## 1 Introduction  
With the development of the world economy, traffic pressure has led to time being wasted and economic loss, as well as serious environmental problems. The development of three-dimensional urban transportation is becoming an important solution to these problems, which are a concern in all countries. NASA and other institutions have put forward the concept of electric vertical take-off and landing aircraft (eVTOL) [1]. eVTOL combines the advantages of both rotorcraft and fixed-wing aircraft, in which the arms of the rotor are set to a certain airfoil shape, and at the same time the power unit is placed at the wing tip. Meanwhile, it also has the vertical take-off and landing function of rotorcraft and uses the wings to generate lift in the horizontal flying stage [2, 3]. Design optimization of the lightweight wing structure of aircraft and wing structure strength involving structure performance and material distribution are the key to improving the vertical take-off thrust-to-weight ratio. Not only can the shape requirements of key components be met and the weight reduced, but it is also more consistent with structural stress characteristics and stress distribution uniformity [4]. Currently, there are two main weight optimization methods: manual setting (based on intuition and experience) and topology optimization.  
A series of classic and breakthrough designs have emerged based on past experience. Data and experience-based manual optimization is quite useful to obtain quick results on some common components with low stress. However, the limitation is that the experience cannot be easily inherited by subsequent researchers, and it does not perform as well in comprehensive optimization for complex equipment [5]. On the other hand, computer-based topology optimization has become more popular in recent years, as it can better satisfy multi-objective optimization. Meanwhile, the analysis system does not need too many settings to obtain optimization results that meet the final design objectives; nevertheless, the process is time consuming and requires expensive devices such as supercomputers to meet the hardware requirements of the operation [6].  
Among all flight states of eVTOL aircraft, the stress condition of the vertical take-off and landing stage is the most complex. The duct is connected to the wing, and it is necessary to design a subsumption component so that a stable and effective connection between the ducted fan and the wing can be established. The design should meet the strength and stiffness requirements [7], as under working conditions, the component mainly undertakes the thrust and torque transmitted from the ducted fan, and then undertakes the gas load, acoustic load, and bending torque between itself and the wings. This paper is focused on the weight reduction of the wingtip component in the design and verification process of a small electric vertical take-off and landing aircraft (eVTOL), and makes a comparison between manual setting and topology optimization calculation of the internal structure. In terms of the manual setting structure, based on the strength characteristics of different internal filling structures of mechanical samples by additive manufacturing and according to the stress characteristics obtained from simulation, the internal structure of the wing tip was changed using a Boolean operation in 3D software so that the balance between the strength and weight of the component could be achieved. In terms of topology optimization, by continuously refining the mesh size, iterative calculation was conducted at different mesh sizes, and finally a lightweight model with the maximum stiffness was obtained. Focusing on this special stage of product design and modification, a method with higher efficiency, simpler calculation, and easier implementation will better meet the needs of the process to accelerate the manufacturing and respond rapidly to the market.