Aerodynamic Performance of Propellers for Multirotor Unmanned Aerial Vehicles: Measurement, Analysis, and Experiment

Key Laboratory of CNC Equipment Reliability, Ministry of Education, School of Mechanical and Aerospace Engineering, Jilin University, Changchun 130025, China College of Electronic Engineering, South China Agricultural University, Guangzhou 510642, China National Center for International Collaboration Research on Precision Agricultural Aviation Pesticides Spraying Technology, Guangzhou 510642, China


Introduction
UAVs (unmanned aerial vehicles) have the characteristics of being aircraft with simple operation, high reliability, good maintainability, high flexibility, and high-performance feedback controllers [1]. UAVs have been widely used in recent years in plant protection operations [2], remote sensing [3][4][5], medical and health care sectors, military reconnaissance, express logistics, emergency rescue, environmental management, mining operations [6], infrastructure development, and other fields [7]. e complex and varied work requires higher performance of UAVs, which makes it essential to analyze the aerodynamic performance of UAVs. e power of multirotor UAVs is derived from the lift generated by the rotation of its propellers. To change the attitude and working state of UAVs, its electronic drivers are used to control the speed and, therefore, the thrust generated in each propeller. Analyze and quantify the controllability about hovering through principle model discussions [8]. Fault diagnosis is performed by applying multilayer adaptation convolutional neural network, and hierarchical representations are learnt from the collected signals [9]. e power of gasoline-powered UAVs is derived from their engines, the output speed of which is essentially constant [10]. e flight attitude of such UAVs is modified by adjusting rotor pitch, thus changing the lift of each rotor [11]. erefore, the propeller is the most important component to determine the performance of the UAVs and the efficiency of its propulsion system [12][13][14]. Monospinner is an aircraft with only one propeller as the moving part. After eliminating the yaw state, its linearization system can be controlled horizontally and vertically in position [15]. In addition to blade radius, the propeller speed and pitch also have a significant impact on the performance of UAVs. For analysis of propeller performance, the classical theory of blade element momentum theory [16,17] has been widely used to evaluate the aerodynamic characteristics of propellers. To guarantee that the theoretical calculations agree closely with the real values, various modified blade element momentum theories have been developed. ese modifications include abandoning the small inflow angle assumption [18,19] and considering the tip vortex [20][21][22]. In addition to computational fluid dynamics (CFD) simulations [23,24], experimental platforms are also selected to determine the aerodynamic performance of propellers. A position estimation method using sensor fusion in a structured environment is developed to obtain the localization states of the UAVs [25]. A simple experimental platform was used to test small two-or three-blade propellers with fixed pitch [26]; a variable-pitch propeller experimental platform has also been used [27]. Furthermore, some researchers have conducted wind tunnel tests [28][29][30][31].
ere has been some analysis of the aerodynamic performance of UAVs, but relatively little analysis related to the influence of propeller parameters on its aerodynamic performance parameters. e rest of the paper is organized as follows. In Section 2, we analyze the aerodynamic performance of propellers based on blade element momentum theory, establish a calculation model for the selection of blade parameters for multirotor UAVs, and then choose symmetric airfoil NACA 0012 as an example to verify the feasibility and rationality of the model. In Section 3, we describe an experimental platform designed to test the aerodynamic performance of small propellers, which is used to explore the variation of thrust, torque, and static thrust coefficient C T0 with diameter, pitch, and speed, and the relationship among these variables. In Section 4, we provide the conclusions along with the significance of this work and future research.

Calculation of Propeller
Aerodynamic Performance 2.1. Blade Element Momentum eory. We use blade element momentum theory as the theoretical basis for studying the aerodynamic performance of UAV propellers. e blade of a propeller can be divided into infinite number of continuous elements along its chord length, and the aerodynamic forces on a blade can be analyzed by analyzing the differential forces of the elements [32,33]. Summing the aerodynamic forces of each blade element gives the aerodynamic force of the whole blade. e parameters, velocities, and forces of a blade element are shown in Figure 1, where V up is the speed at which the drone takes off vertically, V i is the induced velocity, Ω·r is the linear velocity of the blade element at the position where the radius is r, at the axis of the propeller, r � 0, the linear velocity is 0, and the tip of the propeller is r � R, the linear velocity is Ω·R, where Ω is the rotor angular velocity and R is the propeller radius, W is the relative air velocity, α is the angle of attack, φ is the pitch angle, and ε is the inflow angle.
As shown in Figure 1, W can be calculated according to the following equation: (1) Aerodynamic performance mainly refers to the lift, resistance, and power generated by the blade during the working process. e lift, C l , and drag coefficients, C d , are needed to calculate the lift and drag generated by blades according to blade element momentum theory. ey are defined as where ρ is the air density, S is the effective area of the blade element, Y is the airfoil lift, and X is the airfoil drag. S can be calculated according to where b is the length of the chord and Δr is the length of the blade element. e characteristic curves for lift and resistance can be created once the airfoil has been determined. C l and C d are determined for the lift and resistance curves, respectively, according to α and the Reynolds number Re where v f is the freestream velocity, μ is the kinetic viscosity, and l is the characteristic dimension, which is usually defined as either local chord length or chord length at 75% of radius. erefore, the differential lift of the element, dY, and drag of the element, dX, can be calculated according to equations (2) and (3), as Furthermore, the net force on the blade element along the rotation axis, dT, the net force on the blade element along the vertical rotation axis, dQ, and the lift force, T, and drag force, Q, generated by a single blade can be calculated as For calculating the propeller power, the required power, P R , includes the induced rotation resistance power, P i , the airfoil resistance power, P 0 , the waste resistance power, P p , and the amount of power UAVs uses to overcome gravity during vertical takeoff, P m : where n is the number of blades per rotor, L is the distance between the point of action of the rotation resistance and the propeller shaft, Q air is the freestream resistance, V forward (as distinct from v f ) is the cruising speed of the rotary-wing UAVs, and m is the mass of the maximum takeoff weight of the UAVs. Combined with the above theory, we establish a calculation model for the selection of blade parameters for multirotor UAVs by comparing the maximum thrust and the takeoff weight of the UAVs, and write a calculation program in the C language. e model of propeller aerodynamic performance can determine feasible ranges for R and b on the basis of the airfoil and m. e calculation flowchart of the model is shown in Figure 2.
In Figure 2, M is the Mach number and V i can be calculated, based on slipstream theory, as where R 0 is the distance between the propeller shaft and the propeller root and a (as distinct from α) is a correction coefficient to compensate for the loss of thrust due to the presence of a narrow ring at the tip of the paddle.

Example Calculation.
In this paper, NACA 0012 symmetrical airfoil [34] is analyzed to determine whether the preset values of R and b are feasible according to our model. e required airfoil parameters can be found on the NACA website.
e lift characteristic curve of the airfoil under different Reynolds numbers is shown in Figure 3. e atmospheric composition is preset, where ρ � 1.225 kg/m 3 , v f � 2 m/s, and μ � 1.8247 × 10 −5 Pa·s. Re is calculated based on equation (5): Refer to Figure 3 and consider the curve for Re � 100,000.   Setting m � 50 kg, we get the best lift-drag characteristics at α � 5°. When α � 5°, by setting R � 0.4 m, b � 0.04 m, M � 0.513979, and a � 0.99, we can calculate the thrust generated by a single rotor, T single, in the vertical takeoff state as V i in this state can be calculated as and T single as so that the lift of the whole UAV, T tol , is calculated as e calculation results are shown in Table 1. T tol in Table 1 is the lift for the whole UAV. e calculation results show that the selected blade parameters ensure that T tol ≈ mg in the hover state and T tol > mg during vertical takeoff, whether α � 5°or 10.75°. erefore, the preset values of R and b are feasible.

Experimental System
Design. e experimental system for propeller aerodynamic performance that we designed and reported in this paper can collect data on propeller speed, T single , and propeller torque. It requires manual operation via a remote control to vary the rotation speed of the blade and consists of two parts: the experimental platform and the measurement and control system. Figure 4 shows the overall design of the system.
A photograph of the experimental system is shown in Figure 5. e experimental platform built on an aluminum alloy frame is used for installing the propeller under test, comprising a brushless motor, force sensor, infrared speed sensor, and other equipment. e propellers on the multirotor are vertically arranged. e propeller in the experimental system is designed to be placed horizontally to eliminate the ground effect.
We have used two sensors, S-type force sensor and photoelectric speed sensor, and the parameters are shown in Table 2.
As shown in Figure 6, the force measurement sensors are divided into two groups: torque sensors, placed vertically, and thrust sensors, placed horizontally.
As shown in Figure 7, the torque is measured indirectly by sensors arranged symmetrically on the left and right sides. During the experiment, the torque generated by the propeller is transferred to the sensors through the roof plate so that the sensors on each side are subject to the same tension and pressure. e distance between the force sensors on the left and right sides is the length of the moment arm. e product of the measured torsional force and the length of the moment arm (45 mm) is equal to the torque produced by the propeller.
As shown in Figure 8, to measure thrust, the thrust generated by the propeller is transferred to the moving plate through the roof plate. After the moving plate is stressed, it moves backwards along the guide rail and transfers the thrust to the S-type thrust sensor arranged horizontally towards the rear of the experimental platform.
An infrared speed sensor measures the motor speed. In addition, the experimental platform includes a DC stabilized voltage power supply for the brushless motor. e measurement and control system is used to measure the speed of the brushless motor and the thrust according to the force sensor. e measurement and control system communicates with the computer through a serial port using a data acquisition card. e data on propeller speed, T single , and propeller torque are collected and saved by a LabVIEW program as an Excel file.
To further analyze the data obtained from the aerodynamic performance experiment, data processing software is designed, based on Visual Basic.NET, to provide rapid analysis. e software has two functions: (i) Reading the propeller speed, thrust, and torque data collected by the data acquisition card and generating the thrust speed and torque speed graphs for the propeller to show the trends, while the speed is varied (ii) Calculating propeller power, efficiency, and V i , based on user input of the thrust, torque, and speed e user interface of the data processing software is shown in Figure 9.
L is difficult to calculate, so P 0 was not calculated using equation (9). e input torque, T 0 , and speed, n 0 , can be used to calculate P 0 as follows: V i can be calculated by equation (11), and P i can be calculated by equation (8).
e efficiency of the propeller, η, can be calculated as

Analysis of Experimental
Results. e purpose of this experiment is to explore how propeller thrust and torque vary with rotational speed. e following six types of propellers, manufactured by APC [35], with different diameters and pitch, are selected for aerodynamic performance testing: APC 8045, APC 9045, APC 9060, APC 9075, APC 1145, and APC 1245. APC is an American company known for producing high-quality propellers. Taking APC 9045 as an example, the first two digits (90) represent that the diameter     Figure 10. e results of the repeated experiments are almost identical, indicating that the experimental platform has good repeatability.
One of the five experimental results is selected for analysis, and the analysis results are as follows.

Effects of Propeller Speed, Diameter, and Pitch on
rust. Among the propellers tested, those with a pitch of 4.5 inch (114.3 mm), i.e., APC 8045, APC 9045, APC 1145, and APC 1245, were chosen to deduce the effect of diameter on thrust; their parameters are shown in Table 3. Figure 11 shows the effect of diameter on thrust: thrust increases with rotational speed and, for the same speed, thrust increases with diameter. e thrust of APC 8045 has no significant change when the rotational speed is less than 5,000 r/min, the thrust of APC 9045 and APC 1145 has no significant change when the rotational speed is less than 4,000 r/min, and the thrust of APC 1245 has no significant change when the rotational speed is less than 3,000 r/min. When the diameter of propeller varies from 8 inches (203.2 mm) to 9 inches (228.6 mm), the increase of thrust is significant, and the thrust reaches a maximum difference of 7 N at 11,000 r/min; when the diameter of propeller varies from 11 inches (279.4 mm) to 12 inches (304.8 mm), the increase of thrust is not significant, and the difference of thrust remains about 1 N when the rotational speed is more than 6,000 r/min. We can speculate according to the above analysis. For the same rotational speed, when the diameter of propeller increases to a certain extent, the impact of further increase on thrust force is not significant. If more thrust is needed, a more powerful motor will be necessary to produce a higher rotational speed. e thrust of APC 9045 sharply increases and changes irregularly when the rotational speed is ∼7,000 r/min, due to a resonance between the propeller and experimental platform; the other three propellers have no obvious resonances.
Among the propellers to be tested, those with a diameter of 9.0 inches (228.6 mm), i.e., APC 9045, APC 9060, and APC 9075, are chosen to deduce the effect of pitch on thrust; their parameters are shown in Table 4. Figure 12 shows the effect of pitch on thrust. e thrust increases with increasing rotational speed, and at the same speed, the thrust increases with increasing pitch. e thrust of APC 9045, APC 9060, and APC 9075 have no significant change when the rotational speed is less than 4,000 r/min; and the thrust of these three types of propellers are almost the same when the rotational speed is less than 5,000 r/min. e difference of thrust between APC 9045 and APC 9060 remains about 2 N when the rotational speed is more than 7,500 r/min; the difference of thrust between APC 9060 and APC 9075 remains about 1.5 N when the rotational speed is more than 6,500 r/min. e thrust of APC 9045 sharply increases and changes irregularly when the rotational speed is ∼7,000 r/min, due to a resonance between the propeller and experimental platform; the other two propellers have no obvious resonances. In Figure 9, we can see that the only resonance occurs for APC 9045, when the rotational speed is ∼7,000 r/min. Considering the information from Figures 9 and 10 together, we can thus conclude that the rotational speed at which resonance occurs relates to diameter and not pitch.    Shock and Vibration Figure 13 shows the effect of diameter on torque. e torque decreases at first then increases with rotational speed and, for the same speed, the torque increases with diameter. e torque of APC 8045 and APC 9045 decreases slowly with rotational speed when the rotational speed is less than 3,800 r/min and then increases rapidly when the rotational speed is more than 3,800 r/min; the torque of APC 1145 and APC 1245 decreases slowly with rotational speed when the rotational speed is less than 2,800 r/min and then increases rapidly when the rotational speed is more than 2,800 r/min. When the diameter of propeller varies from 8 inches (203.2 mm) to 9 inches (228.6 mm) or varies from 9 inches (228.6 mm) to 11 inches (279.4 mm), the increase of torque is significant; when the diameter of propeller varies from 11 inches (279.4 mm) to 12 inches (304.8 mm), the increase of torque is not significant. is variation of torque is similar to  that of thrust. e speed-torque curve is not smooth. e explanation for this phenomenon is that the torque produced by the propeller is small, so the vibration of the experimental platform has great influence on the torque measurement, which makes the continuity of the curve poor. e propellers with a diameter of 9.0 inches (APC 9045, APC 9060, and APC 9075) are used to deduce the effect of pitch on torque. Figure 14 shows the effect of pitch on torque. e torque decreases at first then increases with rotational speed and, for the same speed, the torque increases with pitch. e torque of APC 9045, APC 9060, and APC 9075 decreases slowly with rotational speed when the rotational speed is less than 3,800 r/min and then increases rapidly when the rotational speed is more than 3,800 r/min. e torque of APC 9060 and APC 9075 is practically the same when the rotational speed is less than 5,200 r/min. e torque difference of each propeller increases gradually, but the increase is not significant. As before, the continuity of the curves is poor. By comparing Figures 13 and 14, it is obvious that the diameter of the propeller has a greater influence on torque, while the pitch of the propeller has a smaller influence on torque.

Effects of Propeller Speed, Diameter, and Pitch on C T0 .
Static thrust coefficient C T0 is a parameter to evaluate the hover performance of propeller. C T0 can be calculated by equation (19), where D is the propeller diameter: e propellers with a pitch of 4.5 inch (APC 8045, APC 9045, APC 1145, and APC 1245) are used to deduce the effect of diameter on C T0 . Figure 15 shows the effect of diameter on C T0 . C T0 of APC 8045 increases with rotational speed, while C T0 of APC 9045, APC 1145, and APC 1245 increases with rotational speed till the speed is ∼6,500 r/min and then remains constant with a slight downward trend. is is because the range of speed is not wide enough to make C T0 of APC 8045 remain constant. C T0 of APC 8045 is the lowest in this group of experiment. When the rotational speed is less than 6,500 r/min, C T0 of APC 9045, APC 1145, and APC 1245 have no significant distinction; when the rotational speed is more than 6,500 r/min, C T0 of APC 9045, APC 1145, and APC 1245 decreases with diameter for the same speed. One possible reason for this phenomenon is that the motor does not provide enough torque to drive the propeller with large diameter. C T0 of APC 9045 sharply increases and changes irregularly when the rotational speed is ∼6,500 r/min, due to    Figure 16 shows the effect of pitch on C T0 . C T0 of APC 9045, APC 9060, and APC 9075 increases with rotational speed when the rotational speed is less than 6,500 r/min and then remains constant when the speed is more than 6,500 r/min. When the rotational speed is less than 6,500 r/min, C T0 of APC 9045, APC 9060, and APC 9075 has no significant distinction; when C T0 of APC 9045, APC 9060, and APC 9075 remains constant, it increases with pitch for the same speed. C T0 of APC 9045 sharply increases and changes irregularly when the rotational speed is ∼6,500 r/min, due to a resonance between the propeller and experimental platform; the other two propellers have no obvious resonances.

Conclusions
(1) We analyze the aerodynamic performance of propellers to calculate their thrust, drag, and power based on blade element momentum theory.
(2) A model of propeller aerodynamic performance is presented and verified by a calculated example. is model of propeller aerodynamic performance can determine feasible ranges of R and b on the basis of the airfoil and m. (3) A design for a propeller aerodynamic performance experiment system is proposed, including its measurement and control system. We construct an experimental platform and test the aerodynamic performance of several propellers. (4) Software is developed to process the data on propeller speed, thrust, and torque. e software can plot graphs of propeller thrust and torque against rotational speed and perform a single-step calculation of three parameters for a propeller: P 0 , V i , and η. We conclude that both thrust and torque increase with increasing speed, propeller diameter, and propeller pitch; C T0 increases with rotational speed and propeller pitch, but not with propeller diameter.
After referring to existing experimental platform, we design a new structure of sensor group including torque sensors, thrust sensors, and an infrared speed sensor. e new structure makes the propeller aerodynamic performance experiment system more accurate in torque measuring and more stable under the existing experimental conditions. e experimental data including propeller speed, thrust, and torque are used to analyze the aerodynamic performance of APC propellers. e propeller chosen for the experiment is the most commonly used type for UAVs, which makes the experimental data more convincing in assisting propeller selection for UAVs. And, we plan to test more types Shock and Vibration of propellers to build an aerodynamic performance database based on existing research, which will provide a higher reference value for propeller selection for UAVs. By analyzing the experimental data, we find the experimental system has some deficiencies, including a certain extent effect from the load cell structure and the vibration of the experimental platform.
So, a few projects and recommendations for future work are listed below: (1) Redesign the force measurement sensors to acquire a smaller structure, aiming at decreasing the blockage of the load cell structure behind the propeller (2) Select sensors with higher accuracy to get more accurate experimental data (3) Redesign the structure of the aluminum alloy frame to reduce the vibration and achieve the purpose of lightweight (4) Design a variable-pitch propeller experimental platform based on this paper and study the effect of pitch on thrust, torque, and C T0 better (5) In order not to be limited to the aerodynamic performance research of UAV in the hovering state, we will conduct further experiment under the condition of the wind tunnel to study the effect of speed, diameter, and pitch on thrust coefficient C T and advance ratio J

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.