1. Introduction
In recent years, unmanned aerial vehicles (UAVs), also known as drones, have seen significant growth and are increasingly being developed across various sectors. These autonomous aerial systems exhibit versatility through various categorizations, catering to specific applications within different domains. Fixed-wing UAVs are effective for conducting remote sensing tasks in remote and extreme environments like the Arctic [
1]. Nonetheless, deploying UAVs in cold climates poses distinctive challenges due to the potential freezing of supercooled droplets upon contact with aerofoil surfaces. Such a process can alter the wing profile, leading to aerodynamic degradation. Atmospheric icing represents a primary concern for fixed-wing UAVs operating in cold weather conditions [
2,
3]. This phenomenon impacts UAVs and has implications for commercial aviation, wind energy infrastructure, and architectural integrity. To delve into this exploration, it is essential to recognize the historical trajectory of UAV icing.
The development of UAVs can be traced back to World War I, when they were used for tactical purposes [
4], but it was not until the late 1920s that research formally addressed the wing icing of aerial vehicles [
5,
6,
7]. Hazards due to ice formation on aerofoil were first reported by Carroll and McAvoy for a VE-7 aircraft in 1929 [
5], and the aerodynamic performance degradation caused by surface roughness and protuberances was investigated by Jacobs [
6] and Jones [
7] in the early 1930s. In 1942, the icing research tunnel (IRT) was built by the National Advisory Committee for Aeronautics (NACA) at the Lewis Flight Propulsion Laboratory [
8]. From 1953, Gary and Von [
9,
10,
11] performed a sequence of tests in which ice was formed under meticulous conditions. The shape of the ice accumulation was recorded, along with alterations in lift, drag, and pitching moment. With the advancement of computational power, early research on computational fluid dynamics (CFD) emphasized computing the flow characteristics and performance of aerofoils with large glaze ice horns [
12]. These calculations were based on the NACA 0012 aerofoil and were compared to experimental data that were available at the time. Potapczuk [
13] was one of the pioneers in conducting these calculations by employing a thin-layer, Reynolds-averaged Navier–Stokes (RANS) method within a body-fitted curvilinear coordinate system. The outcomes of this assessment thus far demonstrate a favorable correlation with experimental findings, particularly in the context of rime and glaze ice formations.
The ATR-72 accident in late 1994 changed the direction of aerodynamic icing research [
14]. Since the cause of the accident was believed to be a unique supercooled large droplet (SLD) ice accretion, unlike any that had been studied up to that point, there was renewed interest in investigating different ice accretion shapes and critical ice accretions [
12]. The accident also increased interest in testing the sensitivity of different aerofoil sections to icing. As a result, iced-aircraft safety and aerodynamics became a significant research topic, leading to the discovery of many new avenues of study and several intriguing findings. However, until the 21st century, most research on wing icing was primarily centered around manned aerial vehicles. Koenig et al. [
15] published an early paper on fixed-wing icing, in which they conducted UAV icing flight simulations using LEWICE2D to predict the rates of ice accumulation and ice formations on an unspecified aerofoil. The formation of ice on aerodynamic surfaces, caused by supercooled cloud droplets, freezing drizzle, and freezing rain, poses a threat to all aircraft, producing a particularly significant issue for UAVs due to their limited power [
15]. The icing phenomenon on UAVs shares similarities with that observed in manned aircraft, albeit characterized by distinctive variations in airframe dimensions, mission profiles, and ice accretion sensitivity [
16]. A significant amount of research is available on the icing of manned aircraft at high Reynolds numbers [
17,
18,
19,
20], ranging from
to
. Nevertheless, this study makes an original contribution to research on low-Reynolds-number, fixed-wing ice accretion and their aerodynamics performance within a temperature range of −10 °C to −30 °C—an area that has been notably underexplored.
Table 1 provides a concise overview of pertinent studies focused on aerofoil investigations at a low Reynolds number from
to
.
The collection of aerodynamic data related to aircraft icing primarily relies on methods including flight tests, wind tunnel experiments, and computational approaches [
27,
28]. The former two methods are less frequently chosen by researchers, considering the substantial cost implications and inherent technical complexity [
28]. For instance, the wind tunnel icing test, despite being indispensable for evaluating running-wet ice protection systems (IPSs) under design scenarios, necessitates complicated scaling laws to ascertain the efficiency of water droplets [
29]. Consequently, an increasing number of researchers favor using computational techniques [
24,
25,
30]. This preference stems from the practicality of computational methods in predicting ice formation, designing IPSs, formulating ice removal strategies, and assessing the decrease in aerodynamic performance caused by icing incidents [
27,
28,
29]. In recent years, researchers have been actively developing innovative IPSs to enhance aircraft performance while addressing concerns regarding weight, cost, and energy efficiency [
31,
32,
33,
34]. These endeavors encompass promising strategies such as integrating anti-icing systems into the primary aircraft structure [
35,
36,
37]. Nevertheless, the scenario diverges when considering smaller-sized UAVs. Such platforms often lack dedicated de-icing or anti-icing systems [
24,
34,
38,
39]. Hann et al. [
34,
39] emphasizes the need for standardized unmanned aircraft system (UAS) certification procedures, particularly addressing the underexplored threat of in-flight icing, especially for medium-sized UAVs with a focus on energy-efficient operation procedures for IPS. In accordance with Szilder and McIlwain’s findings [
40], UAVs are commonly configured for low-altitude cruising due to their operation at reduced flight altitudes. Opting for low-altitude cruising inherently involves lower flight velocities, consequently heightening the potential for surface icing occurrence. Consequently, an augmented comprehension of the influences exerted by icing-accreted aerodynamic degradation for small-size UAVs becomes imperative.
The investigation pertaining to UAVs underscores that Reynolds numbers exert a significant influence on the attributes of icing phenomena and the consequent aerodynamic performance detriments that ensue [
41,
42]. Research activity was undertaken for RG-15 aerofoil by Williams et al. [
23] in 2017 at constant temperatures of −5 °C and −20 °C. The present simulated ice shape was validated through a comparison with the physical one upon the aerofoil. Several existing codes for ice accretion have been developed in the last decade, such as LEWICE [
43], FENSAP-ICE [
44], CIRAML [
45], TRAJICE 2 [
46], and ONERA [
47]. The aerodynamic performance under icing conditions was simulated at specific altitudes and varying temperatures to accurately replicate ice formation in humid environments, such as marine and polar regions. Therefore, FENSAP-ICE, which employs the Eulerian method [
44], was selected due to its feasibility of solving the governing equation using the same grid for airflows and supercooled droplets. The droplet characteristics within FENSAP-Ice were aligned with the reference conditions outlined in the Federal Aviation Regulation (FAR) Appendix C to Part 25 [
48]. This alignment was informed by the fact that the entirety of the investigated icing scenarios in this study were predicated upon assumed conditions conforming to the aforementioned standard. The contribution of this study is an extension of the previous research efforts. Oo et al. [
24] conducted an examination of aerodynamic deterioration in the context of ice-accreted RG-15 aerofoil configurations, utilizing ice shapes sourced from Williams et al. [
23], with investigations performed at −5 °C. In 2023, Lindner et al. [
25] conducted an in-depth numerical inquiry that spanned a temperature spectrum ranging from −2 °C to −6 °C. This temperature interval is noteworthy due to its encapsulation of the critical phase transition from glaze ice to rime ice. The validation of these numerical outcomes was carried out by Hann et al. [
26]. Consequently, this study expands the scope of the investigation to encompass a temperature range spanning from −10 °C to −30 °C, a range pertinent to ceiling altitudes ranging from 1200 m to 6700 m [
48]. This study fills the research gap within this range and underscores the significance of its role as a valuable contribution to the field.