Abstract
The load-bearing antenna approach for state-of-the-art aero-vehicle structure has been made for military aircraft in recent studies. This study presents the structural design and development of multiband aero-vehicle smart skin antenna which has been developed as a simplified structured load-bearing antenna panel compared with recent studies for aircraft. A multifunctional concept of aircraft structure combines structural and electrical functions to single structural component. The structural function of multiband aero-vehicle smart skin antenna is load-bearing member of aircraft, and its electrical function is antenna for communication and navigation of aircraft. The radar cross section and drag reduction could be achieved by using sandwich structure and composite material. Through sequential design and development process, multiband aero-vehicle smart skin antenna successfully demonstrated the design, fabrication, and structural integrity of a load-bearing multifunction antenna component subjected to flight load conditions. This study concentrated on the computational analysis using finite element to validate the structural design of multiband aero-vehicle smart skin antenna structure. In addition, structural test results were briefly introduced to compare with the analysis results. The prototype of multiband aero-vehicle smart skin antenna was fabricated and tested for the verification of each analysis within the desirable tolerance.
Introduction
Load-bearing antenna for military aircraft has been the item of recent research by a number of researchers. Aircraft antenna could be categorized into three levels: (1) Level 0—conventional antenna that has protruded shape on aircraft outer mold line (OML), (2) Level 1—conformal non-load-bearing antenna that constitutes aircraft OML, and (3) Level 2—conformal load-bearing antenna that constitutes aircraft OML. After successful substitution of Level 1 over Level 0 in military aircraft, Level 2 technology has been selected for experimental use and partially deployed in military aircraft (Callus, 2007). Lockyer et al. (1997, 2001) have done Level 2 antenna demonstration on military aircraft.
In this study, the structural design and development of multiband aero-vehicle smart skin antenna (MASSA) have been introduced. When an antenna is loaded in the skin of aircraft, radar cross section (RCS) and drag of the aircraft could be greatly decreased by removing protruded shape. So, MASSA adopts antenna impregnated load-bearing panel for high-performance stealth aircraft. The MASSA is an essential technology for the next-generation stealth aircraft. This overall study proposes MASSA that contains communication and navigation antenna embedded in the skin of aircraft.
This study carried out structural design and analysis on MASSA. Structural configuration adopted the sandwich-structured construction that consists of two outer skins and one core. Structural safety and stability evaluation on the proposed design was done by a commercial finite element analysis code, GENOA. Through design and development process, the final structural design was proposed to meet the design requirements of MASSA.
Structural design
Design outline
The design of MASSA is based on sandwich-structured composite construction for lightweight without sacrificing strength. As shown in Figure 1, face sheet and housing are the outer skin of sandwich construction that withstands tension and compression loading on MASSA. Especially, the face sheet is used as a radome of MASSA. The core supports shear loading and contains radiator that transmits and receives radio frequency (RF) signal for communication and navigation of the aircraft.
Configuration of MASSA.
Figure 1 is a configuration of MASSA for flight demonstration, its dimension (653.5 × 466.0 mm) and curvature shape are tailored for interface of target aircraft. But curved structure cannot carry compressive load, and 570 × 570-mm-flat MASSA panel was adopted for both structural analysis and test.
Design requirements
The design requirements of MASSA are axial/shear load, buckling, impact, and fatigue that simulate flight load conditions of the aircraft as shown in Table 1.
Design requirements of MASSA.
MASSA: multiband aero-vehicle smart skin antenna.
Typical running loads (axial = 315.2 kN/m and shear = 105.1 kN/m) on military aircraft’s dorsal region were applied to meet the various installation requirements on aircraft. These running loads represent almost all strength requirements on fuselage skin of military aircraft. No visible damage from a single 0.553 kg m impact and no functional impairment or water intrusion from 0.830 kg m impact were the specifications that are prescribed in JSSG-2006.
Fabrication process
According to the preliminary prototype fabrication, it was found that the radiator for RF transmission and reception was very susceptible against high temperature. Therefore, the fabrication process was modified by reordering of process.
The manufacturing process of MASSA could be summarized into two stages of curing process. First stage is co-curing process and second stage is secondary bonding process.
Co-curing process consists of a number of subprocesses: (1) the conductive mesh and face sheet are lay-up to mold, (2) the radiator and core are lay-up to face sheet, (3) the housing is lay-up to core, and (4) all parts are fabricated by co-curing. To prevent warping and crack in electrically sensitive area, radiator was substituted by dummy block. The release film was adopted face sheet and around dummy block for release of secondary bonding surfaces (refer to Figure 2).
Co-curing process.
Secondary bonding process consists of a number of subprocesses: (1) dummy block is substituted by radiator, (2) bonding paste is applied to face sheet and around radiator, and (3) released surfaces during co-curing are assembled by secondary bonding (refer to Figure 3). This two-stage fabrication process is established for protection of electrically sensitive area of MASSA.
Secondary bonding process.
Structural analysis
In order to evaluate the structural integrity, the design feature of MASSA was modeled as surfaces and solids from CATIA modeling after final design confirmation. In the finite element structural analysis of MASSA, the static stress analysis under axial/shear load, buckling analysis, durability, impact, and normal mode analysis were performed using the commercial finite element method (FEM) code NASTRAN, LS/DYNA, and GENOA that specialized in composite finite element analysis. Total number of nodes for FEM mesh was 10,549. Total number of elements was 12,812 including solid and shell mesh for MASSA (Figure 4).
Finite element model of MASSA.
Mechanical properties for analysis were obtained from coupon test results of face sheet and housing material. Test matrix and number of batches and coupons were based on MIL-HDBK-17, and each test was performed according to American Society for Testing and Materials (ASTM) standards (Table 2).
Mechanical properties of face sheet and housing.
Axial/shear load
The axial load requirement for MASSA was 315.2 kN/m running load, which is equivalent to 180 kN force. In structural test, axial load would be applied displacement control of universal tensile machine (UTM), 180 kN force was applied by displacement boundary condition for better correlation. As a result, 180 kN force was equivalent to 1.34 mm displacement boundary condition.
As shown in Figure 5, the maximum stress occurred on face sheet, and the margin of safety becomes 0.10. Because the margin of safety was over 3 except face sheet, the overall structural integrity was stable under axial load condition.
Axial load analysis of MASSA.
The shear load requirement for MASSA was 105.1 kN/m running load, which is equivalent to 42.4 kN diagonal force. In structural analysis and test, 42.4 kN was applied by picture frame jig for better correlation (Figure 6). The maximum stress occurred on face sheet, and the margin of safety becomes 3.14. Because the margin of safety was over 3, the overall structural integrity was stable under shear load condition. Through the analyses, the structural integrity of MASSA was verified under axial/shear load conditions.
Shear load analysis of MASSA.
Buckling
To check the structural stability of MASSA, buckling analysis under axial load condition was performed. Because the axial load condition was considered the most critical load condition, analysis similar to axial load analysis was done (Figure 7).
Buckling analysis of MASSA.
As a result, buckling occurs under 2.052 times compressive load condition. So, MASSA is structurally stable under design axial load condition.
Durability
The fatigue life of MASSA was estimated using GENOA based on the face sheet and housing material S-N test data. As shown in Figure 8, fatigue lifetime of 100,000 cycles was confirmed under design limit load of 120 kN. For conservative approach for safety, if the event of design limit load occurs 100,000 times (one design lifetime), it was verified that MASSA has enough fatigue life without repair during one design life. The estimated lifetime was 105 cycles under design limit load condition. So, it meets the requirements for MASSA.
Durability analysis of MASSA.
Impact
For better transmission and reception of RF for radiator, the face sheet is relatively of thin area. Because it constitutes aircraft OML, the face sheet is susceptible to impact. Impact analysis was done to verify impact resistance characteristics under MASSA requirement. Under 0.553 and 0.830 kg m impact conditions, damage of each ply was checked to verify structural rigidity of face sheet. As a result, there was no damage under 0.553 and 0.830 kg m impact conditions (Figure 9).
Impact analysis of MASSA: (a) 0.553 kg m—no damage and (b) 0.830 kg m—no damage.
Normal mode
Through normal mode analysis, the possibility of resonance due to vibration caused by flight environment was investigated. To check the modal characteristics under unrestrained condition, free–free boundary condition was used for analysis. As a result, first mode of MASSA is higher than 200 Hz under free–free boundary condition, and possibility of resonance is very low under flight condition (Figure 10).
Normal mode analysis of MASSA: (a) first mode—206.9 Hz and (a) second mode—341.9 Hz.
Test
To verify the structural integrity of MASSA, structural test was performed by using a full-scale flat-type model. The full-scale model was set on the test jig and loaded by hydraulic actuator with load cell. Figure 11 shows the experimental setup of MASSA. Table 3 shows comparison results of strain values between the test and the analysis on the sensor point of face sheet under design limit load test (DLLT) and design ultimate load test (DULT) conditions. The structural test results were similar to analysis results, and MASSA withstood DULT as well as DLLT condition.
Test of MASSA.
Strain of face sheet (µε).
DLLT: design limit load test; DULT: design ultimate load test.
Conclusion
In this study, structural design, analysis, and test for proposed MASSA were performed. Structural design that adopted the sandwich-structured construction was applied to improve strength weight ratio as well as stiffness weight ratio.
The structural analysis results demonstrated that MASSA design has sufficient strength for given structural requirements. Commercial software such as NASTRAN, LS/DYNA, and GENOA was effectively used for the computational structural analyses. The prototype of MASSA was fabricated and tested for the verification of each analysis within the desirable tolerance.
Footnotes
Declaration of conflicting interests
The authors declare that there is no conflict of interest.
Funding
This research was supported by the Agency for Defense Development (ADD) grant funded by the Korean Government.