## 7 Conclusions  
The design of propulsion systems for HAPSs presents a complex challenge, closely linked to the extreme environmental conditions characterizing the upper atmosphere. Low air density, along with strict constraints on weight and power, necessitates an innovative and highly engineering-focused approach.  
Currently, for low-speed applications, propulsion systems employing propellers remain the most efficient propulsors. This review has shown that optimizing propeller performance is essential to enhancing the endurance and payload capacity of stratospheric platforms. Traditional propeller design methods often result in suboptimal solutions because they neglect Reynolds number effects under HAPS operating conditions.  
After a concise review of the main propulsion systems used on stratospheric platforms and their evolution over time, this study delves into propeller theory, design methodology, and performance estimation for such applications, emphasizing how design methodology is crucial for maximizing efficiency at these altitudes. Optimization results reveal common trends, such as maximizing propeller diameter to improve efficiency and distributing chord length with maximum values between 60% and 80% of the blade span. A significant limitation in the design process is the considerable computational time required. The most effective approach to designing a high-efficiency propeller involves integrating 3D CFD simulations within each step of the iterative process, although this can be time-consuming. To reduce computational effort, employing potential 2D codes or limiting the use of 3D CFD simulations may be viable alternatives, albeit with some loss in accuracy. Among the methodologies proposed in the literature, promising results have been achieved using a multi-fidelity optimization technique which combines low-fidelity BEMT with high-fidelity CFD. Additional methods to enhance performance, such as incorporating Gurney flaps and accounting for wind speed in the design process, have demonstrated significant potential compared to traditional propeller design techniques, which were originally developed for low-altitude propulsion systems. Moreover, various methodologies have been presented for evaluating the performance of high-altitude propellers, ranging from numerical analyses to experimental testing. Among these, scaled-model experiments stand out as particularly advantageous, offering greater reliability and practical insight compared to CFD simulations alone. These experimental approaches also provide essential validation data for advanced optimization techniques, which, from a practical perspective, offer concrete benefits for future HAPS developments. Improved aerodynamic efficiency can directly enhance mission endurance and payload capacity, while simultaneously reducing energy requirements. Furthermore, coupling optimization algorithms with probabilistic wind models could lead to adaptive propeller designs tailored to specific mission profiles and geographic locations. Future research should aim to integrate these optimization methods into early-stage design workflows of HAPSs, supported by experimental validation under low-Reynolds number conditions, to enable robust and cost-effective development of next-generation stratospheric propulsion systems.  
**Author Contributions:** E.R.: Data curation, Investigation, Writing, Original draft preparation. F.A.: Data curation, Investigation, Writing, Original draft preparation. V.R.B.: Supervision, Reviewing and Editing. D.C.: Supervision, Reviewing and Editing. D.C.: Supervision, Reviewing and Editing. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflicts of interest.