FIELD OF THE INVENTION

[0001] The present invention relates to an aircraft, and an aircraft fuselage, with a pressure shell, a fairing, and an unpressurised space between the pressure shell and the fairing.

BACKGROUND OF THE INVENTION

[0002] There is a drive to provide aircraft that can meet future emissions targets. One approach to tackling this challenge is the use of alternative fuels, such as liquid hydrogen. A difficulty in providing aircraft that are powered by some of these alternative fuels is that they need to be stored in pressurised fuel tanks, which are ideally circular or elliptical in order to withstand the pressure forces.

[0003] Currently, conventional fuel tanks are primarily located in the wings, at least partly so that their weight counters the aerodynamic lift generated during flight. However, it will be difficult to accommodate fuel tanks that are big enough for the proposed alternative fuels, such as liquid hydrogen, as the shape of the pressurised tanks does not fit well with the shape of the wings. One option is to locate the fuel tank on the fuselage, but this provides new design challenges.

SUMMARY OF THE INVENTION

[0004] According to a first aspect of the invention, there is provided an aircraft comprising: a fuselage having a pressure shell, a fairing, and an unpressurised space between the pressure shell and the fairing; a fuel tank; an engine; and one or more fuel lines extending inside the unpressurised space from the fuel tank to the engine.

[0005] By providing the fuel lines outside the pressure shell, any leaks that may occur from the fuel lines are contained outside the pressure shell. This is particularly advantageous when the pressure shell contains a passenger cabin.

[0006] The fuel tank may be on top of the pressure shell.

[0007] The fuel tank may be configured to carry a cryogenic fuel.

[0008] The cryogenic fuel may be one of liquid hydrogen, liquid methane, liquid ammonia, and liquid natural gas.

[0009] The fuel tank may be forward and/or aft of a disk burst zone of the engine. The fuel tank may be separated into two or more fuel tank elements to provide a gap/space in the disk burst zone.

[0010] The one or more fuel lines may extend through a disk burst zone of the engine and each fuel line may include a valve adjacent to the disk burst zone configured to selectively restrict the flow fuel to the engine.

[0011] With this arrangement, fuel flowing through the fuel lines in the disk burst zone in the event of engine disk failure can be restricted. This prevents fuel flowing into the critical zone and exacerbating the problem.

[0012] The aircraft may further comprise a window in the pressure shell, and a second window on the fairing in line with the first window.

[0013] According to a further aspect of the invention, there is provided an aircraft fuselage comprising: a pressure shell with a first window; a fairing with a second window in line with the first window; and an unpressurised space between the pressure shell and the fairing. A component may be provided inside the unpressurised space. The component may be a fuel line or a hydraulic line for example.

[0014] With this arrangement, the first window in the pressure shell is not blocked by the fairing. The view of passengers through the first window of the pressure shell is thereby unimpeded.

[0015] The following comments apply to both aspects of the invention.

[0016] A wing may be provided which meets the fuselage at a wing root.

[0017] The wings of aircraft typically generate the majority of the lift of an aircraft in order to maintain the aircraft in flight. In order to generate this lift, the wings have a large planform area.

[0018] As a result of shape of the wings, the cross-sectional area of the aircraft taken in a plane normal to the longitudinal axis of the aircraft increases significantly adjacent to the wings. This can result in a significant increase in transonic wave drag, a phenomenon explained by the transonic area rule, also known as the Whitcomb area rule.

[0019] The wing typically contributes significantly to transonic wave drag by increasing the cross-sectional area of the aircraft rapidly and contrary to the transonic area rule.

[0020] At least a portion of the fairing may be forward or aft of the wing root. In other words, at least a portion of the fairing may have a fore-aft position which is offset from the fore-aft position of the wing root.

[0021] By positioning at least a portion of the fairing forward or aft of the wing root, i.e. not positioning all of the fairing in line with the wing root, the effect of the fairing on transonic wave drag may be reduced, or the fairing may provide a reduction in wave drag.

[0022] The fairing may have an outer (aerodynamic) surface which forms a bulge.

[0023] A cross-sectional area of the unpressurised space may increase then decrease in a fore-aft direction across an apex of the bulge.

[0024] The bulged shape can assist in smoothing the overall change in cross-sectional area of the aircraft, thereby minimising transonic wave drag.

[0025] A cross-sectional area of the unpressurised space may be at a local maximum at an apex of the bulge.

[0026] A cross-sectional area of the fuselage may be at a local maximum at an apex of the bulge.

[0027] At least part of the bulge may have a fore-aft position which coincides with the fore-aft position of the wing root.

[0028] Optionally the bulge comprises a leading portion, a trailing portion and an apex between the leading portion and the trailing portion; the leading or trailing portion of the bulge has a fore-aft position which is offset from the fore-aft position of the wing root; and the trailing or leading portion of the bulge has a fore-aft position which coincides with the fore-aft position of the wing root. By such a positioning of the bulge, the effect of the fairing on transonic wave drag may be reduced, or the fairing may provide a reduction in wave drag.

[0029] Optionally an apex of the bulge has a fore-aft position which is offset from a fore-aft position of the wing root, or coincides with a fore-aft position of a leading or trailing edge of the wing root. By such a positioning of the apex of the bulge, the effect of the fairing on transonic wave drag may be reduced, or the fairing may provide a reduction in wave drag.

[0030] Optionally the bulge terminates at a bulge leading edge or a bulge trailing edge; and the bulge leading or trailing edge has a fore-aft position which coincides with the fore-aft position of the wing root. By such a positioning of the edge of the bulge, the effect of the fairing on transonic wave drag may be reduced, or the fairing may provide a reduction in wave drag.

[0031] The outer surface of the fairing may have two or more bulges.

[0032] A cross-sectional area of the unpressurised space forward or aft of the wing root may be larger than any cross-sectional area of the unpressurised space in line with the wing root.

[0033] As the largest cross-sectional area of the aircraft is typically at the wing root, by providing the largest cross-sectional area of the fairing forward or aft of the wing root, the transonic wave drag can be reduced.

[0034] The wing root may further comprise a wing root leading edge and a wing root trailing edge. A cross-sectional area of the unpressurised space at the wing root leading edge or at the wing root trailing edge may be larger than any cross-sectional area of the unpressurised space between the wing root leading edge and the wing root trailing edge.

[0035] This can help to smooth the overall cross-sectional area of the aircraft, thereby reducing transonic wave drag, as the cross-sectional area of the aircraft typically decreases rapidly forward from the wing root leading edge and aft from the wing root trailing edge.

[0036] The aircraft may further comprise a second external fairing, and a second unpressurised space between the pressure shell and the second external fairing, wherein one or more further fuel lines extend inside the second unpressurised space from the fuel tank to the engine.

[0037] A cross-sectional shape of the pressure shell along the fairing, in a fore-aft direction, may be substantially constant. In other words, the shape of the pressure shell may be substantially constant between a leading edge and a trailing edge of the fairing.

[0038] With this arrangement, the shape of the pressure shell is improved relative to a shell that substantially changes its shape or size along its length.

[0039] The engine may be mounted to a wing of the aircraft.

[0040] The pressure shell may contain a fuselage space which is pressurised at a higher pressure than the unpressurised space, for instance when the aircraft is at altitude. Alternatively the pressure shell may contain a fuselage space which is at the same pressure as the unpressurised space (for instance when the aircraft is grounded) but is pressurisable so that it can be maintained at a higher pressure than the unpressurised space when the aircraft is at altitude.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0042] Figure 1A shows a schematic of an existing aircraft;

[0043] Figure IB shows a plan view of the aircraft of Figure 1A;

[0044] Figure 2 shows a partial cross-section of the fuselage of the aircraft of Figure 1;

[0045] Figure 3 shows a partial cross-section of the fuselage of an aircraft according to a first embodiment of the invention;

[0046] Figure 4 shows an isometric view of the aircraft of Figure 3;

[0047] Figure 5 shows a plan view of the aircraft of Figure 3;

[0048] Figure 6 shows an isometric view of an aircraft according to a further embodiment of the invention;

[0049] Figure 7A shows a plan view of the aircraft of Figure 6; [0050] Figure 7B shows a graphical representation of the cross-sectional area of the aircraft of Figure 6;

[0051] Figures 8A to 8C show a partial cross-section of the fuselage taken at the sections indicated in Figure 7A;

[0052] Figure 9 shows an isometric view of an aircraft according to a further embodiment of the invention;

[0053] Figure 10A shows a plan view of the aircraft of Figure 9;

[0054] Figure 10B shows a graphical representation of the cross-sectional area of the aircraft of Figure 9;

[0055] Figure 11 shows an isometric view of an aircraft according to a further embodiment of the invention;

[0056] Figure 12A shows a plan view of the aircraft of Figure 11 ;

[0057] Figure 12B shows a graphical representation of the cross-sectional area of the aircraft of Figure 11 ;

[0058] Figure 13 shows a cross-sectional view of a fuselage of an aircraft according to a further embodiment of the invention;

[0059] Figure 14 shows an external view part of the aircraft fuselage of Figure 13; and

[0060] Figure 15 shows an isometric view of an aircraft according to a further embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENT(S)

[0061] Figure 1 shows an existing aircraft 1 with port and starboard fixed wings 2, 3, engines 9, a fuselage 4 with a nose end 5 and a tail end 6, the tail end 6 including horizontal and vertical stabilising surfaces 7, 8. The aircraft 1 is a typical jet passenger transonic transport aircraft but the invention is applicable to a wide variety of fixed wing aircraft types, including commercial, military, passenger, cargo, jet, propeller, general aviation, etc. with any number of engines attached to the wings or fuselage.

[0062] Each wing 2, 3 of the aircraft 1 has a cantilevered structure with a length extending in a span- wise direction from a wing root 10 to a wing tip 11, the root 10 being joined to the aircraft fuselage 4. The wing root 10 extends in a fore-aft direction from a wing root leading edge 12 to a wing root trailing edge 13.

[0063] The fore-aft direction of the aircraft is defined by a longitudinal axis of the fuselage, i.e. an axis from the nose 5 to the tail 6. The fore-aft direction of the aircraft 1 is indicated in Figure IB, and the fore-aft position of each element of the aircraft is defined by an x-coordinate in this direction. For example the wing root 10 has a fore- aft position coordinate ranging from xl at the wing root leading edge 12 to x2 at the wing root trailing edge 13.

[0064] The fuselage 4 comprises a pressure shell 14 which contains a pressurised space including a cabin 41 shown in Figure 2 containing passenger seats 40.

[0065] Endeavours to increase aircraft efficiency mean that improvements to existing aircraft are continuously being made, with one such solution being the use of cryogenic fuels such as liquid hydrogen. However, liquid hydrogen has a lower specific energy content (i.e. energy per volume of fuel) and so the fuel tanks required to carry the necessary amount of fuel are correspondingly much larger. The fuel tanks of many of these alternative fuels, such as liquid hydrogen, also need to be stored in pressurised tanks.

[0066] Various embodiments of the invention will now be described. Identical elements are given the same reference number as in Figures 1 and 2, and will not be described again.

[0067] Figures 3-5 show a first example of a hydrogen-fuelled aircraft la in which a hydrogen fuel tank 15 is located on top of a fuselage 4a. The fuel tank 15 is sufficiently thermally insulated to maintain the liquid hydrogen fuel at cryogenic temperatures.

[0068] The fuselage 4a comprises a pressure shell 14a which contains a pressurisable space including a cabin 41a designed to accommodate passengers on seats 40. A cargo bay below the cabin 41a inside the pressure shell 14a may also be pressurisable.

[0069] The fuel tank 15 on top of the pressure shell 14a is covered by a fuel tank fairing 16, such that the fuel tank 15 is enveloped between the top of a pressure shell 14a of the fuselage 4a and the fuel tank fairing 16. The top and bottom of the pressure shell 14a have the same shape as the pressure shell 14 of the conventional aircraft 1, as can be seen by comparing Figures 2 and 3.

[0070] In order to feed fuel from the fuel tank 15 to the engines 9, the engines 9 being located on either wing 2, 3, a network of fuel lines 17a is arranged to feed fuel from the fuel tank 15 to the engines 9. Fuel connectors 23 a, 23b connect the fuel lines 17a to the fuel tank 15 and engines 9 respectively.

[0071] The aircraft la includes a refuel line 18, extending from the rear of the fuel tank 15 to a refuel point 19 located on an external surface of the fuselage, aft of a pressure bulkhead 28. The refuel line 18 allows the fuel tank 15 to be refilled.

[0072] The fuel lines 17a, and the refuel line 18, are located outside the pressure shell 14a. In the case of the refuel line 18, this is due to the pressurised cabin 41a terminating at the aft pressure bulkhead 28, so the refuel line 18 is free to extend directly to the refuel point 19 in the unpressurised space aft of the cabin 41a. This is not possible with the fuel lines 17a of this particular example, as the engines 9 are on the wings 2, 3.

[0073] By placing the fuel lines 17a and the refuel line 18 outside the pressure shell 14a, any fuel leaks that might occur from the lines 17a, 18 are contained outside the pressure shell 14a and prevented from entering the cabin 41a or cargo bay.

[0074] In order to ensure the fuel lines 17a are not exposed to the outside airflow, the fuel lines 17a are covered by a fairing 20a. The fairing 20a has an outer aerodynamic surface exposed to the external airflow over the aircraft la. The fairing 20a defines an unpressurised space 21a between the pressure shell 14a and the fairing 20a, and as shown in Figure 3 the fuel lines 17a extend inside the unpressurised space 21a from the fuel tank 15 to the engine 9, although a section of the fuel lines 17a does extend outside of the unpressurised space 21a of the fairing 20a and through the wing 2. It will be clear to the skilled person that whilst in this example the fuel lines 17a extend outside of the unpressurised space 21a of the fairing 20a and through the wing 2, in alternative examples the fuel lines 17a may extend through other sections of the aircraft 1 other than the wings 2 or may be entirely contained within the unpressurised space 21a of the fairing 20a. [0075] As shown in Figure 4, there are multiple fuel lines 17a in order to make them the smallest possible cross-sectional area to minimise the required depth of fairing 20a, and in order to provide segregation and redundancy.

[0076] The pressure shell 14 of the conventional aircraft has a generally circular cross- section as shown in Figure 2. The top and bottom of the pressurised shell 14a has the same shape as the pressure shell 14, but each side of the pressure shell 14a is formed with a dent or depressed region so that the pressure shell is locally narrowed or waisted. The dent in the pressure shell 14a is covered by the fairing 20a. This enables the fairing 20a to have a conformal shape so that the overall outer cross-section on each side of the fuselage is generally circular as shown in Figure 3, and does not change at the fairing 20a in a fore-aft direction as shown in Figure 5. Thus the conformal fairing 20a ensures that the fuselage 4a has a smooth outer aerodynamic profile.

[0077] When the aircraft is at altitude, the cabin 41a is at a higher pressure than the unpressurised space 21a between the pressure shell 14 and the fairing 20a (the unpressurised space 21a being at a reduced pressure due to the reduction of ambient pressure at altitude). When the aircraft is grounded, the cabin 41a may be unpressurised and at the same pressure as the unpressurised space 21a.

[0078] As a result of the narrowed pressure shell 14a, the space inside it is reduced and this can have significant knock-on effects to internal fuselage configurations. For example the cabin 41a shown in Figure 3 requires the removal of a number of seats 40 to accommodate the fuel lines 17a (in comparison to the conventional fuselage of Figure 2) even though the fuel lines 17a do not require all of the space that would otherwise be occupied by those seats 40.

[0079] The pressure shell 14a returns to its generally circular cross-section forward and aft of the fairing 20a, i.e. where there are no fuel lines 17a. As a result, the fuselage 4a maintains its substantially constant cross-sectional area along a significant portion of its length, and in particular adjacent to the fairing 20a, even though the cross-sectional area of the pressure shell 14a decreases and then increases in a fore-aft direction at the dent which is covered by the fairing 20a. [0080] A result of the narrowed or waisted shape of the pressure shell 14a is that the shell 14a is not idealised as a pressure vessel, the shape allows locally high stresses to develop, and the shape is also more difficult to manufacture than a cylindrical shell.

[0081] Figures 6-8C show a second example of an aircraft lb with a hydrogen fuel tank 15 on a fuselage 4b.

[0082] In this example, the cross-sectional shape of the pressure shell 14b of the fuselage 4b is maintained along its length as indicated by contour lines 55 in Figure 6. As shown most clearly in Figure 8A-C, fuel lines 17b extend through an unpressurised space 21b between a pressure shell 14b and a fairing 20b.

[0083] The outer surface of the fairing 20b forms a bulge as viewed from above, as shown in Figure 7A. Thus the cross-sectional area of the unpressurised space 21b increases then decreases in a fore-aft direction, so at the apex of the bulge there is a local maximum of the fuselage cross-sectional area and a local maximum of the cross- sectional area of the unpressurised space 21b. It will be noted that the cross-sectional area and shape of the pressure shell 14b along the fairing 20b, in the fore-aft direction, is substantially constant.

[0084] In this example, the locations at which the fuel lines 17b run from the fuel tank 15 to the engines 9 are carefully selected, such that the location of the fairing 20b is tailored to provide improved aerodynamic performance.

[0085] The fairing 20b is located adjacent the wing root trailing edge 13, with a leading portion of the fairing 20b forward of the wing root trailing edge 13 and a trailing portion of the fairing 20b aft of the wing root trailing edge 13. This can be seen most clearly in the plan view of Figure 7A, which shows the fore-aft position of the fairing 20b relative to the wing root trailing edge 13. As shown in Figure 7A, the apex of the bulge is positioned close to the wing root trailing edge.

[0086] More specifically: the wing root 10 has a fore-aft position ranging from xl to x2. The bulge of the fairing 20b comprises a tapered leading portion terminating at a bulge leading edge 70b (labelled in Figure 6), a tapered trailing portion terminating at a bulge trailing edge 71b (labelled in Figure 6) and an apex between the leading portion and the trailing portion. The trailing portion of the bulge has a fore-aft position which is offset aft from the fore-aft position x2 of the wing root trailing edge; the leading portion of the bulge (including the bulge leading edge 70b) has a fore-aft position which coincides with the fore-aft position of the wing root (that is, most of the leading portion of the bulge is in line with the wing root); and the apex of the bulge has a fore-aft position x3 which is slightly offset aft from the fore-aft position x2 of the wing root trailing edge (i.e x3>x2). Alternatively the apex of the bulge may have the same fore- aft position as the wing root trailing edge (i.e x3=x2).

[0087] The aircraft lb (including the wings 2, 3) has a cross-sectional area that changes along its longitudinal axis as shown in Figure 7B. In particular, the cross-sectional area around the wing root trailing edge 13 decreases rapidly, and this can cause a high transonic wave drag that is explained by the transonic area rule (also known as the Whitcomb area rule). To reduce the number and power of shock waves (and hence reduce the transonic wave drag) the cross-sectional area should change as smoothly as possible.

[0088] The fairing 20b is positioned and shaped to smooth this decrease in the total cross-sectional area of the aircraft. The effect of the fairing 20b is shown in Figure 7B, in which the x-axis 30 represents the aircraft longitudinal axis and the y-axis 31 represents the aircraft cross-sectional area. The solid line 33 indicates the aircraft cross- sectional area without the fairing 20b and the dashed line 34b indicates the cross- sectional area of the aircraft lb with the fairing 20b. The smoother profile of the dashed line 34b gives a reduction in transonic wave drag.

[0089] Figures 8A to 8C show a partial cross-section of the fuselage taken at the sections indicated in Figure 7A, through the leading portion of the fairing 20b.

[0090] In the example shown in Figures 8 A to 8C, the pressure shell 14b is generally heart-shaped. The heart-shaped profile provides a channel in which to fit the fuel tank 15, thereby decreasing the overall frontal area of the fuselage that would otherwise add a significant drag penalty. In alternative examples, the pressure shell 14b may not be heart shaped, but instead circular (like the pressure shell 14 of the fuselage 4) or elliptical. [0091] Figure 8 A, taken at the section 50a, shows a forward location of the fairing 20b where the area of the unpressurised space 21b is minimised whilst still accommodating the fuel lines 17b.

[0092] Figure 8B, taken at the section 50b, shows a location of the fairing 20b aft of section 50a show in Figure 8 A where the size of the unpressurised space 21b has increased. The unpressurised space 21b is therefore larger than required to accommodate the fuel lines 17b, but the increase in area has drag benefits, as described above.

[0093] Figure 8C, taken at the section 50c, shows the further increased area of the unpressurised space 21b near the trailing edge 13 of the wing root and the apex of the bulge.

[0094] Figure 9 shows a third example of an aircraft lc with a fuselage 4b identical to the fuselage 4b of Figure 6 but with two fairings 20b, 20c rather than one. The first fairing 20b covers the fuel lines 17b and the second fairing 20c covers a second set of fuel lines 17c.

[0095] Like the first fairing 20b, the second fairing 20c has a bulged shape viewed from above as shown in Figure 10A. Thus the cross-sectional area of the unpressurised space increases then decreases in a fore-aft direction, providing a local maximum of the fuselage cross-sectional area at the apex of the bulge in the second fairing 20c.

[0096] The second fairing 20c is located adjacent the wing root leading edge 12, with a leading portion of the fairing 20c (including a bulge leading edge 72c) forward of the wing root leading edge 12 and a trailing portion of the fairing 20c (including a bulge trailing edge 73c) aft of the wing root leading edge 12. This can be seen most clearly in the plan view of Figure 10A, which shows the fore-aft position of the fairing 20c relative to the wing root leading edge 12. As shown in Figure 10A, the apex of the bulge is positioned close to the wing root leading edge 12.

[0097] The bulge of the second fairing 20c comprises a tapered leading portion which terminates at the bulge leading edge 72c, a tapered trailing portion which terminates at the bulge trailing edge 73c and an apex between the leading portion and the trailing portion. The leading portion of the bulge has a fore-aft position which is offset forward from the fore-aft position xl of the wing root leading edge; the trailing portion of the bulge (including the bulge trailing edge 73c) has a fore-aft position which coincides with the fore-aft position of the wing root; and the apex of the bulge has a fore-aft position x4 which is slightly offset forward from the fore-aft position xl of the wing root leading edge (i.e. x4<xl). Alternatively the apex of the bulge may have the same fore-aft position as the wing root leading edge (i.e. x4=xl).

[0098] The second fairing 20c is sized so as to smooth the increase in the total aircraft cross-sectional area at the wing root leading edge 12. This is shown in Figure 10B, in which the solid line 33 indicates the aircraft cross-sectional area without any fairings and the dashed lines 34b and 34c indicate the aircraft cross-sectional area including the first and second fairings 20b, 20c. As a result, transonic wave drag is reduced.

[0099] Figure 9 also shows that the fuel tank is separated into two fuel tank elements 15c. This is to provide a gap between the fuel tank elements 15c in a disk burst zone of each engine 9, which is a zone adjacent to the engine 9 that is at risk of being impacted in the event of an engine failure.

[0100] The disk burst zone 56 of the engine 9 on the port wing 2 is shown in Figure 9. The network of fuel lines 17b, 17c includes a number of shut-off valves 24 adjacent to the disk burst zone 56 that allow the fuel flow to be selectively restricted in the event of a disk burst. The valves 24 segregate the fuel lines 17b covered by the first fairing 20b from the fuel lines 17c covered by the second fairing 20c.

[0101] Figure 11 shows a fourth example of an aircraft Id which is similar to the aircraft lc of Figure 9 except it has a one-piece fairing 20d that extends across the wing root 10.

[0102] The transonic area ruling of the aircraft Id is similarly improved by providing a pair of bulges in the fairing 20d near the leading and trailing edges of the wing root, separated by a waisted part 47 over the wing root as shown in Figure 12A. The cross- sectional area of the unpressurised space at the wing root leading edge 12 and at the wing root trailing edge 13 is larger than the cross-sectional area of the unpressurised space at the waisted part 47 of the fairing 20d. [0103] The waisted part 47 minimises the increase in the maximum cross-sectional area of the aircraft Id, which is located in-line with the wings 2,3, although a small increase in the cross-sectional area is unavoidable. Any associated disadvantages of this increase in the cross-sectional area may be counterbalanced by an increase in the ease of manufacture and assembly as a result of using a one-piece fairing 20d.

[0104] The fairing 20d has an outer surface formed with a pair of bulges which cover the fuel lines 17b and 17c. The edges of the bulges are shown in dashed line in Figure 11. The pair of bulges in the single-piece fairing 20d have a similar profile to the bulges of the fairings 20b, 20c of Figure 9. Thus, similar to fairing 20b, the aft bulge terminates at a leading edge 70d in line with the wing root (i.e. the leading edge 70d has a fore-aft position which coincides with the fore-aft position of the wing root) and a trailing edge 7 Id aft of the wing root. Also, similar to fairing 20c, the forward bulge terminates at a trailing edge 73d in line with the wing root (i.e. the trailing edge 73d has a fore-aft position which coincides with the fore-aft position of the wing root) and a leading edge 72d forward of the wing root.

[0105] The fairing 20d is sized so as to smooth the increase in the total aircraft cross- sectional area at the wing root leading edge and at the wing root trailing edge. This is shown in Figure 12B, in which the solid line 33 indicates the aircraft cross-sectional area without any fairings and the dashed line 34d indicates the aircraft cross-sectional area including the fairing 20d. As a result, transonic wave drag is reduced.

[0106] The fairings 20a-20d described above may provide various additional benefits beyond those already described, including providing a barrier against external threats such as engine debris, or providing additional sound insulation.

[0107] In the examples shown, the fairing extends across a significant portion of the outside surface of the pressure shell. In some aircraft, particularly commercial aircraft, this can present difficulties as the fairing extends over the windows, blocking the view of passengers.

[0108] Figure 13 shows an aircraft le with a first window 42 in a pressure shell 14e, as well as a second window 22 in a fairing 20e in line with the window 42 in the pressure shell 14e. The second window 22 may be a plastic screen (made of, for example, polymethylmethacrylate - PMMA) that maintains a flush outer aerodynamic surface, whilst allowing a passenger 45 to see through the windows 22, 42.

[0109] As with previous embodiments, a space 21e between the pressure shell 14e and the fairing 20e is unpressurised. A space 43 between the windows 22, 42 is also unpressurised and is bounded by walls 44.

[0110] The pressurised cabin 41e retains existing pressurised window assemblies 42. Thus the windows 22 in the external fairing 20e do not need to be designed to maintain cabin pressure. This enables much larger windows 22 to be incorporated in the fairing 20e, and these can be configured to maximise the external field of view seen by the passengers. In this case the window 22 in the external fairing 20e has a larger area, a larger height and a larger width than the window 42 in the pressure shell.

[0111] Optionally a single window 22 in the fairing 20e extends over two windows 42 of the pressure shell, as shown in Figure 14.

[0112] Optionally, as with previous embodiments, the space 21e between the pressure shell 14e and the fairing 20e may contain fuel lines 17e and/or other components such as hydraulic lines or electrical cables. Alternatively, the space 21e may be empty. In this case the fairing 20e may be provided solely to reduce drag, provide a barrier against external threats such as engine debris, or provide additional sound insulation.

[0113] Optionally, if the space 21e between the pressure shell 14e and the fairing 20e contains fuel lines 17e and/or other components, these components should not obstruct the view of passengers through the windows 42, 22. For instance the fuel lines 17e may be positioned (in a fore-aft sense) in the gaps between adjacent fairing windows 22 and/or in the gaps between adjacent fuselage windows 42.

[0114] Similar windows 22 may be incorporated into the fairings 20a, 20b, 20c or 20d of the previously described embodiments.

[0115] Figure 15 shows a fifth example of an aircraft If. In this example, the pressure shell of the fuselage may be the same as the conventional pressure shell 14 of Figure 2 or the heart-shaped pressure shell 14b of Figure 8A. [0116] Fuel lines 17f extend aft of the fuel tank 15, then down through the unpressurised space aft of the bulkhead 28, then forwards under the pressure shell to the engine 9. A belly fairing 20f underneath the pressure shell extends forward from the bulkhead 28 all the way to the wing root 10. The fuel lines 17f run under the pressure shell through the unpressurised space between the pressure shell and the belly fairing 20f. In this particular case, the belly fairing 20f extends forward of the wing root leading edge 12.

[0117] In the examples described, the engines 9 are mounted to a wing 2, 3 of the aircraft. In alternative examples, the engines 9 may be mounted to a different part of the aircraft, for example the engines 9 may be mounted on the fuselage or empennage 7, 8. As a result, the position of the fairing (or fairings) may differ, and correspondingly the shape of the fairing(s) may be adapted to conform better to the transonic area rule.

[0118] A fairing may also be utilised to enclose the refuel line 18, particularly if the refuel line 18 extends forward of the aft pressure bulkhead 28.

[0119] The fuel tank 20 is described in relation to its storage of cryogenic fuels, in particular liquid hydrogen. In alternative examples, the cryogenic fuel is liquid methane, liquid natural gas, liquid ammonia, or any cryogenic fuel known in the art. In further examples the fuel is not a cryogenic fuel, for example the fuel is a kerosene based aviation fuel or other aviation fuel known in the art.

[0120] Where the word 'or' appears this is to be construed to mean 'and/or' such that items referred to are not necessarily mutually exclusive and may be used in any appropriate combination.

[0121] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.