Leading edge structure for a flow control system of an aircraft

A leading edge structure (1) for a flow control system of an aircraft (101) including a double-walled leading edge panel (3) surrounding a plenum (7). The leading edge panel (3) has a first side portion (11) extending to a first attachment end (17), a second side portion (13) extending to a second attachment end (19), an inner wall element (21) facing the plenum (7), an outer wall element (23) for contact with an ambient flow (25), a core assembly (97). The outer wall element (23) includes micro pores (31) forming a fluid connection between the core assembly (97) and the ambient flow (25). The inner wall element (21) includes openings (33) forming a fluid connection between the core assembly (97) and the plenum (7). The outer wall element (23) extends from the first attachment end (17) to the second attachment end (19).

RELATED APPLICATION

This application incorporates by reference and claims priority to German Patent Application DE 10 2021 101 444.1, filed Jan. 22, 2021.

BACKGROUND

The present invention relates to a leading edge structure for a flow control system of an aircraft, in particular for a Hybrid Laminar Flow Control (HLFC) system, where air is sucked in or blown out of a porous surface of a flow body in order to extend the region of laminar flow along the flow body.

Further aspects of the present invention relate to a vertical tail plane comprising such a leading edge structure, an aircraft comprising such a leading edge structure or such a vertical tail plane, and a method for manufacturing such a leading edge structure. Instead of to a vertical tail plane the leading edge structure might also be attached to a horizontal tail plane or to a wing.

The leading edge structure comprises a double-walled leading edge panel that surrounds a common plenum in a curved, i.e. arcuate, manner. The plenum extends in a span direction through the leading edge structure.

When viewed in a cross section across the span direction, the leading edge panel has a first side portion extending from a leading edge point, i.e. from a fore tip of the leading edge structure, to a first attachment end on a first side of the leading edge structure, the first attachment end being configured for attachment to a further structure located downstream from the leading edge. Further, the leading edge panel has a second side portion opposite the first side portion, wherein the second side portion extends from the leading edge point to a second attachment end on a second side of the leading edge structure opposite the first side, the second attachment end being configured for attachment to a further structure downstream from the leading edge.

The leading edge panel comprises an inner wall element facing the plenum and spaced apart from the inner wall an outer wall element for contact with the ambient flow. Between the inner and outer wall elements the leading edge panel comprises a core assembly. The outer wall element comprises a plurality of micro pores, such as perforations, forming a fluid connection between the core assembly and the ambient flow. The inner wall element comprises openings forming a fluid connection between the core assembly and the plenum. The inner wall element may be made of fiber reinforced plastic (FRP) and the outer wall element may be made of titanium sheet.

Similar leading edge structures are known in the art. The known leading edge structures comprise an outer wall element extending around the leading edge point but having only a limited extension downstream from the leading edge point, so that outer wall element does not extend all the way to the first and second attachment ends but rather ends well ahead of the attachment ends, while the attachment ends are formed solely by the inner wall element. This is partly due to the fact that titanium sheet panels with a sufficient width to reach from the first attachment end to the second attachment end are not available on the market and that micro pores in the very downstream regions of the leading edge panel are not absolutely necessary for an effective flow control system, since the effect of flow control through the micro pores in the downstream regions is considerably lower than the effect of flow control through the micro pores in the upstream regions around the leading edge point. However, the known design with an outer wall element ending ahead of the inner wall element brings along a “two step” flow surface with a transition between the end of the outer wall element and the continuation of the inner wall element that forms an obstacle for the ambient flow along the flow surface.

SUMMARY OF INVENTION

The present invention may be configured to provide a leading edge structure with increased flow efficiency by extending the outer wall element from the first attachment end to the second attachment end.

In this way, a leading edge structure with a one-step, continuous and smooth flow surface is provided that does not form an obstacle for ambient flow streaming along the flow surface and, thus, decreases drag and increases flow efficiency of the leading edge structure. Further, the area of the micro pores can be extended further downstream closer to the attachment ends, thereby increasing the overall flow control effectivity. Moreover, the leading edge structure can be optimized in terms of weight and costs since the outer wall element also supports the attachment ends.

According to an embodiment of the invention, the core assembly comprises a plurality of elongate stiffeners connecting the inner and outer wall elements and spaced apart from one another, so that between each pair of adjacent stiffeners a hollow chamber is formed between the inner and outer wall elements. The stiffeners may be formed from FRP integrally with the inner wall element. The plurality of micro pores form a fluid connection between the hollow chambers and the ambient flow, while the openings form a fluid connection between the hollow chambers and the plenum. Each hollow chamber may comprise at least one opening. Such stiffeners and hollow chambers form a simple and effective core assembly through which air flow can pass during suction or blowing operation.

The outer wall element may form a first end edge at the first attachment end and a second end edge at the second attachment end. The first end edge and/or second end edge may extend in parallel to the leading edge and may be formed to extend along or rest against a vertical tail plane box of a related vertical tail plane. This means, the outer wall element extends as far as or even further downstream as the inner wall element at the first attachment end and/or at the second attachment end. In this way, a full outer coverage of the leading edge structure by outer wall element is achieved.

In particular, the first end edge and/or the second end edge may extend in parallel to at least one of and/or all of the stiffeners. This means, the end edges extend in span direction. In this way, the end edges can continuously abut a vertical tail plane box when the leading edge structure is attached to a vertical tail plane.

The outer wall element may comprise a main wall portion as well as a first wall extension and/or a second wall extension. The main wall portion includes the leading edge point. The first wall extension includes the first attachment end and maybe the first end edge. The second wall extension includes the second attachment end and maybe the second end edge. In this way, the outer wall element might be formed by connecting the first and second wall extensions to the main wall portion, so that for the main wall portion a titanium sheet might be used with a width as available on the marked, while the remaining width required to cover the full leading edge panel from the first attachment end to the second attachment end can be covered by the first and second wall extensions.

The first wall extension may be connected to the main wall portion via a straight first welding seam. Alternatively or additionally, the second wall extension is connected to the main wall portion via a welding steam that may be straight. First and/or second welding seams may be butt-welded seams, formed by laser welding. Other forms of welding or alternative forms of connection of the main wall portion to the first and/or second wall extensions may be used.

The first welding seam and/or the second welding seam may be dressed, at least at the outer flow surface in contact with the ambient flow, to form a smooth transition between the main wall portion and the first wall extension and/or between the main wall portion and the second wall extension. In this way, the first and second welding seams do not form a flow obstacle and laminar flow across the welding seams is possible.

The first welding seam, e.g., the entire first wall extension, may be attached, e.g., bonded, directly and planar to the inner wall element with no core assembly in between. Additionally or alternatively, the second welding seam, e.g., the entire second wall extension, may be attached, e.g., bonded, directly and planar to the inner wall element with no core assembly in between. In this way, the first and second welding seams are sufficiently supported by the inner wall element, wherein the bonding forms an additional load path.

There may be no micro pores in the first and/or second wall extensions. The micro pores are present in the main wall portion, such as only in the main wall portion, and are distributed from the leading edge point to the first welding seam and/or to the second welding seam, with a minimum distance from the welding seams. In this way, the entire main wall portion can be used for flow control, thereby increasing overall flow control efficiency of the leading edge structure. A minimum distance of the micro pores from the first and/or second welding seam may be kept to avoid welding influence on the micro pores.

The stiffeners, such as at least some of the stiffeners, are formed integrally with the inner wall element. Integrally in this connection is to be understood as formed in one piece that is not separable or mounted together from separate components. Such a leading edge structure with stiffeners formed integrally with the inner wall element represents a very simple and light weight construction, since fasteners, such as bolts or rivets, can be avoided. Also, the mechanical properties are improved, so that material and, once again, weight can be saved. Additionally, manufacturing can be simplified and expedited, as the inner wall element can be formed together with the stiffeners in one common process step, e.g. by Resin Transfer Molding (RTM).

The leading edge structure may further comprises a back wall, in particular a membrane of Carbon Fiber-reinforced polymers (CFRP) material. The back wall may connect the first attachment end to the second attachment end of the leading edge panel, thereby enclosing the plenum together with the leading edge panel on a side opposite the leading edge point.

The openings may be formed as throttle holes having a predefined diameter adapted for a predefined mass flow rate through the throttle holes in order to achieve a predefined fluid pressure in the hollow chambers. In this way, the mass flow rate through the micro pores can be controlled by the fluid pressure in the hollow chambers and, thus, by the predefined diameter of the throttle holes. Alternatively, the openings might also be formed such that they allow an uncontrolled mass flow rate and are not adapted to control the fluid pressure in the hollow chambers, for example by a number of bores or by one large diameter hole. In this case, the fluid pressure in the hollow chambers corresponds to the fluid pressure in the plenum, so that the mass flow rate through the micro pores can be controlled only by the fluid pressure in the plenum. Whether the openings are formed as throttle hole or as simple openings not adapted for a specific mass flow rate, may vary from chamber to chamber.

The stiffeners may have a solid cross section with a square or trapezoid shape. In this way, the stiffeners provide plane support surfaced for the inner and outer walls elements.

The stiffeners may extend in the span direction, i.e. in parallel to the leading edge point, and may be parallel to one another. In this way, the stiffeners may have a long extension.

The inner wall element may be formed of a Fiber Reinforced Plastic (FRP) material, such as from a Carbon Fiber Reinforced Plastic (CFRP) material. Further, the stiffeners might be formed as sandwich structures, each sandwich structure comprising a core element enveloped on opposite sides by separate layers of FRP of the inner wall element. In other words, the inner wall element splits up in two separate layers. One layer encloses the core element on the side facing the plenum, and thus forms the inner wall element in the region of the stiffeners. The other layer encloses the core element on the side facing the outer wall element or resting against the outer wall element. This layer may be formed in an omega shape, i.e. has an omega-shaped cross section. In this way, a simple, strong and light weight integral construction of the inner wall element and the stiffeners is provided.

The core elements may be formed of a foam material. Foam has a high stiffness at a low weight.

A plurality of support ribs, e.g. frames, may be attached to the inner wall element in this way that they face the plenum and extend across, and possibly perpendicular to, the span direction along the inner wall element. The support ribs stiffen the leading edge structure across the span direction.

The support ribs may be formed integrally, e.g., single piece construction, with the inner wall element. In this way, the inner wall element can be formed as one piece together with both the stiffeners and the support ribs. This further simplifies the entire leading edge structure and reduces additional weight.

The support ribs may be formed of FRP. This allows that the support ribs can easily be formed integrally with the inner wall element.

The outer wall element may be formed as a titanium sheet. Such titanium sheet provides the strength and stiffness required for the outer surface along the leading edge.

The outer wall element may comprise multiple sections, when viewed from a leading edge downstream, i.e. in a chord direction. The porosity varies from one section to another in terms of pore diameter and/or pore pitch. In particular, the diameter decreases and the pitch increasing from the leading edge downstream. In this way, the mass flow rate of the air sucked in or blown out can be adapted to the demand. For example, in sections close to the leading edge point, where a higher mass flow rate is demanded, the pore diameter might be larger and/or the pore pitch might be smaller than in sections further downstream.

The invention may be embodied in a vertical tail plane for an aircraft. The vertical tail plane comprises a vertical tail plane box and a leading edge structure according to any of the afore-described embodiments. The vertical tail plane box has a first lateral panel with a first attachment portion and an opposite second lateral panel with a second attachment portion. Both the first attachment portion and the second attachment portion extend in the span direction. The first attachment end is attached to the first attachment portion, such that the first end edge may extend along, e.g., rests against, the first attachment portion, and the second attachment end is attached to the second attachment portion, such that the second end edge may extend along, e.g., rest against, the second attachment portion, so that the first side portion of the leading edge panel forms a continuous, e.g., smooth, flow surface with the first lateral panel of the vertical tail plane box, and the second side portion of the leading edge panel forms a continuous, e.g., smooth, flow surface with the second lateral panel of the vertical tail plane box. The plenum may be in fluid connection with an air outlet, such as an adjustable outlet flap with a rear-facing opening, for causing a vacuum in the plenum to draw ambient air through the micro pores and the hollow chambers into the plenum. The plenum may be in fluid connection with an air inlet, such as an adjustable inlet flap with a forward-facing opening, for causing an overpressure in the plenum to blow out air from the plenum through the hollow chambers and the micro pores to the ambient. The air outlet and the air inlet may be provided in a cover panel on one side or on opposite sides of the vertical tail plane. The vertical tail plane may further comprise a connection duct connecting a lower end of the plenum to the air outlet and/or to the air inlet. The above explanations with respect to the leading edge structure apply vis-à-vis to the vertical tail plane.

The first attachment end may be attached to the first attachment portion by a first front line of fasteners extending through the main wall portion, and by a first rear line of fasteners extending through the first wall extension, so that the first welding seam extends between the first front line of fasteners and the first rear line of fasteners. Additionally or alternatively, the second attachment end is attached to the second attachment portion by a second front line of fasteners extending through the main wall portion, and by a second rear line of fasteners extending through the second wall extension, so that the second welding seam extends between the second front line of fasteners and the second rear line of fasteners. The fasteners can be e.g. bolts, lockbolts and rivets. In this way, the main wall portion and the first and second wall extensions are each secured to the first attachment portion and the second attachment portion, respectively, by at least one line of fasteners, so that the first welding seam and the second welding seam, respectively, are not necessarily required for structural integrity of the outer wall element.

Yet a further aspect of the present invention relates to an aircraft comprising a leading edge structure according to any of the afore-described embodiments, or comprising a vertical tail plane according to any of the afore-described embodiments. The above explanations with respect to the leading edge structure and the vertical tail plane apply vis-à-vis to the aircraft.

Yet a further aspect of the present invention relates to a method for manufacturing a leading edge structure according to any of the afore-described embodiments comprising the following steps: The inner wall element and the core assembly is produced, wherein the stiffeners and the inner wall element are formed together as an integral part by a Resin Transfer Molding (RTM) process, in particular by a common RTM step. The support ribs may be formed as an integral part together with the inner wall element and the stiffeners by an RTM process. Further, the outer wall element is produced. Then, the outer wall element is connected, e.g., bonded, to the core assembly and/or to the inner wall element, wherein the outer wall element may be bonded against the stiffeners and against the inner wall element at the first and/or second attachment ends. The above explanations with respect to the leading edge structure also apply to the present method.

The outer wall element may be produced using the following steps: The main wall portion is provided, e.g., formed by a titanium sheet of 1.25 meter (m) width and 1 millimeter (mm) thickness. The first wall extension and/or the second wall extension are provided, e.g. titanium sheets of 0.125 m width each and 1 mm thickness. Then, the main wall portion is welded, e.g. butt-welded, to the first wall extension to form the first welding seam, and/or to the second wall extension to form the second welding seam, e.g. by laser welding. By butt-welding the main wall portion to the first and/or second wall extensions a reliable and smooth connection between these parts can be produced.

The first welding seam and/or the second welding seam may be subsequently dressed, at least at the outer flow surface in contact with the ambient flow, to form a smooth transition between the main wall portion and the first wall extension and/or between the main wall portion and the second wall extension. In this way, the welding seams do not form an obstacle to the ambient flow thereby allowing a laminar flow along the flow surface.

The following steps may be carried out to produce the outer wall element: First, a blank of the main wall portion is provided. Then, the micro pores are produced in the blank after which it is sanded and etched. Subsequently, the first and/or second wall extensions are provided and welded to the main wall portion to form the first and/or second welding seams. After welding the first and/or welding seams are dressed to form a smooth and continuous surface. Finally, the outer wall element is formed, in particular bent, to the final curved shape of the leading edge.

DETAILED DESCRIPTION

InFIG.1an aircraft101according to an embodiment of the present invention is shown. The aircraft comprises a fuselage103, wings105, a horizontal tail plane107, and a vertical tail plane109according to an embodiment of the invention. The vertical tail plane109is shown in more detail inFIG.9. The vertical tail plane109comprises a leading edge structure1according to an embodiment of the invention. Various embodiments of the leading edge structure1are shown in more detail inFIGS.2to8.

InFIGS.2and3an embodiment of a leading edge structure1for a flow control system of an aircraft according to the present invention is shown. The leading edge structure1comprises a double-walled leading edge panel3and a back wall5.

The leading edge panel1surrounds a plenum7in a curved manner, wherein the plenum7extends in a span direction9. The leading edge panel3has a first side portion11and an opposite second side portion13. The first side portion11extends from a leading edge point15to a first attachment end17. The second side portion13extends from the leading edge point15to a second attachment end19, as shown inFIG.4.

The back wall5is formed as a membrane of CFRP material and connects the first attachment end17to the second attachment end19of the leading edge panel3. In this way, the back wall5encloses the plenum7together with the leading edge panel3on a side opposite the leading edge point15.

The leading edge panel3comprises an inner wall element21facing the plenum7and an outer wall element23in contact with an ambient flow25. Between the inner and outer wall elements21,23the leading edge panel3comprises a core assembly97comprising a plurality of elongate stiffeners27spaced apart from one another, so that between each pair of adjacent stiffeners27a hollow chamber29is left open between the inner and outer wall elements21,23, as shown inFIG.5. The stiffeners27extend in the span direction9in parallel to the leading edge point15and in parallel to one another.

The outer wall element23comprises a plurality of micro pores31forming a fluid connection between the hollow chambers29and the ambient flow25. The inner wall element21comprises openings33forming a fluid connection between the hollow chambers29and the plenum7. At some of the hollow chambers29, the openings33are formed as throttle holes35having a predefined diameter37adapted for a predefined mass flow rate through the throttle holes35in order to achieve a predefined fluid pressure in the hollow chambers29, as it is shown inFIG.6. However, at others of the hollow chambers29, the openings33are formed to allow a random mass flow rate and are not adapted to control the fluid pressure in the hollow chambers29, as it is the case in the hollow chamber29shown inFIG.5.

The stiffeners27are formed integrally with the inner wall element21. The inner wall element21is formed of a Carbon Fiber Reinforced Plastic (CFRP). The stiffeners27have a solid trapezoid-shaped cross section and are formed as sandwich structures39. Each sandwich structure39comprises a core element41enveloped on opposite sides by first and second separate layers43a,43bof CFRP of the inner wall element21, wherein one layer43aencloses the core element41on the side facing the plenum7, while the other layer43bencloses the trapezoid surface of the core element41on the side facing the outer wall element23by an omega-shape curse. The core elements41are formed of a foam material.

As shown inFIG.2, a plurality of support ribs45are attached to the inner wall element21in this way that they face the plenum7and extend perpendicular to the stiffeners27and to the span direction9along the inner wall element21. The support ribs45are formed integrally with the inner wall element21and are also formed of a CFRP material.

The outer wall element23is formed as a titanium sheet and comprises multiple sections47a,47b,47carranged subsequently in a chord direction49. The porosity varies from one section47ato another section47b,47cin terms of the pore pitch, wherein the pore pitch increases from the leading edge point15downstream.

As shown inFIGS.4,7and8, the outer wall element23extends from the first attachment end17to the second attachment end19. The outer wall element23forms a first end edge51at the first attachment end17and a second end edge53at the second attachment end19. The first end edge51and the second end edge53extend in parallel to the leading edge54and are formed to extend along or rest against a vertical tail plane box55of the associated vertical tail plane109. This means, the outer wall element23extends as far as the inner wall element21at the first attachment end17and at the second attachment end19. The first end edge51and the second end edge extend53in parallel to the stiffeners27. This means, the end edges51,53extend in span direction9.

The outer wall element23comprises a main wall portion59as well as a first wall extension61and a second wall extension63. The main wall portion59includes the leading edge point15. The first wall extension61includes the first attachment end17and the first end edge51. The second wall extension63includes the second attachment end19and the second end edge53. As shown inFIGS.7and8, the first wall extension61is connected to the main wall portion59via a straight first welding seam67. Additionally, the second wall extension63is connected to the main wall portion59via a straight second welding seam69. First and second welding seams67,69are butt-welding seams formed by laser welding. The first welding seam67and the second welding seam69are dressed at the outer flow surface71to form a smooth transition between the main wall portion59and the first wall extension61and between the main wall portion59and the second wall extension63.

As shown inFIG.4, the first welding seam67and the entire first wall extension61are bonded directly and planar to the inner wall element21with no core assembly97in between. Additionally, the second welding seam69and the entire second wall extension63are bonded directly and planar to the inner wall element21with no core assembly97in between. No micro pores31are present in the first and second wall extensions61,63. As indicated inFIG.7, the micro pores31are present only in the main wall portion59and are distributed from the leading edge point15to the first welding seam67and to the second welding seam69with a minimum distance from the welding seams67,69.

FIGS.9to11show a vertical tail plane109for an aircraft101according to the invention. The vertical tail plane109comprises a vertical tail plane box55and a leading edge structure1as described before. The vertical tail plane box55has a first lateral panel75with a first attachment portion77and an opposite second lateral panel79with a second attachment portion81. The first attachment end17of the leading edge panel3is attached to the first attachment portion77such that the first end edge51extends along the first attachment portion77, and the second attachment end19of the leading edge panel3is attached to the second attachment portion81such that the second end edge53extends along the second attachment portion81. The first side portion11of the leading edge panel3forms a continuous flow surface71with the first lateral panel75of the vertical tail plane box55, and the second side portion13of the leading edge panel3forms a continuous flow surface with the second lateral panel79of the vertical tail plane box55. The plenum7is in fluid connection with an air outlet83for causing a vacuum in the plenum7to draw air from the ambient flow25through the micro pores31and the hollow chambers29into the plenum7.

As shown inFIGS.4and8, the first attachment end17is attached to the first attachment portion77by a first front line of fasteners85extending through the main wall portion59, and by a first rear line of fasteners87extending through the first wall extension61, so that the first welding seam67extends between the first front line of fasteners85and the first rear line of fasteners87. Additionally, the second attachment end19is attached to the second attachment portion81by a second front line of fasteners89extending through the main wall portion59, and by a second rear line of fasteners91extending through the second wall extension63, so that the second welding seam69extends between the second front line of fasteners89and the second rear line of fasteners91. The fasteners85,87,89,91can be e.g. bolts, lockbolts, rivets.

As shown inFIGS.10and11, the vertical tail plane109further comprises a connection duct93connecting a lower end95of the plenum7to the air outlet83. In addition to the air outlet flap83the plenum7might also be in fluid connection with an air inlet (not shown) for causing an overpressure in the plenum7to blow out air from the plenum7through the hollow chambers29and the micro pores31to the ambient flow25.

The leading edge structure1shown inFIGS.2to8can be manufactured by a method including the following steps: The inner wall element21and the core assembly97is produced, wherein the stiffeners27and the inner wall element21are formed together as an integral part by a Resin Transfer Molding (RTM) process in a common RTM step. Also the support ribs45are formed as an integral part together with the inner wall element21and the stiffeners27by an RTM process. Further, the outer wall element23is produced. Then, the outer wall element23is bonded to the core assembly97and to the inner wall element21, wherein the outer wall element23is bonded against the stiffeners27and against the inner wall element21at the first and second attachment ends17,19.

The outer wall element23is produced comprising the following steps: The main wall portion59is provided, which is formed by a titanium sheet of 1.25 m width and 1 mm thickness. The first wall extension61and the second wall extension63are provided in the form of titanium sheets of 0.125 m width each and 1 mm thickness. Then, the main wall portion59is butt-welded to the first wall extension61to form the first welding seam67, and to the second wall extension63to form the second welding seam69, by laser welding. The first welding seam67and the second welding seam69are subsequently dressed at the outer flow surface71to form a smooth transition between the main wall portion59and the first wall extension61and between the main wall portion59and the second wall extension63.

By the present invention as described above, a leading edge structure1with a one-step, continuous and smooth flow surface71is obtained that does not form an obstacle for ambient flow25streaming along the flow surface71and, thus, decreases drag and increases flow efficiency of the leading edge structure1. Further, the area of the micro pores31can be extended further downstream closer to the attachment ends17,19, thereby increasing the overall flow control effectivity. Moreover, the leading edge structure1can be optimized in terms of weight and costs since the outer wall element23also supports the attachment ends17,19.