Internal Structural Design Optimization for a Medium-Class Helicopter through the Load Path Prediction = 하중전달 경로 예측을 통한 중형 회전익기 내부 구조물의 최적 설계

국문 초록 (Abstract)

본 논문에서는 하중전달 경로 예측에 기반한 중형급 회전익기 내부 구조물 최적설계 방법론을 제안하였다. 회전익기는 고속으로 회전하는 로터 블레이드로부터 상당한 공기역학적 모멘트, 양력 하중, 저주파 진동 하중이 로터 허브를 통해 기체 구조로 전달되므로, 이에 대한 충분한 구조 안전율을 확보하는 많은 최적설계 연구가 수행되었다.
그러나 고속 복합 회전익기는 추진체계, 동체 형상 및 주요 하중 조건이 기존 회전익기와 상이하여 전통적 설계 접근의 직접 적용이 어렵다. 이에 본 연구는 두께 분포 최적화에 더해 하중을 효율적으로 전달하는 구조 배치까지 함께 탐색하기 위해 하중전달 지수(load transfer index, 𝑈∗) 이론을 바탕으로 MATLAB–ANSYS 연계 프레임워크를 구축하고 UH-60A 항공기에 적용해 효용성을 검증하였다.
정적해석을 통해 초기형상의 하중전달 특성을 분석하고, 균일성 향상을 목표로 형상을 개선하였다. 그 결과, 응력 집중이 저감되고 내하중성이 향상된 구조 형상을 정량적 지표를 통해 도출할 수 있었다. 또한, N/rev 인근 모드에 대한 모드 기반 하중전달 해석을 수행하여 구조 고유 진동수와 추진체계 가진 주파수 간 공진을 효과적으로 회피하였다. 이를 통해 진동하중으로부터의 안전성을 확보하는 동시에 과잉 설계를 방지할 수 있음을 확인하였다.
제안된 프레임워크는 중형 회전익기 특성에 적합한 구조 설계 방안을 제공하며, 안전마진 확보와 공진 회피 등 주요 설계 요구조건 만족을 목표로 하였다. 더 나아가 새로운 하중조건을 요구하는 비행체의 개발에 실질적으로 기여할 수 있을 것으로 기대된다.

다국어 초록 (Multilingual Abstract)

This thesis introduces an internal-structure design optimization methodology for a medium-class rotorcraft based on the load path prediction. Because large aerodynamic moments, lift, and low-frequency vibratory loads generated by rapidly rotating blades are transmitted through the rotor hub to the airframe, many optimization studies have been attempted to ensure sufficient structural safety factors against those loads and vibrations.
However, high-speed compound rotorcraft have propulsion system, fuselage configuration, and major loading conditions that will be different from those for the conventional rotorcraft. Therefore direct application of the traditional design approaches will not be straightforward. Therefore, beyond the thickness optimization, the thesis will further explore efficient structural arrangement that transmits the relevant loads efficiently. A MATLAB–ANSYS integrated framework is developed on the basis of the load transfer index (𝑈∗) theory and is applied to the UH-60A aircraft to verify its effectiveness.
Static analysis is performed to evaluate the initial load transfer characteristics, and the configuration is refined with the goal of improving uniformity. As a result, reduction in the stress concentration and an enhancement in the load-carrying capability are obtained and confirmed in terms of the quantitative indicators. Furthermore, a modal load transfer analysis is carried out near N/rev frequency to effectively avoid the resonance between the main rotor excitation frequencies and the natural frequencies of the primary vibration modes. These results indicate that structural safety against vibratory loads will be improved while preventing unnecessary over-design.
The proposed framework provides a practical design approach tailored to the characteristics of a medium-class rotorcraft, ensuring the safety margin and resonance avoidance, and it is expected to contribute to the development of an aircraft that requires more realistic loading conditions

목차 (Table of Contents)

• Abstract i
• Contents iii
• List of Tables v
• List of Figures vi
• List of Symbols viii
• Chapter 1 Introduction 1
• 1.1 Motivation and Background 1
• 1.2 Previous Research 4
• 1.2.1 Structural Configuration by the DAMVIBS Program 4
• 1.2.2 Structural Optimization Approaches for a Rotorcraft 7
• 1.2.3 Load Transfer Based Evaluation and Visualization Methods 10
• 1.3 Research Objectives and Thesis Outline 13
• Chapter 2 Load Transfer Index Methodology and Framework 15
• 2.1 Load Transfer Index Theory 15
• 2.1.1 Representation using the Index U* 15
• 2.1.2 Representation using the Index U** 22
• 2.2 Load Transfer Framework and Example Analysis 26
• 2.2.1 Load Transfer Framework 26
• 2.2.2 Load Transfer Example Analysis 28
• Chapter 3 Initial Fuselage Layout and Analysis Condition 30
• 3.1 Initial Fuselage Layout and Automation 30
• 3.1.1 Initial Fuselage Layout 30
• 3.1.2 Automated Structural Layout Generation 34
• 3.2 Initial Conditions and Flight Loads 37
• 3.2.1 Modeling of Internal Equipment Mass 37
• 3.2.2 Flight Loads and Boundary conditions 42
• Chapter 4 Static Load Path Analysis Results 45
• 4.1 Static Load Transfer Analysis under Flight Loads 45
• 4.1.1 Static Load Path Analysis Results 45
• 4.1.2 Uniformity Path Assessment 49
• 4.2 Layout Modification and Thickness Optimization 51
• 4.2.1 Structural Layout Modification 51
• 4.2.2 Formulation of the Optimization Task 58
• 4.2.3 Comparison of the Optimization 62
• Chapter 5 Mode Shape and Load Path Analysis Results 66
• 5.1 Modal Analysis Results and Load Path Analysis 66
• 5.1.1 Modal Analysis of the Optimized Configuration 66
• 5.1.2 Mode Shape and Load Path Analysis 68
• 5.2 Layout Modification and Results 72
• Chapter 6 Conclusions 76
• 6.1 Contributions 76
• 6.2 Future Work 77
• References 80
• 국문초록 87