Design, aerodynamic evaluation, and flight test of inflatable-winged aircraft

Abstract

This project presents a comprehensive investigation into the feasibility, aerodynamic characteristics, and performance enhancement strategies of inflatable-winged aircraft, with a specific focus on their application to airborne-deployable high-altitude long-endurance (HALE) unmanned aerial vehicle (UAV) systems. The research is motivated by the inherent structural fragility and deployment limitations of conventional high-aspect-ratio stratospheric aircraft during tropospheric transit. To address these challenges, inflatable wings are explored as a lightweight, compactly foldable structural solution compatible with existing airborne and rocket-based launch platforms, enabling direct deployment into near-space environments.

First, this study addresses system-level feasibility through the design, fabrication, and flight validation of two inflatable-winged UAV prototypes with distinct configurations: a single-fuselage layout equipped with external trailing-edge control surfaces, and a twin-fuselage configuration featuring fully movable control surfaces. Inflatable wings were manufactured using various materials and subjected to comprehensive ground testing evaluations. Two inflatable-winged UAV prototypes, each employing a different configuration and manipulation method, were designed, assembled, and subjected to basic low-altitude flight tests to assess the feasibility of their aerodynamic layouts and control characteristics. The results demonstrated that a segmented wing design with a multi-boom configuration is particularly well-suited for inflatable wings. Furthermore, both proposed control methodologies were validated in flight and shown to deliver effective, reliable control authority.

During prototype development and flight testing, surface undulations and wrinkles inherent to inflatable wings were identified as the primary source of aerodynamic performance degradation. To elucidate the underlying flow physics, systematic wind-tunnel experiments and high-fidelity computational fluid dynamics (CFD) simulations were conducted on both smooth and fully undulated inflatable airfoil configurations at moderate Reynolds numbers ($Re$ in $3\times10^5 \sim 7\times10^5$), representative of low-speed inflatable-winged UAV operational conditions. The results reveal a fundamental trade-off between aerodynamic efficiency and stall behavior: fully undulated surfaces significantly degrade lift and increase drag across the linear aerodynamic regime, but stabilize and delay stall onset, transforming abrupt leading-edge flow separation into a gradual, trailing-edge-dominated stall process.

To mitigate the aerodynamic performance degradation of inflatable wings induced by inherent surface undulations, a Sobol’ global sensitivity analysis (GSA) based aerodynamic enhancement framework was developed and applied to both the cambered NACA 4318 and symmetric NACA 0018 airfoils with tailored regional undulated surface profiles. By evaluating first-order and total-order Sobol’ indices across a range of angles of attack, this study demonstrates that aerodynamic performance is predominantly governed by localized surface regions, with limited higher-order interaction effects between sections. The upper leading-edge region is identified as the critical aerodynamic zone governing lift, drag, and stall characteristics. In contrast, controlled undulations confined to the lower surface, particularly near the trailing edge, can enhance lift production with only moderate drag penalties. Guided by the GSA results, several targeted aerodynamic enhancement strategies were formulated, which deliver significant performance improvements relative to the fully undulated inflatable airfoil.

Finally, informed by these sensitivity analysis findings, selective aerodynamic enhancement configurations were validated through dedicated wind-tunnel testing, including a quantitative assessment of fabrication-induced trailing-edge loss. The experimental results confirm that targeted regional smoothing significantly restores aerodynamic efficiency in the linear regime, while selective lower-surface undulations can outperform the smooth baseline airfoil in lift generation under specific operating conditions. Furthermore, trailing-edge loss segment is shown to generally increase the surface pressure differential for most configurations, with the exception of the fully undulated airfoil, where its influence is negligible.

Overall, this work establishes a closed-loop research methodology that integrates prototype flight demonstration, region-resolved aerodynamic characterization, and sensitivity-driven design enhancement. The findings deliver both fundamental insights into the aerodynamic behavior of inflatable wings and actionable design guidance for inflatable-winged UAV systems. Moreover, this research demonstrates that strategically implemented partial surface undulations can function as an effective passive flow control mechanism for conventional wings operating in moderate-$Re$ regimes. The results also delineate the key technical limitations of inflatable wing technology, namely fabrication precision constraints, material stiffness limits, and gust sensitivity, thereby defining clear research priorities for future materials, manufacturing, and flight control studies aimed at advancing inflatable wing technology towards full operational application.

Date of Award	15 Nov 2026
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Yixiang Xu (Supervisor), Xinmin Chen (Supervisor) & Donglei SUN (Supervisor)