A case study on the modeling and simulation of UAVs

Abstract

Keywords: Flow Analysis; Structural Analysis; Unmanned Aerial Vehicles; Fluent; Design; ANSYS

Introduction

Because of technological advancements in artificial intelligence in recent years, they have enabled unmanned systems to be included in a broad range of application fields, gaining prominence in the defense industry and aviation in particular. The unmanned aerial systems, which

have brought a new dimension to the defense industry sectors of the countries, have caused a revolutionary transformation on the battlefield and in the fight against terrorism. UAVs (unmanned aerial vehicles) are kept at the forefront as unmanned systems in the defense industry. These vehicles, which contain many different models for a wide variety of

*Corresponding author. *E-mail address: dalkilic@yildiz.edu.tr This paper was recommended for publication in revised form by Regional Editor Alibakhsh Kasaeian Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

missions, remove the duty limitations imposed by the human role and play a revolutionary role in wars. In the current study, the structural and mechanical design, structure configurations, flow, and structural analysis, propulsion and firing systems, and prototype production of the UAVs, as well as a design that can fulfill the determined missions and features in the best way. The UAV model utilized in computational fluid dynamic (CFD) and mechanical analysis is demonstrated in Figure 1. The main parts of the aircraft such as the wing, tail, body, and landing gear are examined in detail. Information about the design details, analysis methods, and other elements of these sections is presented. In addition to these, a firing system design is developed for the aircraft to perform various missions during flight, and it is controlled over MATLAB. An aircraft model on the computer starts to be produced in design. Simulations are carried out by using flow and mechanical modeling with the ANSYS Fluent and ANSYS Mechanical programs, respectively. It is an aircraft that does not have a pilot and a passenger in unmanned aerial vehicles (UAV), only carries equipment for the purpose (camera, sensors, etc.), has a remote control or automatic control, and performs its mission [1]. In the UAV Systems Road Map document made by the Turkish Undersecretariat of Defense Industry [2]. the roles of UAV systems are broken down into five groups: Attack, Target Simulation, Electronic Warfare, Reconnaissance/Surveillance Support, and Special/Specific Missions. There are different classifications of military-type UAVs. Military UAV classification can be in more than one class [3]. Raymer [4] studied aircraft design with a conceptual approach that was widely discussed for aircraft. Koc et al. [5] also presented the study as an aerodynamic analysis and aero package optimization for trucks. Bredberg [6] investigated turbulence models for the wall boundary condition. Sforza [7] researched wing design according to commercial airplane design principles. Johnson [8] researched wing loading, icing ansd associated aspects of modern trans-port design. Banal and Ubando [9] investigated the fuzzy programming approach to UAV preliminary sizing in the study. Sforza also [10] emphasized the manned hyper-sonic missions in the atmosphere for their study. Sforza [11]

determined the wing’s properties from the engine selection of UAV design. Sirohi [12] studied bioinspired and biomimetic micro flyers for unmanned aerial vehicles. Sforza [13] obtained the landing gear design for the commercial airplane.. Coban and Oktay [14] conducted tactical unmanned aerial vehicles, according to the literature. Utilizing this study, the physical properties and the autopilot system structure as well as the optimization method, Fatma et al. [15] revealed some of the latest developments in the application of UAVs in three main areas: transport, road safety, traffic moni-toring, and road infrastructure management. Dündar et al. [16] stated that their aim in their work includes the design stages that include the energy consumption of a fixed-wing (FW) vertical take-off and landing (VTOL) unmanned aerial vehicle (UAV), as well as performance analysis. Alzahrani et al. [17] highlighted existing UAV-supported research, such as routing, cellular communications, data collection, and Internet of Things (IoT) networks, as well as disaster management that supports existing facilitating technologies in the study. Deng et al. [18] drew attention to the impact on the ground, which is important for the UAV’s take-off and landing modes during flight, with experiments for both hover and forward flight configurations on the UAV. Tan and Katrancı [19] investigated the effects of unmanned aerial vehicles (UAV) on the battlefield by making determinations about the use of more and widening their usage areas in the coming years. According to the literature, the study determined the effects of unmanned aerial vehicles (UAVs) on the battlefield. A UAV concept design in the mini class with the gun (to shoot) is applied within the electric motor, fixed-wing, and automatic and remote-controlled, capable of reconnaissance, taking off from the runway, surveillance, and attack with this work. Unmanned combat aerial vehicles (UCAV) are systems that contain weapons as a payload and can carry heavy ammunition such as missile systems as well as light ammunition such as hand grenades and light machine guns, according to the class of UCAV. Apart from the fact that these systems are designed more carefully than unarmed UAVs, the weapon systems cause effects that disrupt the movement in the air, apart from creating an additional load for the UAV. Therefore, necessary analyses should be made for ammunition. Mini UCAVs are light unmanned aerial vehicles weighing 2-20 kg, and although they are generally used for reconnaissance and observation as UAVs, they are armed for simpler armed missions. These UCAVs have two different classes fixed-wing and rotary-wing. Rotary-wing ones can use weapons such as machine guns and bombs, while fixedwing-type ones are used only for bomb-throwing. These unmanned aerial vehicles, produced for reconnaissance and observation purposes, are armed with additional equipment and sent on some missions, but their main purpose is not armed use. Preliminary design parameters should be deter-mined to produce of a fixed-wing UCAV. In this study, conceptual design is created by paying close attention to the factors to be considered in UAV design. Mechanical and flow analyses have been employed to

determine the dimensions of the unmanned aerial vehicle's wings, tail, body, and landing gear. The aerodynamic behavior of the related UAV design is evaluated at various altitudes. In addition, mechanical analyses of the UAV's fuselage-engine and landing gearfuselage connections are performed. As a consequence of the analyses, the detailed design of the mini unmanned aerial vehicle has been completed as part of the current research.

Theory

Aerodynamics is the science that studies the forces exerted by the air on an object. Mass effects of air occur because of the movement of air or objects on the object. In inflight aerodynamics, the air affecting the aircraft during flight is examined, and calculations and analyses are made. Air vehi-cles are exposed to more aerodynamic effects than land and sea vehicles, and many forces occur that affect the movement of the aircraft. The aerodynamic forces that UAVs are exposed to are divided into four.categories These are lifting, dragging, and thrust forces [4]. carrying, Aerodynamic forces and moments, Mach, and Reynolds numbers can be determined for UAV design. drag force with transport and the resulting moment value can be calculated with the defined equations [4]. Lift Force (FL), Drag Force (FD), and Moment (M) equations are presented from Eq. (1) to Eq. (3) respectively ,as follows: (1) (2) (3) Aerodynamic coefficients depend on the angle of attack, geometry (profile geometry, wing geometry, aircraft configuration, wet area, etc.), Reynolds number, and Mach number In this study, the coefficients were calculated for wing and tail selection. The Reynolds number is a dimensionless number defined as the ratio of the inertial forces a of fluid to the viscosity forces. The value of the Reynolds number allows comment on the flow regime. To determine the Reynolds number for the wing, it is necessary to calculate the coefficients depending on the flow formed on the surface. The calculation of the Reynolds number depends on the fluid’s velocity, density, hydraulic diameter, and dynamic (or kinematic) viscosity [20]. In this study, coefficients were obtained by performing analyses depending on the Reynolds number (Re) that is given in Eq. (4) as follows:

The Mach number (Ma) is defined as the ratio of the velocity of a moving mass to the velocity of sound in its environ-ment. The flight is named according to the value of the Ma. If the Ma is less than 1 subsonic and greater than 1, it is considered supersonic velocity [6]. Since the design UCAV flies at an altitude of 1000 meters at a velocity of 15 m/s, the Ma is determined to be 0.045, taking the ratio of sound and UCAV velocity. This value is used in the calculations for the UCAV, which would fly at subsonic velocities since the Ma is calculated to be much smaller than 1.

Results And Discussion

The flow analysis and boundary conditions are specified in this section. The design of the unmanned aerial vehicle is completed by determining its aerodynamic configurations. Due to the complexity of the design, its geometry is simpli-fied by preparing for flow analysis. Flow analyses are car-ried out in the ANSYS Fluent program. The flow analyses are compared with the CFD analysis results with the specified configuration. Thus, it could be determined whether the system will work correctly or not. The flow lines that may cause lift, drag force, and turbu-lence were researched using CFD analysis. By checking whether the lift and drag forces are sufficient, it is examined whether the lifting force could meet the vehicle’s weight at cruising velocity. Drag force is kept to a minimum at specified velocities. In the CFD analysis, the aerodynamic forces of the UAV at different heights and temperatures are determined by testing the UAV at different velocities as well as flow analysis at a different densities. In the CFD analysis, the number and quality of mesh elements are also clarified by referring to the accuracy analysis of the solution mesh. The mesh independence analysis is performed with different mesh element numbers. Apart from the optimum number of elements determined independently of the mesh, accurate results are achieved in the fastest and simplest way with mesh quality.

(4) The Reynolds number and values are given as follows; Air density (ρ) 225 kg/m3, velocity (V) 15 m/s, average center length (c) 0.262 m, dynamic viscosity (μ) 0.0000179 kg/(m.s), and Re 269,092.27 were employed in the calculations.

Figure 2. The boundary conditions and fluid zones for CFD analysis.

Table 1. The lift and drag forces of the UAVs parameters Lift force

Figure 3. Mesh structure (a) General view, (b) Inner zone detail, (c) Mesh structure on UAV’s wall detail. 35

Figure 2 illustrates boundary conditions in the fluid domain. While the inlet boundary condition is constant velocity, the outlet boundary condition is atmospheric pressure. To facilitate the number of meshes and resolu-tion, the flow dominated approach s taken by considering half of the UAV. The symmetry boundary condition is applied to the relevant surfaces. A new zone has been created to better examine the flow in regions close to the UAV within the streaming domain. Meshes are engaged in this zone with smaller meshes than in the outer fluid zone. The wall boundary condition is defined for UAV surfaces and other surfaces. While the distance between the input and UAV is as 5 of the chord length, the distance between the output and the UAV is 10 times the chord length. Similarly, the dis-tance between the other walls and the UAV is taken 5 times the chord length. The surface area obtained because of the design affects the lift and drag forces of the drone. Some parameters affecting the lift force and drag force are pre-sented in Table 1. To facilitate the solution, the mesh is created with the sweep method in uncomplicated geometries, as shown in Figure 3 [5]. Although hexagonal mesh is tried to be created in sections where complex geometries occur, tetragonal mesh structure is applied as an alternative and is shown in Figure 3b). This situation causes the number of elements to increase and the analysis to be prolonged. In the boundary layer behaviors, inflation layers in subsonic aerodynamic flow analysis are applied (Figure 3c). In CFD analysis, the solution mesh is of great impor-tance regarding the accuracy of the analysis. Thanks to the optimum number of elements determined independently of the mesh, it has provided accurate results in the fastest and simplest way with mesh quality [21]. With this parametric study, a solution mesh with the least number of elements, which can give the most accurate result, was determined by applying foue different analyses in the range of 2.9-12 mil-lion. It is observed to converge in the range of 6-12 million mesh elements. The analysis with 6,031,055 elements is determined as the optimum solution mesh. FL and FD changes are demonstrated according to the number of meshes created for mesh independence analysis in Figures 4 and 5, respectively.

Figure 4. Number of elements-lift force curve (Independency of mesh).

Figure 5. Number of elements-drag force curve (Independency of mesh).

Method

Aspect ratio (Average-maximum) 4.3072-781.71 Orthogonal quality (Average)

The mesh properties used in the analysis are listed in Table 2. In describing the mesh size for a particular flow pattern, the dimensionless value y+ is commonly employed. In the ANSYS Fluent Setup section, k-ω SST is chosen as the flow turbulence model. In determining the appropriate

size of the cells near the field walls in turbulence modeling, the turbulence model laws have restrictions on the y+ value in the wall [6]. In Figure 6, it is tried to keep the y+ value below 2 to increase the accuracy of the analysis in the k-ω SST model. The number of inflation layers is determined as 15, the growth ratio is 1.2, and the first-layer thickness is

45. N. Thus, a force of 10 N on the front landing gear and

20 N on the rear landing gear is affected. The mesh structure formed in the body material, the number of nodes and elements, the displacement, and the safety coefficient in the body are presented in Figure 14. The knot number of the mesh structure is 256,267, the number of elements is 158,184; the maximum displacement in the body is 1.266 mm; and the material safety factor is 0.495. In Figure 15, the tail and wing areas of the unmanned aerial vehicle are produced by the hot wire cutting method. XPS Styrofoam foam material is used in wings and tail material. Photographs of the design are presented in Figure 15a. A hot wire cutting machine to producte wing and tail structures and a wire machine, as presented in the

figure, are designed and produced to give profile shapes. The power source of the wire-cutting machine is 12 V, and its current rating is 10 A. In Figure 15b, the wing profile, is determined as the tail profile as NACA 0012. These profiles are glued to wood for wing production and then cut with laser cutting, and the geometry of the profile is transferred to the wood to be used as an auxiliary element in produc-tion. Wing and tail geometries made of XPS Styrofoam are designed in Figure 15b. The test has been performed under various loads on the propulsion engine within the scope of the study. The supplier of the electric motor using a brushless electric motor published Sunnysky as thrust provider X3530. A 12x8propeller thrust with a test 6S battery (22V) is to be produced at approximately the maximum thrust force of 4240 g. According to the results of the SunnySky X3530 brushless electric motor impulse test, the voltage, power, current, impulse, and efficiency are determined in Table 4. With the operation of the brushless electric motor, the impulse value that affected the sensitive balance is presented in Table 4. The effect of the assembly on the precision balance with the engine not running is

Table 4. The catalog data of the SunnySky X3530 brushless electric motor impulse test, the voltage, power, current, impulse, and efficiency Prop [inch] Voltage [V] Current [A] Thrust [g]

Efficiency [g/W] Revolution [rpm] Load temperature in 100% throttle

measured as a mass of 594 g. The electric motor is operated with various currents and compared with the data provided by the supplier. While the supplied electric motor, which has a mass of 4240 g, gives the amount of thrust produced at full power, it is noticed that the measured value in the precision balance is 4200 g. The thrust test setup is as shown in Figure 15c. It is thought that the difference might be due to friction in the installed setup. The thrust value remains within acceptable limits, and the test is successfully completed.

Conclusion

According to the detailed design and analysis of the mini-UCAV, the wing, tail, body, landing gear, shooting systems, and engine have been examined. Parameters are deter-mined and the decision method is mentioned. These values are supported by analysis by paying attention to the values found and suggested in the literature. Since the detailed design throughout the study includes a system design, it is determined that the information and values in each section affect other designs. This work required optimization work. Therefore, it led to recalculation and analysis. • The wng design is the first UCAV section to be examined. While designing and analyzing the wing, it is started by making certain acceptances as it is the first design part. These assumptions are changed later in the study, and optimization is achieved again. • While designing and analyzing the tail, it was determined that the tail design was based on the wing design. It ihas been noticed that the wing design and center of gravity directly affect the tail design. The design is completed by examining the important design requirements men-tioned in the literature for a tail design. • In the body design, the most suitable aerodynamic shape is made by taking manufacturability into consideration. In the landing gear design, control and analysis studies were carried out to ensure that the aircraft could move safely.

This study aimed to carry out an appropriate aerodynamic and structural design to perform the current gun release task in the design of the UCAV’s firing system. For the propulsion system, the most suitable propulsion system is determined with the help of the values obtained according to the analyses. The final design was obtained by combining each of the examined UCAV sections, and the detailed design emerged. According to the detailed design determined, wing and tail production is started depending on the analyses’ results. The load test of the produced wing is carried out and the thrust value produced by the engine is measured. These results are found to be compatible with the literature. The flight is successfully completed after all production processes for the UAV.

Nomenclature

Average center length, m Coefficient Force, N Gravity acceleration, m/s2 Moment, N.m Reynolds number Area, m2 Temperature, K Air velocity, m/s

Greek symbols µ Dynamic viscosity, kg/m.s ρ Air density, m3/kg Subscripts D Drag Lift L ∞ Free stream Acronyms UAV Unmanned Aerial Vehicle UCAV Unmanned combat aerial vehicle

Computational Fluid Dynamic Fixed-Wing Internet of Things Reynolds-averaged Navier-Stokes Extruded Polystyrene Vertical Take-off and Landing

Acknowledgment

This study was carried out as the flow analysis of the unmanned aerial vehicle, which was supported by the 2020 TUBITAK (The Scientific and Technological Research Council of Turkey) Unmanned Aerial Vehicle Competition (Project Funding Number 548110 and Date 15-20 September 2020) and was based on the altitude of Gaziantep.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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