Detail and structural design of a fixed-wing BWB UAV

The current study focuses on the development of a prototype BWB UAV for highway traffic monitoring by supporting a Cooperative Intelligent Transport System (C-ITS). This system allows the monitoring of traffic conditions at large roads and highways. Having determined the mission requirements and concluded the aerodynamic conceptual and preliminary design phases, high fidelity CFD simulations are performed, aiming to calculate the key aerodynamic and stability characteristics of the platform and to optimize its performance throughout the mission. More specifically, regarding the aerodynamic vehicle design, results concerning the calculation of stability derivatives, control surfaces sizing, trim analysis and flight envelope (V-n diagram) are presented, along with the respective methodologies. Considering the structural design of the aircraft, a combination of layout, FE simulations and parameterized design tools were employed, allowing the design and sizing of the skin and the internal structural parts. The parts are mainly made of composite and additively manufactured nylon materials. Coupled interaction loops are conducted among the aerodynamic and structural analyses to optimize the overall performance of the aerial vehicle, maximizing the aerodynamic efficiency, and reducing the structural weight. Finally, the study is concluded by the presentation of the manufactured prototype of the UAV, which satisfies all the structural, aerodynamic, stability and performance requirements for the established highway traffic monitoring mission.


Introduction
Intelligent Transport Systems (ITS) are advanced applications of information technology and telecommunications primarily in road transportation.Their goal is to improve the mobility of passengers and goods by providing traffic management services, user information, and the detection of malfunctions and accidents on the road network.Identifying the transportation vehicle volumes of traffic is essential for an intelligent transportation system (ITS) and traffic analyses.The measurement of transportation volume is also one of the basic functions of traffic management and highway monitoring.Various types of sensing systems are employed, including road-embedded pressure sensors, optical sensors, and cameras.Cooperative Intelligent Transport Systems (C-ITS) are advanced applications of information technology and telecommunications primarily used in road transportation.C-ITS improves the mobility of passengers and merchandise by providing traffic management services, user information, and the detection of malfunctions and accidents on the road network, allowing for a safer, more efficient, and coordinated use of the road network while reducing energy consumption and atmospheric pollution.Considering the complexity of the traffic analysis, the integration of Unmanned Aerial Vehicles (UAVs) equipped with a C-ITS is judged as a suitable choice for this mission [1].
Unmanned Aerial Vehicles (UAVs) are of increased interest in many purposes, which has led to the development of innovative UAVs that can perform various operations [2].They can be equipped with multiple sensors to collect and transmit data in real time.Due to the demand for UAVs in various purposes, several types have been developed [3].The UAV presented in the current study is based on the Blended-Wing-Body (BWB) configuration layout, where the wings are smoothly integrated into the center body (fuselage) of the vehicle.It features a larger internal volume to wetter area and a higher aerodynamic efficiency, when compared to a conventional tube-and-wing configuration, resulting in increased flight time and payload capacity [4].All the above make the BWB configuration a suitable candidate for the development of a C-ITS UAV.
The detailed design is conducted considering the use of Carbon Fiber Reinforced Polymers (CFRPs) and Additively Manufactured (AM) materials.CFRPs are lightweight laminated structures that offer high strength and rigidity.It should be noted that their mechanical properties, including the failure criteria, are direction dependent [5].The aerospace sector, which seeks to be at the forefront of technology, is gradually implementing the AM method to its products.Parts fabricated by additive manufacturing offer greater weight reduction due to fewer manufacturing constraints compared to the conventional methods.Considering the UAV assembly, parts manufactured from all described materials, are connected via adhesives, and the sizing of the structure takes place using dedicated Finite Element (FE) tools.
This study presents the structural design procedures of a small-scale Unmanned Aerial Vehicle (UAV), designed to support a Cooperative-Intelligent Transport System (C-ITS) incorporating BWB configuration.A combination of additively manufactured parts and CFRPs is used to achieve lightweight structure.Results show that the structure can withstand all loads acting during the flight mission.

Design methods and tools
The overall design workflow of the project is presented in Figure 1.First, the mission requirements are set and described in detail.Next, the aerodynamic and structural teams are working concurrently, with all three major design stages depicted for each team separately.However, it should be noted, that even though the teams are presented as two individual teams, cooperation and feedback during all design phases is mandatory to increase the design efficiency.Following the structural detailed design, the manufacturing of the prototype vehicle takes place.Finally, the design concludes with the flight test, where all aspects of the design are evaluated (structural integrity, aerodynamic performance, and stability etc.).In the scope of this work, the structural design is presented in detail, along with the tools used in the sizing of the structure.

Mission profile & vehicle characteristics
The mission profile of the vehicle is consisted of seven sections starting with take-off, climb to the design cruising altitude, cruise to the area of interest, loiter and carry out the main part of the mission, cruise back to the base, climb down to the base and finally land.Detailed information about the mission profile is presented in [6].The final parameters considered for the structural design are presented in Table 1.

FE analysis
In all design stages, the sizing of the structural elements is carried out using Finite Element simulations, employing the ANSA [7] and META [8] pre-and post-processor along with ABAQUS [9] solver.Due to symmetry, one half of the vehicle is simulated, and respective symmetry boundary conditions are applied, conserving solution time.The mesh should be fine enough to describe the geometry with accuracy and account for the overall deformation.In terms of simplicity, the mesh size is chosen to be uniform, 5mm length.Surfaces that are going to be glued together using adhesives, are simulated with solid elements for the adhesive (red elements), and Multi Point Constraints (MPCs) (Figure 2) to connect the adhesive with the surfaces due to mesh mismatch.Next, since the masses contained into the aircraft affect the overall structural performance, they are imported in the model using point masses (magenta spheres) and connected with the structure using MPCs.The load cases considered, are fully described by three flight segments and presented in the Table 2.The aerodynamic pressure distribution is extracted from the respective Computational Fluid Dynamics (CFD) simulations conducted for each load case.For each load case, the aerodynamic pressure distribution is applied to the external surfaces in combination with acceleration loads due to maneuvering.The total number of elements is around 1.5 million, including 700k shell and 800k solid elements.The overall time required for solution is 2 hours.

Conceptual & preliminary design
In the preliminary design stage, the materials that are going to be used are determined.In Table 3 the chosen materials and their mechanical properties are presented in detail.The use of carbon fiber reinforced polymers (CFRPs) is extensive on the vehicle.The external skin is completely made of from CFRPs to be as lightweight as possible.Due to the size of the UAV, some of the main structural elements are too small to be manufactured by CFRPs.In these cases, polyamide (PA12) is chosen, which is fabricated using additive manufacturing.The epoxy adhesive is used as a connection element, not only between CFRP elements, but also between the CFRPs and the additive manufactured elements.
Due to packaging limitations, the vehicle will be separated into three different parts, the wings, the winglets, and the main body.Even if the separation of the winglets from the wings introduces an additional weight penalty, it is chosen as a valid option because high maintainability and replacement of potential damaged parts during service life are required.There will be two spars on the vehicle: one for the main body-wing connection and one for the wing-winglet connection.In this vehicle scale, the main aerodynamic and inertial loads acting on the wings will be received from the external skin.Consequently, for each side of the connecting parts, there are going to be two ribs to maintain structural stability, except for the winglet, where there is going to be one wider rib. Figure 3 presents the main structural elements of the vehicle, and they are color coded based on material chosen.The ribs are represented using bulk volumes in this stage; the detailed design will be presented in the next sections.After the determination of the number and the position of the key structural elements, an initial estimation of the position of the Center of Gravity (CoG) and the Moments of Inertia (MoI) for the vehicle takes place using the FE method which offers a high accurate weight estimation.During the internal layout assessment, the parts have been positioned, and their weight and inertia characteristics have been described in detail from datasheets provided by each manufacturer.The combination of the structural and internal layout results in an initial estimation of the vehicle's CoG and MoI.[6]

Detailed design
Using the water-tight surface as an input, two major changes are made considering manufacturing and assembling constrains.At first, the trailing edge of vehicle is trimmed, and a flange is created to glue the upper and lower surfaces of the wing's skin.Next, the leading edge is modified to create a region where the upper and lower skin surfaces will be overlapped and bonded using adhesives.The complexity of the AM parts is so high that they can only be manufactured by using 3D printing.From previous experience, it was shown that the bonding between parts that are manufactured using AM and CFRPs is low.To increase the adhesion bonding between the parts, holes are introduced to the ribs, as shown in Figure 4 where the upper skin of all sub-parts has been removed.
It should be noted that some of the ribs include the support of the servo actuators required for the control surfaces, transforming these ribs into multi-purposed structural elements.

Results
Firstly, for all designed CFRP components, the failure index (Max stress) was used as the criterion of strength.A failure index of 2 describes a structure which undertakes double the loading that it can withstand.For this reason, a maximum value of 0.8 was set as an upper limit for all design stages.require robust sizing tools using the Finite element method.Design loops between the preliminary and detailed stages resulted in weight loss higher than 20% in some cases.The main goals of the project, including the successful manufacturing and flight tests, were accomplished.During the flight tests, all the initial requirements were satisfied and also the stability and flight capability of the UAV was accredited.As a future step, a test flight with the fully equipped UAV, including the C-ITS and a gimbal, will be conducted at first, and the final step is an autonomous flight above a Greek national road.

Figure 4 :
Figure 4: Detailed design of the UAV.

Table 1 .
Vehicle characteristics

Table 2 .
Load cases used in the FE sizing procedure.
Load case description Angle of Attack (°) Load factor (g) Velocity (km/h)

Table 3 .
List of used materials.