Research of a novel aerodynamic evaluation method for fixed-wing UAV

The aerodynamic parameters of unmanned aerial vehicles (UAVs) play an important role in the newly designed drone. For the purpose of the real-time measurement of the aerodynamic performance parameters of the UAV in flight status, based on the self-development connected-wing configuration airplane, a novel airborne aerodynamics test scheme is researched and verified in this work. The findings of this study indicate that the effectiveness of numerical calculations can be verified by comparison with both results of the numerical simulation and airborne flight test in the same conditions. The newly designed UAV equipped with this test system in the future, not only achieves quick and reliable results of aerodynamic performance and powered system efficiency but also can save the expense of wind tunnel experiments, which is of high potential to the UAV application.


Introduction
Unmanned aerial vehicles (UAVs) industry is booming along with its large quantity of engineering applications like hazardous environment monitoring, search and rescue, border surveillance and military community, etc [1].It can be divided into three main streams of UAVs which are fixed-wing type, multirotor type, and hybrid type.In terms of the mission specifications and requirements, the three kinds of aerial vehicles are chosen to fit in different application domains depending on the requirement of performance capabilities including flight speed, flight endurance, payload capacity, etc [2].To keep up with the targets for enhanced aerodynamic maneuverability and performance, novel technologies constantly emerge in the last decades such as novel configuration layouts, flow control techniques, and propulsion architectures, where predicting and obtaining aerodynamic parameters plays a significant role, especially in the investigation of those technologies and aircraft design.Therefore, it is urgently required for rapid and accurate methods to estimate or measure aerodynamic and stability coefficients and facilitate performance calculations.Three conventional methods are capable of providing aerodynamic parameters [3,4].Firstly, the Theoretical calculation method that uses existing analytical theories and equations can provide reliable results for traditional configurations based on ideal hypotheses and proper simplifications during the conceptual design.Secondly, to obtain the final concept constraints, numerical methods are adopted in aerodynamic design like Computational Fluid Dynamics (CFD) simulations to achieve an accurate estimation of the aerodynamic coefficients.Finally, wind tunnel testing is an approach to measure the aerodynamic data in a simulated flying environment [5].Hence, the emphasis of these methods is given to the accuracy and consistency with a real flight of the UAV aerodynamic figures.In the present study, a large quantity of aerodynamic analysis methods has been presented and discussed.Many of them develop diverse models to calculate specific flight features of aircraft [6][7][8].A transition prediction method based on the coupled Michel transition Criterion and γ − Re θ transition model has been proposed by Chen et al. for aerodynamic analysis of low-speed fixed-wing UAVs [9].Shen et al. developed the procedure of computing longitudinal and lateral aerodynamic derivatives of Small-scaled Fixed-wing UAVs, aiming to improve the accuracy of the Aircraft Dynamic Model [10].A simple but more comprehensive aerodynamic force model was developed by Bannwarth et al. to make a balance between high accuracy and computational Complexity [11].Moreover, numerical methods are also used to estimate, identify or optimize aerodynamic parameters [12][13][14][15][16][17].Piedra et al. verified the flight requirements of light sport aircraft numerically with the Vortex Lattice Method and Computational Fluid Dynamics simulations concluded [18].Saderla et al. carried out an online aerodynamic parameter estimation method of UAV, which provide high-accuracy results compared with other methods [2,19].Analytical methods provide reliable results as well as convenient ways for estimating aerodynamic parameters.However, there still exists a non-negligible margin of error when compared with testing data obtained from wind tunnels and flight tests in the real flight environment.Thus, experimental methods are necessarily required to obtain and analyze the aerodynamic performance of UAVs to increase data accuracy.It is previously found in the review that a large diversity of research on the aerodynamic parameter test has been studied currently.Nevertheless, the relevant investigations associated with the airborne test approach have hardly been reported.In this study, a novel airborne aerodynamic estimation system of unmanned aerial vehicles is constructed, which enables aircraft to achieve measurement of aerodynamic data in real flight environment conditions.

Aerodynamic parameter test system
Based on the developed electric propulsion test system, a UAV airborne aerodynamic parameter test system (AAPTS) is proposed and applied.It can not only obtain real-time power system efficiency parameters but also acquire real-time aerodynamic parameters of the aircraft in flight state.

A.Measurement theory equation
A measurement scheme has been implemented for the airborne aerodynamic system based on the 2.4GHz network transmission and real-time data processing.The parameters of the aerodynamic performance and the system efficiency of the aircraft, i.e., lift coefficient (abbr.CL), drag coefficient (abbr.CD), lift-drag ratio (abbr.i) can be derived based on the data provided by the measurement system, and Eq.( 1)- (2).
Table 1.Measurable parameters of the airborne aerodynamic and electric power evaluation system Item Voltage Current Thrust Torque RPM Airspeed The aircraft is set to cruise in a steady state by utilizing a ground control station platform while the measurements are carried out.This assumption allows the establishment of a balance of forces in the direction of flight and the vertical direction, respectively.
Where the weight W is evaluated as a constant value owing to there being no fuel reduction for purely electric-powered aircraft.The thrust T of the propeller is obtained by the aerodynamic parameter test system.
The lift coefficient CL and drag coefficient CD are equal to the lift and drag force divided by the product of the corresponding airflow pressure and reference wing area respectively.The lift-to-drag ratio is calculated by dividing the CL by the CD, which is the same as dividing the lift equation by the drag equation.
Where  is air density, is the airspeed, and S the area of the wing.The ratio is perhaps the most significant consideration in designing a new UAV.And the higher the percentage, the better the aircraft will own aerodynamics.

Tested platform
To verify whether the aerodynamic parameters of the aircraft in flight state for the new proposed airborne aerodynamic parameters test system can be obtained, a self-development connected-wing airplane platform is employed.It has a mass of 12kg, a wingspan of 2.6m, and an area of 1.18 square meters.Detailed data that the aircraft's technical data & specifications are presented in Table 2.Because of the layout of the connected wing, the wing is divided into front and rear wings with the aspect ratio of 8.55 and 8.25 respectively.

Verification and analysis
To verify the validity of the numerical calculation results, the airborne performance test of the UAV is carried out.Figure 1 shows the physical image of the test prototype (the third-generation composite material prototype).At the same time, in order to test the thrust and aerodynamic parameters of the connected wing aircraft in real-time flight conditions, an airborne dynamic tension test system was installed at the concentric location of the fixed-wing motors.The assembly is completed as shown in Figure 2. In order to complete this experiment, the experimental design elements are as follows: 1) Before the flight test, the airborne power test system has been checked and the flight control system has been debugged; 2) The flight path is a fixed-point hovering with a radius of 300 m; 3) In the fixed-height linear cruise state of the aircraft, the thrust is collected by the tension sensor, and the lift of the aircraft is equal to the gravity of the whole aircraft.The other electrical signals and power system parameters: voltage, current, airspeed, speed, torque, and other parameters are also recorded by the data acquisition system; 4) The angle of attack of the aircraft is recorded by the flight control system of the UAV.
Table 3 shows the dynamic pull test conditions of the connected-wing UAV.For the fully assembled UAV, the take-off weight is 12 kg.According to the different angles of attack of the UAV, 4 sets of flight tests are performed, and each set of tests is repeated 5 times.The test site was selected in the yurt airspace of Binhai New Area, Tianjin. Figure 3 is an image of the flight test process.Figure 4 shows the aerodynamic curves of the UAV within the range of the angle of attack.The comparison results show that the lift and drag coefficients of the airborne experiment and the numerical calculation are in good agreement.Therefore, it can also be concluded that the airborne performance test experiment also possible replace the traditional wind tunnel experiment to complete the verification of the numerical solution.

Conclusions
The airborne dynamic tension test system proposed in this paper can conduct real-time online tests on the power system of electric UAVs and finally forms a complete test platform.Through the comparative analysis of numerical calculation and airborne test results, the following conclusions are obtained: A novel aerodynamic evaluation method system of the fixed-wing is put forward in this work, and flight evaluations are performed to investigate the reliability of the newly proposed airborne efficiency system.Airborne experiments were conducted to verify the numerical calculation, which is potentially promising for electric aircraft optimization.

Figure 1 .Figure 2 .
Figure 1.The physical picture of the whole vertical take-off and landing UAV

Figure 3 .
Figure 3. Snapshots of the aircraft during the test.

Figure 4 .
Figure 4.The aerodynamic performance of the connected wing aircraft at various AOA

Table 2 .
Physical specifications of the self-development connected-wing airplane

Table 3 .
Experimental conditions for airborne performance test of connected-wing UAV Takeoff weightTest times at a single angle of attack