HIGH-ALTITUDE OPERATIONS OF UNMANNED AERIAL VEHICLES (UAVs): AERODYNAMICS, PROPULSION, AND SYSTEM-LEVEL PERSPECTIVES

High-altitude operations of unmanned aerial vehicles (UAVs) provide significant advantages suchas extended mission endurance, wide-area coverage, and persistent surveillance capability, therebyrendering Medium Altitude Long Endurance (MALE) and High Altitude Long Endurance (HALE)class platforms strategically important systems. However, the unique physical characteristics ofthe high-altitude environment, which include low air density, varying temperature and pressureprofiles, reduced Reynolds numbers, and moderate Mach numbers, also introduce substantialengineering challenges in terms of aerodynamic, propulsion, and overall system performance. Thisstudy addresses the effects of high-altitude operations on UAVs from a holistic engineeringperspective and aims to evaluate this subject within an interdisciplinary framework, which haspredominantly been examined in a fragmented manner in the existing literature. In this context,the physical characteristics of the high-altitude environment and their effects on aerodynamicforces, flow regimes, and Reynolds and Mach numbers are investigated based on their physicaland mathematical foundations. While the advantages of high-altitude flight, such as low dragpotential, high lift-to-drag ratios, long-endurance loiter capability, and wide-area coverage, arediscussed in detail, the associated disadvantages are analyzed from an engineering standpoint.These include boundary layer behavior, flow separation, reductions in thrust generation, thermalmanagement challenges, structural flexibility, and limitations in energy storage. In addition,engineering strategies aimed at mitigating these limitations are discussed within the framework ofaerodynamic design, propulsion and energy systems, and system-level optimization approaches,supported by computational fluid dynamics (CFD), response surface methodology (RSM), andmultidisciplinary optimization techniques. The present study demonstrates that high altituderepresents not only an operational opportunity for UAVs but also a multidimensional engineeringproblem that must be carefully managed. Accordingly, it provides designers and researchers with a physics-based, comprehensive, and guiding technical framework for the design and analysis ofhigh-altitude UAV systems.