1 Introduction

Unmanned aerial vehicles (UAVs) encompass a spectrum ranging from micro-sized systems to full-sized aircrafts having diverse mission capabilities [1, 2]. UAVs first gained popularity in the military to conduct Dull, Dirty, and Dangerous (DDD) missions such as airborne intelligence, surveillance, reconnaissance, close air support, air combat, and ground strikes, etc., while saving precious and expensive trained human lives [3, 4]. Later, the popularity extended to civilian applications such as meteorological data collection, precision farming, remote aerial mapping, habitat assessment, traffic surveillance, search and rescue, disaster, public infrastructure monitoring, and more [4,5,6]. Military UAVs predominantly fall in the high-altitude long endurance (HALE) and medium-altitude long endurance (MALE) categories, utilizing a combination of composites and alloys for their construction [7, 8]. The aluminum alloy called duralumin, recognized for its versatility and reduced weight, is popular in manufacturing military UAVs [9]. In contrast, UAVs designed for civilian applications belong to the low-altitude long endurance (LALE) category, emphasizing the need for lightweight composites in the manufacturing process [9, 10].

Polymer composites find extensive applications in aerospace, automotive, structural, and UAV applications due to their superior mechanical qualities and properties, ease of fabrication, rapid availability, and cost-effectiveness compared to traditional and old materials [8]. However, to achieve high performance, low weight, and good bedding properties, the requirement exists for various composite materials such as carbon fiber, Kevlar, glass fiber, and aramid–carbon mixture to meet the growing demand for more maneuverable and payload-carrying capacity for UAVs [11]. These composite materials weigh about half as much as different metals and metal alloys but possess about twice Young's modulus [12] and good strength. Another aspect that makes them popular is their availability in various weaves and compositions. For hybrid VTOL UAVs, other characteristics, including stiffness, resistance to corrosion, thermal and acoustic insulation, and vibration damping, become more critical [11].

Composites are mixtures of different materials that make them stronger and more durable, with each phase having a different physical and chemical makeup. Combining all phases results in a new material with properties different from each constituent material. Composite parts are broken down into reinforcement, filler, and matrix based on functionality [13]. Metals, ceramics, polymers, and carbon are some of the most common materials used as a matrix. On the other hand, carbon, glass, boron, and aramid are the common fillers that make things stronger [10, 12].

Polymer composites are easier to manufacture, cost-effective, and more workable than traditional materials [14]. Adding fibers or fillers to the polymer matrix increases its durability and usefulness. Combining composite and filler strengthens the structure's joints or corners. Earlier, researchers studied the impact of adding carbon and glass fiber as padding by examining the tensile properties of the polymer matrix [15]. The volume ratio of fiberglass in the polymer matrix influenced its bending strength, where about 45–55% of the volume filled with fiberglass increased its durability [16, 17]. Finally, researchers [18, 19] documented the design of laminated composite material with supporting filler material to obtain more strength in a localized manner using a layered architecture to the composite structure.

This research focused on identifying a combination of lightweight, cost-effective, easily moldable, and abundantly available materials to manufacture fixed-wing hybrid VTOL UAVs. Our research work utilizes self-generated experimental data of composite materials and specimens. We used dynamic mechanical analysis (DMA) [18] to quantify the performance of the various combinations of fabric-reinforced composite material [19,20,21]. The intended usage of the fixed-wing hybrid VTOL UAV with optical and sensor-based payloads is for observation, reconnaissance, surveillance, and asset monitoring missions in fully autonomous mode with an operational range of 20 km. Before flight testing the composite airframe, we used Matlab (Ver. 2023) and Xplane (Ver.12) software to conduct the flying simulation across different flight regimes, viz., vertical take-off, transition to forward flight, mid-flight hovering, back transition from forward flight, and vertical landing [22].

The organization of this manuscript is as follows. The introduction forms the first section, and the following section explains the methodology and techniques used in our composite manufacturing, including the molding process and experiments. The section following methodology compiles the experimental results, data analysis, and the actual hybrid UAV manufacturing using the chosen composition of materials. The final section concludes this work by documenting significant findings on various combinations of composite materials used to manufacture fixed-wing hybrid VTOL UAVs.

2 Methodology

This section summarizes the design of experiments, test specimens, and the actual experimentation setup with various composite samples. Our first step in the UAV design was the selection of the UAV configuration, propulsion system, and payload capabilities. A fixed-wing hybrid VTOL system is most desirable for our mission due to the long endurance requirements and take-off and landing space constraints. The hybrid UAV could hover, cruise, and climb using its independent electric propulsion systems. The quad-copter propulsion system has four brushless direct current (BLDC) motors with carbon-fiber propellers, ensuring vertical take-off and landing, negating the need for runway infrastructure. Also, a single and separate BLDC pusher motor mated to a composite propeller powered the forward fixed-wing flights. Both independent propulsion systems imparted the VTOL capability and fixed-wing efficiency, making the UAV a genuinely hybrid design. Table 1 summarizes the major specifications of the fixed-wing hybrid VTOL UAV. The maximum take-off weight (MTOW) is  ~ 12 kg, including the payload, avionics with all electrical components, propulsion systems with batteries, and the composite airframe. The wing span and aspect ratio are 2.6 m and eight, respectively, resulting in a 15 kg/m2 wing loading. The UAV exhibited an endurance of more than 50 min in the fixed-wing mode with a 10,000 mAH (10AH) lithium polymer (Li-Po) battery.

Table 1 Important specifications of the LALE fixed-wing hybrid VTOL UAV
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Due to stringent weight restrictions and a higher payload-carrying requirement (~ 25% of MTOW), we decided to manufacture the entire hybrid UAV using composite materials. Considering various loads acting on the UAV, we utilized woven fabrics made from carbon fiber, glass fiber, and Kevlar. These technical textile materials had balanced plain-weave with zero and ninety-degree orientations. Epoxy resin became the matrix material due to its wide availability in the market. We reinforced the carbon fiber fuselage with Kevlar to protect critical electronic components and autopilot from landing impacts. Fiber-reinforced polymer (FRP), carbon fiber, balsa wood, and Styrofoam formed the fixed-wing structure. Figure 1 illustrates the manufacturing process of the hybrid UAV using the vacuum bagging method under room temperature curing. According to Fig. 1, three major steps, viz., (i) mission requirement, (ii) analytical design, and (iii) CAD modeling with flight simulation, generated the necessary information required for the design and manufacturing of the hybrid UAV. The initial prototype used high-density foam (HDF), i.e., extruded polystyrene sheet with a density of 47 kg/m3 for building the airframe, which became the pattern for designing various molds. We constructed the final model using the chosen materials and these molds.

Fig. 1
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Hybrid UAV design and manufacturing process block diagram

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2.1 Molding

Figure 2 depicts the overall molding process. We completed the preliminary design from the mission profile required by the customer, along with parameter estimation, digital design, and simulation steps before mold making. The initial design of the model considered manufacturing the experimental prototype using HDF. This design aimed to experimentally validate the performance parameters of the aircraft obtained from analytical modeling and calculations. HDF model offers the added advantage of quick incorporation of minor modifications.

Fig. 2
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Overall molding process followed in the manufacturing

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After freezing the parametric design using the digital model, we created the first functional prototype using HDF and instrumented it for flight tests to validate the design. We fine-tuned this HDF model with minor tweaks to extract adequate real-time performance. Having all the necessary tweaks, the HDF model became the template for mold creation to manufacture the composite model.

First, we hardened the surface of the tweaked HDF prototype by applying layers of FRP strips and general-purpose resin. After layering, we smoothed the surface using putty and a surfacer to eliminate bubble gaps, as depicted in Fig. 3a and b. Then, the smoothened components were painted, rubbed, and smoothed after applying wax polish and polyvinyl alcohol (PVA). Next, hardener, accelerator, and pigment color were applied to the surface in the said sequence to achieve the results shown in Fig. 3d. A vertically placed 10-mm plywood acted as the mold's foundation. The curing time of the molds took around ≈ 12 h at room temperature. Figure 4 provides illustrative examples of the significant component molds of the fixed-wing hybrid VTOL UAV. We repeated the same process for all three components of the fuselage, six components of the wing (including control surfaces), and seven parts of the empennage (Fig. 5).

Fig. 3
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Actual molding process a Wing surface hardening, b smoothing of wing surface, c fuselage hardening, d elevator casting, e flanging of the wing, and f curing of wing mold at room temperature

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Fig. 4
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Examples of final mold patterns

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Fig. 5
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Composite material samples for experiments

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2.2 Experiments

This research aimed to develop a fully composite fixed-wing hybrid VTOL UAV with a high payload-to-MTOW ratio. The first step focused on identifying appropriate materials to manufacture the composite airframe. The design of experiments was used to identify the best material combination for the UAV to achieve a strong and light structure. For this, we used different material combinations to make composite test specimens to conduct experiments on the universal testing machine (UTM) to determine the value of Young's modulus, yield point, and peak stress. All specimens had three layers, and the thickness depended on the sandwich material properties and the fabric weight (gsm). First, we created rectangular sheets of composite materials with different combinations using vacuum bagging. Then, we converted them into four specimen sizes (120 × 5 mm) using the laser cutting machine, as illustrated in Fig. 6. In this step, we extracted two specimens in the zero (R0) orientation. In contrast, the remaining specimens were at forty-five (R45)-degree orientation. We subjected one pair to the tensile test and the second to the bending test.

Fig. 6
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Sample for testing in zero-degree and 45-degree orientation

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Multiple approaches are available to create composite materials from continuous fiber and non-metallic matrix material. Most of these processes involve vacuum bagging, matching die molding, filament winding, and resin transfer molding. The most crucial component of the hybrid UAV is its fixed wing, made out of two-part composite (male and female) from separate molds using vacuum bagging.

The vacuum bagging manufacturing process requires raw materials such as carbon fiber, aramid fiber, glass fiber, breather cloths, peel ply, resin, sealant tape, thin plastic cover, etc., and equipment such as vacuum motor, releasing agents, wax, etc. The releasing agent is a chemical applied to the tool (mold) surface to prevent the laminate from sticking. Peel ply is a tightly woven fabric, typically made of nylon, having a releasing agent that makes the laminate's surface rough rather than smooth. The release film is a treated thin plastic, eliminating the chances of lamination adhesion. It has small openings that allow the matrix material to pass through. Breather cloths are thick fabric layers that absorb extra resin from the laminate, passing through the peel ply and releasing film.

Further, the breather cloths also ensure the consistent distribution of vacuum. The vacuum bagging film is a relatively thick sheet of plastic used to isolate the laminate and provides necessary vacuum conditions. Sealant tapes facilitate the sticking of the vacuum bagging film to the tool surface to prevent air entry during the generation of vacuum pressure by a vacuum pump. All composite specimens comprised two layers of technical textile cloths, including one Airex sheet used as the sandwich material. A laser machine facilitated the cutting of the test specimens in two different orientations, viz., \(\left({R}_{{0}^{0}}\right)\) and \(({R}_{{45}^{0}})\).

Table 2 summarizes the experimental data against various material weights and overall weight, where CFn stands for carbon fiber with n as the GSM value of cloths, ACFn is the aramid–carbon fiber, AR denotes the Airex for sandwich, and GF denotes glass FRP. The bending experiment is more critical among both bending and tensile strength tests because the force is perpendicular to the fiber orientation, just like on the aircraft wing during flying. We found that the zero-degree oriented test specimens exhibited superior bending strengths than the forty-five-degree (45°) oriented ones. Finally, after conducting a trade-off analysis between weight and strength, we chose the combination of two layers of 93 GSM carbon fiber with a sandwich of 2 GSM carbon fiber to manufacture the airframe (Fig. 7).

Table 2 Experimental data vs. material combination (flexural test, tensile)
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Fig. 7
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Bending experiment of the test specimen on UTM

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2.3 Manufacturing a composite airframe

Vacuum bagging is a straightforward technique for fabricating composite structures. Relevant details and illustrative images of the vacuum bagging process used for manufacturing the composite hybrid UAV airframe are as follows. As shown in Fig. 8a and b, the first step involves thoroughly cleaning the mold's surface to remove foreign items, particles, oil, or other contaminants. This step uses fine-class sandpaper on the internal mold surfaces and wax application on the tool part's cleaned surfaces. As mentioned earlier, wax acts as the releasing agent of the composite part after curing.

Fig. 8
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Composite airframe building a Rubbing for smoothing surface, b waxing the mold surface, c Airex cutting for the sandwich, d applying resin on selected material, e wing vacuum bagging, f fuselage vacuum bagging, g wing component, and h assembled three components for fuselage

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Next, we cut the carbon fiber and Airex sheets in the desired shape for layering on the mold, followed by the uniform distribution of the ultra-clear resin using a roller or brush to create the sandwich skin of the hybrid UAV as illustrated in Fig. 8c and d. We identified 93 GSM carbon fiber as the best choice from the experimental findings to create the composite in sandwich style, as shown in Fig. 8d. The entire sandwich, including the mold, was packed using peel ply, breather cloths, and sealant tape and then connected with the vacuum pump arrangement as pictured in Fig. 8e and f. After properly curing the composite part, we removed it from the mold (tool), and then, both parts were joined at the seam using a specific glue. Figure 8g and h depicts the final composite wing and fuselage, respectively. The circular carbon tubes protruding from the fuselage facilitate the snap-fit of both wings to it.

3 Results

We tested sixteen samples using different composite materials (FRP, CF, and Aramid) on the UTM to quantify the bending and tensile strengths. Table 2 compares the experimental results obtained after testing the designed composite materials. In the tensile experiment, the maximum value of Young's modulus was 7.767 GPa, reported by the composite material created from the combination of two layers of carbon aramid and Airex with epoxy, albeit exhibiting higher weights. The original aim was to build a lightweight hybrid VTOL UAV. Hence, the rows bold values in Table 2 indicate our choice of composite material with comparable Young's modulus while being much lighter, which comprises two layers of 93 GSM carbon fiber and 2 GSM sandwich carbon fiber with ultra-clear e-glass/epoxy laminate. Figure 9a and b illustrates the experimental data plot of specimen weight vs. peak and yield stresses, respectively. Figure 9c and d shows the bending stress and strain plots, respectively. The zero-degree (0°) specimen's peak stress is 12.348 MPa, while forty-five degree (45°) has 10.923 MPa. Hence, we selected the zero-degree orientation for the carbon fiber composite to manufacture the UAV wing, weighing 1097.49 g, resulting in a total empty airframe weight of 2.7 kg.

Fig. 9
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Plot (a) weight vs. peak stress, (b) weight vs. yield stress stress–strain plots, (c) zero-degree orientation \(({R}_{{0}^{0}})\), and (d) forty-five-degree \(({R}_{{45}^{0}})\) orientation

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Figure 10 shows the resultant modular and packable airframe manufactured using the chosen composite. Three Li-Po batteries provide the necessary power for different missions, of which two 10,000 mAH Li-Po batteries mounted on the square tubes power the vertical missions, viz., (i) take-off, (ii) hovering, and (iii) landing, whereas the third 10,000 mAH Li-Po battery inside the fuselage drives the fixed-wing flights (cruise and climb). Also, the Li-Po battery's position inside the fuselage is adjustable to manage the center of gravity (COG).

Fig. 10
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Final carbon composite LALE fixed-wing VTOL UAV

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4 Conclusion

This research presents the fabrication process of a lightweight composite fixed-wing hybrid VTOL UAV parts, viz., wing, tails, and fuselage with control surfaces. The existing literature has very little pertinent data about such manufacturing process, ingredients mixing, layer stacking order, and other environmental factors on the material properties of composites for building such hybrid VTOL UAVs. This necessitated the conceptualization of experimental design using test specimens to quantify mechanical properties to identify appropriate material combinations to provide the best strength at the lowest weight. We used the data from various composite samples to determine the ideal trade-off between weight and strength to achieve a high payload-to-weight ratio. Then, we created a manufacturing setup in our composite fabrication laboratory for creating sub-assemblies using the vacuum bagging process. We used the hand lay-up approach to manufacture the composite parts, which underwent room temperature curing. Another significant contribution of this research is the characterization of composites created using different raw material combinations. Quantifying these composites' mechanical characteristics and associated constraints became crucial for adequately enabling the hybrid VTOL UAV to carry various payloads for diverse missions. We identified that two layers of 93 GSM combined with one layer of 2 GSM carbon fiber (CF93-CF2-CF93-R0) in a sandwich manner provide the best combination for manufacturing the hybrid UAV, achieving the desired payload-to-weight ratio. The final product exhibited sufficient strength while maintaining a total airframe weight of around 2.7 kg with a maximum take-off weight (MTOW) of about 12 kg.