decrease of the airframe download. The system figure of merit for all three in ground cases merely changes.