Experimental and Numerical Investigation for a Rotor Blade Impact and its Post-impact Response = 실험과 수치해석을 통한 로터 블레이드 충돌 응답 특성 및 충돌 후 회전익기 거동 평가

다국어 초록 (Multilingual Abstract)

A helicopter rotor blade impact, as specified in MIL-STD 1290A, poses a catastrophic risk to rotorcraft airworthiness. This dissertation presents a comprehensive analytical approach for evaluating the time-transient response of a rotorcraft, bifurcating the crash event into the following two distinct phases: “during impact” and the subsequent “post-impact” scenarios. Since a direct experimental replication of the crash event is unrealistic, an analytical method will be needed to simulate a crash event correctly, while assessing the analytical result against the experiment.
The dynamic response of a composite rotor blade subject to an impulse load is investigated in an experimental way. An improved impact testing apparatus is developed to accurately measure the impulse response of the rotor blade. The improved design is validated by comparing the measured absorbed energy against that determines solely from the displacement measurement. Following the numerical analysis, the validity of the analytical result is evaluated by comparing it against the test result. The present analytical results are used to simulate the impact situation of a rotating blade and the resulting reaction at each hub component.
The hub imbalance after the impact is evaluated using a whirl tower test facility. An analytical approach is developed to ensure the safety of the test on a dissimilar rotor (outer 10% span removed). To ensure the credibility of such approach, the finite element representation of the rotor stand and hub, and the free wake analysis results are used for each phase. The safety of the rotor stand is assessed based on the oscillatory displacement of the shaft, as specified in ISO 7919-3. Finally, the present analytical approach and the derived reaction are applied to evaluate the hub loads for a full-scale UH-60A rotorcraft beyond the impact. The analysis reveals that during forward flight, aerodynamic dissimilarity becomes dominant as the flight speed increases.

국문 초록 (Abstract)

MIL-STD 1290A에 명시된 헬리콥터 로터 블레이드 충돌은 회전익 항공기의 감항성에 치명적인 위험을 초래한다. 본 논문은 회전익 항공기의 충돌 상황을 “충돌 중”과 “충돌 후”의 두 단계로 구분하여 동적 응답을 종합적으로 규명하고자 하였다. 규정과 동등한 수준의 충돌 상황을 실험적으로 재현하는 데에는 많은 어려움이 따른다. 따라서 해당 충돌 상황을 모사하기 위한 종합적인 해석 기법과 각각의 해석을 실험적으로 평가하는 것이 필수적이다.
우선, 충돌 하중을 받는 복합재 로터 블레이드의 동적 응답을 실험적으로 구현하였다. 블레이드의 충돌 응답을 정확히 계측하기 위해 개선된 충돌 시험 장치를 제안하였으며, 변위 기반으로 산출한 흡수 에너지와 실측 결과와 비교하여 그 타당성을 검증하였다. 아울러 수치해석을 수행한 후, 시험 결과와 비교하여 해석 결과의 타당성을 평가하였다. 이러한 해석 결과는 회전하는 실규모 UH-60A로터 블레이드의 충돌 상황을 모사하는 데에 활용하였으며, 그로 인해 각 허브 구성품에 작용하는 하중을 평가하였다.
충돌 이후 발생하는 허브 불균형은 훨 타워 시험설비를 활용하여 평가하였다. 이를 위해10% 스팬을 제거한 블레이드로 인해 발생하는 허브 불균형 발생 시, 안전한 회전시험 조건을 도출하는 해석적 접근법을 제안하였다. 해당 접근법의 신뢰성을 확보하기 위해 로터 허브의 유한요소 모델과 자유후류 해석 결과를 각 단계에서 평가하였으며, 이후 ISO 7919-3에서 제시한 축의 진동 변위를 기준으로 로터 스탠드의 안전도를 평가하였다. 마지막으로, 해당 해석적 접근법을 바탕으로 충돌 해석 결과를 활용하여 충돌 후 실규모 UH-60A 회전익기의 허브 하중을 평가하였다.

목차 (Table of Contents)

• Abstract i
• Contents iii
• List of Tables vi
• List of Figures vii
• Chapter 1 Introduction 1
• 1.1 Background 1
• 1.2 Literature Survey 2
• 1.2.1 Rotorcraft crash analysis 2
• 1.2.2 Impact response of a composite rotor blade 3
• 1.2.3 Influence of the dissimilar rotor 5
• 1.3 Motivation 7
• 1.4 Scope and Objectives of Thesis 10
• 1.5 Outline of Dissertation 13
• Chapter 2 Impact Response Analysis of a Composite Rotor Blade against Experimental Result 15
• 2.1 Rotor Blade Design 15
• 2.1.1 Material properties identification 16
• 2.1.2 UH-60A rotor blade design 19
• 2.2 Impactor Design 21
• 2.3 Impact Response Measurement using the Test Facility 22
• 2.4 Limitations of Impulse Load Measurement using the Test Facility 24
• 2.5 Improvement of the Test Apparatus 27
• 2.6 Methodology for Validating an Impulse Load Measurement 31
• 2.7 Comparison of test results 34
• 2.8 Analytical Specifications 41
• 2.8.1 Penalty method using Node-to-Surface (NTS) approach 41
• 2.8.2 Numerical setup 42
• 2.9 Comparison Between the Numerical and Experimental Results 46
• 2.10 Crash Analysis of a Rotating Rotor Blade 51
• 2.10.1 Analysis specifications 51
• 2.10.2 Influence of crash event on hub components and impactor 55
• Chapter 3 Analysis of Hub Loads induced at Dissimilar Rotor Blades against Experimental Result 60
• 3.1 Whirl Tower Test on a Dissimilar Rotor 60
• 3.2 Preparation for the Whirl Tower Test 66
• 3.2.1 Blade properties 66
• 3.2.2 Calibration of the blade loads 77
• 3.3 Identification of the Potential Test Condition 79
• 3.3.1 Determination of the nominal rotational speed 79
• 3.3.2 Rotor stand mode analysis 81
• 3.3.3 Determination of the potential test matrix 84
• 3.4 Methodology for Evaluating the Rotor Stand Safety under the Hub Unbalance 85
• 3.4.1 Finite element representation of the rotor system 86
• 3.4.2 Correlation of the mode frequencies and shapes against test result 88
• 3.4.3 Aerodynamic loads analysis 93
• 3.4.4 Safety assessment of the rotor stand under damaged-blade whirl tower test 96
• 3.5 Influence of the Damaged Rotor Blade on the Hub Unbalance 99
• 3.5.1 Comparison of baseline and damaged-blade experimental results 99
• 3.5.2 Comparison of the experimental and analytical hub loads 102
• 3.5.3 Influence of the rotor stand on the hub unbalance 117
• 3.6 Analysis of Post-Impact Rotorcraft Response 125
• 3.6.1 Analysis specifications 125
• 3.6.2 Influence on hub loads resulting from the hub unbalance 129
• 3.6.3 Influence on the hub components resulting from the hub unbalance 133
• Chapter 4 Conclusion 136
• 5.1 Summary and Contribution of the Thesis 136
• 5.2 Limitations and Recommendations for Future Works 141
• References 143
• 국문초록 159