Design method of large-capacity external fuel tank of helicopter

Abstract

The application belongs to the technical field of helicopter aerodynamic design, and particularly relates to a design method of a large-volume external fuel tank of a helicopter. The method comprises the following steps: S1, determining the design constraint conditions of the external fuel tank; S2, setting the length of a fuel arrangement area of the external fuel tank as the length L1 of a straight section of the external fuel tank and the cross-sectional area of the fuel arrangement area as the cross-sectional area S of the straight section of the external fuel tank according to the constraint conditions; S3, keeping the cross-sectional area S of the external fuel tank unchanged and optimizing the cross-sectional shape of the external fuel tank according to the vertical drag coefficient; S4, keeping the length L1 of the straight section of the external fuel tank unchanged and optimizing the longitudinal cross-sectional shape of the external fuel tank according to the forward drag coefficient; S5, constructing the three-dimensional shape of the external fuel tank according to the optimization results of the cross-sectional shape of the external fuel tank obtained in S3 and the optimization results of the longitudinal cross-sectional shape of the external fuel tank obtained in S4; S6, determining the longitudinal moment coefficient, the vertical drag coefficient and the forward drag coefficient corresponding to the three-dimensional shape of the external fuel tank; S7, adjusting the length L1 of the fuel arrangement area and the cross-sectional area S of the external fuel tank and repeating S3-S6; and obtaining the three-dimensional shape of the external fuel tank when the longitudinal moment coefficient, the vertical drag coefficient and the forward drag coefficient are optimal.

Description

Technical Field

[0001] This invention belongs to the field of helicopter aerodynamic design technology, and in particular relates to a design method for a large-volume external fuel tank for helicopters. Background Technology

[0002] Heavy helicopters often employ external fuel tanks for easier fuel system maintenance. Compared to medium and small helicopters, heavy helicopters have a larger fuel capacity, requiring larger external fuel tank volumes. This leads to increased overall aerodynamic drag and greater fuselage weight due to rotor downwash, resulting in higher cruise fuel consumption and reduced takeoff payload. External fuel tanks not only generate significant aerodynamic forces themselves, but their wake can also interfere with the horizontal stabilizer, significantly impacting the helicopter's longitudinal aerodynamic characteristics. Therefore, aerodynamic optimization design for external fuel tanks is essential.

[0003] Existing external fuel tank design methods design an initial envelope based on the fuel tank volume, and then rectify the initial envelope forward and backward to obtain the fuel tank's external shape. However, these methods do not consider vertical aerodynamic drag and cannot perform comprehensive optimization that includes forward, vertical, and longitudinal aerodynamic characteristics, thus failing to meet the overall aerodynamic requirements of helicopters. Summary of the Invention

[0004] This invention proposes a large-capacity external fuel tank for helicopters and its design method. With the three objectives of low drag, low vertical weight gain, and good longitudinal aerodynamic characteristics, the design integrates the aerodynamic shape of the external fuel tank with the fuselage through two-dimensional profile curve optimization and three-dimensional surface optimization. The aerodynamic optimization design has a significant drag reduction effect, effectively improving the helicopter's payload, reducing fuel consumption, and reducing the aerodynamic interference of wake turbulence on the helicopter's horizontal stabilizer and other components, thus enabling the helicopter to have a good flight attitude.

[0005] To achieve the above objectives, the present invention employs the following technical solution.

[0006] A design method for a large-capacity external fuel tank for helicopters, the method comprising:

[0007] S1, Determine the design constraints for the external fuel tank;

[0008] S2, According to the constraints, the length of the fuel arrangement area of ​​the external fuel tank is set to the length L1 of the straight section of the external fuel tank, and the cross-sectional area is the cross-sectional area S of the straight section of the external fuel tank.

[0009] S3, keeping the cross-sectional area S of the external fuel tank constant, optimize the cross-sectional shape of the external fuel tank according to the vertical drag coefficient;

[0010] S4, keep the length L1 of the straight section of the external fuel tank unchanged, and optimize the shape of the longitudinal section of the external fuel tank according to the forward drag coefficient;

[0011] S5. Based on the optimization results of the cross-sectional shape of the external fuel tank obtained in S3 and the optimization results of the longitudinal section shape of the external fuel tank obtained in S4, construct the three-dimensional shape of the external fuel tank.

[0012] S6. Determine the longitudinal moment coefficient, vertical drag coefficient, and forward drag coefficient corresponding to the three-dimensional shape of the external fuel tank;

[0013] S7, adjust the length L1 and cross-sectional area S of the fuel arrangement area of ​​the external fuel tank, and repeat S3-S6; obtain the three-dimensional shape of the external fuel tank when the longitudinal moment coefficient, vertical resistance coefficient, and forward resistance coefficient are optimal.

[0014] Furthermore, S1 specifically refers to:

[0015] The length of the external fuel tank shall not exceed the distance L between the passenger / cargo door and the stern door;

[0016] The height h of the external fuel tank shall not exceed the height H from the bottom of the passenger / cargo cabin window to the lower edge of the fuselage.

[0017] The effective volume of the external fuel tank should not be less than 1.1Ve, where Ve is the required fuel quantity.

[0018] The longitudinal space of the fuel tank inside the external fuel tank coincides with the longitudinal center of gravity of the helicopter.

[0019] The external fuel tank has sufficient space for landing gear placement, and its shape conforms to the requirements for emergency buoy mooring.

[0020] Furthermore, S3 specifically refers to:

[0021] S31 defines multiple component curves of the cross-section of the external fuel tank;

[0022] The cross-sectional geometry of the external fuel tank is formed by five curves connected together, which are, in counterclockwise order, segments I, II, III, IV, and V. Among them, segment I is flush with the fuselage skin and is a straight line; segment II is flush with the bottom surface of the fuselage and is a straight line; segment III uses an elliptical curve for transition; segment IV uses a straight line parallel to the longitudinal plane of symmetry of the fuselage; and segment V is a parabolic curve.

[0023] S32, parameterize each component curve;

[0024] S33, based on the CFD method, conducts two-dimensional aerodynamic characteristic analysis to determine the optimal parameter values ​​of each component curve corresponding to the minimum vertical drag coefficient.

[0025] Furthermore, S32 specifically refers to:

[0026] The height h of the external fuel tank remains constant, and the horizontal distance c between the external fuel tank and the upper intersection point A and the lower intersection point B of the fuselage remains constant; the length of the lower base of the cross-section of the external fuel tank in section II is a; the length of the horizontal axis of the elliptic curve in section III is a1, and the length of the vertical axis is b1; the length of the outer side of the cross-section of the external fuel tank in section IV is b; the vertex of the parabola in section V is point C, and it opens to the left. Therefore, the equation of the standard parabola is:

[0027]

[0028] The cross-sectional area of ​​the external fuel tank is obtained by the following formula.

[0029]

[0030] Furthermore, S33 specifically refers to:

[0031] With segment III of the elliptic curve remaining constant, and a1 and b1 being constants, segment II, IV, and V are optimized. The length of the bottom edge of the external fuel tank cross-section, a, is taken as the independent variable, and b is obtained by using the formula for the cross-sectional area of ​​the external fuel tank, thus obtaining different geometric shapes of the external fuel tank cross-section.

[0032] Determine the optimal parameter values ​​for each component curve when the vertical drag coefficient is minimized.

[0033] Furthermore, S4 specifically refers to:

[0034] S41 defines multiple component curves of the longitudinal section of the external fuel tank;

[0035] The longitudinal section geometry of the external fuel tank is formed by seven curves connected together, which are, in counterclockwise order: segment VI, segment VII, segment VIII, segment IX, segment X, segment XI, and segment XII.

[0036] Among them, segments VI and VII are elliptical curves, segment VIII is flush with the bottom surface of the fuselage and is a straight line, segments IX and XI are parabolic curves, segment X is a circular arc curve, and segment XII is a straight line parallel to the bottom surface of the fuselage.

[0037] S42, parameterize each component curve;

[0038] S43, based on the CFD method, conduct two-dimensional aerodynamic characteristic analysis to determine the optimal parameter values ​​of each component curve corresponding to the minimum forward drag coefficient.

[0039] Furthermore, S42 specifically refers to:

[0040] The tank height h remains constant. The horizontal axis length of the elliptic curve in segment VI is d1, and the vertical axis length is h-h1; the horizontal axis length of the elliptic curve in segment VII is d1, and the vertical axis length is h1; the length of the lower base of the longitudinal section of the external tank in segment VIII is d; the vertex of the parabola in segment IX is point E, opening upwards; the vertex of the parabola in segment XI is point F, opening downwards. Therefore, the equations of the standard parabolas for segments IX and XI are respectively:

[0041]

[0042] Segment X is the inscribed arc of segments IX and XI, with an inscribed circle radius of r; the length of the bottom edge on the longitudinal section of the external oil tank in segment XII is u.

[0043] Furthermore, S43 specifically refers to:

[0044] The length u of the bottom edge on the longitudinal section of the external fuel tank shall not be less than L1; the length d of the bottom edge on the longitudinal section of the external fuel tank shall not be less than L2. Take u = L1 and d = L2 unchanged, and optimize the front and rear edges of the external fuel tank.

[0045] For the front edge segments VI and VII of the external fuel tank longitudinal section, keep h1 / h = 0.45 constant, and optimize with d1 as the independent variable, d1 = 0.5h as the initial value;

[0046] For the trailing edge segments IX and XI of the longitudinal section of the external fuel tank, keep h2 / h = 0.35 constant, and optimize with d2 as the independent variable, where d2 = h is the initial value;

[0047] Multiple optimization schemes were obtained, and the optimal parameter values ​​for each component curve corresponding to the minimum forward drag coefficient were determined.

[0048] Furthermore, S5 specifically refers to:

[0049] After two-dimensional optimization, the cross section is swept along the bottom edge of the longitudinal section by a length d to form surface 1; surface 2 is formed by closing the front edge of the longitudinal section and the front curve of surface 1, and surface 2 is tangent to surface 1; surface 3 is formed by closing the rear edge of the longitudinal section and the rear curve of surface 1, and surface 3 is tangent to surface 1.

[0050] The aerodynamic characteristics of the external fuel tank's three-dimensional shape were evaluated using the CFD method. The length L1 and cross-sectional area S of the fuel arrangement area of ​​the external fuel tank were adjusted, and the two-dimensional and three-dimensional shape optimizations were iteratively performed to obtain the final three-dimensional shape of the external fuel tank.

[0051] The technical solution of this invention selects the main dimensional parameters of the aerodynamic shape of the external fuel tank of a helicopter, designs the external fuel tank shape scheme through parametric modeling, and evaluates the aerodynamic characteristics of the external fuel tank using CFD calculation methods. First, it optimizes the two-dimensional contour curves of the longitudinal section and cross section of the external fuel tank, and then performs comprehensive optimization of the three-dimensional surface. The design concept of integrating the external fuel tank shape with the fuselage can reduce the longitudinal and vertical air resistance of the external fuel tank, effectively improve the helicopter's payload, reduce fuel consumption, reduce the aerodynamic interference of the wake duct on the helicopter's horizontal stabilizer and other components, and enable the helicopter to have a good flight attitude. Attached Figure Description

[0052] Figure 1 This is a schematic diagram (left side) showing the location of the external fuel tank on a helicopter according to the present invention;

[0053] Figure 2 This is a parameterized schematic diagram of the cross-sectional shape of the external fuel tank involved in the present invention;

[0054] Figure 3 This is a schematic diagram of the cross-sectional geometry of the external fuel tank involved in the present invention;

[0055] Figure 4 This is a schematic diagram illustrating the variation of the vertical drag coefficient of the external fuel tank as a function of a, as per the present invention.

[0056] Figure 5 This is a parameterized schematic diagram of the longitudinal section shape of the external fuel tank involved in the present invention;

[0057] Figure 6 This is a schematic diagram of the longitudinal section geometry of the external fuel tank involved in the present invention;

[0058] Figure 7 This is a schematic diagram illustrating the variation of the drag coefficient of the external fuel tank as a function of d1, as per the present invention.

[0059] Figure 8 This is a schematic diagram illustrating the variation of the drag coefficient of the external fuel tank as a function of d2, as per the present invention.

[0060] Figure 9 This is a three-dimensional schematic diagram of the external fuel tank involved in the present invention;

[0061] Figure 10 This is a cross-sectional pressure distribution cloud map of the external fuel tank involved in the present invention. Detailed Implementation

[0062] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0063] This invention provides a large-capacity external fuel tank for helicopters and its design method. The main dimensional parameters of the aerodynamic shape of the external fuel tank are selected, and the external fuel tank shape scheme is designed through parametric modeling. The aerodynamic characteristics of the external fuel tank are evaluated by CFD calculation method. The design concept is to first optimize the two-dimensional contour curves of the longitudinal section and cross section of the external fuel tank, and then optimize the three-dimensional surface. The external fuel tank shape is integrated with the fuselage design.

[0064] 1. Determine the design constraints for the external fuel tank.

[0065] Due to limitations in fuselage passenger and cargo cabin length, door and window arrangement, the length of the external fuel tank should not exceed the distance L between the passenger / cargo cabin door and the aft cabin door, and the height h of the external fuel tank should not exceed the height H from the bottom of the passenger / cargo cabin window to the lower edge of the fuselage. Figure 1 As shown. Due to the fuel quantity requirement of Ve, the effective volume of the external fuel tank should not be less than 1.1Ve; constrained by the range of fuel center of gravity variation, the longitudinal space of the fuel inside the external fuel tank should coincide with the longitudinal center of gravity of the helicopter. In addition, the shape of the external fuel tank should allow for landing gear arrangement space and facilitate emergency float mooring, etc. Based on the above constraints and combined with model design experience, the initial proposed length of the external fuel tank fuel arrangement area is the length L1 of the straight section of the external fuel tank, and the cross-sectional area is the cross-sectional area S of the straight section of the external fuel tank.

[0066] 2. Optimization of the cross-sectional shape of the external fuel tank

[0067] While keeping the cross-sectional area S of the external fuel tank constant, the aerodynamic geometry of the external fuel tank cross-section is optimized.

[0068] The cross-sectional geometry of the external fuel tank is formed by five curves connected together, which are, in counterclockwise order, segments I, II, III, IV, and V. Figure 2 As shown. Section I is flush with the fuselage skin and is basically a straight line; Section II is flush with the lower surface of the fuselage to maintain ground clearance and is basically a straight line; Section III uses an elliptical curve transition; Section IV uses a straight line parallel to the longitudinal symmetry plane of the fuselage to facilitate emergency pontoon mooring; Section V is a parabolic curve to reduce vertical drag.

[0069] The cross-sectional geometry of the external fuel tank is parameterized, such as... Figure 2 As shown. Due to the window arrangement constraints of the fuselage passenger and cargo compartments, the height h of the external fuel tank remains constant. Correspondingly, due to the fuselage skin profile constraints, the horizontal distance c between the fuel tank and the upper intersection point A and the lower intersection point B of the fuselage remains constant, i.e., segment I remains unchanged. The length of the lower base of the external fuel tank cross-section in segment II is a; the length of the horizontal axis of the elliptical curve in segment III is a1, and the length of the vertical axis is b1; the length of the outer side of the external fuel tank cross-section in segment IV is b; the vertex of the parabola in segment V is point C, opening to the left. Therefore, the equation of the standard parabola is...

[0070]

[0071] The cross-sectional area of ​​the external fuel tank can be obtained by the following formula.

[0072]

[0073] In this embodiment, segment III of the elliptic curve is temporarily kept constant, i.e., a1 and b1 are constants. Segments II, IV, and V are optimized. The length of the bottom edge of the external fuel tank cross-section, a, is taken as the independent variable, and b can be obtained through equation (2). Different geometric shapes of the external fuel tank cross-section are thus obtained, such as... Figure 3 As shown.

[0074] Two-dimensional aerodynamic characteristic analysis was conducted using CFD methods to obtain the variation law of the vertical drag coefficient in the vertical direction, such as... Figure 4 As shown in the figure, the vertical drag coefficient is minimized when the length 'a' of the bottom edge of the external fuel tank cross-section is increased by 6%, decreasing by approximately 14.5% compared to the initial state.

[0075] Similarly, parameters for a1 and b1 can be optimized.

[0076] 3. Optimization of the longitudinal section shape of the external fuel tank

[0077] The longitudinal section geometry of the external fuel tank is formed by seven curves connected together, arranged counterclockwise as follows: Figure 5 As shown. Among them, segments VI and VII are elliptical curves, segment VIII is flush with the bottom surface of the fuselage and is basically a straight line, segments IX and XI are parabolic curves, segment X is a circular arc curve, and segment XII is a straight line parallel to the bottom surface of the fuselage.

[0078] The geometry of the longitudinal section of the external fuel tank is parameterized, such as... Figure 5 As shown. Due to the window arrangement constraints in the fuselage passenger and cargo compartments, the fuel tank height h remains constant. The horizontal axis length of the elliptic curve in segment VI is d1, and the vertical axis length is h-h1; the horizontal axis length of the elliptic curve in segment VII is d1, and the vertical axis length is h1. The length of the lower base of the longitudinal section of the external fuel tank in segment VIII is d; the vertex of the parabola in segment IX is point E, opening upwards; the vertex of the parabola in segment XI is point F, opening downwards. Therefore, the equations of the standard parabolas for segments IX and XI are respectively:

[0079]

[0080]

[0081] Segment X is the inscribed arc of segments IX and XI, with an inscribed circle radius of r; the length of the bottom edge on the longitudinal section of the external oil tank in segment XII is u.

[0082] To maintain the fuel tank volume, the length u of the bottom edge on the longitudinal section of the external fuel tank must not be less than L1; for the installation and rectification of the landing gear struts, the length d of the bottom edge on the longitudinal section of the external fuel tank must not be less than L2. In this example, we temporarily take u = L1 and d = L2 to remain unchanged and optimize the front and rear edges of the external fuel tank.

[0083] In this embodiment, for the front edges VI and VII of the longitudinal section of the external fuel tank, h1 / h = 0.45 is kept constant, and d1 / h = 0.5 is used as the initial value, with d1 as the independent variable for optimization. For the rear edges IX and XI of the longitudinal section of the external fuel tank, h2 / h = 0.35 is kept constant, and d2 / h = 1 is used as the initial value, with d2 as the independent variable for optimization. The optimization scheme is as follows: Figure 6 As shown.

[0084] Two-dimensional aerodynamic characteristic analysis was conducted using CFD methods. The results show that for the leading edge of the external fuel tank, the drag is minimized when d1 / h = 0.75, achieving a drag reduction of approximately 5.2% compared to the initial design with d1 / h = 0.5. Figure 7 As shown. For the trailing edge of the external fuel tank, as d2 increases, the resistance of the external fuel tank decreases, and the drag reduction effect gradually weakens. Due to the limitation of the length of the external fuel tank, obtaining d2 = Ld - d1 can reduce the drag by about 18% compared to the initial scheme d2 / h = 1. Figure 8 As shown.

[0085] Similarly, parameters for h1 and h2 can be optimized.

[0086] 4. Comprehensive optimization of the three-dimensional shape of the external fuel tank

[0087] Based on the two-dimensional optimization results, the three-dimensional shape of the external fuel tank is constructed, such as... Figure 9 As shown. After two-dimensional optimization, the cross section is swept along the bottom edge of the longitudinal section by a length d to form surface 1; surface 2 is formed by closing the front edge of the longitudinal section and the front curve of surface 1, and surface 2 is tangent to surface 1; surface 3 is formed by closing the rear edge of the longitudinal section and the rear curve of surface 1, and surface 3 is tangent to surface 1.

[0088] The aerodynamic characteristics of the external fuel tank design were evaluated using CFD methods, followed by analysis based on flow field pressure and velocity, such as... Figure 10 As shown, drag is reduced by minimizing the area of ​​the high-pressure zone facing the external fuel tank, alleviating airflow separation at the rear, and minimizing the outline of the external fuel tank as much as possible, thus optimizing the three-dimensional shape of the front and rear edges of the external fuel tank. The length and cross-sectional proportions of the external fuel tank can be further adjusted, and the two-dimensional and three-dimensional shape optimizations can be iteratively performed to obtain the final external fuel tank design.

[0089] The calculation scheme of this invention 1) takes low drag, low vertical weight gain, and good longitudinal aerodynamic characteristics as the comprehensive design goals to achieve the purpose of increasing helicopter payload, reducing fuel consumption, and achieving good flight attitude; 2) adopts an iterative optimization design method from two-dimensional parametric optimization design to three-dimensional comprehensive optimization design; 3) for the longitudinal section of the external fuel tank, the leading edge of the external fuel tank is formed by connecting 1 / 4 of two different minor axis elliptical curves, and the trailing edge of the external fuel tank is formed by two parabolas with different focal lengths and a circle inscribed in both, which facilitates the optimization of the diverse shapes of the leading and trailing edges of the external fuel tank; 4) for the cross section of the external fuel tank, it is surrounded by five curves, including straight lines, elliptical curves, and parabolas, which can accurately calculate the cross-sectional area and facilitate parametric optimization with area constraints; 5) the external fuel tank in this embodiment has a large volume and takes into account the landing gear and emergency float arrangement.

[0090] This invention proposes a large-capacity external fuel tank for helicopters and its design method. The optimized external fuel tank scheme is compared with the original scheme using CFD method. The results show that compared with the original scheme, the optimized external fuel tank scheme significantly reduces aerodynamic drag, with the drag coefficient reduced by 7.9% and the vertical drag reduced by 7.4%. It effectively improves the helicopter's payload, reduces fuel consumption, and reduces the aerodynamic interference of wake turbulence on the helicopter's horizontal stabilizer and other components, enabling the helicopter to have a good flight attitude.

Claims

1. A design method for a large-capacity external fuel tank for helicopters, characterized in that, The method includes: S1, Determine the design constraints for the external fuel tank; S2, According to the constraints, the length of the fuel arrangement area of ​​the external fuel tank is set to the length L1 of the straight section of the external fuel tank, and the cross-sectional area is the cross-sectional area S of the straight section of the external fuel tank. S3, keeping the cross-sectional area S of the external fuel tank constant, optimize the cross-sectional shape of the external fuel tank according to the vertical drag coefficient; S4, keep the length L1 of the straight section of the external fuel tank unchanged, and optimize the shape of the longitudinal section of the external fuel tank according to the forward drag coefficient; S5. Based on the optimization results of the cross-sectional shape of the external fuel tank obtained in S3 and the optimization results of the longitudinal section shape of the external fuel tank obtained in S4, construct the three-dimensional shape of the external fuel tank. S6. Determine the longitudinal moment coefficient, vertical drag coefficient, and forward drag coefficient corresponding to the three-dimensional shape of the external fuel tank; S7, adjust the length L1 and cross-sectional area S of the fuel arrangement area of ​​the external fuel tank, and repeat S3-S6; obtain the three-dimensional shape of the external fuel tank when the longitudinal moment coefficient, vertical resistance coefficient, and forward resistance coefficient are optimal.

2. The design method for a large-capacity external fuel tank for helicopters according to claim 1, characterized in that, S1 specifically refers to: The length of the external fuel tank shall not exceed the distance L between the passenger / cargo door and the stern door; The height h of the external fuel tank shall not exceed the height H from the bottom of the passenger / cargo cabin window to the lower edge of the fuselage. The effective volume of the external fuel tank should not be less than 1.1Ve, where Ve is the required fuel quantity. The longitudinal space of the fuel tank inside the external fuel tank coincides with the longitudinal center of gravity of the helicopter. The external fuel tank has sufficient space for landing gear placement, and its shape conforms to the requirements for emergency buoy mooring.

3. The design method for a large-capacity external fuel tank for helicopters according to claim 2, characterized in that, S3 specifically refers to: S31 defines multiple component curves of the cross-section of the external fuel tank; The cross-sectional geometry of the external fuel tank is formed by five curves connected together, which are, in counterclockwise order, segments I, II, III, IV, and V. Among them, segment I is flush with the fuselage skin and is a straight line; segment II is flush with the bottom surface of the fuselage and is a straight line; segment III uses an elliptical curve transition; segment IV uses a straight line parallel to the longitudinal symmetry plane of the fuselage; and segment V is a parabolic curve. S32, parameterize each component curve; S33, based on the CFD method, conducts two-dimensional aerodynamic characteristic analysis to determine the optimal parameter values ​​of each component curve corresponding to the minimum vertical drag coefficient.

4. The design method for a large-capacity external fuel tank for helicopters according to claim 3, characterized in that, S32 specifically refers to: The height h of the external fuel tank remains constant, and the horizontal distance c between the external fuel tank and the upper intersection point A and the lower intersection point B of the fuselage remains constant; the length of the lower base of the cross-section of the external fuel tank in section II is a; the length of the horizontal axis of the elliptic curve in section III is a1, and the length of the vertical axis is b1; the length of the outer side of the cross-section of the external fuel tank in section IV is b; the vertex of the parabola in section V is point C, and it opens to the left. Therefore, the equation of the standard parabola is: The cross-sectional area of ​​the external fuel tank is obtained by the following formula.

5. The design method for a large-capacity external fuel tank for helicopters according to claim 4, characterized in that, S33, specifically: With segment III of the elliptic curve remaining constant, and a1 and b1 being constants, segment II, IV, and V are optimized. The length of the bottom edge of the external fuel tank cross-section, a, is taken as the independent variable, and b is obtained by using the formula for the cross-sectional area of ​​the external fuel tank, thus obtaining different geometric shapes of the external fuel tank cross-section. Determine the optimal parameter values ​​for each component curve when the vertical drag coefficient is minimized.

6. The design method for a large-capacity external fuel tank for helicopters according to claim 5, characterized in that, S4 specifically refers to: S41 defines multiple component curves of the longitudinal section of the external fuel tank; The longitudinal section geometry of the external fuel tank is formed by seven curves connected together, which are, in counterclockwise order: segment VI, segment VII, segment VIII, segment IX, segment X, segment XI, and segment XII. Among them, segments VI and VII are elliptical curves, segment VIII is flush with the bottom surface of the fuselage and is a straight line, segments IX and XI are parabolic curves, segment X is a circular arc curve, and segment XII is a straight line parallel to the bottom surface of the fuselage. S42, parameterize each component curve; S43, based on the CFD method, conduct two-dimensional aerodynamic characteristic analysis to determine the optimal parameter values ​​of each component curve corresponding to the minimum forward drag coefficient.

7. The design method for a large-capacity external fuel tank for helicopters according to claim 6, characterized in that, S42, specifically: The tank height h remains constant. The horizontal axis length of the elliptic curve in segment VI is d1, and the vertical axis length is h-h1; the horizontal axis length of the elliptic curve in segment VII is d1, and the vertical axis length is h1; the length of the lower base of the longitudinal section of the external tank in segment VIII is d; the vertex of the parabola in segment IX is point E, opening upwards; the vertex of the parabola in segment XI is point F, opening downwards. Therefore, the equations of the standard parabolas for segments IX and XI are respectively: Segment X is the inscribed arc of segments IX and XI, with an inscribed circle radius of r; the length of the bottom edge on the longitudinal section of the external oil tank in segment XII is u.

8. The design method for a large-capacity external fuel tank for helicopters according to claim 7, characterized in that, S43, specifically: The length u of the bottom edge on the longitudinal section of the external fuel tank shall not be less than L1; the length d of the bottom edge on the longitudinal section of the external fuel tank shall not be less than L2. Take u = L1 and d = L2 unchanged, and optimize the front and rear edges of the external fuel tank. For the front edge segments VI and VII of the external fuel tank longitudinal section, keep h1 / h = 0.45 constant, and optimize with d1 as the independent variable, with d1 = 0.5h as the initial value; For the trailing edge segments IX and XI of the longitudinal section of the external fuel tank, keep h2 / h = 0.35 constant, and optimize with d2 as the independent variable, where d2 = h is the initial value; Multiple optimization schemes were obtained, and the optimal parameter values ​​for each component curve corresponding to the minimum forward drag coefficient were determined.

9. The design method for a large-capacity external fuel tank for helicopters according to claim 8, characterized in that, S5 specifically refers to: After two-dimensional optimization, the cross section is swept along the bottom edge of the longitudinal section by a length d to form surface 1; surface 2 is formed by closing the front edge of the longitudinal section and the front curve of surface 1, and surface 2 is tangent to surface 1; surface 3 is formed by closing the rear edge of the longitudinal section and the rear curve of surface 1, and surface 3 is tangent to surface 1. The aerodynamic characteristics of the external fuel tank's three-dimensional shape were evaluated using the CFD method. The length L1 and cross-sectional area S of the fuel arrangement area of ​​the external fuel tank were adjusted, and the two-dimensional and three-dimensional shape optimizations were iteratively performed to obtain the final three-dimensional shape of the external fuel tank.

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