A hydraulic expansion forming method for reverse double curvature fairing skin

Abstract

The present invention discloses a method for hydraulic bulging of a reversed hyperbolic fairing skin, comprising the following steps: constructing a hydraulic bulging process surface for the fairing skin; determining the dimensions of the expanded blank; designing the hydraulic bulging die structure for the fairing skin in stages; performing hydraulic bulging; and trimming the outer shape. Based on the reversed hyperbolic structure of the fairing skin, the present invention designs hydraulic bulging process surfaces for different skins with opposite overall curvatures. This increases the plastic deformation area of ​​the material during hydraulic bulging, addressing issues uncontrolled by conventional drop-press forming of reversed hyperbolic skins, such as severe orange peel defects on the material surface, penetration of the aluminum cladding layer of the duralumin material, and surface quality that fails to meet customer requirements.

Description

Technical Field

[0001] The invention relates to a sheet metal parts forming technology in the field of aircraft manufacturing, in particular to a hydraulic expansion forming method for a reverse double curvature fairing skin. Background Art

[0002] The function of an aircraft's skin is to maintain its shape, ensure optimal aerodynamics, and minimize drag. Therefore, the skin material must be high-strength, highly ductile, and possess a smooth surface after forming. Complex skin shapes are often found on aircraft wingtips, fairings, nose cowls, and tail cowls. The fairing skin is located at the front of the fuselage, with the left and right fairings covering both sides of the nose and protecting the radar antenna. These skins feature complex curvatures, thin walls, and multi-section structures. By regulating airflow over the surface during flight, they reduce drag, improve the aircraft's aerodynamic performance, and enhance its flight capabilities.

[0003] The fairing skin is made of duralumin, with a thickness of no more than 1.6mm, to meet the requirements of lightweight aircraft design. The fairing skin presents a smooth transition of hyperbolic streamlines on both sides of the central axis. The curvature of the left and right fairing skins changes in opposite directions, and the curvature of the right fairing skin changes more rapidly than that of the left fairing skin. This protects the front end of the aircraft from protruding radars and related discontinuities, while improving the aircraft's aerodynamic performance. In the past, fairing skin structural parts were manufactured using a progressive forming process using a skin drop press process. However, the streamlined characteristics of the reverse hyperbolic curvature increased the difficulty of skin drop presses, resulting in serious orange peel defects on the material surface during forming, and also caused the aluminum layer of the duralumin material to penetrate, resulting in surface quality that failed to meet customer requirements.

[0004] Orange peel defects are a surface roughening phenomenon of metal materials that not only affects the appearance of the skin, but also reduces the skin's strength, plasticity, and toughness, seriously affecting the fatigue life of the aircraft. In order to avoid surface wrinkles or dead folds on the fairing skin, the amount of pressure drop is often reduced during forming, and multiple passes of pressure drop forming are used. During the pressure drop forming process, manual trimming and repeated polishing are used to improve its surface morphology, resulting in reduced part forming accuracy and penetration of the aluminum cladding, affecting the appearance and assembly of the aircraft. Due to the long-standing bottleneck problem of difficult-to-control surface and internal damage and extremely unstable product quality in pressure drop forming, rejection and scrapping occur frequently, causing significant economic losses to companies.

[0005] The fairing skin is about 1.6m long and 1.3m wide, with a maximum arch height of nearly 300mm. The orange peel phenomenon cannot be avoided by drop forming. The open fairing skin is formed using the traditional hydraulic bulging process. Due to the large hydraulic bulging height of the skin, the deformation trends on both sides of the central axis of the shape are opposite, and the deformation amounts are unequal, which makes the skin prone to material accumulation and wrinkles. Due to the uneven plastic deformation, the skin rebounds severely after forming and cannot reach the design drawing size. It is often necessary to form the skin into sections according to the positive and negative curvatures. For the thin-walled sheet metal fairing skin with reverse double curvature, large size, multiple cross-sections, and streamlined shape, there is an urgent need for a mechanized and precise processing method to achieve orange peel-free parts, uniform deformation, high quality, and integrated controllable forming. Summary of the Invention

[0006] In order to solve the problems of severe orange peel on the surface of the reverse hyperbolic thin-walled large-size fairing skin, penetration of the aluminum layer and high scrap rate after forming during the drop-press forming of the reverse hyperbolic fairing skin, the purpose of the present invention is to provide a hydraulic expansion forming method for the reverse hyperbolic fairing skin.

[0007] The present invention provides a hydraulic expansion forming method for a reverse hyperbolic curvature fairing skin, comprising the following steps:

[0008] Step 1: constructing a hydraulic bulging process surface of the fairing skin;

[0009] Step 2: Determine the size of the unfolded blank;

[0010] Step 3: Determine the hydraulic bulging die structure of the fairing skin according to the hydraulic bulging process surface;

[0011] Step 4: Laying the skin blank that is larger than the unfolded blank size on the hydraulic bulging die structure for hydraulic bulging;

[0012] Step 5: Trim the shape.

[0013] Optionally, the step 1 is to construct a hydraulic bulging process surface of the fairing skin, and the specific process is as follows:

[0014] Step 1-1: Define the hydroforming direction

[0015] Using the XY plane in the CATIA environment as the design horizontal plane H, the fairing skin is rotated to ensure that the distance h between the two end points P and Q of the fairing skin and the design horizontal plane is equal, thereby reducing the fairing skin forming depth h, and the fairing skin bulging direction F is perpendicular to the design horizontal plane H;

[0016] Step 1-2: Add the fairing skin

[0017] Extract the upper surface of the fairing skin, select the vertices A and G of the elliptical notch on the upper surface of the skin, construct the spline AG with the side contour lines on both sides of the notch as tangents, fill the spline AG, the elliptical notch contour line, and the oblong notch contour line to establish the fairing skin surface;

[0018] Step 1-3: Design the transition extension surface

[0019] Select two high points A and B of the fairing skin, and establish a reference plane with reference to the ZX plane in the CATIA environment. Establish design planes M and N by offsetting the reference plane by 200 mm in the opposite direction. Extract the contour lines of the fairing skin surface W at both ends. Design splines on M and N based on the contours of the fairing skin surface at both ends. Bridge planes M and N and the corresponding splines. Stretch the fairing skin at both ends by 200 mm along the curvature of the bridge surface to establish the transition flange extension surface.

[0020] Step 1-4: Design the flange extension surface

[0021] Select a midpoint C at the notch in the middle of the fairing skin, establish a design plane K with reference to the XY plane in the CATIA environment, design a spline curve in K at a distance of 300 mm from the fairing skin contour line curvature, project the spline curve onto the transition flange extension surface to obtain the flange contour line, and use the flange contour line to cut the transition flange extension surface to establish the flange extension surface;

[0022] Step 1-5: Design the transition guide line of the hydraulic bulging process surface

[0023] Extract the four endpoints D1, D2, D3, and D4 of the fairing skin contour, extract the four points E1, E2, E3, and E4 on the fairing skin surface, and fit the connecting lines D1 E1, D2E2, D3E3, and D4E4. Construct points F1, F2, F3, and F4 at the closest distances between the four endpoints of the fairing skin contour on the inner contour line of the flange extension surface. According to the four endpoints D1, D2, D3, and D4 of the fairing skin contour, the connecting lines D1 E1, D2E2, D3E3, and D4E4, and the construction points F1, F2, F3, and F4, fit the transition guide lines D1 F1, D2F2, D3F3, and D4F4 of the hydraulic bulging process surface. The tension of the guide lines is defined as 1.

[0024] Step 1-6: Design the transition surface

[0025] Extract the inner contour line of the flange extension surface, project points A and G onto the inner contour line of the flange extension surface to obtain points A1 and G1, cut the inner contour line of the flange extension surface at points A1 and G1, retain the inner contour line A1 G1, bridge the inner contour line A1 G1 and the spline AG to obtain the transition surface Y1 between the flange extension surface and the fairing skin surface, extract the contour lines AA1 and GG1 at both ends of the transition surface Y1, fill the contour line GG1 and the transition guide line D1 F1, the transition guide line D1 F1 and the transition guide line D2F2, the transition guide line D2F2 and the transition guide line D3F3, the transition guide line D3F3 and the transition guide line D4F4, the transition guide line D4F4 and the cross contour line AA1 to obtain transition surfaces Y2, Y3, Y4, Y5, and Y6, and join Y1, Y2, Y3, Y4, Y5, and Y6 to obtain the transition surface;

[0026] Step 1-7: Design the hydroforming process surface

[0027] The fairing skin surface, flange extension surface and transition surface are joined, and the flange extension surface and the transition surface are transitioned with a fillet radius R to obtain a hydraulic bulging process surface.

[0028] Optionally, the step 2 of determining the size of the unfolded blank is as follows:

[0029] Import the fairing skin surface into the finite element analysis software, use the blank inverse calculation function to calculate the unfolded shape of the hydroforming, and obtain the unfolded blank and the outer edge line of the unfolded blank;

[0030] Determine the size of the unfolded blank based on the outer edge line of the unfolded blank.

[0031] Optionally, the step 3 determines the hydraulic bulging die structure of the fairing skin according to the hydraulic bulging process surface, and the specific process is as follows:

[0032] Step 3-1: Design the die structure

[0033] The flange extension surface of the hydraulic bulging process surface is extended in all directions within the hydraulic bulging horizontal plane XY. The expanded hydraulic bulging process surface is used as the die working surface. The outer edge line of the unfolded blank is projected onto the corresponding position of the die working surface to obtain the outer contour line of the unfolded blank on the die. The die working surface is adjusted according to the outer contour line of the unfolded blank. A drainage hole with a diameter of 10 mm is set on the die.

[0034] Step 3-2: Design the blank holder structure

[0035] Generate the blank holder working surface based on the flange extension surface, transition surface and gap;

[0036] Gap g = (0.95-1.0) × δ;

[0037] The outer edge line of the unfolded blank is projected onto the corresponding position of the working surface of the blank holder to obtain the outer contour line of the unfolded blank on the blank holder; the working surface of the blank holder is adjusted according to the outer contour line of the unfolded blank; a water inlet hole with a diameter of 10 mm is provided on the blank holder;

[0038] Where δ is the material thickness of the fairing skin.

[0039] Optionally, adjust the die working surface according to the outer contour of the unfolded blank, including:

[0040] Determine whether the distance between the outer edge line of the unfolded blank and the outer contour of the die working surface remains relatively uniform, and the distance between them is ≥80mm. If not, adjust the die working surface.

[0041] Optionally, in step 4, the skin blank that is larger than the unfolded blank size is laid on a hydraulic bulging die structure for hydraulic bulging. The specific process is as follows:

[0042] Step 4-1: Install the hydraulic bulging die

[0043] Install the die and blank holder on the working table of the hydraulic bulging machine. The die is fixed to the bed of the hydraulic bulging machine. The hydraulic bulging outer surface of the die corresponds to the working surface of the blank holder. The blank holder can move up and down through the movable ejector. Apply lubricating oil to the forming working surfaces of the die and blank holder.

[0044] Step 4-2: Expand the blank and position it

[0045] Attach plastic film to the upper surface of the unfolded blank for protection, and place it on the working surface of the die according to the outer contour of the unfolded blank on the die. Apply lubricating oil to the corresponding area of ​​the lower surface of the unfolded blank and the working surface of the die.

[0046] Step 4-3: Hydraulic bulging

[0047] The blank holder moves downward along the hydraulic bulging direction F, and after touching the die, it clamps the expanded blank with the die. Fluid is introduced from the water inlet of the blank holder, and the fluid medium acts as a punch to apply hydraulic pressure P to the expanded blank, causing the expanded blank to undergo plastic deformation and form a part against the die cavity.

[0048] Optionally, the hydraulic pressure is adjusted by an overflow valve, and hydraulic bulging is performed according to the shape of the die. The unit blank holding force setting range is 8MPa to 9MPa.

[0049] Optionally, the step 5 of trimming the fairing skin parts is as follows:

[0050] The cutting is completed according to the fairing skin outline on the hydraulic bulging surface of the die, and the fairing skin parts with shape accuracy and surface quality meeting the requirements of the drawing are obtained.

[0051] The beneficial effects of the present invention are:

[0052] 1) Based on the reverse hyperbolic structure of the fairing skin, the present invention designs hydraulic bulging process surfaces with opposite overall forming curvatures for different skins. This increases the plastic deformation area of ​​the material during the hydraulic bulging of the part, solving the problems of severe orange peel defects on the material surface, penetration of the aluminum coating of the duralumin material, and surface quality that cannot be controlled by conventional drop forming of the reverse hyperbolic skin.

[0053] 2) An innovative design method for hydraulic bulging process surfaces was established. By prioritizing the creation of flange extension surfaces that conform to the part surface structure, a bridge-fill method was used to construct the transition surface between the part surface and the flange extension surface. This design method significantly reduces the difficulty of process surface design in the CATIA environment and can easily reduce the hydraulic bulging depth of large-sized, multi-section, and multi-curvature parts, achieving high forming accuracy and good versatility.

[0054] 3) An innovative design method for hydraulic bulging process surfaces was established. This surface design method can replace the traditional drop forming process, which can only form two skins with opposite curvatures in sections due to the inability to control wrinkles, with an integrated forming method. This reduces the weld seam at one end, significantly improving the overall strength of the fairing and enhancing the overall aerodynamic performance of the aircraft.

[0055] 4) Innovatively adopt hydraulic bulging to replace traditional drop forming. Hydraulic bulging only uses a single-sided die, and the other half is replaced by liquid medium, which reduces mold costs. Generally, mold costs can be reduced by more than 30%, making the production of complex skin parts simpler and more flexible. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is a schematic diagram of the fairing skin structure;

[0057] Figure 2 It is a schematic diagram of the hydraulic expansion direction of the fairing skin;

[0058] Figure 3 This is a schematic diagram of the fairing skin surface design process;

[0059] Figure 4 It is a schematic diagram of the fairing skin process surface structure;

[0060] Figure 5 This is a schematic diagram of the fairing skin die structure;

[0061] Figure 6 This is a schematic diagram of the fairing skin pressure ring structure;

[0062] Figure 7 This is a schematic diagram of the unfolded blank structure of the fairing skin;

[0063] Figure 8 This is a schematic diagram of the hydraulic expansion die structure for the fairing skin;

[0064] Explanation of the accompanying drawings: 1 fairing skin, 2 fairing skin surface, 3 flange extension surface, 4 transition surface, 5 hydraulic bulging process surface, 6 unfolded blank, 7 unfolded blank outer edge line, 8 die, 9 blank holder, 10 die working surface, 11 elliptical notch, 12 transition fillet surface, 13 hydraulic bulging shape surface, 14 blank holder working surface, 15 blank holder unfolded blank outer edge line, 17 die unfolded blank outer contour line, 18 die drainage hole, 19 blank holder water inlet hole. DETAILED DESCRIPTION

[0065] The present application is described in further detail below with reference to the accompanying drawings of the embodiments.

[0066] Referring to the accompanying drawings, the aircraft sheet metal parts provided in the embodiment are as follows Figure 1 As shown, the fairing skin is made of duralumin, about 1.6m long and 1.3m wide, with a maximum bow height of nearly 300mm. The fairing skin presents a smooth transition of hyperbolic streamlines on both sides of the central axis, and the curvatures of the left and right fairing skins change in opposite directions, while the curvature of the right fairing skin changes more rapidly than that of the left fairing skin. In the past, the fairing skin structural parts were manufactured by progressive forming using the skin drop pressing process, but the streamline characteristics of the reverse hyperbolic curvature increased the difficulty of the skin drop pressing, and the material surface had serious orange peel defects during forming, which caused the aluminum layer of the duralumin material to penetrate, and the surface quality could not meet customer requirements.

[0067] like Figures 2 to 8 As shown, a hydraulic expansion forming method for a reverse double curvature thin-walled large-size fairing skin comprises the following steps:

[0068] Step 1: Construct the hydraulic bulging process surface of the fairing skin 5

[0069] Step 1-1: Define the hydroforming direction

[0070] With the XY plane in the CATIA environment as the design horizontal plane H, the fairing skin 1 is rotated to ensure that the distances h between the two end points P and Q of the fairing skin and the design horizontal plane are equal, thereby reducing the fairing skin forming depth h, and the fairing skin bulging direction F is perpendicular to the design horizontal plane H;

[0071] Step 1-2: Add the fairing skin

[0072] Extract the upper surface of the fairing skin, select the vertices A and G of the elliptical notch 11 on the upper surface of the skin, construct the spline AG with the side contour lines on both sides of the notch as tangents, fill the spline AG, the elliptical notch contour line, and the oblong notch contour line to establish the fairing skin shape surface 2;

[0073] Step 1-3: Design the transition extension surface

[0074] Select the two endpoints P and Q of the fairing skin, and establish a reference plane with reference to the ZX plane in the CATIA environment. Establish design planes M and N by offsetting the reference plane by 200 mm in the opposite direction. Extract the contour lines of the fairing skin surface W at both ends. Design splines on M and N based on the contours of the fairing skin surface at both ends. Bridge planes M and N and the corresponding splines. Stretch the two ends by 200 mm along the curvature of the bridge surface to establish the transition flange extension surface.

[0075] Step 1-4: Design flange extension surface 3

[0076] Select a midpoint C at the notch in the middle of the fairing skin, establish a design plane K with reference to the XY plane in the CATIA environment, design a spline curve in K at a distance of 300 mm from the fairing skin contour line based on the curvature, project the spline onto the transition extension surface to obtain the flange contour line, and use the flange contour line to cut the transition extension surface to establish flange extension surface 3;

[0077] Step 1-5: Design the transition guide line of the hydraulic bulging process surface

[0078] Extract the four endpoints D1, D2, D3, and D4 of the fairing skin contour, extract the four points E1, E2, E3, and E4 on the fairing skin surface, and fit the connecting lines D1 E1, D2E2, D3E3, and D4E4. Construct points F1, F2, F3, and F4 at the closest distances between the four endpoints of the fairing skin contour on the inner contour line of the flange extension surface. According to the four endpoints D1, D2, D3, and D4 of the fairing skin contour, the connecting lines D1 E1, D2E2, D3E3, and D4E4, and the construction points F1, F2, F3, and F4, fit the transition guide lines D1 F1, D2F2, D3F3, and D4F4 of the hydraulic bulging process surface. The tension of the guide lines is defined as 1.

[0079] Step 1-6: Design transition surface 4

[0080] Extract the inner contour line of the flange extension surface 3, project points A and G onto the inner contour line of the flange extension surface 3 to obtain points A1 and G1, cut the inner contour line of the flange extension surface 3 at points A1 and G1, retain the inner contour line A1 G1, bridge the inner contour line A1 G1 and the spline AG in step 1-2, obtain the transition surface Y1 between the flange extension surface 3 and the fairing skin surface 2, extract the contour lines AA1 and GG1 at both ends of the transition surface Y1, fill the contour line GG1 and the transition guide line D1 F1, the transition guide line D1 F1 and the transition guide line D2F2, the transition guide line D2F2 and the transition guide line D3F3, the transition guide line D3F3 and the transition guide line D4F4, the transition guide line D4F4 and the transition contour line AA1 to obtain transition surfaces Y2, Y3, Y4, Y5, and Y6, and join Y1, Y2, Y3, Y4, Y5, and Y6 to obtain the transition surface 4;

[0081] Step 1-7: Design the hydraulic bulging process surface 5

[0082] The fairing skin surface 2, the flange extension surface 3, and the transition surface 4 are joined, and the flange extension surface 3 and the transition surface 4 are transitioned with a fillet radius R to obtain a hydraulic bulging process surface 5.

[0083] Step 2: Determine the size of the unfolded blank

[0084] Import the fairing skin surface 2 into the finite element analysis software, use the blank inverse calculation function to calculate the unfolded shape of the hydraulic bulge, and obtain the unfolded blank 6 and the unfolded blank outer edge line 7. Determine the unfolded blank size based on the unfolded blank outer edge line 7.

[0085] Step 3: Design the hydraulic bulging die structure of the fairing skin

[0086] According to the hydraulic bulging process surface 5, the hydraulic bulging die structure of the fairing skin is determined, including the concave die 8 and the blank holder 9.

[0087] Step 3-1: Design the die 8 structure

[0088] The flange extension surface 3 of the hydraulic bulging process surface 5 is extended in all directions within the hydraulic bulging horizontal plane XY, and the expanded hydraulic bulging process surface 5 is used as the die working surface 10. The outer edge line 7 of the unfolded blank is projected onto the corresponding position of the die working surface to obtain the unfolded blank outer contour line 17 on the die 9. The die working surface 10 is adjusted according to the unfolded blank outer contour line 17, and a drainage hole 18 with a diameter of 10 mm is set on the die;

[0089] Step 3-2: Design the structure of the blank holder 9

[0090] Generate the blank holder working surface 14 according to the flange extension surface 3, the transition surface 12 and the gap;

[0091] Gap g = (0.95-1.0) × δ;

[0092] The outer edge line 7 of the unfolded blank is projected to the corresponding position of the working surface 14 of the blank holder to obtain the outer contour line 15 of the unfolded blank on the blank holder; the working surface 14 of the blank holder is adjusted according to the outer contour line 15 of the unfolded blank; a water inlet hole 19 with a diameter of 10 mm is provided on the blank holder; wherein δ is the material thickness of the fairing skin.

[0093] In addition, it is necessary to determine whether the distance between the outer edge line 15 of the unfolded blank and the outer contour of the working surface of the die 8 and the pressure ring 9 is relatively uniform, and the distance between them is ≥80mm. If not, adjust the working surfaces 10 and 14 of the die and the pressure ring.

[0094] Step 4: Hydraulic bulging

[0095] Step 4-1: Install the hydraulic bulging die

[0096] Install the die 8 and the blank holder 9 on the working surface of the hydraulic bulging machine. The die 8 is fixed to the hydraulic bulging machine bed. The hydraulic bulging outer surface 13 of the die 8 corresponds to the working surface 14 of the blank holder. The blank holder 9 can move up and down through the movable ejector. Apply lubricating oil to the forming working surfaces of the die 8 and the blank holder 9.

[0097] Step 4-2: Expand the blank and position it

[0098] The upper surface of the unfolded blank 6 is protected with a plastic film, and is placed on the die working surface 10 according to the outer contour line 17 of the unfolded blank on the die 9, and lubricating oil is applied to the corresponding area of ​​the lower surface of the unfolded blank and the die working surface 10;

[0099] Step 4-3: Hydraulic bulging

[0100] The blank holder 9 moves downward along the hydraulic bulging direction F. After touching the die 8, the die 8 and the blank holder 9 clamp the expanded blank 6. Fluid is introduced from the water inlet 19 of the blank holder. The fluid medium acts as a punch to apply hydraulic pressure P to the expanded blank 6, causing the plate to undergo plastic deformation and form the part against the die cavity. The hydraulic pressure is adjusted by the overflow valve and hydraulic bulging is performed according to the shape of the die 8. The unit blank holder force setting range is 8MPa~9MPa.

[0101] Step 6: Trim the shape

[0102] The cutting is completed according to the fairing skin outline 20 on the hydraulic bulging outer surface 10 of the die 8, so as to obtain a fairing skin part whose outer shape accuracy and surface quality meet the requirements of the drawing.

[0103] It should be noted that the present invention is suitable for aluminum skins with opposite overall forming curvatures, complex cross-sections, and an initial overall structure within the range of an open structure; after completing the hydraulic bulging die structure design and unfolding blank calculation, the hydraulic bulging effect can be predicted through hydraulic bulging finite element simulation. The ideal range of variation in the material thickness of the skin is 8%δ to 18%δ. When the material thickness variation range is greater than 18%δ, the hydraulic bulging die structure can be optimized by adjusting the liquid pressure P and adjusting the transition surface curvature; when hydraulic bulging skin parts, the unit clamping force can be adjusted according to the actual machine tool conditions.

Claims

1. A method for hydraulic expansion forming of a reverse double curvature fairing skin, characterized in that , including the following steps: Step 1: constructing a hydraulic bulging process surface of the fairing skin; Step 2: Determine the size of the unfolded blank; Step 3: Determine the hydraulic bulging die structure of the fairing skin according to the hydraulic bulging process surface; Step 4: Laying the skin blank that is larger than the unfolded blank size on the hydraulic bulging die structure for hydraulic bulging; Step 5: trim the shape; The fairing skin presents a smooth transition of hyperbolic streamlines on both sides of the central axis. The curvature of the fairing skin on the left and right sides changes in opposite directions, and the curvature of the fairing skin on the right side changes more rapidly than that on the left side. The step 1 constructs the hydraulic bulging process surface of the fairing skin, and the specific process is as follows: Step 1-1: Define the hydroforming direction Using the XY plane in the CATIA environment as the design horizontal plane H, the fairing skin is rotated to ensure that the distance h between the two end points P and Q of the fairing skin and the design horizontal plane is equal, thereby reducing the fairing skin forming depth h, and the fairing skin bulging direction F is perpendicular to the design horizontal plane H; Step 1-2: Add the fairing skin Extract the upper surface of the fairing skin, select the vertices A and G of the elliptical notch on the upper surface of the skin, construct the spline AG with the side contour lines on both sides of the notch as tangents, fill the spline AG, the elliptical notch contour line, and the oblong notch contour line to establish the fairing skin surface; Step 1-3: Design the transition extension surface Select two high points A and B of the fairing skin, and establish a reference plane with reference to the ZX plane in the CATIA environment. Establish design planes M and N by offsetting the reference plane by 200 mm in the opposite direction. Extract the contour lines of the fairing skin surface W at both ends. Design splines on M and N based on the contours of the fairing skin surface at both ends. Bridge planes M and N and the corresponding splines. Stretch the fairing skin at both ends by 200 mm along the curvature of the bridge surface to establish the transition flange extension surface. Step 1-4: Design the flange extension surface Select a midpoint C at the notch in the middle of the fairing skin, establish a design plane K with reference to the XY plane in the CATIA environment, design a spline curve in K at a distance of 300 mm from the fairing skin contour line curvature, project the spline curve onto the transition flange extension surface to obtain the flange contour line, and use the flange contour line to cut the transition flange extension surface to establish the flange extension surface; Step 1-5: Design the transition guide line of the hydraulic bulging process surface Extract the four endpoints D1, D2, D3, and D4 of the fairing skin contour, extract the four points E1, E2, E3, and E4 on the fairing skin surface, and fit the connecting lines D1E1, D2E2, D3E3, and D4E4. Construct points F1, F2, F3, and F4 at the closest distances between the four endpoints of the fairing skin contour on the inner contour line of the flange extension surface. According to the four endpoints D1, D2, D3, and D4 of the fairing skin contour, the connecting lines D1E1, D2E2, D3E3, and D4E4, and the construction points F1, F2, F3, and F4, the transition guide lines D1F1, D2F2, D3F3, and D4F4 of the hydraulic bulging process surface are fitted. The guide line tension is defined as 1. Step 1-6: Design the transition surface Extract the inner contour line of the flange extension surface, project points A and G onto the inner contour line of the flange extension surface to obtain points A1 and G1, cut the inner contour line of the flange extension surface at points A1 and G1, retain the inner contour line A1 G1, bridge the inner contour line A1 G1 and the spline AG to obtain the transition surface Y1 between the flange extension surface and the fairing skin surface, extract the contour lines AA1 and GG1 at both ends of the transition surface Y1, fill the contour line GG1 and the transition guide line D1 F1, the transition guide line D1 F1 and the transition guide line D2F2, the transition guide line D2F2 and the transition guide line D3F3, the transition guide line D3F3 and the transition guide line D4F4, the transition guide line D4F4 and the cross contour line AA1 to obtain transition surfaces Y2, Y3, Y4, Y5, and Y6, and join Y1, Y2, Y3, Y4, Y5, and Y6 to obtain the transition surface; Step 1-7: Design the hydroforming process surface The fairing skin surface, flange extension surface and transition surface are joined, and the flange extension surface and the transition surface are transitioned with a fillet radius R to obtain a hydraulic bulging process surface.

2. The method for hydraulic expansion forming of a reverse double curvature fairing skin according to claim 1, characterized in that: The step 2 above determines the size of the unfolded blank. The specific process is as follows: Import the fairing skin surface into the finite element analysis software, use the blank inverse calculation function to calculate the unfolded shape of the hydroforming, and obtain the unfolded blank and the outer edge line of the unfolded blank; Determine the size of the unfolded blank based on the outer edge line of the unfolded blank.

3. The method for hydraulic expansion forming of a reverse double curvature fairing skin according to claim 2, characterized in that: The step 3 determines the hydraulic bulging die structure of the fairing skin according to the hydraulic bulging process surface. The specific process is as follows: Step 3-1: Design the die structure The flange extension surface of the hydraulic bulging process surface is extended in all directions within the hydraulic bulging horizontal plane XY. The expanded hydraulic bulging process surface is used as the die working surface. The outer edge line of the unfolded blank is projected onto the corresponding position of the die working surface to obtain the outer contour line of the unfolded blank on the die. The die working surface is adjusted according to the outer contour line of the unfolded blank. A drainage hole with a diameter of 10 mm is set on the die. Step 3-2: Design the blank holder structure Generate the blank holder working surface based on the flange extension surface, transition surface and gap; Gap g = (0.95-1.0) × δ; The outer edge line of the unfolded blank is projected onto the corresponding position of the working surface of the blank holder to obtain the outer contour line of the unfolded blank on the blank holder; the working surface of the blank holder is adjusted according to the outer contour line of the unfolded blank; a water inlet hole with a diameter of 10 mm is provided on the blank holder; Where δ is the material thickness of the fairing skin.

4. The method for hydraulic expansion forming of a reverse double curvature fairing skin according to claim 3, characterized in that: Adjust the die working surface according to the outer contour of the unfolded blank, including: Determine whether the distance between the outer edge line of the unfolded blank and the outer contour of the working surface of the die and the blank holder is relatively uniform, and the distance between them is ≥80mm. If not, adjust the working surface of the die and the blank holder.

5. The method for hydraulic expansion forming of a reverse double curvature fairing skin according to claim 1, characterized in that: In step 4, the skin blank that is larger than the unfolded blank size is laid on the hydraulic bulging die structure for hydraulic bulging. The specific process is as follows: Step 4-1: Install the hydraulic bulging die Install the die and blank holder on the working table of the hydraulic bulging machine. The die is fixed to the bed of the hydraulic bulging machine. The hydraulic bulging outer surface of the die corresponds to the working surface of the blank holder. The blank holder can move up and down through the movable ejector. Apply lubricating oil to the forming working surfaces of the die and blank holder. Step 4-2: Expand the blank and position it Attach plastic film to the upper surface of the unfolded blank for protection, and place it on the working surface of the die according to the outer contour of the unfolded blank on the die. Apply lubricating oil to the corresponding area of ​​the lower surface of the unfolded blank and the working surface of the die. Step 4-3: Hydraulic bulging The blank holder moves downward along the hydraulic bulging direction F, and after touching the die, it clamps the expanded blank with the die. Fluid is introduced from the water inlet of the blank holder, and the fluid medium acts as a punch to apply hydraulic pressure P to the expanded blank, causing the expanded blank to undergo plastic deformation and form a part against the die cavity.

6. The method for hydraulic expansion forming of a reverse double curvature fairing skin according to claim 5, characterized in that: The hydraulic pressure is adjusted by the overflow valve, and hydraulic bulging is performed according to the shape of the die. The unit blank holding force setting range is 8MPa ~ 9MPa.

7. The method for hydraulic expansion forming of a reverse double curvature fairing skin according to claim 1, characterized in that: The step 5 is to trim the fairing skin parts, and the specific process is as follows: The cutting is completed according to the fairing skin outline on the hydraulic bulging surface of the die, and the fairing skin parts with shape accuracy and surface quality meeting the requirements of the drawing are obtained.

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