# Simulation, Fabrication and Testing of UAV Composite Landing Gear
## 2 Materials and Methods
### Landing Gear Models Manufactured by FFF Process  
The landing gear models (Figure 2a) were manufactured on an Ultimaker S5 3D printer (Ultimaker, Utrecht, The Netherlands), from several types of filaments (Figure 2b), as follows:  
* Polylactic acid matrix reinforced with short glass fibers (PLA GF) has good chemical and mechanical resistance, is mixed up to 20% with short glass fibers, has high durability and can sustain temperatures of up to 100 \({}^{\circ}\)C. The fiberglass-reinforced filament is significantly stronger, harder and more heat resistant than regular PLA [31]. * Polypropylene matrix reinforced with short glass fibers (PP GF30) is made of polypropylene, is reinforced with 30% glass fiber content and has a high heat resistance and an improved UV resistance. Due to its exceptional rigidity, this material is highly suitable for demanding applications [32]. * Polyethylene terephthalate matrix reinforced with short carbon fibers (PET CF15) is a carbon-fiber-reinforced PET that has good strength and stiffness, high heat resistance, high dimensional stability and low abrasiveness. Due to its properties, this material can be used for a wide range of technical applications [33]. * Polyamide matrix reinforced with short carbon fibers (PAHT CF15) is a thermoplastic reinforced with 15% carbon fibers that combines resistance to high temperatures and chemicals with special mechanical properties. With a peak temperature of 180 \({}^{\circ}\)C, this material can withstand constant temperatures of 150 \({}^{\circ}\)C [34]. * High-quality carbon-fiber-reinforced polymer (CFRP) is a material in which carbon fiber is used as the reinforcement element in a polymer matrix [35]. When compared to any conventional polymer or metal, CFRP has excellent strength and low specific weight characteristics, damage tolerance and fatigue resistance [36]. An important advantage of using a high-quality carbon fiber-reinforced polymer (CFRP) for the landing gear is that this material has a very good mechanical strength resistance and a low mass. The carbon fiber landing gear, without mounting holes, is wrapped in a 3K \begin{table}
\begin{tabular}{c c c} \hline \hline
**Abbreviation** & **Name** & **Dimensions (mm)** \\ \hline A & Wheel Mounting Surface & 20 \\ B & Fuselage Width & 145 \\ C & Leg Height & 145 \\ D & Wheel Track & 315 \\ T & Thickness & 5 \\ W & Width Landing Gear & 28 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Landing gear dimensions (3D printed and commercial aircraft landing gear). Figure 1: Landing gear: (**a**) position on the UAV; (**b**) dimensions. cloth layup and pressed in a polished mold. The CFRP landing gear was manufactured using 6 layers of carbon fiber, with 0- and 90-degree fiber orientations. The mechanical and thermal properties of materials used to obtain the landing gear models, manufactured by the FFF process, are described in Table 2.  
All filaments used in the printing process have the manufacturing parameters presented in Table 3 and were utilized in the state delivered by the manufacturer without being subjected to thermal procedures. By exception, for the FFF process of the landing gear manufactured with PAHT CF15 filament, it was pre-dried at 70 \({}^{\circ}\)C for 12 h, according to the manufacturer's specifications, using the Wanhao Box 2 dryer (Wanhao, Jinhua, China). The properties of the materials used are given by the filament suppliers. In order to present a more realistic picture of the mechanical performances, in the field of aviation, tests are carried out on real products. The thickness in the case of the 5 landing gear models was the same. Landing gear models were manufactured by the FFF process from filaments reinforced with short glass/carbon fibers that have high mechanical characteristics and are used in the aviation field for the manufacture of UAVs or components for small aircraft. Landing gear models were manufactured with 100% infill density because they represent vital components of UAVs and require the highest strength possible.