Field of the Invention This invention relates to a method and apparatus for forming a compound curvature metal skin and in particular to a method and apparatus for forming a nacelle cowl leading edge (hereinafter referred to as a "lipskin") or a sector thereof from a conic metal blank. Background of the Invention

A typical nacelle lipskin comprises a thin, aerodynamically shaped metal skin covering a jet engine of an aircraft. The front region of the nacelle comprises a lipskin defining the inlet of the engine, which may be comprised of a single piece or multiple sectors. The main features of a lipskin are a smooth outer trailing edge, which must be free from irregularities and discontinuities to reduce drag and to avoid the creation of turbulence and an inner inlet edge, which is typically shaped to attenuate noise from the engine by shielding fan noise and to guide the flow of air into the engine, and a leading edge or lip, which provides a smooth transition between the outer trailing edge and the inner inlet edge while creating a small frontal area to reduce drag. The lipskin, and in particular the leading edge thereof, is prone to damage from debris kicked up during take off and landing and by bird strike. While composite materials can be used for many parts of the nacelle, the lipskin must generally be made from a metal, such as aluminium or titanium, to be able to withstand impacts upon the leading edge or lip thereof. However, the complex three dimensional compound curvature shape of the lipskin typically requires a complex multi-stage forming process, often requiring intermediate heat treatments. Typically lipskins are produced by multiple stage deep drawing or spin forming processes, requiring complex and costly tooling and time consuming multi-step processing with intermediate heat treatments. Super plastic forming processes can only be used with specific metals that exhibit strain rate sensitivity under high temperature deformations. Additionally, the process requires the production of expensive heated tooling which necessitates initial heating and subsequent cooling cycles, significantly increasing the production cycle time.

Stretch forming is traditionally applied to deep double curvature components that have a shallow secondary curvature. The secondary curvature is the cause of major problems for this traditional process. The grips are usually straight or segmented, but limited in their adjustment. As the degree of secondary curvature increases, the parts become more difficult to manufacture in a single 'hit'. Typical failures in stretching double curvature components include tearing and buckling, caused by excessive tensile and compressive stresses. Many of these problems are 'solved' through the introduction of inter-stage heat treatments. The requirements for new energy efficient aircraft designs, including laminar flow nacelles, require new materials and component designs that cannot be manufactured using traditional processing routes.

Deep drawing of these components is extremely difficult. The very deep sections require multi-stage forming operations with inter-stage heat treatment processes. There can be as many as five stages in production of a single part. The inter-stage heat treatments often lead to part rejections on the basis of grain growth and degradation of the metallic structure.

Lip-skins are therefore viewed as the most difficult parts to produce for an aircraft. Only a few specialist companies have the expertise to use the processes listed above to produce these components. Consequently the unit cost of these components tends to be high.

Summary of the Invention

According to a first aspect of the present invention there is provided a method of forming a compound curvature metal skin from a sheet metal blank comprising the steps of :- providing a blank defining at least a sector of a truncated cone having a central axis of symmetry, a leading edge and a trailing edge; providing an arcuate or annular punch substantially corresponding in shape to an inner surface of the compound curvature metal skin and having an outer surface, a leading edge and an inner surface;

gripping the leading edge of the blank between mating faces of a first blank holding means located on a first side of the punch adjacent the inner face of the punch; gripping the trailing edge of the blank between mating faces of a second blank holding means located on a second side of the punch, opposite to said first side and adjacent the outer face of the punch;

displacing the first and second blank holding means with respect to the punch in a single direction to draw the blank over the punch while permitting the blank to flow between mating surfaces of at least one of the first and second blank holding means, comprising the steps of displacing each of the first and second blank holding means with respect to the punch by different amounts and at different rates. Preferably the first and second blank holding means are displaced in a direction substantially aligned with the central axis of the blank.

In a first step, both the first and second blank holding means may be displaced with respect to the punch, the second blank holding means being moved faster than the first blank holding means, drawing material from between the mating faces of the first blank holding means and over an outer surface of the punch.

Preferably the surface of the punch is lubricated to minimise friction between the blank and the surface of the punch.

Preferably the second blank holding means reaches its limit of travel as the first blank holding means becomes aligned with the leading edge of the punch.

In a second step, the first blank holding means may be moved past the leading edge of the punch and over the inner surface of the punch to redraw the blank through the first blank holding means and over the surface of the punch.

Preferably the leading edge of the blank flows between the mating surfaces of the first blank holding means as the blank is drawn over the punch. The blank may be drawn over a trailing edge of a portion of the first blank holding means as the first blank holding means is displaced past a leading edge of the punch. The second blank holding means may grip the trailing edge of the blank substantially without slippage as the blank is drawn over the punch.

Preferably the mating surfaces of the first and second blank holding means are at least initially aligned with one another to define at least part of a frustoconical surface adapted to match the shape of the blank. In a preferred embodiment the mating faces of the first blank holding means grip the leading edge of the blank with sufficient force to permit the blank to flow therebetween in a controlled manner without tearing or wrinkling such that the blank is drawn between the mating surfaces of the first blank holding means as the blank is drawn over the punch during relative displacement of the first and/or the second blank holding means relative to the punch.

The method may comprise an initial step of forming the blank to define at least a sector of truncated cone. Such initial forming step may comprise a rolling and/or pressing process.

According to a further aspect of the present invention there is provided an apparatus for forming a compound curvature metal skin from a sheet metal blank comprising :- a blank defining at least a sector of a truncated cone having a central axis, a leading edge and a trailing edge;

an arcuate or annular punch substantially corresponding in shape to an inner surface of the compound curvature metal skin having an outer surface, a leading edge and an inner surface;

a first blank holder having a support surface for supporting the leading edge of said blank;

a second blank holder having a support surface for supporting the trailing edge of said blank;

wherein said punch is located between said first and second blank holders; a first die member for retaining the leading edge of the blank against the support surface of the first blank holder;

a second die member for retaining the trailing edge of the blank against the support surface of the second blank holder;

wherein the first and second blank holders and the punch are displaceable with respect to one another in a single direction to draw the blank over the punch and wherein the first and second blank holders are displaceable with respect to the punch by different amounts and at different rates. Preferably the first and second blank holders and the punch are displaceable with respect to one another in a direction substantially aligned with said central axis of symmetry of the blank.

Preferably the first and second blank holders are displaceable with respect to the punch in a direction substantially aligned with the central axis of the blank.

The support surfaces of the first and second blank holders are preferably at least initially aligned with one another to define at least part of a frustoconical surface adapted to match the shape of the blank.

The first blank holder and first die member are preferably adapted to grip said leading edge of the blank with sufficient force to permit the blank to flow therebetween in a controlled manner without tearing or wrinkling, such that the blank is drawn between the mating surfaces of the first blank holder and the first die member as the first and second blanks holders and the punch are displaced with respect to one another to draw the blank over the outer surface of the punch. A trailing or lower edge of the first die member may define a forming surface over which the blank is drawn as the first die member moves past a leading edge of the punch during the forming operation. The second blank holder and the second die member may be adapted to hold the trailing edge of the blank substantially without slippage as the blank is drawn over the outer surface of the punch. In one embodiment the punch may comprise an annular body, replicating an entire nacelle lipskin. Alternatively the punch may comprise an arcuate sector corresponding to a sector of the lipskin to be formed. A tool base may be provided, upon which the punch, and first and second blank holders are mounted. The first and second blank holders may be mounted on the tool base to permit relative displacement between the first and second blank holders and the punch to draw the blank over the punch. Preferably the first and second blank holders are independently moveable with respect to the punch by respective actuating means. Said actuating means may comprise hydraulic or pneumatic rams, more preferably double acting rams, or cam dies.

One or both of the mating surfaces of the first blank holder and first die member may be provided with a replaceable wear plate against which the leading edge of the blank may slide as it is drawn between the first blank holder and first die member.

One or both of the mating surfaces of the second blank holder and the second die member may be textured or otherwise formed to ensure that the trailing edge of the blank can be gripped therebetween without slippage.

By utilising a conic metal blank, a compound curvature metal skin by relative motion between the first and second blank holders and the punch in a single direction. Brief Description of the Drawings

A metal forming apparatus in accordance with an embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is an exploded perspective view of the main components of a metal forming apparatus in accordance with an embodiment of the present invention; Figure 2 is a perspective view of the apparatus of Figure 1 in an initial closed configuration;

Figure 3 is a perspective view of the apparatus of Figure 1 at the end of a first stage of the forming process completing an external draw of the lipskin;

Figure 4 is a perspective view of the apparatus of Figure 1 at the end of a second stage of the forming process completing an internal draw of the lipskin; and Figures 5 to 7 show a workpiece during the two stages of the forming process.

Detailed Description of the Drawings

The present invention provides a method and apparatus for forming a sector of a nacelle lipskin which overcomes the disadvantages of the prior art and is of particular benefit for nacelle repairs by facilitating the quick and easy creation of a replacement sector of a lipskin to replace a damaged sector. While the present invention is particularly described in relation to nacelle lipskins for aircraft, the method and apparatus according to the invention can also be used for the manufacture of other laminar flow leading edges for a variety of applications.

The apparatus is adapted for shaping a conic blank 1 , having a leading or upper edge (which will form an inner wall of the resulting lipskin), a trailing or lower edge(which will form an outer wall of the resulting lipskin) and a central axis.

As schematically illustrated in Figure 1 , the main components of an apparatus for forming a nacelle lipskin in accordance with an embodiment of the present invention comprise a tool base (schematically shown by reference numeral 2 in Figure 1 ) for supporting the other components of the tool, a annular punch 4 having the shape of an inner surface of the metal article to be formed an first and second blank holders 6,8. The components shown in Figure 1 are not meant to be representative of the actual tooling, but rather to show the design intent of the surfaces that are in contact with the blank. The annular punch 4 is supported on the tool base 2 to extend substantially vertically therefrom. The annular punch 4 has an outer surface, a leading edge and an inner surface having a shape corresponding to the inner surface of the finished lipskin. The outer surface of the punch 4 is designed to match the inner surface of the lipskin component with a slight extension in height to compensate for springback due to material contraction post forming. The punch 4 and first and second blank holders may correspond to a portion or sector of the lipskin, for example a 120° sector, or may comprise a full 360° annulus corresponding to the entire lipskin. Preferably the apparatus is intended to form a sector up to one third (120°) of a complete lip skin.

The first and second blank holders 6,8 are mounted on either side of the punch 4 for respectively holding the leading and trailing edges of the blank 1 , the blank 1 being pre-formed to define at least a sector of a truncated cone. Preferably the blank has a taper angle of at least 30°, more preferably between 40° to 45°, to the central axis of the blank. This minimises thinning and wrinkling of the blank during the forming process of the resulting lip skin sector.

Each blank holder 6,8 has a respective blank support surface for supporting the blank. A first die member 10 is associated with the first blank holder 6 for retaining the leading edge of the blank 1 against the support surface of the first blank holder 6. A second die member 12 is associated with the second blank holder 8 for retaining the trailing edge of the blank 1 against the support surface of the second blank holder 8. The first and second blank holders 6,8 and the punch 4 are displaceable with respect to one another in a direction parallel to the axis of the blank when mounted in the apparatus, such that the blank 1 can be drawn over the punch 4 as will be described below in more detail.

The mating surfaces of the first and second blank holders 6,8 and first and second die members 10,12 are designed to conform to the same conic angle as the blank 1 so that they can grip the blank without deformation of the blank when the blank 1 is initially loaded into the apparatus.

The first and second blank holders 6,8 are initially located in a rest position, as shown in Figure 2, wherein the support surfaces of the first and second blank holders 6,8 define at least part of a frustoconical surface adapted to match the initial shape of the blank 1 . In such rest position, a leading edge of the punch 4 may be aligned with said frustoconcial surface such that the leading edge of the punch 4 contacts and supports the blank 1 . The first and second die members 10,12 are moveable to operative positions against the respective support surfaces of the first and second blank holders 6,8 with sufficient clearance for the material being formed.

The conic blank 1 will first need to be pre-formed to the conic section and inserted into the apparatus, with the leading edge of the blank supported on the first blank holder 6 and the trailing edge of the blank supported on the second blank holder 8.

The first blank holder 6 and first die member 10 are shaped to match the conic section of the blank 1 and are adapted to permit the leading edge of the blank 1 to flow in a controlled fashion between the mating faces of the first blank holder 6 and first die member 10.

A lower edge 14 of the first die member 10 is radiused allow to the blank 1 to be drawn over such lower edge 14 as the first blank holder 6 is moved past the leading edge of the punch 4, as will be described below in more detail.

The second blank holder 8 and second die member 12 are adapted to effectively grip the trailing edge of the blank 1 substantially without slippage, while permitting some minor draw-in at the very outer edge to avoid fracture.

Figures 5 to 7 illustrate the forming process stages as the first and second blank holders are displaced with respect to the punch.

In the initial forming stage (shown in Figures 2 and 6), the second blank holder 8 is moved faster than the first blank holder 6, thus drawing material from between the first blank holder 6 and first die member 10 and over the outer surface of the punch 4. The punch 4 may be lubricated to minimise the friction during the forming process. Such motion is controlled such that the second blank holder 10 reaches the full depth of the punch 4 (i.e. the lowermost position of the second blank holder 10 with respect to the tool base 2) as the lower edge 14 of the first die member 10 becomes aligned with the leading edge (uppermost point) of the punch 4. As the first die member 10 moves past the leading edge of the punch 4, the additional friction of the lower edge of the first die member 10 and the action of the leading edge of the punch 4 against the blank act to reduce the draw from between the mating surfaces of the first blank holder 6 and first die member 10, thus increasing the stretching of the outer surface of blank 1 . While some stretching is desirable, excessive stretching will result in a fracture of the component. This differential stretch/draw must therefore be controlled by careful and coordinated control of the timing, amount and rate of movement of the first and second blanks holders 6,8 with respect to the punch 4 and the control of the clamping force between first and second blank holders 6,8 and the first and second die members 10,12.

Once the second blank holder 8 has reached the full depth of the punch 4 (i.e. the lowermost position of the first blank holder 8 with respect to the tool base 2) the first blank holder 6 (and first die member 10) continues to move with respect to the punch 4, as illustrated in Figures 3 and 7, thus redrawing the blank 1 through the first blank holder 6 and first die member 10, around the lower edge of the first die member 10 and over the surface of the punch 4, such that the leading edge of the blank 1 is formed into the inner wall of the finished lipskin.

A final stretch may be carried out by further displacement of the first and second blanks holders 6,8 with respect to the punch 4 without slippage of the blank between either of the first and second blank holders 6,8 and the respective first and second die members 10,12. Such slippage may be prevented by the engagement of the trailing edge 14 of the first die member 10 with a stop of clamp member (not shown).

Once the finished lipskin has been formed, as shown in Figure 7, the first and second die members 10,12 can be raised or released from the first and second blank holders 6,8 and the formed blank can be removed from the apparatus. The remaining selvedge edges 20 of the formed blank can be cut away to leave the finished lipskin 22. This process thus combines elements of stretch forming and deep drawing in a completely novel way.

The apparatus may be attached to a standard single or double acting press. This enables the use of current equipment, reducing the cost of implementation for Original Equipment Manufacturers (OEMs) and Maintenance Repair Overhaul (MROs), making the resulting implementation much more attractive.

The invention is not limited to the embodiment(s) described herein but can be amended or modified without departing from the scope of the present invention.