Unmanned aerial vehicle

Abstract

The present patent disclosure concerns an unmanned aerial vehicle comprising a frame comprising a central body section and three or more outer body sections, wherein the frame is made of a composite material comprising a polymer matrix and graphene based material, wherein the graphene based material is dispersed within the polymer matrix, wherein the composite material comprises 1 to 20 wt% graphene based material; and a propulsion system comprising three or more propulsors, wherein each of the three or more outer body sections accommodates a propulsor of the three or more propulsors, characterized in that the frame both provides primary structural integrity and is an outer shell of the unmanned aerial vehicle Further discloses are a method of manufacturing an unmanned aerial vehicle and a method of manufacturing a polymer part.

Description

[0001] UNMANNED AERIAL VEHICLE

[0002] The present patent disclosure concerns an unmanned aerial vehicle, a method of manufacturing an unmanned aerial vehicle, and a method of manufacturing a polymer part. Particular embodiments concern unmanned aerial vehicles with a lowered radar footprint.

[0003] The present patent disclosure relates to the field of unmanned aerial vehicles (UAVs), commonly known as drones, and specifically to reducing their electromagnetic footprint, such as for radar signals, in other words lowering the radar cross section of the UAV / drone. A lowered radar cross section may be beneficial for applications where lowered interference is desired, such as, for example, radar for air traffic controllers, weather monitoring or maritime navigation, less interference when using a drone for observing wildlife equipped radar-sensitive tracking devices, and certain radar sensitive scientific experiments. Reducing the electromagnetic footprint in general, beyond radar frequencies, may also be desired in these or other applications of drones.

[0004] Various methods of reducing radar signatures exist, such as providing a coating onto the outer shell of an unmanned aerial vehicle. These methods include the addition of graphene to a polymer-based fluid, and then coating that fluid onto the outer shell. Such coating fluids may be limited in the amount of graphene they can contain in order to be usable as a coating fluid. Such coatings may also have limited adhesion to the outer shell, and are subject to wear and other damage, exposing the outer shell. In addition, the coatings provide additional weight to the unmanned aerial vehicles.

[0005] It is an object, among objects, to provide an improved unmanned aerial vehicle and method of manufacturing thereof.

[0006] To this end, there is provided in accordance with a first aspect an unmanned aerial vehicle comprising a frame comprising a central body section and three or more outer body sections, wherein the frame is made of a composite material comprising a polymer matrix and a graphene-based material, wherein the graphene -based material is dispersed within the polymer matrix, wherein the composite material comprises 1 to 20 wt% graphene-based material; and a propulsion system comprising three or more propulsors, wherein each of the three or more outer body sections accommodates a propulsor of the three or more propulsors, wherein the frame is configured to both provide primary structural integrity to the unmanned aerial vehicle and be an outer shell of the unmanned aerial vehicle. The above UAV offers several advantages by integrating radar-absorbing materials directly into the frame of the UAV. This approach not only enhances lowering of the radar cross section of the UAV by increasing the use of radar-absorbing material but also contributes to the structural integrity and durability of the aerial vehicle. The use of a composite material comprising a polymer matrix and a graphene-based material ensures that the UAV frame remains lightweight while providing the necessary strength and rigidity. In many polymers, the mechanical properties of the composite comprising the polymer and the graphene-based material are enhanced. Additionally, the inherent electrical conductivity of the graphene -based material improves the UAV's resistance to electromagnetic interference, enhancing the reliability and resistance to jamming of internal electronics. This integrated approach both improves the UAV's stealth capabilities and provides a multifunctional frame that supports a range of operational requirements, making it a versatile and efficient solution for modem UAV applications. The frame can also be manufactured using common forming techniques, such as but not limited to injection moulding, compression moulding, extrusion and various additive manufacturing techniques Fused Deposition Modelling (FDM). Either the whole frame, or sections of the frame, can thus be formed in one step while providing the beneficial properties listed above, thus simplifying the production.

[0007] The UAV having a frame configured to both provide primary structural integrity to the unmanned aerial vehicle and be an outer shell of the unmanned aerial vehicle, may be referred to as an integral frame. In an integral frame, the body / outer shell of the UAV and the frame (for providing primary structural integrity), are combined into a single, unified structure.

[0008] The unmanned aerial vehicle being unmanned is an optional feature. In other words, the various aspects and embodiments described above and below also concern “aerial vehicles” in general, and are not necessarily limited to unmanned aerial vehicles. As will be understood, the combination of features is, however, especially beneficial when applied to unmanned aerial vehicles like drones.

[0009] Optionally, in some examples, the frame is made for at least 80 wt%, for at least 90 wt% or exclusively of the composite material. This beneficially simplifies the manufacturing process of the unmanned aerial vehicle the most.

[0010] Optionally, in some examples, a number of the three or more outer body sections is the same as a number of the three or more propulsors. The outer body sections are each configured to have a propulsor attached to that outer body section. The outer body sections may each comprise an opening for receiving the propulsor. Optionally, in some examples, the unmanned aerial vehicle is a drone, wherein the propulsors are lift-generating rotors, wherein a lift-generating rotor of the lift-generating rotors is attached to each of the outer body sections, wherein optionally the outer body sections are arranged with equal angular spacing around the central body.

[0011] Optionally, in some examples, the lift-generating rotors are made of the composite material. Beneficially, in this way, the radar cross section of the drone is lowered even further, since even more of the parts of the drone are made of the radar absorbing composite material.

[0012] Optionally, in some examples, the composite material comprises 1 to 15 wt%, 1 to 12 wt%, or 1 to 10 wt% of the graphene-based material per weight of the composite material.

[0013] Optionally, in some examples, an average thickness of the frame multiplied with the graphene content of the composite material lies in a range between 0.7 to 12, with the average thickness in millimetres and the graphene content in wt% per weight of the composite material.

[0014] Optionally, in some examples, when the polymer matrix of the composite material is or comprises a PVDF-based polymer matrix, a product of the graphene-based material concentration in wt% and the thickness in mm lies in the range of 0.7 to 12, or 0.75 to 11.2, or 0.75 to 9.

[0015] Optionally, in some examples, when the polymer matrix of the composite material is or comprises PETG-based polymer matrix, a product of the graphene-based material concentration in wt% and the thickness in mm beneficially is at least 3.7, at least 4 or at least 5.

[0016] Optionally, in some examples, the frame has an average thickness in the range of 0.5 to 5 mm or 0.5 to 4 mm, wherein the composite material comprises the graphene-based material in the range of 1 to 5 wt%, or 1 to 4 wt%, or 1.5 to 3.5 wt% per weight of the composite material. Beneficially, in this range of thicknesses, the graphene-based material content can be lowered while radar absorptance in the X-band (approximately 8-12 GHz) is substantially increased. The average thickness here may be referred to as an average structural thickness. Additionally, or alternatively, the average thickness may apply to any one or all of an average wall thickness, an average web thickness, an average skin thickness, an average panel thickness. A lower concentration of the graphene-based material content is furthermore beneficial because the reflectance of the radar signals from the composite material is also reduced compared to higher graphene-based material contents (e.g. 20 wt%). This combination of thickness and the graphene-based material range thus provides an overall better performing frame for the UAV. The thicknesses are furthermore also such that the frame is still strong enough to perform in the UAV. Logically for larger or heavier UAVs the thickness of the frame can be at the higher end of this range. Also, the frame thickness may vary within this range, depending on the location within the frame and the load carried by the frame at that location.

[0017] Optionally, in some examples, when the polymer matrix of the composite material is or comprises a PVDF -based polymer matrix, the composite material comprises the graphenebased material in the range 1.5 wt% to 3.5 wt%, or 2.5 wt% to 3.5 wt%, wherein optionally the average frame thicknesses ranges between 0.5 mm and 3.2 mm, preferably at most (about) 2.5 mm, beyond which the absorptance decreases and the reflectance increases to above 0.5. The combination of these graphene based material concentrations with the average thickness provide especially beneficial combinations of radar absorptance and reflectance.

[0018] Optionally, in some examples, when the polymer matrix of the composite material is or comprises a PVDF -based polymer matrix, the composite material comprises the graphenebased material in the range of 2.5 wt% to 4 wt%, wherein optionally the average frame thicknesses is at least 1.5 mm, 1.5 mm or 2 mm. The combination of these graphene based material concentrations with the average thickness provide especially beneficial combinations of radar absorptance and reflectance.

[0019] Optionally, in some examples, the unmanned aerial vehicle comprises internal components, wherein the frame encloses the internal components. Tn this way, the internal components, which may comprise electronics and other parts, are shielded from incoming radar signals, since they are inside the frame which is made of the composite material.

[0020] Optionally, in some examples, the propulsion system comprises one or more power sources configured to supply power to the lift-generating rotors; and wherein the internal components comprise an electronic control system configured to control the propulsion system.

[0021] Optionally, in some examples, the unmanned aerial vehicle comprises one or more power sources, each housing a power source of the one or more power sources wherein the power source housings are made of the composite material. The power sources, which may be electrical motors, are beneficially radar shielded by the power source housings being made of the composite material. The term “power source housing” may be the same as or refer to “nacelle”. Optionally, in some examples, the one or more power source housings are integral with the frame. In this way, beneficially, the manufacturing of the UAV is simplified even further. Together with the previously mentioned advantages, it becomes possible to manufacture the UAV with high speed, enabling mass production.

[0022] Beneficially, the composite materials exhibit mechanical properties that are suitable for application in frames for aerial vehicles according to the present patent disclosure. For example, for PETG and PVDF based composite materials comprising the graphene based material, the flexural moduli of the materials are lower than 5 GPa, in most cases even lower than 3 GPa, indicating that the materials are not excessively brittle. The flexural strengths range between 50 and 100 MPa, such as in a range of 75 MPa to 80 MPa. In addition, these materials have reasonable to good impact strength and tensile properties. The notched Charpy impact strengths are found to lie in a range of 0.9 - 3.8 kJ / m2. For PETG based composite materials, the Charpy impact strength lies between about 0.9 and about 1.1 kJ / m2while for PVDF based composite materials the range is between about 1.6 and about 3.8 kJ / m2. The tensile modulus of the composite materials are found to lie between 500 and 3000 MPa, depending on the polymer material and amount of the graphene based material. The ultimate strength of the composite material was found to vary between 15 and 50 MPa, depending on the polymer material and the amount of the graphene based material. The elongation at break was found to lie between about 2 and about 17%.

[0023] The composite material may comprise one or more additives. The additives may include one or more of flame retardants, impact modifiers, polymer stabilizing compounds, processing aids, colour pigments, injection moulding supporting compounds for better release from moulds. Processing aids may help the flow of the polymer during moulding to avoid shearing. The polymer of the polymer matrix may, for example, comprise 0.5 wt% to 1 wt% of processing aid, in weight per weight of the polymer matrix. The polymer of the polymer matrix may, for example, comprise 0. 1 -5 wt% of impact modifier.

[0024] Optionally, in some examples, the polymer matrix may be a grade specific to the forming technique used to manufacture the frame. The polymer matrix may, for example, comprise an injection moulding grade polymer, an additive manufacturing grade polymer, such as a 3D- printing grade polymer (e.g. FDM-grade), or a thermoforming grade polymer.

[0025] Optionally, in some examples, the polymer matrix is a PVDF -based matrix. The PVDF-based matrix may be a grade specific to the forming technique used to manufacture the frame. The PVDF-based matrix may, for example, be an injection moulding grade PVDF-based matrix, or an additive manufacturing grade PVDF-based matrix or a 3D-printing grade PVDF-based matrix.

[0026] In general, the PVDF-matrix has improved mechanical properties due to the addition of graphene-based material, thereby providing a stronger frame with larger load bearing capacity.

[0027] Optionally, in some examples, the polymer matrix is a PETG-based matrix. The PETG-based matrix may be a grade specific to the forming technique used to manufacture the frame. The PETG-based matrix may, for example, be an injection moulding grade PETG-based matrix, an additive manufacturing grade PVDF-based matrix or a 3D-printing grade PVDF-based matrix. The PVDF-based matrix may be suitable for FDM, for example.

[0028] Optionally, in some examples, the graphene-based material is one or more selected from the group consisting of single layer graphene, few layers graphene, multi-layered graphene, reduced graphene oxide (rGO) and graphene nanoplatelets (GNP), “one or more” indicating a single material of these graphene-based materials or any mixture of two or more thereof.

[0029] Optionally, in some examples, the graphene-based material in the composite material comprises, or is, rGO. The rGO may have a carbon content of at least 80 at%, 85 at%, 90 at%, 95 at% or 97 at%. Often a higher carbon content is associated with an increased conductivity of the rGO.

[0030] Optionally, in some examples, the composite material of the frame comprises a multilayer structure comprising a first layer and a second layer, wherein the first layer comprises the polymer matrix as a first polymer matrix and the graphene based material as a first graphene based material, and wherein the second layer comprises a second polymer matrix, different from the first polymer matrix, comprising 1 to 20wt% of a second graphene based material per a total weight of the second polymer matrix and the second graphene based material. The second graphene based material may be the same as the first graphene based material or it may be different. The first and second graphene based materials may both be rGO, for example.

[0031] The interface between the layers of the multilayer may be non-flat / uneven, and may comprise a periodic structure, such as a jagged, or sawtooth, or wavelike structure, in order to reduce specular reflection and to increase a pathlength of the radiation within the composite material. This beneficially increases the absorption of the incoming radiation. Optionally, in some examples, when the composite material comprises the multilayer structure, the first polymer matrix is a PVDF-based matrix, and the second polymer matrix is a PETG- based matrix.

[0032] Optionally, in some examples, the multilayer structure comprises one or more layers with a first graphene-based material concentration and one or more layers with a second graphene-based material concentration, different from the first graphene-based material concentration. In this way, the radar signal is consecutively reflected by the material with a lower graphene based material concentration and then absorbed by the material with higher concentration graphenebased material.

[0033] Optionally, in some examples, the above and below noted preferred thicknesses and / or graphene based material concentrations of the frame or composite material may be respective thicknesses of one of the layers of the multilayer, when the frame comprises a multilayer structure. A total (average) frame thickness would then be the sum of the (average) thicknesses of each layer.

[0034] Optionally, in some examples, the frame comprises one or more hollow frame sections, wherein respective inner surfaces of each of the hollow frame sections are interfacing with a dielectric material. This beneficially improves the electromagnetic radiation absorption, such as radar absorption, by the frame.

[0035] According to a second aspect of the present patent disclosure, there is provided a method of manufacturing a frame for an unmanned aerial vehicle, the frame comprising one or more frame sections, the method comprising: providing a composite material comprising a polymer matrix and a graphene based material, wherein the graphene based material is dispersed within the polymer matrix, wherein the composite material comprises 1 to 20 wt% graphene based material; forming the composite material into the one or more frame sections of the frame using a forming technique; assembling the unmanned aerial vehicle using the formed one or more frame sections.

[0036] It will be understood that the technical advantages of the UAV described above and below apply to the method of manufacturing of the frame of the UAV, since many or most advantages described are associated with the frame of the UAV. Optionally, in some examples, the unmanned aerial vehicle is an unmanned aerial vehicle according to the first aspect above, or any examples associated with the first aspect, or according to any other aspect or example described in the present patent disclosure.

[0037] Optionally, in some examples, the method comprises forming the composite material into liftgenerating rotors of the unmanned aerial vehicle, wherein the assembling of the unmanned aerial vehicle comprises using the formed lift-generating rotors.

[0038] Optionally, in some examples, the providing the composite material comprises: providing a first mixture of neat polymer, or a polymer grade suitable for the forming technique, and a graphene based material in an amount of 3-20 wt% of the first mixture, and diluting the first mixture with the polymer grade suitable for the forming technique when the first mixture comprises the neat polymer, and with the neat polymer when the first mixture comprises the polymer grade suitable for the forming technique, and optionally an impact modifier, thereby forming the composite material with a reduced amount of graphene based material compared to the first mixture.

[0039] In this way, since the polymer grade for the forming technique comprises additives suitable for the forming technique, the graphene based material is added to that polymer grade, simplifying the composite material design process. Since the addition of graphene based material generally changes properties of the polymer such as the melt flow rate (MFR), the addition of neat polymer is then done in order to again increase the melt flow rate again to stay within the desired window for processing with the specific forming technique.

[0040] Optionally, in some examples, the polymer grade suitable for the forming technique comprises additives suitable for the forming technique.

[0041] Optionally, in some examples, the polymer grade suitable for the forming technique comprises the neat polymer.

[0042] Optionally, in some examples, the forming technique is injection moulding, wherein the polymer grade suitable for the forming technique is an injection moulding grade of the polymer.

[0043] Optionally, in some examples, the forming technique is thermoforming, wherein the polymer grade suitable for the forming technique is a thermoforming grade of the polymer. Optionally, in some examples, the forming technique is additive manufacturing, wherein the polymer grade suitable for the forming technique is an additive manufacturing grade of the polymer, wherein the composite material comprises 2 - 8 wt% graphene based material.

[0044] Optionally, in some examples, the polymer matrix is a PVDF-based polymer matrix or a PETG- based polymer matrix.

[0045] According to a second aspect of the present patent disclosure, there is provided a method of manufacturing a polymer part, such as a polymer part of an unmanned aerial vehicle, by a forming technique, the method comprising: providing a first mixture of neat polymer, or a polymer grade suitable for the forming technique, and a graphene based material in an amount of 3-20 wt% of the first mixture; diluting the first mixture with the polymer grade suitable for the forming technique when the first mixture comprises the neat polymer, and with the neat polymer when the first mixture comprises the polymer grade suitable for the forming technique, and optionally an impact modifier, thereby forming a second mixture with a reduced amount of graphene based material compared to the first mixture; forming of the part with the forming technique using the second mixture.

[0046] In this way, since the polymer grade for the forming technique comprises additives suitable for the forming technique, the graphene based material is added to that polymer grade, simplifying the composite material design process. Since the addition of graphene based material generally changes properties of the polymer such as the melt flow rate (MFR), the addition of neat polymer is then done in order to again increase the melt flow rate again to stay within the desired window for processing with the specific forming technique.

[0047] Optionally, in some examples, the forming technique is injection moulding, wherein the polymer grade suitable for the forming technique is an injection moulding grade of the polymer.

[0048] Optionally, in some examples, the forming technique is additive manufacturing, wherein the wherein the polymer grade suitable for the forming technique is an additive manufacturing grade of the polymer.

[0049] Optionally, in some examples, the forming technique is thermoforming, wherein the polymer grade suitable for the forming technique is a thermoforming grade of the polymer. Optionally, in some examples, the forming technique is hot pressing, wherein the polymer grade suitable for the forming technique is a hot pressing grade of the polymer.

[0050] Optionally, in some examples, the forming technique is blow moulding, wherein the polymer grade suitable for the forming technique is a blow moulding grade of the polymer.

[0051] Optionally, in some examples, the forming technique is rotational moulding, wherein the polymer grade suitable for the forming technique is a rotational moulding grade of the polymer.

[0052] Optionally, in some examples, the polymer part is a frame or a section of the frame of an unmanned aerial vehicle, such as the unmanned aerial vehicle is an unmanned aerial vehicle according to any of the above or below described aspects, examples, and / or embodiments.

[0053] Optionally, in some examples, the polymer part is at least a section of a frame of the unmanned aerial vehicle.

[0054] Optionally, in some examples, the polymer is a PVDF-based polymer or a PETG-based polymer.

[0055] Optionally, in some examples, the forming technique is injection moulding, wherein the polymer grade suitable for the forming technique is a polymer grade suitable for injection moulding.

[0056] Optionally, in some examples, the forming technique is additive manufacturing, wherein the polymer grade suitable for the forming technique is a polymer grade suitable for additive manufacturing.

[0057] Optionally, in some examples, the neat polymer is a neat PVDF-based polymer or a neat PETG- based polymer.

[0058] Optionally, in some examples, the polymer grade suitable for the forming technique is a PVDF- based polymer grade suitable for the forming technique or a PETG-based polymer grade suitable for the forming technique.

[0059] The present patent disclosure, in one or more other aspects, further relates to an unmanned aerial vehicle comprising a frame, wherein the frame is made of a composite material comprising a polymer matrix and a graphene based material, wherein the graphene based material is dispersed within the polymer matrix, wherein the composite material comprises 1 to 20 wt% graphene based material; and a propulsion system comprising one or more propulsors, wherein the frame is configured to both provide primary structural integrity to the unmanned aerial vehicle and be an outer shell of the unmanned aerial vehicle.

[0060] It will be apparent that any examples, features and / or advantages of the UAV according to the first aspect apply to the UAV of the present aspect.

[0061] The frame of the UAV may alternatively be configured as an airplane and may comprise two or more wings, a tail section, a vertical stabilizer, a horizontal stabilizer, and / or various control surfaces, such as ailerons, elevators and / or a rudder.

[0062] In accordance with another aspect, there is provided a frame for an unmanned aerial vehicle comprising a central body section and three or more outer body sections, wherein the frame is made of a composite material comprising a polymer matrix and a graphene based material, wherein the graphene based material is dispersed within the polymer matrix, wherein the composite material comprises 1 to 20 wt% graphene based material, wherein the frame is configured to carry or have mounted thereon a propulsion system comprising three or more propulsors, wherein each of the three or more outer body sections is configured to accommodate or have mounted thereon a propulsor of the three or more propulsors, wherein the frame is configured to both provide primary structural integrity to the unmanned aerial vehicle and be an outer shell of the unmanned aerial vehicle. The frame may be for providing an unmanned aerial vehicle with a lowered radar cross section.

[0063] It will be understood that the technical advantages of the UAV described above and below apply to the frame for the unmanned aerial vehicle, since many or most advantages described are associated with the frame of the UAV. Any embodiments of the frame UAV according to the first aspect apply to the UAV of the present aspect.

[0064] It will be understood that technical advantages and effects associated with features, examples and / or embodiments of one aspect, apply to the corresponding, similar or equivalent features, examples and / or embodiments the other aspects. It will also be apparent that the features of the various aspects, examples and / or embodiments thereof may be applied to the other aspects, examples and / or embodiments thereof.

[0065] Brief Description of the Drawings The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of devices of the present disclosure. The above and other advantages of the features and objects of the disclosure will become more apparent, and the aspects and embodiments will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

[0066] Fig. 1 illustrates a schematic diagram representing an unmanned aerial vehicle according to some examples of the present patent disclosure;

[0067] Fig. 2 illustrates a perspective view of an unmanned aerial vehicle according to some examples of the present patent disclosure;

[0068] Fig. 3 illustrates a perspective view of an unmanned aerial vehicle according to some examples of the present patent disclosure;

[0069] Fig. 4 illustrates a schematic diagram representing an unmanned aerial vehicle according to some examples of the present patent disclosure;

[0070] Fig. 5 illustrates a method according to some examples of the present patent disclosure;

[0071] Fig. 6 illustrates a method according to some examples of the present patent disclosure;

[0072] Fig. 7 illustrates a method according to some examples of the present patent disclosure;

[0073] Fig. 8 illustrates a cross section of an outer body section of an unmanned aerial vehicle according to some examples of the present patent disclosure;

[0074] Fig. 9 illustrates a perspective view of an unmanned aerial vehicle according to some examples of the present patent disclosure;

[0075] Fig. 10 is a graph indicating electrical impedance of a PVDF-based composite material according to some examples of the present patent disclosure;

[0076] Fig. 11 is a graph of surface resistance of a PETG-based composite material according to some examples of the present patent disclosure;

[0077] Fig. 12a-d are graphs of obtained real and complex part of permittivity versus frequency for a PVDF reference and PVDF-based composite materials according to some examples of the present patent disclosure;

[0078] Figs. 13a-d are graphs of obtained real and complex part permittivity versus frequency for a (a) PETG reference and (b-d) PETG-based composite materials according to some examples of the present patent disclosure;

[0079] Figs. 14a-b are graphs of calculated absorption coefficients versus graphene concentration of (a) PVDF-based composite materials (b) PETG-based composite materials according to some examples of the present patent disclosure

[0080] Fig. 15 is a graph of calculated electromagnetic material properties, including reflectance, transmittance and absorptance, versus sample thickness of a PVDF-based composite material comprising 3.2 wt% rGO according to some examples of the present patent disclosure; Figs. 16a-f are graphs of calculated electromagnetic material properties, including reflectance, transmittance and absorptance, versus concentration for various thicknesses of PVDF-based composite materials according to some examples of the present patent disclosure; and

[0081] Figs. 17a-f are graphs of calculated electromagnetic material properties, including reflectance, transmittance and absorptance, versus concentration for various thicknesses of PETG-based composite materials according to some examples of the present patent disclosure.

[0082] Detailed Description of Preferred Embodiments

[0083] Fig. 1 shows an unmanned aerial vehicle (UAV) 1 comprising a frame 10 comprising a central body section 20 and a first 31, a second 32, a third 33, and a fourth 34 outer body section. The UAV comprises a propulsion system 40 that comprises a first 41, a second 42, a third, and a fourth 44 propulsor. The fourth outer body section 34 and the fourth propulsor 44 are indicated as optional in Fig. 1 by dashed lines. The present disclosure in addition concerns single or double propulsor UAVs having any number of outer body sections. An example of such a UAV is shown in, and described in relation to, Fig. 9. Referring again to Fig. 1, each of the outer body sections 31 -34 accommodates a corresponding propulsor 41 -44.

[0084] The frame 10 is made of a composite material comprising a polymer matrix and graphene based material. The graphene based material is dispersed within the polymer matrix and the composite material comprises 1 to 20 wt% graphene based material per weight of the composite material. The frame 10 is configured to both provide primary structural integrity to the unmanned aerial vehicle 1 and be an outer shell of the unmanned aerial vehicle 1 . The frame 10, may, in other words, be referred to as an integral frame 10.

[0085] The frame 10 may be made for at least 80 wt% or at least 90 wt% of the composite material.

[0086] It is also an option that the frame 10 is made exclusively of the composite material. The frame 10 may comprise one or more frame sections 1 1-15, as shown, for example, in the UAV 1 of Fig. 2. The frame sections may be produced separately with a suitable forming technique, as further described below, and may thereafter be appropriately attached to each other to form the frame 10. The frame sections 11-15 may each be made for at least 80 wt%, at least 90 wt% or exclusively of the composite material. The frame 10 may be comprised of a plurality of frame sections each having the same amount of composite material or one or more of the frame sections having a different amount of composite material.

[0087] In Fig. 2, the propulsors of the unmanned aerial vehicle 1 comprise lift-generating rotors 51-54 and respective electrical motors 71-74, configured to supply power to the lift-generating rotors 51-54. A first lift-generating rotor 51 is arranged in outer body section 31. Second 52, third 53, and fourth 54 lift-generating rotors are arranged respectively in the second 32, third 33, and fourth 34 outer body sections. In this example, the outer body sections 31-34 are arranged with equal angular spacing around the central body 20, since this makes the control of the UAV 1 simpler, but this is optional as the control can be adapted to each specific design if necessary. The lift-generating rotors 51-54 may be made of the composite material. The composite material of the lift-generating rotors 51 -54 may comprise a different amount of graphene based material

[0088] The frame 10 of UAV 1 of Fig. 2 comprises a first frame section 11 and a second frame section 12, which were manufactured separately and attached to each other when assembling the UAV 1. The UAV 1 of Fig. 3 shows another example of how the frame 10 is built. The frame 10 in this example comprises a first frame section 11, which comprises the central body section 20. To the first frame section 11 are connected a second frame section 12, a third frame section 13, a fourth frame section 14, and a fifth frame section 15. These second to fifth frame sections 12-15 correspond to, or comprise, the outer body sections 31-34. The dashed lines between the first 1 1 and each of the second 12 to fifth 15 frame sections indicate where these frame sections are respectively attached.

[0089] The unmanned aerial vehicle 1 as depicted in Fig. 4 comprises internal components 60, wherein the frame 10 encloses the internal components. In Fig. 4, the internal components are part of the central body section 20, but there may be additional internal components in the outer body sections, such as outer body sections 1 -33, for example. The internal components 60 comprise an electronic control system 80 configured to control the propulsion system 40.

[0090] The UAV 1 of Fig. 4 furthermore indicates that the electronic control system 80 is arranged to control the propulsion system 40, as indicated by the dotted arrow. Although this UAV 1 is configured with three propulsors, it may have any suitable number of propulsors, such as one, two, four, six, eight, etc. The propulsion system 40 in this example comprises battery 46 which, as indicated by the dashed arrows, is arranged to provide electricity to the first 71, second 72. and third 73 electrical motors, which in turn drive, respectively, the first 51, second 52, and third 53 propellers. As indicated with the dash-dotted arrows, the first 51, second 52, and third 53 propellers are positioned respectively in the first 31, second 32, and third 33 outer body sections.

[0091] The unmanned aerial vehicle 1 comprises one or more power source housings 90, 92, 94 each covering or housing a power source 71-74 of the one or more power sources 71-74. The power source housings 90, 92, 94 are made of the composite material, and may optionally be integral with the frame 10. In the example of Fig. 8, an outer body section 31 of a UAV is shown, in which a power source 71. is attached. The power source 71 is arranged to power the rotor 51, which is, in this cross-section view, visible as a rotor hub. The power source 71 comprises a rotatable axle 76 and windings 78 but may comprise additional components, such as connecting wires and further electrical connections, which are not shown in Fig. 8. The power source housing 90 covers a lower end of the electrical motor 71, in this example. The power source housing 92 and 94 are made to abut against each other and comprise a suitable shape so that axle 76 is free to move while covered to a large extent by the power source housings 92 and 94. Although the edges of the parts are drawn as lines, being actual parts, they will have a certain thickness like 2 mm, 3 mm, 4 mm, or 5 mm, for example.

[0092] Fig. 9 shows a UAV 1 with a single propulsor, here embodied as a motor (not shown) with a propeller 51 , placed in the outer body section 31. The outer body section comprises a power source housing 90 covering the motor. The UAV 1 in this example comprises a right wing 102, a left wing 104, and undercarriage 108, which may be foldable. The frame 10 of the UAV 1 in this example further comprises a tail section 106, comprising a first 108 and a second 109 vertical stabilizer and a horizontal stabilizer 110 in this example. In this example, the frame 10 comprises a first frame section 10 comprising the central body section 20 and the first outer body section 31, a second frame section 11 comprising the tail section 106 and a third frame section 12 comprising the right 102 and left 104 wings. The frame 10 of the UAV 1 is suitably made of the composite material, such that it is a UAV 1 with a lowered radar cross section. The propeller 51 may also be made of the composite material. The UAV 1 of Fig. 9 may further comprise additional parts, which are not shown, but are known in the art, such as ailerons, flaps, elevators, a rudder, etc. Although the propulsor in this case is a propeller, the UAV 1 may alternatively comprise a different type of propulsor, like a jet engine. The UAV 1 may also comprise more than one propulsor.

[0093] The frame 10 may comprise one or more internal reinforcing elements, such as ribs, placed on suitable places on in inner side thereof, for providing increased strength, improved stiffness, and / or increasing of load-bearing capacity of the frame without significantly increasing its thickness and weight, and thus lowering production costs. Typically, these elements may be provided perpendicular to the outer surface of the frame 10. Alternatively, or additionally, the frame 10 may have curved outer and inner surfaces for providing similar or additional beneficial effects on the strength, stiffness and / or load-bearing capacity.

[0094] Method(s) of preparing the composite material The composite material, comprising a polymer-based matrix and a graphene-based material, may be prepared by blending or compounding, for example with a twin screw extruder, of a powder or granulate of the polymer and the graphene -based material, rGO in the examples 1 and 2 below, in powder form. The compounding may be performed at elevated temperature, such as in the range of 150 to 300 °C. Respective filaments for use in additive manufacturing equipment like 3D-printers were produced by an extruder thereafter, using the composite materials. Parts for material evaluation were prepared by FDM printing at 240-300 °C.

[0095] The composite materials according to the examples below were processed in a 24-mm twin screw extruder where the rGO was fed through a side feeder to be mixed with the neat polymer. The compounding process may be conducted in two steps, including a first step to make a masterbatch of the composite material with a relatively high loading (e.g. 2-12 wt%, for example a 6.5 wt% masterbatch for PVDF and a 3 wt% masterbatch for PETG) of the reduced graphene oxide in the neat polymer, and a second step was dedicated to extruding the dilution of the masterbatch with the neat polymer and obtaining the final compound with the desired concentration of the graphene. Unless stated otherwise, the composite materials in the examples below were produced in a single step procedure in which the respective polymer is compounded with an amount of the rGO that results in the wanted concentration in the composite material.

[0096] In Fig. 5 is shown an example method 500 of manufacturing a frame 10 for an unmanned aerial vehicle 1, such as the UAV 1 according to any of the examples described above, the frame 10 comprising one or more frame sections 11-15. The method 500 comprises a step of providing 510 a composite material comprising a polymer matrix and a graphene based material. The graphene based material is dispersed within the polymer matrix and comprises 1 to 20 wt% graphene based material relative to the weight of the composite material. The method 500 further comprises a step of forming 520 the composite material into the one or more frame sections 1 1-15 of the frame using a forming technique. The method 500 further comprises a step of assembling 530 the unmanned aerial vehicle 1 using the formed one or more frame sections 1 1-15.

[0097] Method 500 comprises the optional step of forming 522 the composite material into liftgenerating rotors 51-54 of the unmanned aerial vehicle 1. Then, the assembling 530 the unmanned aerial vehicle 1 comprises using the formed lift-generating rotors 51-54 for assembling the UAV 1.

[0098] According to some examples, as shown in Fig. 6, the providing 510 the composite material may comprise a step of providing 612 a first mixture of neat polymer, or a polymer grade suitable for the forming technique, and the graphene based material in an amount of 6-10 wt% of the first mixture. The providing 510 may further comprise a step of diluting 614 the first mixture with the polymer grade suitable for the forming technique when the first mixture comprises the neat polymer, and with the neat polymer when the first mixture comprises the polymer grade suitable for the forming technique, and optionally an impact modifier and / or other additives, thereby forming the composite material with a reduced amount of graphene based material compared to the first mixture.

[0099] For example, when the forming technique is injection moulding, wherein the polymer grade suitable for the forming technique is an injection moulding grade of the polymer.

[0100] In another example, when the forming technique is additive manufacturing, the polymer grade suitable for the forming technique is an additive manufacturing grade of the polymer. In this case, to allow for suitable processing of the material with the additive manufacturing technique, the composite material may suitably comprise 2 - 8 wt% graphene based material, such as rGO. A higher percentage of the graphene based material may cause the material to exhibit properties that are unsuitable for the technique, such as a high melting temperature or a too low melt flow rate, for example.

[0101] Example composite materials

[0102] Example 1 - PVDF-based composite material

[0103] The composite material may comprise a PVDF-based polymer matrix, further also referred to as PVDF or the PVDF-based polymer, and a graphene-based material, in this example embodied as reduced Graphene Oxide (rGO). The rGO in this example 1 and the other examples had a carbon content > 97 at%.

[0104] The PVDF-based polymer matrix is mixed with rGO by extrusion as described above such that the composite material comprises 1-20 wt% rGO relative to the total weight of the composite material. Several composite materials have been manufactured, namely with 2 wt% rGO, 3.2 wt% rGO and 6.5 wt% rGO. These samples are sometimes indicated herein as “PVDF low”, “PVDF medium” and “PVDF high”, respectively. Unless mentioned otherwise, the respective samples were formed by 3D-printing (Bambulab PIS, Prusa MK3 or Ultimaker S5) as rectangular plates with varying thicknesses, ranging between 0.5 and 5 mm.

[0105] In the present patent disclosure, the PVDF-based polymer may be a PVDF-based polymer for a specific forming technique, such as injection moulding, thermo forming, hot pressing, or FDM. The PVDF-based polymer may comprise additives, such as one or more impact modifiers, for example hexafluoropropylene, chlorotrifluoroethylene, and / or acrylic-based impact modifiers. The PVDF-based polymer may comprise the impact modifiers in a concentration of 1 -30 wt%, for example 5-25 wt% or 10-25 wt%, per total weight of the PVDF- based polymer. The PVDF-based polymer (without the graphene-based material) may have a melt flow rate (MFR) in the range of 0.5- 40 g / 10 min, when measured according to ISO 1133- 1 with a weight of 5 kg and at a temperature of 230 °C. The addition of graphene may lower the MFR.

[0106] The PVDF-based polymer used in this example was the product known as Kynar flex 2750-01 from Arkema. Commercially available examples of other suitable grades of PVDF include, but are not limited to, the grades known on the market by the trademark Kynar from Arkema, for example Kynar flex 2750, Kynar flex 2950. These may be suitable for various forming techniques, including injection moulding, thermoforming, FDM (especially when mixed with a graphene-based material) and FDM. Alternatively, the polymers known on the market by the trademark Zheflon, such as Zheflon FL2008, FL2011 and FL2013 may be suitable grades of PVDF as well. Other examples are Solvay’s “Solexis” PVDF grades, which are known to be suitable for thermoforming, and Nile Polymers’ “Fluorinar” PVDF grades.

[0107] Example 2 - PETG-based composite material

[0108] The composite material may comprise a PETG-based polymer matrix, further also referred to as PETG or the PETG-based polymer, and a graphene-based material, in this example embodied as reduced Graphene Oxide (rGO).

[0109] The PETG-based polymer matrix is mixed with rGO such that the composite material comprises 1-20 wt% rGO relative to the total weight of the composite material. Several composite materials have been manufactured, namely with 2 wt% rGO, 3 wt% rGO and 3.5 wt% rGO. These samples are sometimes indicated herein as “PETG low”, “PETG medium” and “PETG high”, respectively. The PETG-based composite materials having 2 wt% rGO were prepared by the two-step process mentioned above, starting from a 3 wt% rGO masterbatch. Unless mentioned otherwise, the respective samples were formed by 3D-printing (Bambulab PIS, Prusa MK3 or Ultimaker S5) as rectangular plates with varying thicknesses, ranging between about 0.5 mm and about 5 mm.

[0110] One advantage of PETG is that it has a relatively low density, at least compared to PVDF, and therefore the UAV will weigh less. In the present patent disclosure, the PETG-based polymer may be a PETG-based polymer for a specific forming technique, such as injection moulding, thermo forming, hot pressing or FDM. The PETG-based polymer may comprise additives, such as a stabilizer, and / or one or more impact modifiers, such as ethylene terpolymer. The PETG-based polymer may comprise the impact modifiers in a concentration of 1-30 wt%, for example 5-25 wt% or 10-25 wt%, per total weight of the PETG-based polymer. The PETG-based polymer (without the graphenebased material) may have a melt flow rate (MFR) in the range of 0.5- 40 g / 10 min, when measured according to ISO 1133-1 with a weight of 5 kg and at a temperature of 230 °C. The addition of graphene may lower the MFR. The PETG-based polymer may have an intrinsic viscosity in the range of 0.7-0.85 dl / g when measured according to ISO 1628-5. Commercially available examples of suitable grades of PETG include, but are not limited to Selenis Mimesis DP 301, Selenis Selekt XP 210 or TDS Selenis Selekt XP 210 Plus.

[0111] Example 3 - additive manufacturing of parts made of the composite material

[0112] Fused deposition modelling, FDM, was used to print parts based on the composite material for evaluation of radar absorption, electrical properties and mechanical properties. FDM is a 3D printing technique used to produce parts from polymers such as PETG and PVDF, and polymer composite materials such as PVDF-graphene and PETG-graphene described in Example 1 and 2 respectively. The material used for printing is in the shape of a continuous filament thread and is extruded through a moving heated nozzle onto the build volume. Temperatures are typically 180-300 °C. The diameter of the filament was either 2.85 mm or 1.75 mm, and the diameter of the nozzle lay in the range of 0.2-1 mm, such as 0.4 mm.

[0113] Mechanical property testing

[0114] Charpy notched impact strength was evaluated according to ISO 179-2:2020. The test specimens were subjected to conditioning for at least 24 h at (23 ±2) °C with (50 ±10) % RH using a climate chamber before the test. The test pieces were notched according to TSO 179- 1 / leA. The impact testing was performed according to ISO 179-1 / leA: Test piece 1, edgewise, notched, with 10 replicates. A 25 J impact was used. The equipment used was a Tinius Olsen IT 503 Pendulum Impact Tester.

[0115] Tensile properties were measured according to ISO 527-2:2012 with type IB test specimens. The equipment used was a Tinius Olsen 1 OST with extensometer, load cell 0,2-10 kN and Tinius Olsen Horizon software. The test specimens were subjected to conditioning for at least 24 h at (23 ±2) °C in (50 ±10) % RH using a climate chamber before the test. The tensile testing was performed according to ISO 527-2:2012, with five replicates. The test specimens were mounted between the grips and put in place with air pressure to avoid slipping while testing. A 50 mm distance was used on the extensometer to analyse the elongation at break. The tensile test was performed with a speed of 10 mm / min.

[0116] Mechanical properties of materials according to example 1 and 2

[0117] Table 1 below lists the obtained median values of Tensile Modulus, Ultimate Strength, and Elongation at Break for the samples described above. Table 2 below lists the obtained impact strengths for respective PVDF-based and PETG-based composite materials according to examples 1 and 2 respectively. It is noted that the measured samples were made using FDM which may result in different values than for other forming techniques. For example, the tensile modulus may be higher in other techniques, such as injection moulding.

[0118] Table 1: Obtained tensile properties for the PVDF and PETG-based composite materials

[0119] It can be seen that the addition of the rGO to the materials affects the tensile properties. Tn the PETG-based samples, the eeffect is less apparent, which may be due to the rGO concentrations being relatively low. For example, the PETG samples are electrically resistive, while the PVDF samples “medium” and “high” are found to be electrically conductive, as described below. The addition of rGO to PVDF increases the tensile modulus to such an extent, that it becomes better suited for use in a frame such as the frame 10.

[0120] Table 2: Obtained Impact Strength for the PVDF and PETG-based composite materials

[0121] The impact strengths for the PETG-based samples are roughly the same for each rGO concentration. For the PVDF-based samples, the impact strength lowers due to the rGO.

[0122] It is noted that the neat polymers of PETG and PVDF may have literature values that are different since the values are for different forming techniques, not AM. For example, as a comparison, a reported tensile strength of PETG made with 3D-printing could be as low as 70% of the tensile strength obtained for an injection moulded part, an elongation at break of about 20% of the injection moulding value, an impact strength as low as 50% of the injection moulding value and a flexural strength as low as 60% of the injection moulding value. There are multiple ways to improve the mechanical performance of the composite materials, such as by improving the AM parameters or by changing the forming technique, for example to injection moulding.

[0123] In summary, it can be concluded that the composite materials perform sufficiently for use in a frame of a (U)AV such as the frame 10.

[0124] Electromagnetic material properties at radar frequencies

[0125] The complex electrical permittivity (s) and complex magnetic permeability (p) of samples according to example 1 and example 2 were obtained. Samples made by additive manufacturing using the composite materials according to examples 1 and 2 were placed in a waveguide and their respective reflection and transmission of microwaves at different frequencies in the so- called X-band (8.2 - 12.4 GHz) were recorded. Additionally, samples of neat PVDF and PETG without graphene were also measured in the same way. From these data, the electrical permittivity and magnetic permeability can be calculated using known physical relations (see e.g. Antenna-Theory, “Introduction to waveguides, " antenna-theoiy.com, 2024-08-19}, the results of which are shown in Figs. 12a-d (PVDF) and 13a-d (PETG). The estimated error in the obtained values was 10%. The real part of the magnetic permeability of all samples was close to or equal to 1, and the imaginary part close to or equal to 0, which is expected since the samples are made of the composite materials which are non-magnetic. Therefore, the figures and discussion herein concern the permittivity.

[0126] Respective absorption coefficients at various frequencies (8.2, 10 and 12.4 GHz) were calculated for PVDF-based materials (Fig. 14a) and PETG-based materials (Fig. 14b). In addition, values for the electromagnetic material properties Reflectance (R), Transmittance (T) and Absorbance (the fraction of light absorbed, commonly indicated by a, but here indicated by A) where the sum of R, T, and A is equal to 1 were calculated as a function of sample thickness using relations obtained in the book of Kristensson mentioned below. An example thereof of a PVDF-based sample with 3.2 wt% rGO is shown in Fig. 15. Dashed lines are indicated at six thicknesses (0.5, 0.8, 1.6, 2.4, 3.2 and 4.0 mm) and labelled with a-f respectively, which corresponds to the subfigures a-f of Figs. 16 and 17, in which each subfigure relates to the corresponding thickness or each reference sample (no rGO) and the composite material for both PVDF (Fig. 16) and PETG (Fig. 17) based samples. Each of the plots in Figs. 16 and 17 plot the calculated R, T, and A values for a specific thickness versus rGO concentration in wt%, the values at 0 wt% being a of a reference sample made of PVDF without rGO (Fig. 16) and PETG without rGO (Fig. 17). The value of permittivity used for the calculations of R, T and A was the ten nearest neighbour smoothed value for a frequency of 10 GHz.

[0127] In the calculations, it is assumed the sample is situated between two air layers. It is further assumed that incident radiation is perpendicular to the sample. The results of these calculations are therefore conservative in the sense that in such an arrangement, the reflection will all be back towards the source of the incident radiation. In practice, the surface would not always be normal to the signal, and would also have some structure and therefore not reflect all the radiation back to the source. The surfaces of the frame / outer shell can be shaped with such a topography that the signal is mainly reflected in directions not normal to the source. This would yield less detected reflected signal and potentially reabsorbed signal. Reflectance does relate directly to the RCS, however the relation is complex as it depends on factors like drone orientation, surface structure, internal components of the drone, etc. Also, the transmittance (e.g. a value proportional to the square of the transmittance) may contribute to the RCS in practice, because transmitted radiation may be reflected by internal components, especially if they are made of metal. In other words, it can be said that increased absorbance reduces RCS by dissipating incident energy within the material and that increased reflectance contributes to RCS but only in directions that include the radar receiver. The procedure used to calculate R, T, and A from permittivity, permeability and thickness was done according to Kristensson, G. (1999), “Elektromagnetiskvdgutbrednin Studentlitteratur

[0128] AB, for example as published online by Lund University, Sweden: https: / / portal.research.lu.se / files / 5570175 / 4239042.pdf. This publication is incorporated herein by reference in its entirety. Chapter 4, in particular section 4.2, thereof concerns calculations of R, T, and A. The imaginary part of the magnetic permeability was set to 0 and the real part of the permeability was set to 1 in the calculations. The absorption coefficients are calculated based on equations 1.19 and 1.20 of “Solid State Physics, Part II, Optical Properties of Solids”,

[0129] M.S. Dresselhaus, which may be found at https: / / web.mit.edU / 6.732 / www / 6.732-pt2.pdf.

[0130] The frame, being made of the composite material, may have an average thickness in the range of 0.5 to 4 mm, wherein the composite material comprises the graphene based material in the range of 1 to 20 wt% per weight of the composite material. Radiation, like radar, travels through the frame and may be reflected by internal components of the UAV, and then the reflected radiation travels through the frame a second time. The attenuation is therefore occurring by twice the thickness of the frame.

[0131] Referring again to Fig. 15, concerning data for a PVDF-based composition with 3.2 wt% rGO. it can be seen that, surprisingly, there is a maximum of the absorptance as high as about 0.45 at a thickness of approximately 0.8 mm. The reflectance is also not very high in that region, and ranges between about 0.15 to 0.45 in a thickness range of 0.5 to 1.5 mm, while the absorptance lies in a range between 0.42 and 0.45. Fig. 16 shows this trend for PVDF, but for a single thickness between 0.5 mm and 4 mm in each respective subplot a) to 1). It is found beneficial to use composite materials based on PVDF having a graphene-based material concentration in a range of 1.5 wt% to 3.5 wt%, or preferably 2.5 wt% to 3.5 wt%. These concentrations are especially beneficial for thicknesses ranging between 0.5 mm and 3.2 mm, preferably at most (about) 2.5 mm, beyond which the absoiptance decreases and the reflectance increases to above 0.5. A product of the graphene-based material concentration in wt% and the thickness in mm beneficially lies in the range of 0.7 to 12, or 0.75 to 11.2, or 0.75 to 9 (mm wt%).

[0132] Fig. 17 shows the same data for PETG, that is, for a single thickness between 0.5 mm and 4 mm in each respective subplot a) to f). It is found beneficial to use composite materials based on PETG having a graphene-based material concentration of at least 2.5 wt%. Beneficially, the thickness is at least about 1.5 mm, such as 1.6 mm, or 2 mm, in this case. A product of the graphene-based material concentration in wt% and the thickness in mm beneficially is at least 3,7, 4 or 5. Inferring from the plots of Fig. 17, it is clear that PETG will have increased absorptance at higher graphene-based material concentrations. The tested materials, however, are still performing better than PETG without any added graphene-based material. In addition, it is furthermore beneficial in frames comprising multilayers to add a less reflective material, for example, a PETG-based composite, on either an inside or outside surface of the frame, and a composite material with a higher absorptance (and reflectance) respectively on the outside or inside surface of the frame, since in this way still a good total performance can be obtained. This also allows for the shaping of the interface(s) between the two (or more) composite material layers in the frame, in order to reduce the reflection towards the radar source, to increase the attenuation of the signal within the frame, or both.

[0133] Electrical conductivity / resistivity

[0134] Fig. 10 shows the modulus of impedance (|Z|) as a function of graphene (rGO) concentration for the PVDF-based composite materials according to example 1, while Fig. 11 concerns the PETG-based composite materials according to example 2. For each rGO concentration, there are three samples SI (thinnest), S2 (intermediate thickness) and S3 (thickest). For each respective sample, the thickness for each sample is indicated in the legend of the plots of Figs. 10 and 11. The impedance of each sample was measured using an LCZ meter (Keithley 3322) unless stated otherwise at a frequency of 1 kHz and a voltage of 1 V. Contacts were prepared using silver paint. A plurality of measurements was performed on each sample.

[0135] As can be seen in Fig. 10, the PVDF based composite material shows an electrical impedance in the range of 20 ohm (medium and high rGO concentrations) to 10 kohm (low rGO concentration) with an impedance angle which approximates to 0 degrees. It can therefore be considered as a perfect resistor, classifying the material as conductive. A PVDF reference sample without rGO behaves as an insulator (higher than 1012ohm).

[0136] Results for PETG are indicated in Fig. 11, and were recorded using a surface resistance meter (SRM110 by Wolfgang Warmbier), since the samples were exhibiting a capacitive behaviour when electrical impedance was measured. A clear effect on the measured surface resistance of the addition of rGO can be seen. The PETG-rGO composite materials behave as insulators which is beneficial because if some parts of the aerial vehicle are insulating but still absorb / attenuate radar signal while other parts to be electrically conductive or dissipative and at the same time absorb some radar signal. A frame in which PETG and PVDF-composite materials are combined is useful because aerial vehicles with varying electrical conductivity and increased radar absorption can be obtained.

[0137] Further methods of preparing composite materials and polymer parts made thereof

[0138] Fig. 7 shows a method 700 of manufacturing a polymer part, such as the frame 10, any of the frame sections 11-15, any of the propellers 51-54, or any of the power source housings 90, 92, 94, of the UAV 1 according to any of the examples provided herein, by a forming technique. The method comprises: providing 710 a first mixture of neat polymer, or a polymer grade suitable for the forming technique, and the graphene based material in an amount of 6-20 wt% of the first mixture: diluting 714 the first mixture with the polymer grade suitable for the forming technique when the first mixture comprises the neat polymer, and with the neat polymer when the first mixture comprises the polymer grade suitable for the forming technique, and optionally an impact modifier, thereby forming a second mixture with a reduced amount of graphene based material compared to the first mixture; and forming 720 of the part with the forming technique using the second mixture.

[0139] Beneficially, this method allows for the tuning of properties of the produced composite material, the tuneable properties including, for example, the electrical resistivity, radar absorption, the Melt Flow Rate, and the impact strength (e.g. Charpy notched impact strength).

[0140] For example, when the forming technique is injection moulding, wherein the polymer grade suitable for the forming technique is an injection moulding grade of the polymer.

[0141] In another example, when the forming technique is additive manufacturing, the polymer grade suitable for the forming technique is an additive manufacturing grade of the polymer. In this case, to allow for suitable processing of the material with the additive manufacturing technique, the composite material may suitably comprise 2 - 8 wt% graphene based material, such as rGO. A higher percentage of the graphene based material may cause the material to exhibit properties that are unsuitable for the technique, such as a high melting temperature or a too low melt flow rate, for example.

[0142] Although the present invention has been described with reference to specific embodiments, also shown in the appended drawings, it will be apparent to those skilled in the art that many variations and modifications can be done within the scope of the invention as described in the specification and defined with reference to the claims below.

Claims

CLAIMS1. Unmanned aerial vehicle (1) comprising: a frame ( 10) comprising a central body section (20) and three or more outer body sections (31-34); and a propulsion system (40) comprising three or more propulsors (41-44), wherein each of the three or more outer body sections (31-34) accommodates a propulsor (41-44) of the three or more propulsors (41-44), characterized in that the frame (10) is made of a composite material comprising a polymer matrix and a graphene based material, wherein the graphene based material is dispersed within the polymer matrix, wherein the composite material comprises 1 to 20 wt% of a graphene based material per weight of the composite material; and in that the frame (10) is configured to both provide primary structural integrity to the unmanned aerial vehicle (1) and be an outer shell of the unmanned aerial vehicle (1).

2. Unmanned aerial vehicle (1) according to claim 1, wherein the frame (10) is made for at least 80 wt%, or at least 90 wt%, of the composite material.

3. Unmanned aerial vehicle (1) according to claim 2, wherein the frame (10) is made exclusively of the composite material.

4. Unmanned aerial vehicle (1) according to any one of the preceding claims, wherein the unmanned aerial vehicle ( 1 ) is a drone, wherein the propulsors (41 -44) are lift-generating rotors (51-54), wherein a lift-generating rotor (51-54) of the lift- generating rotors (51-54) is attached to each of the outer body sections (31-34), wherein optionally the outer body sections (31-34) are arranged with equal angular spacing around the central body (20).

5. Unmanned aerial vehicle (1) according to claim 4, wherein the lift-generating rotors (51-53) are made of the composite material.

6. Unmanned aerial vehicle (1) according to any one of the preceding claims, wherein the frame has an average thickness in the range of 1 to 4 mm, wherein optionally the composite material comprises the graphene based material in the range of 1 to 4 wt% per weight of the composite material.

7. Unmanned aerial vehicle (1) according to any one of the preceding claims, wherein the unmanned aerial vehicle (1) comprises internal components (60), wherein the frame encloses the internal components.

8. Unmanned aerial vehicle (1) according to claim 7, in dependence of claim 4, wherein the propulsion system (40) comprises one or more power sources (71-73) configured to supply power to the lift-generating rotors (51 -54); and wherein the internal components (60) comprise an electronic control system (80) configured to control the propulsion system (40).

9. Unmanned aerial vehicle (1) according to claim 8, wherein the unmanned aerial vehicle (1) comprises one or more power source housings (90, 92, 94) each covering or housing a power source (71-74) of the one or more power sources (71-74), wherein the power source housings (90, 92, 94) are made of the composite material, and optionally wherein the one or more power source housings (90, 92, 94) are integral with the frame (10).

10. Unmanned aerial vehicle (1) according to any one of the preceding claims, wherein the polymer matrix is a PVDF-based matrix, wherein optionally the composite material comprises 1 to 7 wt% o or 2 to 6.5 wt% of the graphene based material per weight of the composite material.

11. Unmanned aerial vehicle (1) according to any one of the preceding claims, wherein the polymer matrix is a PETG-based matrix, wherein optionally the composite material comprises 1 to 7 wt%, 2 to 6.5 wt%, 1 to 4 wt%, or 2 to 3.5 wt% of the graphene based material per weight of the composite material.

12. Method (500) of manufacturing a frame (10) for an unmanned aerial vehicle (1) according to any one of claims 1 to 10, the frame (10) comprising one or more frame sections (11- 15), the method (500) comprising: providing (510) a composite material comprising a polymer matrix and a graphene based material, wherein the graphene based material is dispersed within the polymer matrix, wherein the composite material comprises 1 to 20 wt% graphene based material; forming (520) the composite material into the one or more frame sections (11-15) of the frame using a forming technique; assembling (530) the unmanned aerial vehicle (1) using the formed one or more frame sections (11-15).

13. Method (500) according to claim 12, comprising forming (522) the composite material into lift-generating rotors (51-54) of the unmanned aerial vehicle (1), wherein the assembling (530) the unmanned aerial vehicle (1) comprises using the formed lift-generating rotors (51-54).

14. Method (500) according to claim 12 or 13, wherein the providing (510) the composite material comprises: providing (612) a first mixture of neat polymer, or a polymer grade suitable for the forming technique, and a graphene based material in an amount of 3-20 wt%, or 3-10 wt%, of the first mixture, and diluting (614) the first mixture with the polymer grade suitable for the forming technique when the first mixture comprises the neat polymer, and with the neat polymer when the first mixture comprises the polymer grade suitable for the forming technique, and optionally an impact modifier, thereby forming the composite material with a reduced amount of graphene based material compared to the first mixture.

15. Method (500) according to claim 12, 13, or 14, wherein the forming technique is injection moulding, wherein the polymer grade suitable for the forming technique is an injection moulding grade of the polymer.

16. Method (500) according to claim 12, 13, or 14, wherein the forming technique is additive manufacturing, wherein the polymer grade suitable for the forming technique is an additive manufacturing grade of the polymer, wherein the composite material comprises 1 - 8 wt% graphene based material.

17. Method (500) according to any one of claims 12 to 16, wherein the polymer matrix is a PVDF-based polymer matrix or a PETG-based polymer matrix.

18. Method (700) of manufacturing a polymer part, such as a polymer part (1 1-15, 51 -54) of an unmanned aerial vehicle (1), by a forming technique, the method comprising: providing (710) a first mixture of neat polymer, or a polymer grade suitable for the forming technique, and a graphene based material in an amount of 3-20 wt% of the first mixture; diluting (714) the first mixture with the polymer grade suitable for the forming technique when the first mixture comprises the neat polymer, and with the neat polymer when the first mixture comprises the polymer grade suitable for the forming technique, and optionally an impact modifier, thereby forming a second mixture with a reduced amount of graphene based material compared to the first mixture; and forming (720) of the part with the forming technique using the second mixture.

19. Method (700) according to claim 18, wherein the forming technique is injection moulding, wherein the polymer grade suitable for the forming technique is a polymer grade suitable for injection moulding.

20. Method (700) according to claim 18, wherein the forming technique is additive manufacturing, wherein the polymer grade suitable for the forming technique is a polymer grade suitable for additive manufacturing.

21. Method (700) according to claim 18, 19, or 20, wherein the polymer part is a frame (10) or a section (11-15) of the frame of an unmanned aerial vehicle (1), wherein preferably the unmanned aerial vehicle (1) is an unmanned aerial vehicle (1) according to any one of claim 1 to 11.

22. Method (700) according to any one of claims 18 to 21, wherein the neat polymer is a neat PVDF-based polymer or a neat PETG-based polymer; and / or the polymer grade suitable for the forming technique is a PVDF-based polymer grade suitable for the forming technique or a PETG-based polymer grade suitable for the forming technique.

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