Design and thermal protection performance analysis of insulated wing storage box for hypersonic variable-sweep aircraft

Utilizing variable-sweep wing technology can further enhance the flight performance of hypersonic aircraft in a wide range of high-altitude, high-speed environments. Existing research on hypersonic variable-sweep flying wings has mainly focused on aerodynamic shape design, with limited research on the internal thermal protection of the aircraft. This study firstly investigates the performance improvement of a hypersonic flying wing using variable-sweep technology through CFD simulations. Secondly, to address the high-temperature issues induced by variable-sweep wings, an insulated wing storage box with a corrugated structure is designed, incorporating a thermal insulation layer made of corrugated webs and insulating materials. Heat transfer simulations are conducted by applying thermal loads to the wing storage box to study the temperature distribution during the variable-sweep process. Finally, a comparison between the corrugated structure design and the non-corrugated structure design of the wing storage box is performed to analyze the thermal insulation performance of the insulation layer. The results show that the heated area of the wing storage box is primarily influenced by the wing’s sweep angle, and the corrugated thermal insulation layer can effectively reduce heat transfer efficiency, resulting in a 30% reduction in the external temperature of the wing storage box.


Introduction
Hypersonic aircraft refers to aircraft that can fly at speeds exceeding Mach 5.They possess advantages such as faster speed, greater maneuverability, and higher penetration capability.Morphing aircraft refers to aircraft that can change their shape either locally or globally during flight [1] , in order to maintain good flight performance and efficiency in different flight conditions, thus meeting various mission requirements.The morphing aircraft technologies that have been put into practical application include variable-sweep wings, wing folding, and so on.In recent years, with the rapid development of emerging smart material science, such as shape memory alloys [2] and metamaterials, various forms such as variable wing curvature and variable wing area have also been extensively studied.Among the numerous morphing aircraft technologies, variable-sweep technology has been widely used in traditional supersonic aircraft, with the F-14 fighter jet being a notable example of mature variable-sweep technology [3] .
In recent years, in pursuit of breakthroughs in the performance of hypersonic aircraft in wide-speedrange and large-scale airspace, variable-sweep technology has gradually been applied to hypersonic aircraft.Combining variable-sweep wing technology with hypersonic aircraft design has become a new research focus in the aerospace field.Yuki et al. studied the ability of supersonic forward-swept wings IOP Publishing doi:10.1088/1742-6596/2764/1/012043 2 to reduce aerodynamic drag under cruising conditions by optimizing the wing planform distribution [4] .By obtaining the airfoil distribution of forward-swept and rear-swept wings and using the multi-fidelity Efficient Global Optimization (EGO) method, they solved the problem of spanwise airfoil design under low drag conditions for both forward-swept and rear-swept wings.Peng et al. investigated three deformation modes in hypersonic aircraft: telescoping wing, variable-sweep wing, and folding wing [5] .By analyzing and comparing the lift-to-drag ratio, wing efficiency, and static stability of the three different deformation modes, they proposed a hypersonic conceptual aircraft based on a variable-sweep wing.A multi-objective optimization algorithm was used to optimize the flight distance and total heat at the leading edge of the wing.Liu et al. designed a sliding shear variable-sweep aircraft that can fly at supersonic speeds by using a combination of variable-sweep design and plasma flow control [6] , with flight speeds ranging from Mach 0.25 to 3 and sweep angles ranging from 20 to 70 degrees.Based on the SBO algorithm, they conducted optimization of the wing aerodynamic shape and plasma control to achieve improvements in overall performance.Dai et al. designed a variable-sweep wing shockwave riding missile [7] and performed aerodynamic performance simulations.By calculating and comparing different sweep angles and angles of attack under subsonic, supersonic, and hypersonic flight conditions, they obtained improvements in the aerodynamic performance of the variable-sweep wing shockwave riding missile in both low-speed takeoff and high-speed cruising.
In summary, current research on hypersonic variable-sweep aircraft mainly focuses on the aerodynamic performance analysis and external shape design optimization of conceptual variable-sweep aircraft.There is limited research on the internal structural design of practical hypersonic variable-sweep aircraft and thermal protection structures.Therefore, this paper first performs computational fluid dynamics (CFD) calculations on a hypersonic aircraft to explore the influence of variable-sweep technology on its flight performance.To address the high-temperature issue faced by wings in hypersonic environments, a variable-sweep wing storage box based on a corrugated insulation structure is designed.A comparative analysis is conducted between the heat simulation results of the corrugated insulated wing storage box and the non-corrugated insulated design to verify the thermal insulation benefits of the corrugated insulated wing storage box.Meanwhile, the thermal protection characteristics of the corrugated insulated wing storage box within the variable-sweep range of the wing are analyzed.
The paper is organized as follows.Section 2 discusses the benefits of variable sweep and points out the high-temperature problem.Section 3 introduces the structural design of an insulated wing storage box.Section 4 analyzes the thermal protection performance of the corrugated insulated wing storage box in detail.Finally, Section 5 provides a summary concluding the paper.

The CFD modeling of aircraft
Hypersonic aircraft typically adopt a configuration of wave rider design [8] or blended wing-body design [9] , and a high-speed aircraft model is established based on existing aircraft shapes.The aircraft has a length of 4.5 meters, a height of 1.5 meters, and a wingspan of 3 meters.It consists of a fuselage, strake wing, vertical tail, and wing sections.The maximum thickness of the wing is 54 mm, and the initial sweep angle is 45 degrees, as shown in Figure 1.When flying at high Mach numbers, hypersonic aircraft constantly face complex flow field problems such as high aerodynamic forces and high-temperature effects.The variation of the wing sweep angle inevitably affects the aerodynamic characteristics of the aircraft.Moreover, the harsh high-temperature environment during supersonic flight can cause an increase in surface temperature, interfering with normal aircraft flight.
To investigate the effective range of variable sweep for the aircraft and observe the aerodynamic heating issues faced under hypersonic conditions, Fluent is used to conduct CFD simulations on the aircraft.Density-based solver is used, the turbulence model is k-ω-SST, and the fluid properties are ideal gas.The range of courant numbers is from 0.8 to 5. The aircraft surface is set as a non-slip adiabatic wall, the outer boundary of the flow field is set as a far-field pressure, and the symmetry plane is set as a symmetric boundary.The aircraft's half-model non-structured meshes are created by using ICEM, with to ensure a dimensionless wall distance of 1 y   .The halfmodel mesh of the aircraft is shown in Figure 2.This mesh meets the requirement of mesh independence.

Analysis of aerodynamic simulation results
The flight speed range of the aircraft is 2-8 Mach, with an altitude range of 20-30 kilometers.CFD analysis is conducted for two operating conditions: flying at 2 Mach at an altitude of 20 kilometers and flying at 8 Mach at an altitude of 30 kilometers.For the aircraft, the use of variable-sweep technology allows it to adapt to different flight conditions and improve its flight performance.Common performance indicators for evaluating flight performance include flight speed, climb rate, and range.
To analyze the performance benefits brought by variable sweep, the aircraft with a weight of 2000 kg is taken as a benchmark.However, the issue of high temperatures caused by aerodynamic heating is very severe.Figure 4 shows the temperature distribution on the wing surface during flight at 8 Mach and 8 degrees angle of attack.Due to the simulation setting of adiabatic walls, the theoretical temperature on the wing surface can reach 3000 K.In actual high-speed cruise flight at 8 Mach, the overall structural temperature of the wing increases.As the high-temperature wing enters the interior of the aircraft during the variable sweep process, the diffusion and conduction of heat carried by the wing can significantly affect the structural lifespan and the safe use of equipment inside the aircraft.The variable-sweep structure consists of an inner wing storage box, an outer wing, and a pivot structure.To address the thermal damage caused by high-temperature wings, a thermal protection design is needed for the inner wing storage box.

Structural design of thermal insulation wing storage box
To address the internal heat dissipation damage issue of the high-temperature variable-sweep wing, a corrugated insulation structure is used for the inner wing storage box design.The upper and lower parts of the thermal insulation wing storage box utilize corrugated insulation structures, as shown in Figure 5. From the inside to the outside, it consists of a carbon-carbon composite material on the inside with a thickness of 5 mm, an intermediate insulation layer composed of supporting webs, and silica aerogel with a thickness of 12 mm (with a horizontal angle of 56.3 degrees for the webs), and an outer layer made of high-temperature nickel-based alloy with a thickness of 3 mm.The material parameters used for the wing storage box are shown in Tables 1-3.The wing storage box is located inside the aircraft body, with a length of approximately 1200 mm and a total thickness of 112 mm.The space between the upper and lower parts of the box is used for wing storage.Two rails are designed to restrict and fix the variable sweep of the wing, with a distance of 54 mm between the rails, as shown in Figure 6 (a).Common corrugated structures use a linear arrangement for the webs [10] .However, considering that the variable sweep is a rotational motion of the wing, the supporting webs of the thermal insulation wing storage box are arranged in a concentric circular arc distribution.The box stores the variable-sweep wing internally, while the intermediate insulation layer acts as a barrier for heat protection from the high-temperature wing.The overall structure model of the variable sweep is shown in Figure 6 (b), where the wing shape is an approximate trapezoidal wing with an optimized double-wedge profile, and the variable-sweep wing can fully rotate into the wing storage box.
Where rad q is the surface heat flux,  is the thermal radiation coefficient,  is the Stefan-Boltzmann constant, t T is the radiative equilibrium temperature, and amb T is the ambient temperature.The final temperature of the model surface under the heat flux is approximately equal to the radiative equilibrium temperature.Under a heat flux of 2 1 W/mm , the model surface temperature is 2134.5 K, which is similar to the high-temperature wing heat transfer environment faced by the actual use of the insulated wing storage box.
The corrugated insulation structure (CIS) is used for the insulation wing storage box, and the load and boundary conditions of this structure [11] can be simplified as shown in Figure 7 (a).The carboncarbon composite material on the inside receives the heat flux load and undergoes surface radiation.The outer surface of the metal is set as an adiabatic boundary.
. Two insulation structures of the wing storage box.Figure 8 shows the temperature distribution of the cross-section of both models at 200 s.Due to the different thermal conductivities of the webs in the corrugated insulation model and the insulation material, the temperature distribution along the horizontal direction of the model is uneven.However, the non-corrugated insulation model uniformly conducts heat toward the outside.The inside part has a temperature of 2120 K.However, due to the different structures of the intermediate layer, the temperature of the metal part is significantly different.After 200 s, the average temperature decreases from 441 K to 330 K compared to the non-corrugated insulation model, as shown in Figure 9.The corrugated insulation intermediate layer reduces the external temperature by 25%, demonstrating a good insulation effect.Therefore, the corrugated insulation structure performs better for hypersonic variablesweep aircraft.

Thermal simulation settings for corrugated insulated wing storage box
In order to investigate the overall thermal insulation performance of the corrugated insulated wing storage box, a thermal conduction simulation of the wing storage box is carried out.The corrugated insulated wing storage box model is divided into different parts, with the corrugated supporting webs using 20-node hexahedral temperature elements (SOLID90), and the remaining parts using 10-node tetrahedral temperature elements (SOLID87).The simulated heat transfer duration is set to 1000 s.The internal surface of the corrugated insulated wing storage box is loaded with a heat flux, which gradually decreases from 2 1 W/mm to 2 0.2 W/mm within 1000 s, as shown in Figure 10 (a), to simulate the weakening of heat dissipation of actual high-temperature wings.The heated area is related to the volume of the wing storage box into which the sweep wing is incorporated.Figure 10 (b) shows the schematic of the heated area inside the wing storage box when the wing sweep is 55°.The surface thermal radiation coefficient is set to 0.85.The default temperature in the simulation environment is 295.15K.The temperature distributions of the internal structure of the two designs at 1000 s are shown in Figure 11.From Figure 11 (a), it can be observed that the CIS wing storage box has an uneven temperature distribution inside due to the heat mainly being transferred to the outside through the supporting webs, exhibiting a multi-arc arrangement.On the other hand, the NCIS wing storage box has a higher heat transfer efficiency due to the better thermal conductivity of the intermediate layer.And, the average temperature inside the NCIS wing storage box is lower than that inside the CIS wing storage box.Figure 12 shows the variation curve of the average temperature inside the wing storage box.The temperature distribution of the outside part of the CIS wing storage box and the NCIS wing storage box at different times is compared in Figure 13.From Figure 13 (a), it can be observed that the temperature distribution pattern of the outside part of the CIS wing storage box is influenced by the corrugated webs at 400 s.Analyzing the temperature nephogram in Figure 13 (b), it is evident that as the heat flux is continuously applied, the difference in the temperature range and distribution between the outside part of the CIS wing storage box and the NCIS wing storage box becomes more pronounced.At 1000 s, the average temperature on the outside part of the NCIS wing storage box is 791 K, while the average temperature on the outside part of the CIS wing storage box is 554 K.The insulation structure reduces the average temperature on the outside part of the wing storage box by 30%, as shown in Figure 14.Taking the midpoint position on the outer surface of the wing storage box as the reference for surface temperature, the temperature at that point decreases from 938 K under NCIS conditions to 593 K, with a reduction of 36.7%, as shown in Figure 15.From the comprehensive analysis of Figures 13-15, it can be concluded that the corrugated insulation intermediate layer structure has a good heat insulation effect.The outer temperature of the CIS wing storage box is much lower than that of the NCIS wing storage box.This demonstrates that using a corrugated insulation structure for the wing storage box can effectively alleviate the problem of heat transfer to the inside during the variable sweep process of high-temperature wings, thereby preventing direct heat diffusion into the aircraft's interior and avoiding high-temperature damage.

Thermal transfer study of corrugated insulated wing storage box during variable sweep
The variable-sweep angle of this hypersonic aircraft design ranges from 45° to 75°.When the wing sweep angle is set to the initial state of 45°, it does not enter the interior of the wing storage box, and the influence on the internal temperature environment of the aircraft can be neglected.During the variable sweep process, the volume of the wing that is incorporated into the wing storage box varies with different sweep angles, resulting in changes in the heat transfer area of the wing storage box.For the entire variable sweep process with a sweep range of 30 degrees, three key sweep angles are selected for thermal simulation calculations: 55°, 65°, and 75°.The thermal protection characteristics of the corrugated insulated wing storage box during the variable sweep process are analyzed.12 4.3.2.Analysis of web temperature in the wing storage box.For the insulation layer of the wing storage box, silica aerogel exhibits excellent insulation performance, with a thermal conductivity much lower than that of the supporting webs.Therefore, the heat inside the wing storage box is mainly transferred to the metal outer layer through the supporting webs.The temperature distribution of a section of arcshaped webs is shown in Figure 17.The lower surface of the webs faces heat transfer from the interior of the wing storage box, resulting in higher temperatures, up to 1370 K.However, the temperature gradually decreases along the vertical direction of the web cross-section, and the thermal insulation capability of carbon-carbon composite materials effectively mitigates further heat diffusion to the outside.Figure 18 shows the variation curve of the maximum temperature of the webs in the CIS wing storage box with a sweep angle of 55°.In the first 200 s, the heat flux remains constant, resulting in continuous heating of the wing storage box and a rapid increase in the temperature of the webs.From 200 s to 800 s, the heat flux gradually decreases, but the web temperature reaches its maximum value of 1840 K at 370 s.After that, the temperature gradually decreases.Through the analysis of heat conduction in the CIS wing storage box at three different sweep angles, it is found that the distribution of high-temperature regions in the wing storage box is significantly influenced by the wing sweep angle.When the wing sweep angle changes, the corresponding hightemperature region increases.Due to the design of the corrugated insulation layer in the wing storage box, the average temperature of the metal part is reduced to within 600 K, effectively improving the high-temperature issue of the variable-sweep wing.Therefore, the arc-shaped corrugated insulation structure enables the wing storage box to maintain stable and excellent thermal insulation performance, within the design range of variable-sweep angles from 45° to 75°.Through the outstanding thermal protection performance of the CIS wing storage box, hypersonic aircraft can effectively utilize variablesweep technology to enhance its flight performance, giving it practical and reference value.

Conclusion
This study conducts a CFD analysis on a hypersonic variable-sweep aircraft.To address the heat dissipation issue of the high-temperature wing during the variable sweep process, a corrugated insulated wing storage box is established, and its thermal protection characteristics are analyzed.The following conclusions are drawn: (1) For hypersonic aircraft, variable-sweep technology can effectively reduce aerodynamic drag and enhance the maneuverability of the aircraft, thereby extending the flight range and endurance.
(2) The corrugated insulation structure (CIS) can effectively reduce the heat transfer efficiency of the wing storage box and has good insulation performance.The average temperature of the outside part of the corrugated insulated wing storage box is reduced to below 600 K. Compared to uniform heat transfer of the non-corrugated insulated (NCIS) wing storage box, the average temperature is reduced by 30%.
(3) The high-temperature region is correlated with the wing sweep angle.Within the entire range of 45°-75° sweep angles, the corrugated insulated wing storage box has good thermal insulation properties.
(4) The design of the corrugated insulated wing storage box provides internal thermal protection for the overall structure of the variable-sweep wing, offering important insights for the further application of hypersonic variable-sweep aircraft.

Figure 1 .
Figure 1.Variable sweep process of the aircraft.Figure 2. CFD mesh of the aircraft semi-model.

Figure 2 .
Figure 1.Variable sweep process of the aircraft.Figure 2. CFD mesh of the aircraft semi-model.
Figure 3 shows the relationship between the lift-to-drag performance of the aircraft and the wing sweep angles at an angle of attack of 8 degrees.As the sweep angle increases, the lift-to-drag ratio of the aircraft continuously decreases.For the flight condition of 2 Mach, 20 kilometers, and 8 degrees angle of attack, the wing sweep angle needs to be less than 55 degrees to obtain a lift of 20 kN to meet the weight balance.After the aircraft accelerates to 8 Mach at an altitude of 30 kilometers, a range of 45-75 degrees for the sweep angle can meet the lift requirement.But as the sweep angle increases and the windward area of the wing decreases, the aerodynamic drag of the wing can be significantly reduced.When the sweep angle increases to 75 degrees, the overall drag of the aircraft can be reduced by 24%.On one hand, under the condition of constant engine thrust, reducing the flight drag can improve the maximum climb rate of the aircraft, thereby enhancing the aircraft's maneuverability.On the other hand, the required thrust and fuel consumption are related to flight drag.Reducing drag can increase the flight range and endurance time.Hypersonic aircraft can use variable-sweep technology to effectively enhance the flight performance of the aircraft.

Figure 3 .
Figure 3. Relation between lift-drag performance of aircraft and sweep angle.

Figure 4 .
Figure 4. Temperature distribution of the wing surface.
Figure 7 (b) presents the schematics of a noncorrugated insulation structure (NCIS), where the intermediate layer does not use supporting webs and silica aerogel, but instead uses carbon-carbon composite material internally, allowing heat to be directly conducted uniformly through the interior and the intermediate layer to the outer metal.The calculated heat transfer duration is set to 200 s.The internal surface of the model is subjected to a thermal load with a constant heat flux of 2 1 W/mm .The thermal radiation coefficient on the internal surface is set to 0.85, while the remaining surfaces are treated as adiabatic walls.The non-corrugated insulation structure follows a similar approach.(a) Corrugated insulation structure (CIS) (b) Non-corrugated insulation structure (NCIS)

Figure 8 .
Figure 8. Temperature distribution of the two insulation models.

Figure 9 .
Figure 9. Average temperature variation curve of metal outside part.

Figure 10 .
Figure 10.(a) The history of the applied thermal load; (b) The area with applied heat flux.
To compare and analyze the thermal insulation performance of the wing storage box using a corrugated insulated structure (CIS wing storage box), similar thermal simulation calculations are performed on the wing storage box structure of the non-corrugated insulation model (NCIS wing storage box).The size and shape of the NCIS wing storage box are kept consistent with the CIS wing storage box.The comparison is made for the case where the wing fully incorporates into the wing storage box with a maximum sweep angle of 75 degrees.

Figure 11 .
Figure 11.Temperature distribution of inside part of CIS and NCIS wing storage box.

Figure 12 .
Figure 12.Comparison of average temperature of inside part of CIS and NCIS wing storage box.The temperature distribution of the outside part of the CIS wing storage box and the NCIS wing storage box at different times is compared in Figure 13.From Figure13(a), it can be observed that the temperature distribution pattern of the outside part of the CIS wing storage box is influenced by the corrugated webs at 400 s.Analyzing the temperature nephogram in Figure13(b), it is evident that as the heat flux is continuously applied, the difference in the temperature range and distribution between the outside part of the CIS wing storage box and the NCIS wing storage box becomes more pronounced.At 1000 s, the average temperature on the outside part of the NCIS wing storage box is 791 K, while the average temperature on the outside part of the CIS wing storage box is 554 K.The insulation structure reduces the average temperature on the outside part of the wing storage box by 30%, as shown in Figure14.Taking the midpoint position on the outer surface of the wing storage box as the reference for surface temperature, the temperature at that point decreases from 938 K under NCIS conditions to 593 K, with a reduction of 36.7%, as shown in Figure15.

Figure 13 . 10 Figure 14 .Figure 15 .
Figure 13.Temperature distribution of outside part of CIS and NCIS wing storage box.

1 .Figure 16 .
Figure 16.Temperature distribution of inside part of CIS wing storage box.

Figure 17 .
Figure 17.Temperature distribution of the webs of CIS wing storage box.

Table 1 .
Properties of carbon-carbon composite material.

Table 2 .
Properties of high-temperature nickel-based alloys.

Table 3 .
Properties of silica aerogel.Research on performance of the corrugated insulation structureIn order to explore the thermal protection performance of the insulated wing storage box, this study first conducts a thermal conduction analysis of the insulation properties of the wing storage box.ANSYS transient module is used to solve the differential equation for general three-dimensional heat conduction problems.During the aircraft's 8 Mach cruise, the wing remains at a high temperature, and heat is transferred to the interior of the wing storage box.Assuming a constant heat flux of 2 1 W/mm and a surface radiation coefficient of 0.85, with an initial ambient temperature of 295.15 K, the radiative equilibrium temperature can be calculated as 2134.5 K by using Equation (1).