In both scientific and industrial fields, there has been a notable increase in attention toward High-Altitude Pseudo-Satellites (HAPSs) in recent years. This surge is driven by their distinct advantages over traditional satellites and Remotely Piloted Aircraft Systems (RPASs). These benefits are particularly evident in critical areas such as intelligent transportation systems, surveillance, remote sensing, traffic and environmental monitoring, emergency communications, disaster relief efforts, and the facilitation of large-scale temporary events. This review provides an overview of key aspects related to the propellers and propulsion systems of HAPSs. To date, propellers remain the most efficient means of propulsion for high-altitude applications. However, due to the unique operational conditions at stratospheric altitudes, propeller design necessitates specific approaches that differ from those applied in conventional applications. After a brief overview of the propulsion systems proposed in the literature or employed by HAPSs, focusing on both the technical challenges and advancements in this emerging field, this review integrates theoretical foundations, historical design approaches, and the latest multi-fidelity optimization techniques to provide a comprehensive comparison of propeller design methods for HAPSs. It identifies key trends, including the growing use of CFD-based simulations methodologies, which contribute to notable performance improvements. Additionally, the review includes a critical assessment of experimental methods for performance evaluation. These developments have enabled the design of propellers with efficiencies exceeding 85%, offering valuable insights for the next generation of high-endurance, high-altitude platforms.

In both scientific and industrial fields, there has been a notable increase in attention toward High-Altitude Pseudo-Satellites (HAPS) in recent years. This surge is driven by their distinct advantages over traditional satellites and Remotely Piloted Aircraft Systems (RPAS). These benefits are particularly evident in critical areas such as intelligent transportation systems, surveillance, remote sensing, traffic and environmental monitoring, emergency communications, disaster relief efforts, and the facilitation of large-scale temporary events. This review provides an overview of key aspects related to the propellers and propulsion systems of HAPS. Firstly, an analysis of the propulsion systems proposed in literature or employed by HAPS is presented, focusing on both the technical challenges and advancements in this emerging field. Given that propellers remain the most efficient propulsor for such applications, the discussion then shifts to the fundamental principles of propeller theory, followed by an overview of innovative design methodologies proposed in literature for propellers intended for high-altitude operations and a discussion concerning methodologies for evaluating their performance. The unique atmospheric conditions at high altitudes result in distinct characteristics for the propellers of stratospheric airships compared to conventional ones.

The Italian Aerospace Research Centre (CIRA) is developing an unmanned stratospheric platform for Earth observation and telecommunications, known as a High-Altitude Pseudo-Satellite (HAPS). This paper presents an aerodynamic and stability analysis of a new closed-wing HAPS configuration. The design uses a hybrid approach, combining aerodynamic and aerostatic forces to achieve weight balance, with the stability analysis accounting for the buoyancy force applied at the center of volume of the structure. Following the initial design phase, which aims for an altitude of 20 km, a speed of 16 m/s, and a payload capacity of 20 kg, a suitable configuration using a NACA 0018 airfoil is selected. The aircraft lift–drag curve is evaluated using a stationary, incompressible Reynolds-Averaged Navier–Stokes (RANS) analysis with a k-$\omega$ SST turbulence model in OpenFoam. A detailed longitudinal and lateral-directional stability analysis is also conducted using OpenFOam and AVL software.

This work presents a computational-fluid-dynamics-based methodology for identifying acceleration-dependent aerodynamic derivatives and equivalent added-mass effects in hybrid buoyant--lifting vehicles for high-altitude pseudo-satellite (HAPS) applications. The approach combines unsteady Reynolds-averaged Navier--Stokes simulations of prescribed harmonic rigid-body motions with Fourier-based system identification to extract both velocity- and acceleration-dependent aerodynamic coefficients.The methodology is first validated on a prolate spheroid, for which analytical potential-flow solutions are available. The CFD-identified acceleration derivatives show very good agreement with theoretical predictions, supporting the accuracy of the proposed approach.The method is then applied to a hybrid closed-wing HAPS configuration to obtain a complete six-degree-of-freedom set of aerodynamic derivatives and equivalent added-mass coefficients. Owing to the lifting nature of the configuration, the acceleration-dependent terms cannot be interpreted as purely inertial contributions, but rather as equivalent terms including both fluid-inertial and unsteady aerodynamic effects.The results reveal significant translational and rotational contributions, together with non-negligible coupling terms arising from the interaction between buoyant volume and lifting surfaces. Comparison with potential-flow estimates confirms the overall structure of the equivalent inertial couplings, while highlighting deviations associated with the lifting character of the configuration.A simplified longitudinal flight-dynamics analysis shows that acceleration-dependent terms do not alter the qualitative stability characteristics of the HAPS, but significantly affect its modal properties, including natural frequencies and damping.