Modular aircraft structural support

Abstract

A low-wing modular aircraft (fig.3,10) has a removeable propulsion system 2a mounted externally, below a fuselage body (fig.3,12) section 26, along a centre-line of the aircraft. A removeable support structure 102a is mounted extending between the fuselage section and a central wing structure 14 below it, transferring load between them. The support includes an interface for interfacing with part of an intake duct 16. In a first embodiment, the intake duct extends between, and the support is located between, an air intake opening of the aircraft and an engine of the propulsion system. In further embodiments, the intake duct is located between the fuselage body and the central wing structure, and forms part of a modular aircraft assembly kit having a number of interchangeable supports, and a number of alternative propulsion systems, each with a respective intake duct portion forming part of the intake duct structure. The support’s intake duct interfaces interface with an intake duct portion of respective propulsion systems when it is mounted to the fuselage body. A method of reconfiguring the aircraft, involves removing a first support and mounting a second support.

Description

FIELD OF THE INVENTION The present invention relates to modular aircraft and airframe structures for modular aircraft. In particular the present invention relates to apparatus for transferring load in a modular aircraft, a modular aircraft assembly kit, and methods of reconfiguring a modular aircraft. BACKGROUND TO THE INVENTION The present invention relates to aircraft and airframe structures, and in particular to reconfigurable military jet aircraft. Military aircraft having different capabilities may be required for performing different roles. For example, a more basic relatively low-powered trainer aircraft may be used for pilot training, whilst more powerful and / or more manoeuvrable aircraft may be used in combat roles. In order to provide such different aircraft having different capabilities and performance, it is known to provide a modular, reconfigurable jet trainer aircraft which can be reconfigured by fitting different propulsion units and / or wing structures externally to a common fuselage body in order to provide these different types of aircraft. Such modular aircraft are disclosed in the present applicant’s granted European Patent No. EP 3 411 293 B1. An advantage of such a modular aircraft system is that an organisation such as an air force need not purchase different types of aircraft separately from different manufacturers, but instead a single modular aircraft system can provide all of the required aircraft types, which can have an economic advantage, and also provides continuity and familiarity with the aircraft systems for pilots and engineers. To provide a conveniently reconfigurable aircraft, there is a need to provide an arrangement to mount a propulsion unit or propulsion system externally of a fuselage body of the aircraft, so that the propulsion system can be interchanged without modification to primary, safety-critical airframe structure of the aircraft. One difficulty associated with such arrangements is that, in order for a propulsion system to be readily removable without significant disassembly of the airframe structure, the propulsion system itself and any associated structures may contribute little, if any, structural support to the airframe structure and structural integrity of the aircraft. It is against this background that the present invention is devised. It is an aim of the present invention to overcome at least one problem associated with the prior art, whether referred to herein or otherwise. SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided apparatus for transferring load in a low-wing modular aircraft having a removably mountable propulsion system, said aircraft comprising: a fuselage body, a central wing structure disposed below a section of the fuselage body, and an intake duct structure arranged to extend between an air intake opening of the aircraft and an engine of the propulsion system, and wherein the propulsion system is removably mountable externally of the fuselage body below said section of the fuselage body and along a centre-line of the aircraft; the apparatus comprising: a support structure configured to be removably mounted between the central wing structure and the fuselage body, the support structure being arranged to locate between the engine and the air intake opening, and the support structure comprising an intake duct interface configured to interface with part of the intake duct structure; wherein the support structure is arranged to extend between the central wing structure and the fuselage body to transfer load between the central wing structure and the fuselage body. In this way, load can be transferred from the central wing structure to the fuselage body and / or to an airframe structure of the aircraft without significant transfer of load to the propulsion system or through other structures around the propulsion system or associated with the propulsion system or engine. This arrangement allows different propulsion systems to be interchanged while maintaining structural rigidity of an airframe structure of the aircraft. In this way, in a modular aircraft in which different propulsion systems may be used, load (e.g. in-flight load) is transferred between the central wing structure and the fuselage body) by the support structure, meaning that the structure of the propulsion system, and / or structures extending around the propulsion system, need not transfer any significant loads in use, and so may be configured to be conveniently removed from the aircraft and interchanged. In particular the apparatus is arranged to transmit vertical load between the central wing structure and fuselage body. The present invention also enables an aircraft from which the propulsion system has been removed to be a self-supporting structure, for example whilst being re-configured or transported. The centre-line of the aircraft may also be defined as, or referred to as, a central plane or lateral plane of the aircraft. The central plane or lateral plane extends vertically and longitudinally and may be a plane of symmetry of the aircraft. Preferably the support structure is arranged to extend vertically. Preferably the support structure is arranged to extend substantially vertically between the central wing structure and the fuselage body. Preferably the support structure defines a load path between the central wing structure and the fuselage body. Preferably the support structure is arranged to extend along a load path between the central wing structure and the fuselage body. Preferably the support structure is arranged to extend vertically to define a load path between the central wing structure and the fuselage body. Preferably the support structure is arranged to support part of the intake duct structure. For example to support and / or secure the intake duct structure within the aircraft. Preferably the apparatus comprises mounting means for (removably) mounting the support structure between the central wing structure and the fuselage body. Preferably the apparatus comprises mounting means provided on the fuselage body for removably mounting the support structure to the fuselage body. These mounting means may be referred to as upper mounting means or upper mounting members, and may comprise brackets. Preferably the apparatus comprises mounting means provided on the central wing structure for removably mounting the support structure to the central wing structure. These mounting means may be referred to as lower mounting means or lower mounting members, and may comprise brackets. Preferably the or each support structure comprises mounting means for removably mounting the support structure to the aircraft. Preferably the support structure is removably mountable between the central wing structure and the fuselage body to enable the support structure to be removed and replaced with an alternative support structure. Preferably the apparatus comprises mounting members provided on the fuselage body and mounting members provided on the central wing structure for mounting the support structure between the fuselage body and the central wing structure. Preferably the support structure comprises at least one aperture or recess shaped to engage with part of the intake duct structure. Preferably the intake duct interface is provided at least in part by a surface defining the aperture or recess. Preferably the support structure comprises a plate arranged to extend between the central wing structure and the fuselage body. With this arrangement, the support structure is particularly compact, which helps to reduce the volume occupied by the support structure and to provide space within the aircraft for other components such as the intake duct structure. A plate configuration is also particularly suitable to handle load in variety of different directions, such as tensile and compressive loads, torsion, and shearing-type loads in a fore-aft direction. Preferably the plate extends in a substantially vertical plane. The central wing structure is preferably for securing wings thereto. The central wing structure preferably comprises part of an airframe structure of the aircraft to which wings or wing structures (e.g., lateral outer wing assemblies) may be secured. Preferably the intake duct structure comprises a forward (e.g., first or upstream) intake duct portion which extends from the air intake opening towards the propulsion system, and a rear (e.g., second or downstream) intake duct portion which extends between the engine and the forward intake duct portion, and the forward intake duct portion is arranged to be connected in fluid communication with the rear intake duct portion; and preferably the intake duct interface is configured to interface with the rear intake duct portion. The propulsion system may comprise the rear intake duct portion. The support structure may be configured to interface with the forward intake duct portion, thereby to connect the forward intake duct portion in fluid communication with the rear intake duct portion. In this way the support structure may be arranged to connect the forward and rear portions of the intake duct structure together. For example when a propulsion system is installed in the aircraft. The intake duct interface may be a first intake duct interface and the support structure may comprise a second intake duct interface configured to interface with the forward intake duct portion. In this way the support structure may provide an adapter for connecting the propulsion system with the intake duct. The support structure may comprise, or function as, an adapter for connecting the propulsion system (or an intake of the propulsion system) in fluid communication with the intake duct structure. According to a second aspect of the invention there is provided an aircraft assembly comprising the fuselage body, the intake duct structure, the central wing structure and apparatus according to the first aspect of the invention, and in which a propulsion system is removably mountable externally of the fuselage body between the fuselage body and the central wing structure and along a centre-line of the aircraft. The aircraft assembly may comprise the propulsion system. The apparatus may comprise a plurality of interchangeable support structures arranged to be mounted between the central wing structure and the fuselage body to transfer load between the central wing structure and the fuselage body. The or each support structure may be configured to interface with a respective propulsion system, thereby to connect the propulsion system in fluid communication with part of the intake duct structure. The or each support structure may be configured to interface with the rear intake duct portion, thereby to connect the rear intake duct portion in fluid communication with the forward intake duct portion. In this way the support structure may be arranged to connect the portions of the intake duct structure together. According to a third aspect of the invention there is provided a modular aircraft assembly kit for a low-wing modular aircraft, said aircraft comprising: a fuselage body to which each of a plurality of alternative propulsion systems may be removably mounted externally below a section of the fuselage body and along a centre-line of the aircraft; a central wing structure disposed below said section of the fuselage body; and an intake duct structure arranged to locate between the fuselage body and the central wing structure; each propulsion system being provided with a respective intake duct portion configured to form part of the intake duct structure; said kit comprising: a plurality of interchangeable support structures arranged to be removably mounted between the central wing structure and the fuselage body, each support structure being arranged to extend between the central wing structure and the fuselage body to transfer load between the central wing structure and the fuselage body when the support structure is mounted between the central wing structure and the fuselage body, the plurality of support structures comprising: a first support structure comprising a first intake duct interface configured to interface with a respective intake duct portion of a first removably mountable propulsion system when the first support structure is mounted between the central wing structure and the fuselage body and the first propulsion system is mounted to the fuselage body; and a second support structure comprising a second intake duct interface configured to interface with a respective intake duct portion of a second removably mountable propulsion system when the second support structure is mounted between the central wing structure and the fuselage body and the second propulsion system is mounted to the fuselage body. The kit may comprise the fuselage body, the central wing structure, and / or the intake duct structure. Each respective intake duct portion may be a rear intake duct portion. The first support structure may comprise a plate having a central aperture shaped to receive part of a respective rear intake duct portion of the first propulsion system. Preferably the central aperture is a single central aperture. The aperture may be substantially circular. A surface defining the aperture may provide at least part of the intake duct interface. The modular aircraft assembly kit may comprise the first propulsion system, and preferably the first propulsion system is a single-engine propulsion system. The second support structure may comprise a plate having a pair of recessed edge portions arranged on opposing lateral sides of the plate. In this case preferably each recess is arranged to receive part of the propulsion system. Each recessed portion may comprise a curved surface or concave surface. For example each recessed portion may comprise a part-circular recess. The modular aircraft assembly kit may comprise the second propulsion system, and preferably the second propulsion system is a twin-engine propulsion system. Preferably the second propulsion system is an alternative propulsion system having different propulsion properties to the first propulsion system. For example the second propulsion system may comprise a more powerful propulsion system than the first propulsion system. The second propulsion system may comprise a different type of propulsion system to the first propulsion system. Preferably the modular aircraft assembly kit comprises a third support structure configured to interface with a respective intake duct portion of a third removably mountable propulsion system when the third support structure is mounted between the central wing structure and the fuselage body and the third propulsion system is mounted to the fuselage body. The kit may comprise the third propulsion system. The third support structure may comprise a plate having a (single) central aperture shaped to receive part of the third propulsion system. The third propulsion system may comprise an engine having a larger fan diameter than a fan diameter of an engine of the first and / or second propulsion systems. In this case the central aperture of the third support structure may have a larger diameter than the central aperture of the first support structure. Preferably each support structure is arranged to extend vertically. Preferably each support structure defines a load path between the central wing structure and the fuselage body. Preferably each support structure is arranged to extend along a load path between the central wing structure and the fuselage body. Preferably each support structure comprises a plate arranged to extend in a substantially vertical plane. Preferably the or each support structure is arranged to extend in a plane which bisects a flow path through the intake duct structure. Preferably the or each support structure is arranged to extend in a plane perpendicular to an axis of the intake duct structure. The central wing structure may be arranged below the section of the fuselage body. The central wing structure may be spaced below the section of the fuselage body. Preferably each support structure is arranged to support part of the intake duct structure. For example to mount the intake duct structure in the aircraft. The intake duct structure may be arranged to secure the intake duct structure, or at least part of The or each support structure may be configured to interface with a respective propulsion system, thereby to connect the propulsion system in fluid communication with the intake duct structure. The or each support structure may be configured to connect a respective intake duct portion in fluid communication with another part of the intake duct structure. The or each support structure may be configured to interface with a rear intake duct portion of a respective propulsion system, thereby to connect the rear intake duct portion in fluid communication with a forward intake duct portion of the intake duct structure. In this way the support structure may be arranged to connect portions of the intake duct structure together. With this arrangement the or each support structure may be arranged to connect a propulsion system (or an intake portion of a propulsion system) with a forward portion of the intake duct structure. Each propulsion system may comprise its respective intake duct portion. Preferably common mounting means are provided on the fuselage body and on the central wing structure, and preferably each of the support structures is arranged to be removably secured to the common mounting means such that the support structures may be mounted interchangeably between the fuselage body and the central wing structure. Preferably the kit comprises mounting means provided on the fuselage body for removably mounting the support structure to the fuselage body. These mounting means may be referred to as upper mounting means or upper mounting members, and may comprise brackets. Preferably the kit comprises mounting means provided on the central wing structure for removably mounting the support structure to the central wing structure. These mounting means may be referred to as lower mounting means or lower mounting members, and may comprise brackets. The or each support structure may be arranged to connect the parts of the intake duct structure together. For example when a propulsion system is installed in the aircraft. In this way the or each support structure may provide an adapter for connecting the or each propulsion system with the forward intake duct portion. The or each support structure may comprise, or function as, an adapter for connecting the propulsion system (or an intake of the propulsion system) in fluid communication with the intake duct. The or each support structure may comprise, or be referred to as, an adapter plate or load-bearing sealing plate. At least one surface of the or each plate may be arranged to form a seal with the forward intake duct portion and / or a respective rear intake duct portion. Preferably the intake duct structure comprises a forward portion arranged to extend from an air intake opening of the aircraft towards the propulsion system and each propulsion system comprises, or is provided with, a rear intake portion arranged to extend between an engine of the propulsion system and the forward intake portion. The present invention also provides a modular aircraft comprising apparatus according to the first aspect of the invention, or an aircraft assembly according to the second aspect of the invention, or the kit according to the third aspect of the invention. According to another aspect of the invention there is provided a method of transmitting load in a low-wing modular aircraft, the aircraft comprising a fuselage body, an intake duct structure and a central wing structure, and wherein each of a plurality of alternative propulsion systems is removably mountable externally of the fuselage body, below a section of the fuselage body and along a centre-line of the aircraft, the method comprising: providing a support structure between the central wing structure and the fuselage body, the support structure being arranged to extend between the central wing structure and the fuselage body to transfer load between the central wing structure and the fuselage body when the support structure is mounted between the central wing structure and the fuselage body, and the support structure comprising an intake duct interface configured to interface with the intake duct structure. According to a fourth aspect of the invention, there is provided a method of reconfiguring a low-wing modular aircraft, the aircraft comprising a fuselage body, an intake duct structure and a central wing structure disposed below a section of the fuselage body, and wherein each of a plurality of alternative propulsion systems is removably mountable externally of the fuselage body, below said section of the fuselage body and along a centre-line of the aircraft, the method comprising: removing from the aircraft a first support structure mounted between the central wing structure and the fuselage body, the first support structure comprising a first intake duct interface for interfacing with the intake duct structure; mounting a second support structure between the central wing structure and the fuselage body, the second support structure comprising a second intake duct interface for interfacing with the intake duct structure; wherein each of the first support structure and the second support structure is arranged to extend between the central wing structure and the fuselage body thereby to transfer load between the central wing structure and the fuselage body. In this way the first support structure may be replaced with the second support structure. Preferably each propulsion system is provided with a respective intake duct portion arranged to form part of the intake duct structure. Preferably the first support structure is configured to interface with an intake duct portion of the first propulsion system. Preferably the second support structure is configured to interface with an intake duct portion of the second propulsion system. Replacement of the first support structure with the second support structure may facilitate replacement of the first propulsion system with the second propulsion system, since the second support structure is preferably configured to interface with an intake duct portion of the second propulsion system. Preferably each support structure is arranged to extend substantially vertically between the central wing structure and the fuselage body. Preferably common mounting means are provided on the fuselage body and the central wing structure, and preferably the method comprises removing the first support structure from the common mounting and securing the second support structure to the common mounting means. Preferably the method comprises removing the first propulsion system from the aircraft and replacing the first propulsion system with the second propulsion system. Preferably the method comprises removing the first propulsion system from the aircraft. Preferably the method comprises mounting the second propulsion system to the aircraft. Preferably the method comprises replacing the first propulsion system with the second propulsion system. Preferably the method comprises removing the first propulsion system prior to removing the first support structure. Preferably the method comprises mounting the second support structure between the central wing structure and the fuselage body before mounting the second propulsion system to the aircraft. Preferably the or each support structure is arranged to be mounted between the central wing structure and a mid section of the fuselage body. The or each support structure may form part of an airframe structure of the aircraft when the support structure is mounted between the fuselage body and the central wing structure. The or each support structure may form part of a primary airframe structure of the aircraft when the support structure is mounted between the fuselage body and the central wing structure. Alternatively, the or each support structure may be arranged to be mounted externally of an airframe structure of the fuselage body and / or the central wing structure. The or each support structure may be arranged to be mounted externally of an aerodynamic skin of the fuselage body and / or the central wing structure. Preferably the or each support structure comprises a plate. The support structure, or at least some of the support structures, may comprise a central aperture shaped to receive part of the intake duct structure therethrough. The or each support structure may be arranged to extend between a rear support member of the central wing structure and the fuselage body. The or each support structure may be arranged to extend between a rear portion of the central wing structure and a lower side of the section of the fuselage body. The or each support structure may be arranged to extend between a rear spar of the central wing structure and the fuselage body. The or each support structure may be arranged to be mounted to the rear wing structure or rear wing spar. The or each support structure may be arranged to extend between a mid support member or mid spar of the central wing structure and the fuselage body (or the section of the fuselage body). The mid spar may be disposed between a front portion of the central wing structure and a rear portion of the central wing structure. A forward portion of the central wing structure may be secured to, or integral with, a forward section or cockpit section of the fuselage body. With this arrangement, a space provided between the central wing structure and the fuselage body allows the propulsion system to be easily removed and replaced, while the support structure of the present invention provides structural support to the central wing structure, which helps to prevent undesirably large forces and moments from being transmitted through structures where the central wing structure meets the forward fuselage section. Preferably the or each support structure is arranged to be mounted behind a front portion of the fuselage body. For example, the or each support structure may be located behind a front portion of the fuselage body comprising a cockpit of the aircraft. Preferably the intake duct structure is disposed above the central wing structure. Preferably the or each propulsion system is arranged to locate above the central wing structure. Preferably the or each propulsion system is arranged to locate between a section of the fuselage body and the central wing structure. Preferably the central wing structure is disposed below a midline of the fuselage body. Preferably the central wing structure is disposed below (a mid section of) the fuselage body. The central wing structure may comprise landing gear. Landing gear may be mounted to the central wing structure. Preferably the or each support structure is a unitary body. In this way the aircraft can be reconfigured by removal and replacement of a single structure. The or each support structure may comprise support structure mounting members for mounting the support structure between the central wing structure and the fuselage body. Preferably the mounting members comprise mounting apertures. The or each support structure may comprise at least one upper mounting member and at least one lower mounting member. Preferably the or each support structure comprises a pair of upper support structure mounting members. Preferably the or each support structure comprises a pair of lower support structure mounting members. Preferably the support structure mounting members comprise lug portions or mounting portions configured to engage with respective mounting members provided on the fuselage body and central wing structure. The or each support structure may comprise two, three, four, five or six support structure mounting members, preferably four. For example preferably two upper support structure mounting members (lug portions) and two lower support structure mounting members (lug portions). In this way preferably the or each support structure may be installed or removed using (only) four fasteners. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which like numerals indicate like features and in which: Figure 1 is a perspective view of apparatus according to a preferred embodiment of the invention, together with parts of an aircraft in which the apparatus may be used, showing a support structure of the apparatus mounted between parts of the aircraft; Figure 2 is an alternative perspective view of the apparatus and parts of the aircraft of Figure 1; Figure 3 is a perspective view of the apparatus and parts of the aircraft of Figure 1, together with a propulsion system and a rear portion of an intake duct structure which may be mounted to the aircraft; Figure 4 is an alternative perspective view of the apparatus, parts of the aircraft, and the propulsion system and rear intake duct portion of Figure 3; Figure 5 is a top view of the support structure of the apparatus of Figure 1, showing the support structure engaged with a forward portion of an intake duct structure with which the apparatus may be used; Figure 6 is a perspective view of the portion of the intake duct structure of Figure 5; Figure 7 is a perspective view of the support structure and portion of the intake duct structure of Figure 5; Figure 8 is a perspective view of mounting means forming part of the apparatus of Figure 1, and parts of the aircraft in which the apparatus may be used; Figure 9 is an alternative perspective view of the apparatus and parts of the aircraft of Figure 1; Figure 10 is a perspective view of the apparatus of Figure 1 and parts of the aircraft, showing a variant of the support structure, and an alternative propulsion system with which the apparatus may be used; Figure 11 is a rear perspective view of the apparatus of Figure 10, showing a rear portion of an intake duct structure engaged with the support structure; Figure 12 is an front perspective view of the apparatus of Figure 10, showing the rear intake portion engaged with the support structure; Figure 13 is a perspective view of part of the apparatus of Figure 1 and parts of the aircraft, together with an alternative support structure forming part of a kit according to a preferred embodiment of the invention; Figure 14 is a perspective view of the apparatus of Figure 13, and part of a propulsion system with which the apparatus may be used; Figure 15 is a perspective view of the apparatus of Figure 13, and part of a propulsion system and rear portions of an intake duct structure with which the apparatus may be used; and Figure 16 is a perspective view of an alternative support structure forming part of a kit according to a preferred embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides apparatus for providing structural support in a reconfigurable modular aircraft and methods of reconfiguring such an aircraft. The present invention also provides a modular aircraft assembly kit which enables the interchanging of alternative aircraft propulsion systems. The present invention provides one or more support structures which function to transmit load between parts of the airframe structure whilst also functioning as an adapter to allow the interfacing or connection of different interchangeable propulsion systems with an intake duct structure of the aircraft. In particular, the apparatus of the present invention provides a support structure which is arranged to transmit load between a central wing structure and part of a fuselage body of the aircraft. The present invention is particularly applicable to types of aircraft (which may be referred to as low-wing aircraft) in which wings of the aircraft are disposed proximate a lower side of the fuselage body. Apparatus 100 according to a preferred embodiment of the invention is shown in Figures 1 to 5, 6 and 7, together with parts of a modular aircraft 10 in which the apparatus 100 may be used. The apparatus 100 comprises a support structure 102a which is arranged to be mounted between parts of the aircraft 10. The apparatus 100 also forms part of a modular aircraft assembly kit or system according to a preferred embodiment of the present invention, which is illustrated in Figures 1 to 16. The modular aircraft assembly kit comprises a plurality of support structures 102 (including the first support structure 102a) and mounting means for removably securing the support structures 102 interchangeably to the aircraft 10. Figure 1 shows parts of a fuselage body 12 of the aircraft 10, a central wing structure 14 of the aircraft 10, and the apparatus 100 comprising a first support structure 102a. Figure 2 shows part of the fuselage body 12, the central wing structure 14, and part of an air intake duct structure 16 of the aircraft 10, together with the apparatus 100 of the present invention. The aircraft 10 comprises a modular jet aircraft which can be reconfigured for different roles. The aircraft 10 comprises the fuselage body 12 which provides a main structural body or airframe of the aircraft 10. A forward section 18 of the fuselage body 12 comprises a cockpit 20 and an aft section 22 of the fuselage body 12 extends to an empennage 24 towards the aft end or tail end of the aircraft 10. A central or mid fuselage section 26 is elongate to extend between the forward and aft fuselage sections 18, 22. The forward fuselage section 18 extends forwards and downward from the mid fuselage section 26 towards a forward end or nose end of the aircraft 10. In this embodiment, the fuselage body 12 is of a construction (e.g., a stressed skin structure) comprising internal frame structures (frames, ribs, longerons, stringers and the like) covered by an external skin (not shown). The central wing structure 14 is disposed generally below the mid fuselage section 26. A forward portion of the central wing structure 14 may be secured to, or integral with, the forward section 18 of the fuselage body 12. In this example, the central wing structure 14 extends rearwards from a lower portion of the forward fuselage section 18 towards a rear edge 28 of the central wing structure 14. The rear edge 28 of the central wing structure 14 is disposed below a forward end portion of the mid fuselage section 26. With this arrangement, the central wing section 14 is arranged below the mid fuselage section 26 and below an aft portion of the forward fuselage section 18 to define a space 30 between the fuselage body 12 and the central wing structure 14. This space 30 is arranged to accommodate part of the intake duct structure 16 of the aircraft 10, as can be seen most clearly in Figure 2. Moreover, the space provided between the central wing structure and the fuselage body, and below the mid fuselage section rearwardly of the central wing structure, also facilities the convenient removal and replacement of the propulsion system to allow the modular aircraft to be reconfigured. This arrangement means that the central wing structure requires sufficient structural support and bracing to prevent undesirably large forces and moments from being transmitted through airframe structure where the central wing structure meets the forward fuselage section proximate a lower side of the aircraft. Referring additionally to Figures 3 and 4, the modular aircraft 10 is configured for each a plurality of interchangeable propulsion systems 2 to be removably mounted to the aircraft 10. Each propulsion system 2 comprises at least one engine 4. In this specification, the term “propulsion system” refers to means for providing propulsive force or thrust to an aircraft. A propulsion system may be referred to as, or may comprise, a powerplant. The propulsion system or powerplant preferably comprises at least one engine. The propulsion system or powerplant may provide electrical and / or hydraulic power in addition to thrust. The apparatus 100 is configured such that a central longitudinal axis of each propulsion system 2 is arranged to extend parallel to the centre-line of the aircraft (e.g., along a central plane of the aircraft) when mounted to the aircraft. In this embodiment, the mid fuselage section 12 comprises propulsion system mounting members (not shown) for removably suspending a propulsion system 2 such as a podded engine from the mid fuselage section 26. In this embodiment, the propulsion system mounting members are common mounting members to which each of a plurality of interchangeable propulsion systems may be mounted alternatively. In this way, the aircraft 10 may be reconfigured by mounting different types of propulsion system 2 to the fuselage body 12. As described further below, the present invention also provides a modular system or kit comprising a plurality of alternative support structures, each configured to interface with a respective alternative propulsion system 2. With these arrangements, as shown in Figures 3 and 4, a first propulsion system 2a is arranged to be suspended below the mid fuselage section 26, rearward of the central wing structure and support structure 102a. In use, when a respective one the propulsion systems 2 is mounted to the aircraft mounted in this position, the intake duct structure 16 is arranged to extend between an intake opening 34 of the aircraft 10, and an engine 4a of the respective propulsion system 2a. The intake duct structure 16 comprises a first, forward (upstream) intake portion 36 which extends rearwards from the intake opening 34 towards the propulsion system 2a, and a second, rear (downstream) intake portion 38a which extends between the engine 4 and the forward intake portion 36. Preferably each propulsion system 2 comprises a corresponding rear intake portion, so that interchanging the propulsion system 2 also comprises interchanging the rear intake portion. The intake duct structure 16 therefore comprises the forward intake portion 36 and a respective one of the rear intake portions. Referring additionally to Figures 5, 6 and 7, in this embodiment the forward intake portion 36 comprises a bifurcated, or Y-shaped duct, having first and second lateral duct portions 40 which open on opposing lateral sides of the aircraft 10 and which converge rearwardly to merge towards a rear end 42 of the forward intake duct portion 36. Each lateral duct portion 40 opens forward at an intake opening 34 above a respective wing root of the aircraft 10. The open rear end 42 of the forward intake portion 36 has a substantially circular shape and is arranged to be connected in fluid communication with the rear intake portion 38 of a propulsion system 2. In the present embodiment, the rear end 42 of the forward intake duct 36 is arranged to engage with the support structure 102a, as described further below. In this embodiment, the aircraft 10 is configured such that the central wing structure 14, and wings 15 mounted to the central wing structure 14, are disposed proximate a lower side of the aircraft 10 and the intake duct structure 16 is disposed between the central wing structure 14 and the mid fuselage section 26. A lower surface of the central wing structure 14 provides a lower external surface of the aircraft 10. This ‘low-wing’ arrangement may be particularly suitable for reducing drag, for example where the aircraft is configured to operate at transonic speeds. As mentioned above, the apparatus 100 and kit of the present embodiments comprise mounting means for mounting the or each support structure 102 to the aircraft 10. As shown most clearly in Figures 8 and 9, in this embodiment, the mounting means comprises upper mounting members or fuselage mounting members 104 provided on a lower side of the mid fuselage section 26, and lower mounting members or wing structure mounting members 106 provided on an upper side of the central wing structure 14. The upper mounting members 104 comprise a pair of lugs or clevis brackets 104 which extend downwardly from the mid fuselage section 26 proximate opposite lateral sides of the mid fuselage section 26. Each upper mounting member 104 provides a slot 108 which extends transversely (e.g., in a direction between lateral sides of the aircraft) and which opens downwards and which is arranged to receive part of a support structure 102. Mounting apertures 110 extend through each of the upper mounting members 104 and open on front and rear sides of the upper mounting members 104 and into the slot 108. The upper mounting members 104 are disposed proximate the forward end portion of the mid fuselage section 26. The lower mounting members 106 comprise a pair of lugs or clevis brackets 106 which extend upwardly from proximate a rear side 28 of the central wing structure 14. In particular, the lower mounting members 106 are provided on a rear spar 25 of the central wing structure 14. Each lower mounting member 106 provides a transversely extending slot 116 which opens upwards and which is arranged to receive part of the support structure 102. Mounting apertures 115 extend through each of the lower mounting members 106 and open on front and rear sides of the mounting member and into the slot 116. In other embodiments (not shown), the lower mounting members may be provided on a mid portion of the central wing structure part way between front and rear portions of the central wing structure, for example on a mid spar of the central wing structure. In such embodiments, the upper mounting members may be disposed further forward, substantially vertically above the lower mounting members. In this preferred embodiment, the first support structure 102a comprises a support plate 102a. The first support plate 102a comprise a forward face 118 and an aft face 120. The forward face 118 is arranged to face forwards and towards the forward intake portion 36 when the plate 102a is mounted in the aircraft 10. The aft face 120 of the plate, opposite to the forward face 118, is arranged to face rearwardly towards a propulsion system 2 when the support plate 102a is mounted in the aircraft 10. The first support plate 102a is arranged to interface with the rear intake portion 38a of a propulsion system 2a so as to connect the propulsion system 2a with the forward intake portion 16, to provide the intake duct structure 16. In particular, in this embodiment, the first support plate 102a is arranged to interface with the first propulsion system 2a. The first support plate 102a can be considered to function as an adapter for connecting the rear intake portion 38a of the propulsion system 2a to the forward intake portion 16. As shown most clearly in Figures 7, 9, the first support plate 102a comprises a generally quadrilateral plate having an upper edge 122 and an opposite lower edge 124 extending parallel to the upper edge 122. Lateral side edges 126 extend between lateral ends of the upper and lower edges 122, 124. Lower comers of the plate are truncated such that the lower edge 124 is shorter than the upper edge 122. Each lateral side edge 126 therefore comprises an upper portion 128 and a lower portion 130, each lower portion 130 being provided by an edge of one of the truncated lower corners. The upper portion 128 extends downwards from the upper edge 122 and at an angle away from the opposing lateral side edge 126. The lower portion 130 is angled to extend between a lower end of the upper portion 128 and the lower edge 124. Each upper portion 128 has a greater length than each lower portion 130. Each lower portion 130 is provided with an elongate slotted bracket 132 providing a slot which extends along the lower portion 130 and opens at an angle downwards and away from the plate 102a. Each upper edge portion 128 is provided with a reinforcing flange 138 which extends upwards from a respective slotted bracket 132 and along the upper portion 128. As can be seen most clearly in Figure 7, upper mounting apertures 134 extend through corner portions or lug portions 136 of the first support plate 102a proximate lateral ends of the upper edge 122. Each upper mounting aperture 134 is arranged to align with the apertures 110 of a respective upper mounting member 104 when the first support plate 102a is mounted to the fuselage body 12. In particular, the lug portions 136 of the first support plate 120a are arranged to be received in the slots 108 provided by the upper mounting members 104. Fasteners (such as bolts or pins; not shown) are arranged to insert through the aligned apertures 110, 134, to secure the first support plate 102a to the mid fuselage section 26. In this example, a pair of generally triangular mounting plates 140 are provided for securing the first support plate 102a to the lower mounting members 106. A base of each mounting plate 104 is arranged to be received in the slotted bracket 132 of a respective lower edge portion 130 of the first support plate 102a. Each mounting plate 140 is arranged to be secured in the slotted bracket 132 by fasteners. An aperture 142 extends through an apex portion 144 of each mounting plate 140 (as can be seen in Figure 7). The apex portion 144 is arranged to locate in the slot 116 of a respective lower mounting member 106 such that the aperture 142 through the apex portion 144 aligns with the apertures 115 of the lower mounting member 106. In this way, each mounting plate 140 can be secured to a lower mounting member 106 so as to secure the first support plate 102a to the central wing structure 14. In other embodiments, the mounting plates may not be provided, or they may be integral with the support plate, such that the support plate may be shaped to engage directly with the lower mounting members. The first support plate 102a is provided with a substantially circular central aperture 146 which passes through the plate 102a between the forward and aft faces 118, 120 of the plate 102a. The central aperture 146 is defined by a curved wall 148. The curved wall 148 protrudes from the forward and aft faces 118, 120 of the plate 102a to provide a collar 150 which extends circumferentially around the central aperture 146. The central aperture 146 is shaped to receive part of the rear intake portion 38a of the propulsion system 2a. In this embodiment, the first propulsion system 2a comprises a single-engine propulsion system 2a. The first propulsion system 2a may be used, for example, in a basic training configuration of the modular aircraft 10. Referring additionally to Figures 10 to 16, the modular aircraft assembly kit of the present invention comprises the apparatus 100 described above, and further comprises a plurality of support structures 102. In this preferred embodiment, the plurality of support structures 102 comprises a plurality of interchangeable support plates 102. The plurality of support plates 102 comprises the first support plate 102a, a second support plate 102b and a third support plate 102c. In this embodiment, each support plate 102 is arranged to interface or engage with a respective rear intake portion 38 of a respective propulsion system 2, and with the forward intake portion 36, so as to support the propulsion system 2 and / or the intake duct structure 16 and, in this embodiment, to connect the propulsion system 2 with the forward intake structure 36. At least part of the curved wall 148 and / or surfaces of the forward and aft faces 118, 120 may be configured to interface with parts of the intake duct structure 16. In particular, in this embodiment, the first support plate 102a is arranged to interface with the rear intake portion 38a of the first propulsion system 2a as described above. The second propulsion system 102b is arranged to interface with a rear intake portion 38b of a second propulsion system 2b and the third support plate 102c is arranged to interface with a rear intake portion 38c of a third propulsion system (not shown) as described further below. In this way, each support plate 102a, 102b, 102c can be considered to function as an adapter for connecting each propulsion system 2a, 2b, 2c to the forward intake portion 36. In use, the first support plate 102a is mounted to the fuselage body 12 and central wing structure 14 by aligning the mounting apertures 134, 142 of the first support plate 102a with the apertures 110, 115 of the respective upper and lower mounting members 104, 106. In particular, as can be seen most clearly with reference to Figures 8 and 9, the lug portions 136 of the first support plate 102a are inserted into the slots 108 of the upper mounting members 104 to align the upper mounting apertures 134 with the apertures 110 of the upper mounting members 104. Each mounting plate 140 is secured in a respective slotted bracket 132 of the first support plate 102a, and each mounting plate 140 is inserted into the slot 116 of a respective lower mounting member 106 to align the lower mounting apertures 142 with the apertures 115 of the lower mounting members 106. Fasteners (not shown) are secured in the aligned apertures 134, 110 and 142, 115 to secure the plate 102a between the central wing structure 14 and fuselage body 12. In this way the support plate 102a extends vertically between the central wing structure 14 and the fuselage body 12 to define a load path between the central wing structure 14 and the fuselage body 12 thereby to transfer load between the central wing structure 14 and the fuselage body 12. With this arrangement, the support plate 102a is secured between the central wing structure 14 and fuselage body 12, externally of an airframe structure of the aircraft. A variant 102d of the first support plate 102a is shown in Figures 10, 11 and 12. The variant 102d of the first support plate is substantially similar to the first support plate 102a described above and shown in Figures 1 to 5, 7 and 9, except that the central aperture 146 of the variant plate 102d has a larger diameter than the central aperture 146 of the first support plate 102a described above. The variant 102d of the first support plate is shaped to interface with an alternative first propulsion system 2d, as can be seen in Figures 10 to 12. In particular, a rear intake portion 38d of the alternative first propulsion system 2d is arranged to engage with the variant 102d of the first support plate. The alternative first propulsion system 2d may be used in an advanced trainer configuration of the aircraft 10 which is more powerful than the basic training configuration. With reference to Figures 13, 14 and 15, the second support plate 102b comprises an upper edge 152 and an opposite lower edge 154 extending parallel to the upper edge 152. Lateral side edges 156 extend between lateral ends of the upper and lower edges 152, 154. Each lateral side edge 156 of the second support plate 102b comprises a curved portion 158 providing a concave or recessed edge portion 158 of the second support plate 102b. Each recessed edge portion 158 comprises a curved wall 160. In this way, the second support plate 102b has the appearance of having a part-circular cut-out on each lateral side of the second support plate. Edges of each curved wall 160 protrude from the forward face (not visible in Figures 13 to 15) and aft face 120b of the second support plate 102b to define or lip 162 which extends along each recessed portion 158. In this embodiment, the second propulsion system 2b comprises a twin-engine propulsion system 2b, as can be seen in Figures 14 and 15. Each recessed portion 158 of the second support plate 102b is arranged to engage with a respective part of the twin-engine propulsion system 2b. In particular each recessed portion 158 is shaped to match the external curvature of a respective one of a pair of rear intake portions 38b of the second propulsion system 2b, as shown in Figure 15. In some cases an alternative forward intake portion (not shown) may be used with the second support plate 102b and second propulsion system 2b. For example, the first and second lateral duct portions may not merge into a common duct as described above but instead may not be in fluid communication with one another. Each lateral duct portion may thus connect separately with a respective rear intake portion of the twin engine propulsion system. Upper lug portions 164 of the second support plate 102b proximate lateral ends of the upper edge 152 are arranged to be received in the slots 108 of the upper mounting members 104. Lower lug portions 166 of the second support plate 102b proximate lateral ends of the lower edge 154 are arranged to be received in the slots 116 of the lower mounting members 106. Accordingly, the lug portions 164, 166 are provided with mounting apertures (obscured in Figures 13 to 15). In this way, the second support plate 102b comprises a pair of upper mounting apertures arranged to align with the apertures 110 of the upper mounting members 104, and a pair of lower mounting apertures arranged to align with the apertures 115 of the lower mounting members 106. Fasteners (not shown) are arranged to extend through the aligned apertures to secure the second support plate 102b to the fuselage body 12 and central wing structure 14. Referring additionally to Figure 16, the third support plate 102c is generally similar to the first support plate. The third support plate 102c is generally trapezium shaped, comprising an upper edge 168 and an opposite lower edge 170 having a greater length than the upper edge 168. Lateral side edges 172 extend between lateral ends of the upper and lower edges 168, 170. Each lateral side edge 172 is provided with a reinforcing flange 174 which extends along the lateral side edge 172 and projects from forward and aft faces of the plate 102c. Similarly to the first support plate 102a, the third support plate 102c is provided with mounting apertures proximate lateral ends of the upper and lower edges 168, 170. In particular, upper mounting apertures 176 extend through upper corner portions or upper lug portions 178 of the third support plate 102c proximate lateral ends of the upper edge 168. Each upper mounting aperture 176 is arranged to align with the aperture 110 of a respective upper mounting member 104 when the third support plate 102c is mounted to the fuselage body 12. The upper lug portions 178 of the third support plate 102c are arranged to be received in the slots 108 provided by the upper mounting members 104. Lower mounting apertures 180 extend through lower corner portions or lower lug portions 182 of the third support plate 102c proximate lateral ends of the lower edge 170. Each lower mounting aperture 180 is arranged to align with the aperture 115 of a respective lower mounting member 106 when the third support plate 102c is mounted to the fuselage body 12. In this way, the mounting apertures 176, 180 are arranged to align with the respective apertures 110, 115 of the upper mounting members 104 and lower mounting members 106, when parts of the third support plate 102c are received in the slots 108, 116 provided by the upper mounting members and lower mounting members 104, 106. The third support plate 102c is provided with a substantially circular central aperture 146c which passes through the plate 102 between the forward and aft faces 118c, 120c, of the plate 102c. The central aperture 146c is defined by a curved wall 148c which protrudes from the forward and aft faces 118c, 120c of the plate 102c to provide a collar 150c which extends circumferentially around the central aperture 146c. Reinforcing ribs 184 extend radially between the edges 168, 170, 172 and the collar 150c. The central aperture 146c of the third support plate 102c has a diameter greater than a diameter of the central aperture 146 of the first support plate 102a. The central aperture 146c of the third support plate 102c is shaped to receive part of a third propulsion system (not shown). In this embodiment, the third propulsion system comprises a single-engine propulsion system having a larger fan diameter than the first propulsion system 2a. The third propulsion system may be used, for example, in a combat configuration of the modular aircraft 10 having a more powerful propulsion system than the first propulsion system 2a. The lower edge 170 of the third support plate 102c is curved outwardly to accommodate the diameter of the enlarged central aperture 146c. In this way, the lower edge 170 comprises a middle portion 186 having a convex curvature and lateral portions 188 on either side of the middle portion having a concave curvature, as can be seen in Figure 16. Each support plate 102a, 102b, 102c is configured to connect a respective alternative propulsion system 2 in fluid communication with the forward intake portion 36. In this way, different propulsion systems may be used interchangeably with a common (shared) fuselage body and forward intake portion 36. In use, a respective one of the support plates 102a, 102b, 102c is secured in place by securing the lug portions of the support plate 102a, 102b, 102c to the upper mounting members 104 and lower mounting members 106 using suitable fasteners (not shown). Preferably, relatively few mounting points or mounting members are used to secure the support plate, so that the operation of installing and removing the support plate can be done quickly and conveniently. For example, four mounting members are provided in the examples described above. In some embodiments, more or fewer mounting points or mounting members may be provided. It will also be appreciated that other shapes and configurations of the support plates are possible and may be used to interface with different types of propulsion systems and / or intakes duct structures. With one of the support plates 102 (e.g., 102a, 102b, 102c), mounted to the upper mounting members 104 and lower mounting members 106, the support plate 102 extends vertically between the central wing structure 14 and mid fuselage section 26, as can be seen most clearly in Figures 1 to 4 and 9. In this way, each respective support plate 102 acts as a structural support or bracing member to transmit loads between the central wing structure 14 and the fuselage body 12 and to provide structural integrity to the airframe structure of the aircraft 10. As mentioned above, in some embodiments, the upper and lower mounting members may be located further forward with respect to the central wing structure and fuselage body, such that each support structure is arranged to extend between a mid portion (e.g., a mid spar) of the central wing structure and the fuselage body. For example, in flight, forces acting on wings of the aircraft secured to the central wing structure are transferred from the central wing structure to the fuselage body via the support plate. For instance, lift forces acting upwardly on the wings are transmitted from the central wing structure to the fuselage body via the support plate. In some embodiments, landing gear may be mounted to the central wing structure, in which case compressive forces (e.g., when landing) are transmitted upwardly to the fuselage body via the support plate. Shear forces (e.g. drag) acting rearwardly on the wings are also transmitted via the support plate to the fuselage body. Moreover, the support plate is also arranged to transmit tensile loads between the central wing structure and fuselage body, for example to support the weight of the central wing structure and wings. In this way, the support structure allows a space to be provided between the central wing structure and the fuselage body for accommodating the removable propulsion system and intake duct structure, whilst maintaining structural integrity of the aircraft. In particular, the support structure helps to prevent excessive forces acting on parts of the airframe structure proximate where the central wing structure is joined to the forward fuselage section. With the arrangements described above, since each of the propulsion systems is configured to be readily removable from the fuselage body, each propulsion system is not surrounded by significant load-bearing airframe structure. Similarly, each propulsion system itself does not comprise any load-bearing structures suitable for transmitting significant loads between parts of the airframe structure of the aircraft, such as between the central wing structure and the mid fuselage section. By contrast, in prior art non-modular jet aircraft in which the propulsion system is not intended to be removed without considerable disassembly of the airframe structure of the aircraft, the propulsion system is surrounded by and / or fully integrated within the airframe structure. In these arrangements, loads may be transmitted by parts of the airframe structure which extend around and / or mount the propulsion system in the aircraft. Also, in modular jet aircraft in which a removable propulsion system is mounted below a central wing structure of the aircraft, the propulsion system and / or air intake duct structure are not disposed in a load path between the central wing structure and fuselage body and the central wing structure may be integral with a mid-section of the fuselage body. In such cases load from the wing structure is transmitted directly through airframe structure between the central wing structure and fuselage body and need not be passed through any structure which supports the propulsion system. The present invention therefore provides arrangements to transfer load between the central wing structure and fuselage body in such a way that permits the convenient removal of the propulsion system and / or the interchanging of different propulsion systems. It will be appreciated therefore that the present invention also provides methods of reconfiguring a modular aircraft. In general, to reconfigure the aircraft, the method may comprise removing a respective one of the support structures from the aircraft and replacing the support structure with an alternative one of the support structures. For example, an aircraft may be in a first configuration (e.g., a single-engine training configuration) in which the first support plate and the first propulsion system are fitted to the aircraft. In this case, the method may comprise removing the first propulsion system and removing the first support structure, and mounting the second support structure between the central wing structure and the fuselage body. The second propulsion system may then be mounted / suspended from the fuselage body and arranged to interface with the second support structure to connect the second propulsion system with the forward intake portion. In this way the first support structure may be replaced with the second support structure to facilitate replacement of the first propulsion system with the second propulsion system, since the second support structure is configured to interface with the second propulsion system. Accordingly, the aircraft may be reconfigured to a second configuration, which in this example is a twin-engine configuration. With the arrangements described above, the present invention enables the convenient interchanging of alternative propulsion systems in a modular aircraft, without significant disassembly of the aircraft.

Claims

1. Apparatus for transferring load in a low-wing modular aircraft having a removably mountable propulsion system, said aircraft comprising: a fuselage body, a central wing structure disposed below a section of the fuselage body, and an intake duct structure arranged to extend between an air intake opening of the aircraft and an engine of the propulsion system, and wherein the propulsion system is removably mountable externally of the fuselage body below said section of the fuselage body and along a centre-line of the aircraft; the apparatus comprising:a support structure configured to be removably mounted between the central wing structure and the fuselage body, the support structure being arranged to locate between the engine and the air intake opening, and the support structure comprising an intake duct interface configured to interface with part of the intake duct structure;wherein the support structure is arranged to extend between the central wing structure and the fuselage body to transfer load between the central wing structure and the fuselage body.

2. Apparatus according to Claim 1, in which the support structure is arranged to extend vertically to define a load path between the central wing structure and the fuselage body.

3. Apparatus according to any preceding claim, in which the support structure comprises at least one aperture or recess shaped to engage with part of the intake duct structure.

4. Apparatus according to any preceding claim, in which the support structure comprises a plate arranged to extend between the central wing structure and the fuselage body.

5. Apparatus according to any preceding claim, in which the support structure isarranged to extend between a rear portion of the central wing structure and a lower side of the section of the fuselage body.

6. Apparatus according to any preceding claim, in which the intake duct structure comprises a forward intake duct portion which extends from the air intake opening towards the propulsion system, and a rear intake duct portion which extends between the engine and the forward intake duct portion, and the forward intake duct portion is arranged to be connected in fluid communication with the rear intake duct portion; and wherein the intake duct interface is configured to interface with the rear intake duct portion.

7. Apparatus according to Claim 6, in which the support structure is configured to interface with the forward intake duct portion, thereby to connect the forward intake duct portion in fluid communication with the rear intake duct portion.

8. An aircraft assembly comprising the fuselage body, the intake duct structure, the central wing structure and apparatus according to any preceding claim, and in which a propulsion system is removably mountable externally of the fuselage body between the fuselage body and the central wing structure and along a centre-line of the aircraft.

9. A modular aircraft assembly kit for a low-wing modular aircraft, said aircraft comprising: a fuselage body to which each of a plurality of alternative propulsion systems may be removably mounted externally below a section of the fuselage body and along a centre-line of the aircraft; a central wing structure disposed below said section of the fuselage body; and an intake duct structure arranged to locate between the fuselage body and the central wing structure; each propulsion system being provided with a respective intake duct portion configured to form part of the intake duct structure; said kit comprising:a plurality of interchangeable support structures arranged to be removably mounted between the central wing structure and the fuselage body, each support structure being arranged to extend between the central wingstructure and the fuselage body to transfer load between the central wing structure and the fuselage body when the support structure is mounted between the central wing structure and the fuselage body, the plurality of support structures comprising:a first support structure comprising a first intake duct interface configured to interface with a respective intake duct portion of a first removably mountable propulsion system when the first support structure is mounted between the central wing structure and the fuselage body and the first propulsion system is mounted to the fuselage body; anda second support structure comprising a second intake duct interface configured to interface with a respective intake duct portion of a second removably mountable propulsion system when the second support structure is mounted between the central wing structure and the fuselage body and the second propulsion system is mounted to the fuselage body.

10. A modular aircraft assembly kit according to Claim 9, comprising the fuselage body, the central wing structure, and the intake duct structure.

11. A modular aircraft assembly kit according to Claim 9 or 10, in which the first support structure comprises a plate having a central aperture shaped to receive part of a respective intake duct portion the first propulsion system.

12. A modular aircraft assembly kit according to Claim 11 and comprising the first propulsion system, and in which the first propulsion system is a single-engine propulsion system.

13. A modular aircraft assembly kit according to any of Claims 9 to 12, in which the second support structure comprises a plate having a pair of recessed edge portions arranged on opposing lateral sides of the plate.

14. A modular aircraft assembly kit according to Claim 13, comprising the second propulsion system, and in which the second propulsion system is a twin-enginepropulsion system.

15. A modular aircraft assembly kit according to any of Claims 9 to 14, comprising a third support structure configured to interface with a respective intake duct portion of a third removably mountable propulsion system when the third support structure is mounted between the central wing structure and the fuselage body and the third propulsion system is mounted to the fuselage body.

16. A modular aircraft assembly kit according to any of Claims 9 to 15, in which each support structure is arranged to extend vertically along a load path between the central wing structure and the fuselage body.

17. A modular aircraft assembly kit according to any of Claims 9 to 16, in which each support structure comprises a plate arranged to extend in a substantially vertical plane.

18. A modular aircraft assembly kit according to any of Claims 9 to 17, in which each support structure is arranged to extend between a rear portion of the central wing structure and a lower side of the section of the fuselage body.

19. A modular aircraft assembly kit according to any of Claims 9 to 18, in which each support structure is arranged to support part of the intake duct structure, to mount the intake duct structure in the aircraft.

20. A modular aircraft assembly kit according to any of Claims 12 to 19, in which common mounting means are provided on the fuselage body and on the central wing structure, and each of the support structures is arranged to be removably secured to the common mounting means such that the support structures may be mounted interchangeably between the fuselage body and the central wing structure.

21. A method of reconfiguring a low-wing modular aircraft, the aircraft comprising afuselage body, an intake duct structure and a central wing structure disposed below a section of the fuselage body, and wherein each of a plurality of alternative propulsion systems is removably mountable externally of the fuselage body, below said section of the fuselage body and along a centre-line of the aircraft, the method comprising:removing from the aircraft a first support structure mounted between the central wing structure and the fuselage body, the first support structure comprising a first intake duct interface for interfacing with the intake duct structure;mounting a second support structure between the central wing structure and the fuselage body, the second support structure comprising a second intake duct interface for interfacing with the intake duct structure;wherein each of the first support structure and the second support structure is arranged to extend between the central wing structure and the fuselage body thereby to transfer load between the central wing structure and the fuselage body.

22. The method of Claim 21, in which common mounting means are provided on the fuselage body and the central wing structure, and in which the method comprises removing the first support structure from the common mounting and securing the second support structure to the common mounting means.

23. The method of claim 21 or Claim 22, comprising removing the first propulsion system from the aircraft and replacing the first propulsion system with the second propulsion system.

24. The method of any of Claims 21 to 23, comprising removing the first propulsion system prior to removing the first support structure.

25. The method of any of Claims 21 to 24, comprising mounting the second support structure between the central wing structure and the fuselage body before mounting the second propulsion system to the aircraft.

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