GB2544745A

GB2544745A - Aircraft fuel system

Info

Publication number: GB2544745A
Application number: GB1520731.9A
Authority: GB
Inventors: Poad Michael; Edwards Henry; Morton Nicholas
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2015-11-24
Filing date: 2015-11-24
Publication date: 2017-05-31
Also published as: GB201520731D0

Abstract

An aircraft fuel system comprising: a fuel tank; an outlet (50, fig 7) in the fuel tank for feeding liquid fuel out of the fuel tank; and an expandable gas-container in the fuel tank. The gas-container encloses a mass of gas which is sealed within the gas-container; the gas-container is exposed to ambient pressure and the weight of the head of fuel above the gas-container. A drop in ambient pressure and the weight of fuel during flight of the aircraft causes the gas-container to expand. The expansion of the gas-container causes it to displace the liquid fuel towards the outlet. The mass of gas is a fixed mass of gas - thus the amount of gas in the gas-container does not change as the gas-container expands and its pressure decreases in inverse proportion to the change of volume. The gas-container may comprise a sealed-bladder 60 and may further be filled with a gas containing substantially no oxygen. Alternatively, the gas-container may comprise a flexible diaphragm (10, fig 1) disposed at least partially between a pair of stringers (3, 4, fig 1) in the fuel tank.

Description

Aircraft Fuel System

FIELD OF THE INVENTION

[0001] The present invention relates to an aircraft fuel system.

BACKGROUND OF THE INVENTION

[0002] Conventional aircraft fuel systems suffer from the problem of unusable fuel (also known as residual fuel) - that is, fuel which cannot be fed out of the fuel tank because it is trapped in a low or inaccessible region. The weight of unusable/residual fuel is included in the empty weight of the aircraft. This extra empty weight reduces the aircraft operator's permitted usage of the aircraft in terms of range, passengers and cargo.

SUMMARY OF THE INVENTION

[0003] A first aspect of the invention provides an aircraft fuel system comprising: a fuel tank; an outlet in the fuel tank for feeding liquid fuel out of the fuel tank; and a gas-container in the fuel tank, wherein the gas-container encloses a mass of gas which is sealed within the gas-container, the gas-container is positioned in the fuel tank so that it comes into contact with liquid fuel which exerts a pressure on the gas-container, and the gas-container is expandable so that a reduction in the pressure exerted by the liquid fuel causes the gas-container to expand and displace the liquid fuel towards the outlet.

[0004] The expansion of the gas-container causes it to displace the liquid fuel towards the outlet, thereby reducing a volume of unusable liquid fuel in the fuel tank which cannot be fed out of the fuel tank via the outlet.

[0005] The gas-container is positioned in the fuel tank so that it comes into contact with liquid fuel when the fuel tank is in use (i.e. when it contains liquid fuel) and this liquid fuel exerts a pressure on the gas-container. During flight of the aircraft the pressure exerted by the liquid fuel reduces, and this pressure reduction causes the gas-container to expand. The pressure reduction is typically caused by a reduction in ambient pressure as the altitude of the aircraft increases, and/or by a reduction in an amount of liquid fuel in the fuel tank above the gas-container as the fuel tank empties.

[0006] Typically the outlet is arranged to feed the liquid fuel out of the fuel tank to an engine - alternatively it may be arranged to feed the liquid fuel out of the fuel tank to another fuel tank.

[0007] Typically the mass of gas is a fixed mass of gas - thus the amount of gas in the gas-container does not change as the gas-container expands, and following Boyle's Law its pressure decreases in inverse proportion to the change of volume.

[0008] Unlike a fuel bladder which encloses a variable mass of liquid fuel, the gas-container encloses a fixed mass of gas. An inner face of the gas-container contacts the mass of gas and an outer face of the gas-container contacts the liquid fuel. Thus the expansion of the gas-container causes its outer face to displace the liquid fuel towards the outlet.

[0009] The gas-container may be positioned at a bottom of the fuel tank, or at any location in the fuel tank where fuel becomes trapped as the fuel level decreases so that it cannot flow to the outlet.

[0010] Typically at least part of the gas-container is fixed to the fuel tank - for instance by a clamp or an adhesive.

[0011] Optionally a pair of stringers is carried by a bottom wall of the fuel tank, and the gas-container is positioned at least partially between the pair of stringers.

[0012] The mass of gas is typically air, or a gas (such as pure nitrogen) containing substantially no oxygen.

[0013] In a first embodiment of the invention, part of the gas-container comprises a wall of the fuel tank - for instance its bottom wall.

[0014] In a second embodiment of the invention, the gas-container comprises a sealed bladder. Optionally the sealed bladder has an inner face which contacts the mass of gas and an outer face which contacts a wall of the fuel tank - typically a bottom wall of the fuel tank.

[0015] Optionally at least part of the gas-container comprises a flexible diaphragm or sealed bladder which is arranged to flex to enable the gas-container to expand. Alternatively the gas-container may be made entirely of rigid parts which slide or pivot relative to each other to enable the expansion. For instance the gas-container may comprise a steel bellows.

[0016] Typically the gas-container is arranged so that as it expands, an outer face of the gas-container changes from a planar or concave shape (preferably with no convex portions) to a convex shape (preferably with no concave portions).

[0017] Further preferred features are set out in the dependent claims.

[0018] A further aspect of the invention provides an aircraft comprising an aircraft fuel system according to the first aspect of the invention. Typically the fuel tank is a centre tank of the aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the invention will now be described with reference to the accompanying drawings, in which: [0020] Figure 1 shows an aircraft fuel system according to a first aspect of the invention, with a flexible diaphragm in its contracted state; [0021] Figure 2 shows the system with the flexible diaphragm in its expanded state; [0022] Figure 3 is an exploded view showing the diaphragm, clamping ring and end caps; [0023] Figure 4 is a cross-sectional view with the flexible diaphragm in its contracted state; [0024] Figure 5 is a cross-sectional view with the flexible diaphragm in its expanded state; [0025] Figure 6 is a plan view of an aircraft incorporating the system of Figures 1-5; [0026] Figures 7 and 8 are cross-sectional views of an aircraft fuel system, not according to the invention, illustrating the problem of unusable fuel; [0027] Figure 9 is a cross-sectional view showing an aircraft fuel system according to a second aspect of the invention, with the sealed bladder in its contracted state; [0028] Figure 10 shows the sealed bladder in its expanded state; and [0029] Figure 11 is a cross-sectional view showing an aircraft fuel system according to a third aspect of the invention, with the flexible diaphragm in its contracted state. DETAILED DESCRIPTION OF EMBODIMENT(S) [0030] An aircraft fuel system according to a first aspect of the present invention is illustrated in Figures 1-5. The system comprises a fuel tank with a bottom wall 1 provided by a lower skin of an aircraft, and a top wall (not shown).

[0031] The bottom wall 1 of the fuel tank carries stringers, a pair 3, 4 of such stringers being shown in Figures 1-3. Each stringer has a flange 5, 6 attached to the bottom wall 1 and a blade 7, 8 extending away from the bottom wall 1. Each stringer also passes through a respective "mouse-hole" recess 9 in a rib 2. If the rib 2 provides a sealed wall of the fuel tank then the recess 9 is sealed to prevent fuel from flowing across the rib. Alternatively the rib 2 may be a baffle rib which allows fuel to flow across it, and in this case the recess 9 is not sealed.

[0032] A flexible diaphragm 10 is installed in the fuel tank adjacent the bottom wall 1. The flexible diaphragm is shown most clearly in Figure 3. It has a peripheral edge consisting of edges 11-14. The bottom wall 1 of the fuel tank carries a first end cap 15 and a second end cap 16 which extend between the stringers, each end cap having a curved concave upper edge 16a. The peripheral edge of the diaphragm 10 is clamped to the fuel tank around its full circumference by a clamping ring 20-23 comprising a clamping member 20 which clamps the edge 11 of the diaphragm to the stringer flange 5, a clamping member 21 which clamps the edge 12 of the diaphragm to the stringer flange 6, a clamping member 22 which clamps the edge 13 of the diaphragm to the curved upper edge of the first end cap 15, and a clamping member 23 which clamps the edge 14 of the diaphragm to the curved upper edge 16a of the second end cap 16. The clamping ring 20-23 is fixed to the stringer flanges by fasteners (not shown) which pass through the clamping members 20, 21 of the clamping ring and the edges 11, 12 of the diaphragm.

[0033] As shown in the cross-section of Figures 4 and 5, the diaphragm 10 and the bottom wall 1 of the fuel tank provide the upper and lower walls respectively of a gas-container which encloses a fixed mass of air in a sealed cavity 30. The diaphragm 10 has a lower (inner) face 10a which contacts the air in the cavity 30 and an upper (outer) face 10b opposite the lower face which contacts the liquid fuel above the diaphragm. The upper surface of the liquid fuel is indicated at 31 in Figure 5 but is not shown in Figure 4 because it is above the top of the drawing. A top of the cavity 30 is bounded by the lower face 10a of the diaphragm and a bottom of the cavity 30 is bounded by the bottom wall 1 of the fuel tank. The side walls of the gas-container are provided by sides of the stringer flanges 6, 7 and end caps 15, 16. Thus the gas-container has a rigid part (lower) comprising the bottom wall 1 and the sides of the stringer flanges 6, 7 and end caps 15, 16; and a flexible (upper) part comprising the diaphragm 10. Since the diaphragm 10 is sealed to the rigid part around the full circumference of its peripheral edge by the clamping ring 20-23, the gas-container is air-tight so the air cannot escape from the cavity 30.

[0034] The forces acting on the diaphragm 10 are shown in Figures 4 and 5: an upward force FI acting on the lower face 10a and a downward force F2 acting on the upper face 10b. The upward force FI is provided by the pressure of the air in the cavity 30, and the downward force F2 is provided by a combination of the ambient pressure from the gas in the ullage 32 above the fuel 31 and the weight of the head of liquid fuel above the diaphragm 10 (this pressure being given by rho*g*h where rho is the density of the fuel, g is the acceleration due to gravity, and h is the height of the head of liquid fuel). These forces FI, F2 balance so that when the ambient pressure is relatively high (for instance 0. IMPa with the aircraft on the ground or at low altitude) and the fuel tank is full, then the cavity 30 adopts a contracted state as shown in Figures 1 and 4. At this stage the upper face 10b of the diaphragm has a concave shape with no convex portions as shown in Figures 1 and 4.

[0035] As the aircraft gains altitude in flight, the associated drop in ambient pressure causes a drop in the pressure in the ullage 32 and a consequential pressure differential across the diaphragm 10 which causes it to flex and rise up to the expanded state shown in Figures 2 and 5, thereby expanding the volume of the gas-container and the cavity 30. This expansion causes the gas-container to displace an increased volume of the liquid fuel 31 in the fuel tank. As the diaphragm 10 rises up, its upper face 10b changes from a concave shape (Figure 4) to a convex bulged shape with no concave portions (Figure 5).

[0036] A similar effect arises from the reduction in pressure applied by the height (h) of liquid fuel, as the height (h) reduces due to the fuel being fed out of the fuel tank to feed an aircraft engine. This reduction in pressure is typically of a similar order of magnitude to the reduction in pressure caused by the change of altitude.

[0037] The diaphragm 10 is formed from an elastomeric material such as fluorosilicone rubber which is fuel resistant. The diaphragm is unfolded when in its contracted state, and stretches as it flexes and rises up to its expanded state.

[0038] The mass of gas in the cavity 30 is typically air which becomes trapped as the diaphragm is installed. Optionally a valve (not shown) may be provided to remove the air and then inject an inert gas (such as pure nitrogen) containing substantially no oxygen into the cavity. An inert gas is preferred because it will cause less corrosion, and present less ignition risk in the event of the diaphragm 10 puncturing and releasing the gas.

[0039] Figure 6 is a plan view of an aircraft incorporating the fuel system of Figures 1-5. The aircraft has a fuselage 40 and a pair of wings 41, 42. Each wing contains two fuel tanks: an inboard fuel tank next to the fuselage and an outboard fuel tank remote from the fuselage. The aircraft also has a centre tank at the bottom of the fuselage. The centre fuel tank is typically emptied first, followed by the inboard fuel tanks and then the outboard fuel tanks. The outboard fuel tanks are emptied last because the weight of fuel in the outboard fuel tanks is more effective in opposing the upward bending moment generated by the lift forces on the wings.

[0040] Although the flexible diaphragm 10 may be installed in any of the fuel tanks of the aircraft, it is most effective in the centre tank for two reasons. Firstly, the diaphragm serves its purpose only when the fuel tank becomes empty, and the centre tank is the first tank to empty during flight of the aircraft (the outboard wing tank never becoming empty except in an emergency). Secondly, the dihedral angle of the wings means that fuel tends to collect an inboard end of the wing tanks so that the problem of unusable fuel is less acute in the wing tanks than in the centre tank.

[0041] The liquid fuel in the centre fuel tank is fed out of the centre fuel tank into one of the engines 43, 44 via an outlet pipe 50 shown in Figures 4 and 5 which leads to a pump (not shown). Note that the outlet pipe 50 may or may not be directly above the diaphragm 10 as shown in Figures 4 and 5. Figures 7 and 8 are cross-sectional views of the centre fuel tank in the absence of the diaphragm 10, illustrating the problem of unusable fuel. When the level of the fuel 31 is above the outlet pipe 50 as shown in Figure 7, the fuel can be fed to the engines via the outlet pipe 50. However, when the fuel level drops below the outlet pipe 50 as shown in Figure 8, a pool 52 of unusable liquid fuel is left which cannot be picked up. The volume of this pool 52 is reduced by the expansion of the gas-container as shown in Figure 5. When the aircraft is on the ground and the fuel tank is full then the gas-container adopts the contracted shape of Figure 4 and the effective internal volume of the fuel tank is maximised, thereby maximising the amount of fuel which can be loaded into the tank. When the fuel tank empties at cruise altitude, it eventually reaches a fully expanded state shown in Figure 5 in which the diaphragm 10 has risen up so that the gas-container displaces an increased volume of liquid fuel, thereby reducing the volume of the pool 52 of unusable liquid fuel compared to Figure 8.

[0042] In the embodiment described above, the gas-container enclosing the cavity 30 has a rigid base and sides provided by the bottom wall 1 of the fuel tank and the sides of the stringer flanges 6, 7 and end caps 15. In the embodiment of Figures 9 and 10, the gas-container is provided instead by a sealed elastomeric (e.g. fluorosilicone) bladder 60, the interior of the bladder 60 providing a sealed cavity 61 which contains the fixed mass of gas. The top of the cavity 61 is provided by the lower face of an upper wall 62 of the bladder, and the bottom of the cavity 61 is provided by the upper face of a lower wall 63 of the bladder opposite the upper wall 62. The upper face of the lower wall 63 contacts the gas in the cavity 61, and the lower face of the lower wall 63 is bonded to the bottom wall 1 of the fuel tank by a layer of adhesive (not shown).

[0043] In the alternative embodiment of Figure 11, a box 70-73 is installed at the bottom of the fuel tank between the pair of stringers. The box has two parts: a rigid part with a base 70, sides 71 and an overhanging lip 72; and a flexible fluorosilicone diaphragm 73 (shown in dashed lines) which is bonded around its full periphery to the overhanging lip 72 to seal a mass of gas in a sealed cavity 74. The lower face of the base 70 is bonded to the bottom wall 1 of the fuel tank by a layer of adhesive (not shown).

[0044] An advantage of the box 70-73 of Figure 11 compared with the bladder 60 of Figure 9 is that the rigid part 70-72 of the box can be made of a material (such as Aluminium) which is less dense than fluorosilicone rubber and hence the box 70-73 can be made lighter as a whole than the bladder 60.

[0045] The advantages of using a sealed bladder or box as in Figures 9-11, rather than a diaphragm as in Figures 1 to 5, are that no clamping ring is required in order to seal the cavity 61, 74; and that a precise mass of inert gas (such as nitrogen) can be injected into the cavity 61, 74 before the bladder 60 or box 70-73 is installed in the fuel tank. The disadvantage is that the lower wall 62 of the bladder (and equivalently the rigid part 70-72 of the box) add weight, and a valve must be provided in the bladder or box in order to inject the inert gas into the cavity 61, 74.

[0046] 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.

Claims

1. An aircraft fuel system comprising: a fuel tank; an outlet in the fuel tank for feeding liquid fuel out of the fuel tank; and a gas-container in the fuel tank, wherein the gas-container encloses a mass of gas which is sealed within the gas-container, the gas-container is positioned in the fuel tank so that it comes into contact with liquid fuel which exerts a pressure on the gas-container, and the gas-container is expandable so that a reduction in the pressure exerted by the liquid fuel causes the gas-container to expand and displace the liquid fuel towards the outlet.

2. The system of claim 1 wherein the gas-container is positioned at a bottom of the fuel tank.

3. The system of any preceding claim further comprising a pair of stringers carried by a bottom wall of the fuel tank, wherein the gas-container is positioned at least partially between the pair of stringers.

4. The system of any preceding claim wherein the mass of gas contains substantially no oxygen.

5. The system of any preceding claim wherein at least part of the gas-container comprises a wall of the fuel tank.

6. The system of any preceding claim wherein at least part of the gas-container comprises a flexible diaphragm or sealed bladder which is arranged to flex to enable the gas-container to expand.

7. The system of claim 6 wherein the flexible diaphragm or sealed bladder is formed from an elastomeric material.

8. The system of any preceding claim wherein the gas-container is arranged so that as it expands an outer face of the gas-container changes from a planar or concave shape to a convex shape.

9. The system of any of preceding claim wherein the gas-container comprises a sealed bladder.

10. The system of claim 9 wherein the sealed bladder has an inner face which contacts the mass of gas and an outer face which contacts a wall of the fuel tank.

11. The system of claim 9 or 10 wherein the sealed bladder is formed from an elastomeric material.

12. The system of any of claims 1 to 8 wherein the gas-container comprises a rigid part and a flexible diaphragm, and wherein the flexible diaphragm has a peripheral edge which is sealed to the rigid part of the gas-container around a full circumference of the peripheral edge to prevent the mass of gas from escaping the gas-container.

13. The system of claim 12 wherein the peripheral edge of the flexible diaphragm is clamped to the rigid part of the gas-container by one or more clamping members.

14. The system of claim 12 or 13, wherein the rigid part of the gas-container comprises a pair of stringers carried by a bottom wall of the fuel tank, each stringer of the pair of stringers has a flange attached to the bottom wall of the fuel tank and a blade extending away from the bottom wall of the fuel tank; and the flexible diaphragm has a pair of edges which are each sealed to the flange of a respective one of the pair of stringers.

15. An aircraft comprising an aircraft fuel system according to any preceding claim.