Topology optimization and 3D printing of a unibody quadcopter airframe

The performance of any unmanned aerial vehicle largely depends upon its weight as this characteristic dictates its payload capacity and flight duration. This paper presents a multiphysics simulation method for designing a unibody quadcopter airframe with optimum weight in SolidWorks. By adopting the computer-aided topology optimization concept, the mass of the frame is decreased by 91% (from 1558.44 to 134.738 grams) after getting rid of the unwanted elements, while not compromising its structural rigidity. The efficacy of the model is assessed by finite element analysis and it is found that the stress would remain within the acceptable limit. The optimized structure shows an operating life of 1.6×105 cycles in the fatigue analysis and experiences significantly less drag force in the computational fluid dynamics test than the original airframe for several angles of attack, thus proving the superiority of the optimized frame over the initial model. Finally, additive manufacturing is performed to fabricate the optimized structure using PLA material. 3D printing through the fused deposition modeling technique is chosen since it is ideal for fabricating intricate engineering prototypes compared to orthodox manufacturing processes. While creating the frame, a 45° overhang angle is maintained to ensure the usage of less support material.


Introduction
Among the various types of multirotor unmanned aerial vehicles (UAV), the quadcopter is extensively used for surveillance, search and rescue purposes, parcel delivery, and numerous other applications due to its hovering capabilities, high mobility, and vertical take-off and landing potential in tight areas.The main objective of a quadcopter structure design should be to restrict overall weight while maintaining adequate structural integrity [1].Hence, a trade-off between weight and strength is essential to provide design optimization.A robust and lightweight UAV airframe allows for an increase in flight time while maintaining the carrying capacity.Therefore, structural weight optimization is of great significance to boost flight time and payload capacity.Topology optimization is the logical tool to employ in this regard.Leon et al. [2] illustrated topology optimization methodology for a quadcopter's central airframe structure.The work included material selection, structural-mechanical optimization, and an analytical calculation of the carrying capacity of the optimized frame.Nvss et al. [1] presented a quadcopter frame weight reduction method involving topology optimization and additive manufacturing through fused filament fabrication.The study re-engineered the frame as a monocoque structure and validated the outcome by modal analysis and fluid flow testing.Rayed et al. [3] studied the finite element analysis 1305 (2024) 012021 IOP Publishing doi:10.1088/1757-899X/1305/1/012021 2 (FEA) of a UAV structure and performed topology optimization to reduce its mass without compromising the structural strength.Conventional manufacturing techniques such as lay-up composite processes and computer numerical control (CNC) have been used to manufacture UAV parts [4], which require arduous efforts and are tedious.Moreover, these are process-oriented, thus restricting their ability to handle intricate designs, while decreasing the weight of components that can be effectively acquired by additive manufacturing.Adaptability is one of the benefits of additively manufactured quadcopters as these structures can be customized according to their usage [5].Fused deposition modeling (FDM), one of the 3D printing techniques, is widely used in several engineering applications owing to its availability, good workability, and low cost.Venegas et al. [6] developed an additively manufactured drone for agricultural purposes with the time saving of the proposed drone technology being a minimum of 74.72%.Brischetto and Torre [7] adopted the fused filament fabrication approach to additively manufacture a modular drone and conducted flight simulations and preliminary FEA.Furthermore, computational fluid dynamics (CFD) analysis has been carried out for understanding the flow pattern, and lift and drag force generation of UAV quadcopters [8].The quadcopter airframe constitutes the bulk of the weight of the whole structure.The primary focus of the present research work is to suggest a computer-aided design methodology in SolidWorks using multiphysics simulation for developing a unibody quadcopter airframe with a focus on keeping its weight as scarce as possible.The topology optimization technique is adopted to decrease the mass of the initially modeled frame while maintaining its structural stiffness.The optimized model is validated through FEA, ensuring whether the values of stress and deformation are within the allowable limits.Also, fatigue life analysis and fluid flow simulations are performed to prove the success of the optimized structure compared to the original airframe.Topology optimization typically results in uneven and complex shapes.It is a cumbersome task to manufacture those parts with conventional fabricating methods.As a result, the proposed work attempts to utilize the concept of 3D printing, which is a form of additive manufacturing, in producing the quadcopter airframe.

Design and Topology Optimization of Quadcopter Airframe
Several failure modes need to be considered while selecting the material for designing a drone frame such as elastic deformation, buckling, yielding, fatigue, and fracture [2].Polylactic acid (PLA), which is a thermoplastic material, is chosen for the construction and finite element analysis (FEA) of the UAV airframe.The necessary material properties to define PLA are presented in Table 1.

Design of quadcopter airframe structure
The quadcopter frame design is conducted using the commercially available software, SolidWorks.All the geometric characteristics and dimensions are taken into account while designing with a focus on decreasing the weight and increasing the strength of the UAV frame.The geometry of the airframe is symmetrical with the dimensions of 200 mm × 200 mm × 40 mm.A central slot is provided for the bottom plate to be used for landing and accommodating electronic devices.Also, the frame has four slots for the installation of motors and propellers, which is illustrated in Figure 1.The mass of the frame is 1.55844 kg.The model is partitioned into the design and the non-design spaces [9].Design spaces are areas required to be optimized, while non-design spaces are the regions that are not to be modified.Only the elements that are subjected to loading and boundary constraints (four propeller slots and the bottom plate) are considered non-design spaces.All the other elements are regarded as design spaces.Figure 2 represents the non-design space areas that need to be preserved after the optimization process is complete.A thrust force of 15 N is applied to the four propeller slots and a fixed boundary condition is applied to the central slot, as shown in Figure 3. Quarter symmetry is assigned so that the optimized structure remains as similar as possible on the four sides after the FEA.Fine mesh is assigned to the geometry for better optimization results.

Topology optimization
FEA of the drone frame is followed by topology optimization to find the optimal material distribution by removing the unwanted material, allowing it to find the optimum weight with enough strength to withstand the loading conditions.This process reduces the weight and develops the drone's structural efficiency and aerodynamic performance, thus helping to achieve a higher thrust-to-weight ratio.As the permissible (yield) stress of PLA is 38 MPa, the stress on the airframe structure should be limited to 3.8 MPa (considering 10 as the factor of safety).The computer-aided topology optimization problem is formulated as shown in Table 2. SolidWorks topology optimization tool was used to obtain an approximation of the optimal design of the quadcopter frame based on the picked preserved regions on the model, which is presented in Figure 5 (a).The optimized airframe weighs 116.84 grams.
Table 2. Topology optimization problem definition.

Objective function Minimization of mass and maximization of stiffness
Constraints Airframe mass: ≤ 1.55844 kg von Mises stress: ≤ 3.8 MPa

Final optimized quadcopter frame design
The optimized structure given in Figure 5 (a) is rough and organic.As a result, the design is modified, as shown in Figure 5

Results and Discussion
The main characteristics of the original frame and the final optimized model are compared in Table 3.
The mass of the optimized structure is decreased by about 91% and the specific strength is enhanced notably as well.Also, to validate the optimized frame, an FEA has been performed to ensure that the stress is within allowable limits.Static structural analysis is performed by considering the same load and boundary constraints applied to the initial airframe.Table 4 shows the comparison of mass reduction of the quadcopter frame after performing topology optimization.

Validation of the optimized airframe through static structural finite element analysis
The results represented in Table 3 reveal a maximum deformation of 5.005×10 -2 mm and a maximum von Mises stress of 1.253 MPa encountered in the optimized frame having a minimum factor of safety equivalent to 30.The von Mises stress is below the acceptable stress limit, thus rendering the success of the optimized model, and fulfilling the objectives and constraints given in Table 2.The stress and deformation of the re-designed drone frame are given in Figures 6 (a

Fatigue Life Analysis
UAV structures undergo cyclic loads because of the propeller rotations which are transmitted to the unibody airframe.Fatigue life analysis is carried out to identify the number of cycles a structure can

Current work Nvss et al. [1] Rayed et al. [3]
Mass of original airframe experience before it fails.Therefore, to understand the structural integrity of the optimized quadcopter airframe under cyclic loads, FEA is performed using the SolidWorks platform to analyze its operating life.The S-N curve is generated by using the material properties given in Table 1.A high-cycle fatigue test is performed for 1×10 7 cycles to determine the fatigue life of the quadcopter structure.The results show that the airframe can withstand 1.6×10 5 cycles under cyclic loading, which represents the operating life of the model, as demonstrated in Figure 7.

CFD analysis of the quadcopter airframes in case of forward flight conditions
SolidWorks is used to study the fluid dynamics behavior of the original and optimized airframes.A computational domain is set up for conducting computational fluid dynamics (CFD) analysis in a condition where the drone moves against the wind, i.e., the forward motion.This is simulated in SolidWorks by copying the wind tunnel environment where the frame structure is fixed and the wind is blown at a specific speed.CFD analysis is carried out for both the original and the optimized frame by varying angles of attack (AoA) at 2000 mm/sec airspeed.The original quadcopter airframe generates much more drag force compared to the optimized airframe, thus showing an improvement in the optimized design structure, as demonstrated in Table 5. Fluid flow simulation for 3º AoA is shown in Figures 8 (a) and 8 (b) for demonstration purposes.

Additive manufacturing of the optimized quadcopter airframe
Additive manufacturing (AM) is a computer-controlled process that creates 3D objects by adding materials, and when the fabrication is specifically carried out by building layers of materials, it is known as 3D printing.The optimized quadcopter airframe is fabricated using continuous filament of PLA on a 3D printer adopting the fused deposition modeling printing technique.FDM is one of the most widely used layer-by-layer 3D printing processes for creating common engineering prototypes.Utmost importance is given to decreasing the amount of support material needed because this is directly related to the production cost and time.It took approximately 15 hours to 3D print the optimized airframe.The SolidWorks model of the drone frame is sliced using the Simplify3D software.45º overhang angle is maintained during the printing process to ensure that each successive layer has enough support on it, allowing to save a substantial amount of support material.The specifications of the 3D printer (Anycubic Mega X) are given in Table 6.Also, the printing parameters maintained for 3D printing of the optimized unibody airframe are represented in Table 7.

Conclusions
In this study, a unibody quadcopter airframe is designed and its topology is optimized for additive manufacturing purposes by implementing the knowledge of multiphysics simulation.The FEA results validate that the stress of the optimized structure is within acceptable limits, and the factor of safety is found to be 30.During the computer-aided optimization process, the mechanical strength of the quadcopter airframe is not compromised.• Fatigue life analysis shows that the optimized airframe would have an operating life of 1.6×10 5  cycles.On the other hand, CFD analysis proves the efficiency of the optimized model by revealing that the structure would experience less drag force than the original airframe for the corresponding angles of attack.• The final optimized drone frame is additively manufactured through fused deposition modeling (FDM).Polylactic acid (PLA) is selected for the 3D printing of the drone structure.It is made sure that as little support material as possible is used while fabricating the airframe by maintaining an overhang angle of 45º.Overall, the study contains a methodology necessary for fabricating a quadcopter airframe prototype with the 3D printing technique, which is more convenient than regular manufacturing processes.

Figure 2 .
Figure 2. Non-design spaces/preserved regions (all the other elements are design spaces).

4 Figure 3 .Figure 4 .
Figure 3. Applied loads and boundary constraints in the original airframe.Static structural analysis is carried out, and the results, as presented in Figures 4 (a) and 4 (b), show a maximum von Mises stress and deformation of 9.403×10 4 N/m 2 and 4.705×10 -3 mm respectively.

Figure 6 .
Figure 6.FEA results of the optimized frame: (a) Von Mises stress state, (b) Deformation state.

Figure 7 .
Figure 7. Fatigue life analysis of the optimized airframe.

Figure 9 (Figure 9 .
Figure 9 (a) shows the support generation for the model sliced in Simplify3D with a 45º overhang angle.Finally, the 3D-printed quadcopter airframe structure is shown in Figure 9 (b).

Table 3 .
Comparison between the original and optimized airframe.

Table 4 .
Comparison of mass reduction after topology optimization.

Table 5 .
Drag force of the quadcopter airframes for various AoA during forward flight.
The symmetrical UAV airframe is designed in the SolidWorks platform.Design and non-design spaces are defined to optimize the frame's topology after thrust loads and boundary constraints are applied.The mass of the optimized frame model is reduced by 91% compared to the initial model.
The following points have been deduced based on the results and observations of this study: 10 •