Unmanned aerial vehicle (UAV) is an aerodynamic aircraft that does not require personnel to be carried. It can fly automatically and guide remotely, and can be recovered or used once. It has been successfully used in civilian fields for disaster patrol, environmental monitoring, aerial photography, forest fire prevention, and meteorological observation. In military fields, it has developed into unmanned combat systems for military reconnaissance, air power suppression, electronic warfare, and deep interception. As a new type of aviation weapon, it will become the dominant force in air combat!. Due to the fact that drones only have aircraft destruction and no casualties, their development has been very rapid. The key aspects of drone body design are structural form and material selection, and a reasonable structural form is a prerequisite for the drone structure to meet design requirements. The advantage of designing the structure of a drone is that it does not require consideration of the physiological endurance of humans during flight, nor does it require special consideration of the structure and materials for stealth and anti ballistic capabilities. Only considering the structural performance of the drone can ensure the installation of advanced airborne equipment!
Weight reduction is an eternal theme in drone body design. Compared to aluminum profiles and steel, the unique lightweighting effect of carbon fiber composite materials has also become the mainstream trend in drone lightweighting, which means it can extend travel time or increase mission payload. At present, the amount of carbon fiber composite materials used in advanced unmanned aerial vehicles in the world is generally 60% to 80% of the total weight of ancient aircraft structures, and even reaches over 90%. The weight reduction of unmanned aerial vehicle structures can reach 30% to 5%. Carbon fiber composite materials can be optimized and designed according to the strength and stiffness requirements of unmanned aerial vehicles, meeting the characteristics of integral molding of components such as the drone fuselage and wings. The corrosion resistance of the resin matrix of composite materials can enable drones to be used in harsh environments for a long time, making them easy to maintain and repair
In the development process of various types of drone structures, they have gained unanimous recognition from all parties and have been widely used, becoming the main material of many aircraft models. This plays a crucial role in the lightweighting, miniaturization, and high-performance of drone structures. Next, based on our past work experience and the current manufacturing conditions of carbon fiber composite materials, we will introduce the lightweight technology for drone bodies.
Design technology of composite material drone body
1. Analysis of Carbon Fiber Composite Structural Units for Drones
This model of drone belongs to the small aircraft category and can withstand low loads during flight. Its structural design facilitates the extensive use of lightweight carbon fiber composite materials and their sandwich structures, and strives for a concise structure to reduce manufacturing costs.
The body of this model of drone mainly adopts a Nomex honeycomb sandwich structure, and the basic structural form is shown in the following figure.
This sandwich structure is composed of high-strength carbon fiber composite inner and outer panels and low-strength lightweight core materials, which have the advantages of light weight, high bending stiffness and strength, strong anti instability ability, fatigue resistance, sound absorption and heat repair. The upper and lower panels bear the main tensile and compressive stresses, while the core material mainly bears shear stress. The core material is used to make the upper and lower panels into an integral component: the thin panel does not bend when subjected to high tensile stress, and the shear force is transmitted from the panel to the inner panel, which can enable the panel and core to function as a whole, fully realizing the high specific strength and stiffness characteristics of the layered structure, depending on the performance of the panel, core, and core bonding.
Due to the good flexural stiffness of the sandwich structure, which can effectively coordinate its critical stress level for instability and the allowable stress level for static strength, unmanned aerial vehicles can be designed based on static strength. The outer panel of the drone sandwich structure is designed as a two-layer carbon fiber cloth laminated board, which can withstand in-plane loads and wing aerodynamic forces. The selection of core materials takes into account weight reduction and molding processability, choosing small cell low-density Nomex honeycomb; The inner panel is designed as a layer of carbon fiber cloth, which is very thin and uneven due to the influence of honeycomb cells during the molding process. The in-plane static strength only considers the load-bearing capacity of the outer panel, while the honeycomb and inner panels are designed based on stability.
The load-bearing beams and walls in the composite material body structure of unmanned aerial vehicles are designed as composite laminates, and the wing ribs are made of aviation laminates and their sandwich structures. Considering the characteristics of the overall molding process of composite materials, the sandwich structure skin, load-bearing beams, walls, and wing ribs of the aircraft body are designed as composite material co bonded wing surfaces, that is, the molding of load-bearing beams, walls, and wing ribs and the bonding of the inner panel of the sandwich structure are completed simultaneously. This can eliminate the need for adhesive assembly of components, which is quite effective in reducing the weight of drones, streamlining production processes, and improving the quality of component assembly.
2. Design of drone body based on composite materials
The design of composite material structures for unmanned aerial vehicles requires a concise structure, with multiple components designed as a whole structure as much as possible, significantly reducing the number of connectors and fasteners, thereby reducing the weight of the structure and the number of stress concentration areas caused by assembly, simplifying the maintenance and repair of the aircraft structure; Adopting a holistic structural design can simplify the force transmission relationship of the aircraft, facilitate reasonable force transmission, ensure the continuity of structural strength and stiffness characteristics, and facilitate overall adjustment and improvement of structural design. Implementing overall structural design while taking into account the molding process of composite materials can improve the structure
In the development process of a certain model of drone body, we adopted carbon fiber composite structure for all body components to achieve greater precision. Design materials and their sandwich structures, and integrate the concept of overall co curing and co bonding throughout the entire design process. The carbon fiber composite body components of this model of drone include seven parts: fuselage, fuselage cover, wings, ailerons, vertical tail, flat tail, and tail support. The outline diagram of the drone is shown in the following figure.
The specific design scheme for the carbon fiber composite body components of unmanned aerial vehicles is as follows
1) The fuselage structure is the torso of a drone, used for carrying equipment, installing engines, and carrying payloads. The fuselage structure consists of four longitudinal load-bearing Ω - shaped beams, fuselage skin, and eight parallel arranged transverse reinforcement ribs, as shown in the cross-section diagram below. The load-bearing beam is a composite laminated plate structure. The fuselage skin adopts composite material inner and outer panels, with a sandwich structure in the middle, and the sandwich material is Nomex honeycomb. The reinforced frame ribs are layered with composite materials, and the sandwich material is made of aviation plywood. The four beams and the skin are assembled and connected by adhesive bonding, and the reinforced rib frame is used to support the cross-sectional shape of the fuselage and transmit concentrated loads.
2) Fuselage cover
The fuselage cover is a separate component on the fuselage skin, and the structural form of the material is the same as that of the fuselage skin. It is a carbon fiber composite material with upper and lower panels and a sandwich structure in the middle, and the sandwich material is Nomex honeycomb. The thickness of the hood is also the same as the skin of the fuselage here. The body cover is connected to the body using snap on bolts and can be disassembled repeatedly. The cross-section of the fuselage cover is shown in the figure.
3) Wing structure
The wing is the main lifting surface of the drone, symmetrical on both sides, connected to the fuselage, bearing aerodynamic loads and generating the required lift force for the drone's movement. The wing adopts a carbon fiber composite sandwich structure to ensure sufficient strong support and light weight. Smooth streamline and accurate shape can be obtained through mold forming, thereby improving the structural efficiency, aerodynamic elasticity and control characteristics of the drone.
The wing of the drone consists of upper and lower skins, front and rear U-shaped beams, and 16 transverse ribs, as shown in the diagram. The bending and shear loads of the wing are mainly transmitted by the front and rear U-shaped beams, while the torque is transmitted by the structure composed of the upper and lower skins and the front and rear U-shaped beams. The transverse ribs support the skin and beam web and transmit localized concentrated loads. The cross-sectional view of the wing is shown in the figure.
The skin mainly bears shear stress, and the forming material adopts carbon fiber composite inner and outer panels, with a sandwich structure in the middle, and the sandwich material is Nomex honeycomb. The wing ribs adopt a carbon fiber composite sandwich structure, and the sandwich material is made of aviation plywood.
The front and rear U-shaped beams adopt a pre formed carbon fiber composite laminated plate structure. The beams, wing ribs, and upper and lower skins are all assembled by bonding, simplifying the assembly process and avoiding the need to open assembly holes for fastener assembly.
4) Aileron structure
Aileron is small in size, and its shape is shown in Figure 8.35. The metal parts of the rotating shaft need to be fixed inside, so foam sandwich structure is selected. This structure is composed of upper and lower skins filled with rigid polyurethane foam and two wing ribs. Its transverse section is shown in Figure. The load is mainly transmitted by the skin, and the hard foam core plays a supporting role. The skin is designed as a composite panel with only one layer of carbon fiber cloth, which can bear both normal stress and shear stress under the dense support of foam core. The wing rib is an aviation layer board, mainly used to position the metal rotation axis.
5) Vertical tail structure
The horizontal dimension of the vertical tail is small, and the foam sandwich structure is selected. This structure is directly composed of upper and lower skins filled with rigid polyurethane foam. The load is mainly transmitted by the skin, and the hard foam core plays a supporting role. The skin is designed as a composite panel with single-layer carbon fiber cloth. With the dense support of foam core, it can bear both normal stress and shear stress
6) Flat tail structure
The flat tail is composed of upper and lower skins, walls, and 4 wing ribs. The bending load is mainly transmitted by the skin, the shear load is transmitted by the wall, and the torque is transmitted by the structure composed of the skin and the wall. The wing ribs support the skin wing surface and transmit locally concentrated force loads. In this structure, the skin needs to withstand both normal stress and shear stress, so it is designed as a composite sandwich structure that can withstand in-plane normal stress. The sandwich material is Nomex honeycomb. The wall is a composite sandwich structure, and the sandwich material is selected as aviation plywood. The wing rib material is aviation laminated board.
7) Tail support structure
Tail strut is a load-bearing component that connects the wing and tail fin. Plays the role of transferring all loads from the tail wing to the wing. The load-bearing form is similar to a cantilever beam, which needs to withstand bidirectional shear force, bidirectional bending moment, and torque. Therefore, the tail brace adopts carbon fiber composite material circular tube. This round tube is manufactured by stretch extrusion winding, which can produce longer pipe fittings and cut them according to the design dimensions.
3. Design and analysis of carbon fiber composite materials for unmanned aerial vehicles
1) Structural design analysis shows that carbon fiber composite materials can enhance the fatigue performance of structures. Therefore, structural design analysis mainly focuses on structural static strength analysis and stability analysis, and using engineering calculation methods in design is very effective.
2) Structural static strength analysis
The static strength analysis and calculation of the structure are carried out according to the engineering beam theory. Structural strength is controlled by allowable strain, and structural stiffness is controlled according to deformation requirements
3) Skin stability analysis
The stability design calculation of honeycomb sandwich structure skin can be carried out according to the corresponding methods in the composite material design manual. The stability of the skin of foam sandwich structure needs to be analyzed and calculated by deflection curvature equation and energy method
4) Stability analysis of load-bearing beams
The overall critical stress for the instability of load-bearing beams is generally calculated in the form of a tension rod. However, due to the fact that in drone structures, the load-bearing beam is usually bonded to the skin and supported by the skin, the critical stress for instability calculated in the form of an Euler rod is relatively conservative.