Aerodynamic Characteristics of a Micro Multi-Rotor Aircraft with 12 Rotors Considering the Horizontal Wind Disturbance

Wind disturbance posed difficulties for the stability of the micro air vehicles (MAVs) with attitude variation. In this paper, the aerodynamic performance of a MAV with six coaxial rotor pairs considering the horizontal wind is investigated by both experiments and numerical simulations. First, the effect of the horizontal wind on the multi-rotor aircraft is analyzed in detail. Then, low-speed wind tunnel tests were performed to obtain the thrust and power consumption and the aerodynamic performance of the multi-rotor aircraft (l/D = 1.2 and h/D = 0.19) with the rotational speed of 1500–2300 r/min in the horizontal wind ranged from 0 to 5 m/s. Finally, the distribution of streamline, the pressure of the blade tip, and the velocity and the vortices in the flow field of a multi-rotor aircraft with horizontal wind disturbance, were simulated and studied using the computational fluid dynamics (CFD) method. Through the comparison of experimental results and simulation results, it can be seen that the horizontal wind disturbance will increase power consumption to weaken the aerodynamic performance at higher rotor speeds. However, larger thrust and better hover performance are obtained at lower rotational speeds with good wind resistance. Additionally, due to the mutual induction between rotor wakes, the interactions of downwash flows become more intense at higher rotational speeds or larger wind speeds where the vortexes at the blade tip deformed and moved along with the wind.

Small-Scale Rotor Design Variables and Their Effects on Aerodynamic and Aeroacoustic Performance of a Hovering Rotor

With the increased prominence of multicopter micro-aerial vehicles, more importance has been placed on the aerodynamic and acoustic performance of these systems, as their small-scale and lower Reynolds number regime provide results that are different from full-scale rotors. A computational methodology was employed in order to study the aerodynamic and aeroacoustic performance from different small-scale rotors used in a multicopter configuration. Three rotor design variables (twist, taper, and pitch) were investigated in order to understand their influence on aerodynamic and acoustic performance of a hovering rotor. Variables such as rotor rotation rate and rotor radius were kept constant. Common aerodynamic performance metrics such as the ratio of coefficient of thrust to coefficient of power and figure of merit (FM) were used to assess aerodynamic hover performance of the designed rotors. Acoustic performance was assessed by recording acoustic pressure in the far-field at two separate receivers. Acoustic results are presented in the frequency domain as one-third octave band data and as overall sound pressure level (SPL). Flow fields and pressure contours were calculated and displayed in order to help explain aerodynamic and acoustic results. From the results, insights are provided for rotor designs that are more aerodynamically and acoustically efficient in hover. Specifically, rotors that provided lower values of disk loading and higher values of power loading were typically more acoustically efficient. Using greater rotor twist and taper increased both aerodynamic and acoustic performance.

Hover Performance of Corotating and Counterrotating Coaxial Rotors

Hover testing was performed on a 2-m-diameter coaxial corotating (stacked) rotor system to investigate the effect of azimuthal spacing, or index angle, on performance. Individual rotor thrust and power measurements were performed as a function of the index angles. It was found that the contributions of individual rotors to the total stacked rotor performance were remarkably dependent on the index angle. For an index angle of 10°, that is, when the lower rotor trails the upper rotor by 10°, a nearly 3.6% increase in total system power loading was observed compared to that for an index angle of –10°. Numerical calculations using the U.S. Army's Rotorcraft Comprehensive Analysis System (RCAS) were used to better understand this behavior in terms of self-induced and interference power constituents. For the coaxial, counterrotating (CCR) rotor, the upper rotor performance was close to that of an isolated two-bladed rotor. The lower rotor operating in the wake of the upper rotor showed a relatively poor performance, requiring 70% more induced power than the upper rotor. The improvement in performance for a CCR rotor results from a larger thrust share and better performance of the upper rotor as compared to the lower rotor.

Aerodynamic Performance of Hex-Rotor UAV Considering the Horizontal Airflow

In this paper, the aerodynamic performance of a Hex-rotor unmanned aerial vehicle (UAV) with different rotational speeds (1500–2300 RPM) considering the horizontal airflow conditions is analyzed by both simulations and experiments. A low-speed wind tunnel experiments platform is applied to measure the thrust, torque, and power consumption of a Hex-rotor UAV with different rotational speeds in horizontal airflow, which varied from 0 m/s–4 m/s. First, this paper introduces the effect of horizontal airflow on a UAV. Then, the low-speed wind tunnel experiments were carried out on a Hex-rotor UAV (D/L = 0.56) with different horizontal velocities to determine the hover performance. Finally, numerical simulations were obtained with the streamline distributions, pressure distributions, velocity contour, and vortex distributions at different horizontal airflow conditions to describe the aerodynamic interference effect of different horizontal airflows. Combined with the experimental results and numerical simulations results, the horizontal airflow proved to have a significant influence on the aerodynamic performance of the Hex-rotor UAV with an increase in thrust and power. Indeed, the streamlines in the flow field were coupled to each other at the presence of the incoming airflow. Especially when the incoming airflow was larger, the Hex-rotor UAV could properly use low-speed flight to maintain high power loading. Finally, it is inferred that the aerodynamic performance of the Hex-rotor UAV is also related to the movement and deformation of the vortex at the tip of the rotor.

Investigation of Rotor Tip Vortex in Hover Based on IDDES Methods

A calculation and analysis program of high-precision numerical simulation for rotor blade tip vortex in hovering state was developed. The fifth order Roe-WENO scheme was carried out in order to reduce the numerical dissipation of the rotor wake region. The rotary motion of the rotor was realized by using the dynamic patched technology of structured grids. And at the same time, the technology also helped to avoid the tremendous increase of grid number of the far-field due to the refined grids of the flow region where emphasis was placed on. Hybrid RANS/LES approach was investigated based on the issues about inadequate capabilities of simulations of complex turbulent flows, and IDDES approach was developed. The numerical simulation of the tandem cylinder was carried out firstly to verify the reliability of the IDDES method and the patched grid technology. Then the RANS and IDDES approaches were used to simulate the flow field of the rotor in hover performance, respectively. The analysis of the vortex magnitude, vortex core position and diameter as well as the velocity profiles of the rotor tip vortex were made comparatively in detail. The numerical results showed that the resolutions obtained through IDDEES approach agreed with the experimental results much better than that of the RANS approach with the same gird scales. Meanwhile, the IDDES results can capture the tiny worm vortex structures and vortex paring phenomena in accordance with the practical status, which contributes to study the flow mechanism of rotor and related problems.