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Multicopter Aerodynamics: Characterizing Thrust on a Hexacopter

Overview

Source: Prashin Sharma and Ella M. Atkins, Department of Aerospace Engineering, University of Michigan, Ann Arbor, MI

Multicopters are becoming popular for a variety of hobby and commercial applications. They are commonly available as quadcopter (four thrusters), hexacopter (six thrusters), and octocopter (eight thrusters) configurations. Here, we describe an experimental process to characterize the multicopter performance. A modular small hexacopter platform providing propulsion unit redundancy is tested. The individual static motor thrust is determined using a dynamometer and varying propeller and input commands. This static thrust is then represented as a function of motor RPM, where the RPM is determined from motor power and control input. The hexacopter is then mounted on a load cell test stand in a 5’ x 7’ low-speed recirculating wind tunnel, and its aerodynamic lift and drag force components were characterized during flight at varying motor signals, free-stream flow speed, and angle of attack.

A hexacopter was selected for this study because of its resilience to motor (propulsion unit) failure, as reported in Clothier¹. Along with redundancy in the propulsion system, the selection of high-reliability components is also required for safe flight, particularly for missions over-populated regions. In Ampatis², the authors discuss the optimal selection of multicopter parts, such as motors, blades, batteries, and electronic speed controllers. Similar research has also been reported in Bershadsky³, which focuses on the proper selection of a propeller system to satisfy mission requirements. Along with redundancy and reliability of components, understanding vehicle performance is also essential to assure flight envelope limits are respected and to select the most efficient design.

Procedure

This protocol characterizes hexacopter thrust and aerodynamics. For this experiment, we used commercially available, off-the-shelf components for the hexacopter, and the details are provided in Table 2. For the flight controller, we selected an open-source autopilot, Librepilot,⁹ as it provided flexibility to control individual motor commands issued to the hexacopter.

The test stand for mounting the load cell and hexacopter was fabricated in-house using laminated plywood

Results

Dynamometer Tests

In Figures 5-6, the plots illustrate the variation of thrust and torque, respectively, with increasing motor RPM. From these plots, the minimum motor RPM required for the multicopter to hover can be determined. A plot showing data from multiple propellers can be obtained from Sharma¹². Further, the quadratic relations between thrust vs. RPM and moment vs. RPM can be clearly observed, which are described in Equations (1) and (2). Using this

Application and Summary

Here we describe a protocol to characterize the aerodynamic forces acting on a hexacopter. This protocol can be applied to other multirotor configurations directly. Proper characterization of aerodynamic forces is needed to improve control design, understand flight envelope limits, and estimate local wind fields as in Xiang¹³. The presented protocol for determining motor RPM based on power consumption and throttle command has direct applications to estimate RPM and thrust when low-cost electronic speed control

References

Clothier, R.A., and Walker, R.A., “Safety Risk Management of Unmanned Aircraft Systems,” Handbook of Unmanned Aerial Vehicles, Springer, 2015, pp. 2229–2275.
Ampatis, C., and Papadopoulos, E., “Parametric Design and Optimization of Multi-rotor Aerial Vehicles,” Applications of Mathematics and Informatics in Science and Engineering, Springer, 2014, pp. 1–25.  
Bershadsky, D., Haviland, S., and Johnson, E. N., “Electric Multirotor UAV Propulsion System Sizing for Performance Prediction and Design Optimization,” 57th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conf., 2016.
Bangura, M., Melega, M., Naldi, R., and Mahony, R., “Aerodynamics of Rotor Blades for Quadrotors,” arXiv preprint arXiv:1601.00733, 2016
Ducard, G., and Minh-Duc Hua. "Discussion and Practical Aspects on Control Allocation for a Multi-rotor Helicopter." Conf. on Unmanned Aerial Vehicle in Geomatics, 2011.
Powers C., Mellinger D., Kumar V. “Quadrotor Kinematics and Dynamics” In: Handbook of Unmanned Aerial Vehicles. Springer, 2015
McClamroch, N. Harris. “Steady Aircraft Flight and Performance.” Princeton University Press, 2011.
Quan, Q., “Introduction to Multicopter Design and Control”, Springer Singapore, 2017.
LibrePilot, https://www.librepilot.org/site/index.html
Foster, J. and Hartman, D., “High-Fidelity Multi-Rotor Unmanned Aircraft System Simulation Development for Trajectory Prediction under Off-Nominal Flight Dynamics,” Proc. Air Transportation Integration & Operations (ATIO) Conference, AIAA, 2017.
Russell, Carl R., et al. "Wind Tunnel and Hover Performance Test Results for Multicopter UAS Vehicles," 2016.
Sharma, P. and Atkins, E., “An Experimental Investigation of Tractor and Pusher Hexacopter Performance,” Proc. AIAA Aviation Conference, AIAA, June 2018. (to appear)
Xiang, X., et al. "Wind Field Estimation through Autonomous Quadcopter Avionics." 35^th AIAA/IEEE Digital Avionics Systems Conference (DASC), IEEE, 2016.
Kamel, M., et al. "Model Predictive Control for Trajectory Tracking of Unmanned Aerial Vehicles using Robot Operating System." Robot Operating System (ROS). Springer, Cham, 2017, 3-39.