## 4 Results
### Thematic Analysis  
Figure 4 displays a set of word clouds of patent titles across the seven categories and one combined category. These word clouds highlight the core focus areas of innovation within each invention category. In the category _fuel cell integration_, prominent terms such as \"clean electric\", \"lightweight\", and \"power density\" reflect efforts to embed compact, efficient, and fault  
Figure 4: Word cloud of patent titles across invention categories. tolerant fuel cells into aircraft architectures. The category of _fuel delivery_ emphasizes terms like \"solid storage\", \"intercooled turbine\", and \"engine device\", consistent with innovations in modular refueling, safe cryogenic handling, and efficient reactant delivery. The category of _fuel management_ centers on \"high altitude\", \"electric energy\", and \"oxygen supply\", highlighting the real-time optimization of energy flow, altitude-based control, and hybrid system coordination. In the category of _fuel storage_, terms like \"blended wing\", \"cryogenic\", and \"cell device\" reflect the structural integration of tanks and systems that enhance thermal stability and aircraft balance. The category of _hybrid propulsion_ features terms like \"hybrid electric\", \"solid oxide\", and \"cell assembly\", highlighting powertrain flexibility and the integration of advanced fuel cells. The category of _turbine enhancement_ includes \"steam injected\", \"turbine engine\", and \"combustion chamber\", pointing to efficiency gains via thermal recovery and steam cycles. The category of _safety enhancement_ is marked by terms like \"leak detection\", \"pressurized\", and \"prevention\", emphasizing hazard mitigation and sensor-based controls. The _Combined Categories_ word cloud blends features from all the categories, reinforcing cross-functional innovations with terms such as \"steam injected\", \"hybrid electric\", and \"intercooled turbine\". Together, these word clouds validate the relevance of these patents to hydrogen-powered aviation and confirm the quality of their classification into the seven categories. Figure 5 shows a term co-occurrence network generated from the TF-IDF transformation of patent claims. The size of a circle indicates the relative frequency of a term in the corpus, and the thickness of the arcs indicates the relative frequency of the co-occurrence. The colors indicate that the terms cluster into three main term relationships: red (control, power, and cell), green (engine, turbine, and compressor), and blue (tank, fluid, and pressure). The red cluster reflects patents in the categories of fuel cell integration, fuel management, and hybrid propulsion, emphasizing energy control, electric power, and cell-based systems. The green cluster aligns with the categories of turbine enhancement and hybrid propulsion, centered on combustion, compression, and propulsion components. The blue cluster represents the categories of fuel storage and fuel delivery, focusing on tanks, fluid flow, and cryogenic or pressurized systems. Central linking terms like \"supply\", \"heat\", and \"flow\" confirm the cross-functional nature of many patents. The distinct but interconnected clusters validate the relevance of the selected documents and the robustness of the category classification. This is an efficient method of visualizing all the topics in the huge corpus of patent claims. This visualization supports the quality of the dataset and confirms the key technical concepts distributed across the identified innovation domains. The themes identified through word clouds and the co-occurrence network reveal not only discrete areas of technical innovation but also an ecosystem of interdependent subsystems. For example, advancements in fuel storage directly impact the effectiveness of fuel delivery and fuel management, since pressurization and thermal stability are prerequisites for consistent reactant flow and control.