By the beginning of the 21st century, more researchers started working on icing-related issues of UAVs. Most of them were concentrated on studying the sensitivity of nature and the shapes of accreted ice to environmental and geometric conditions. The nature of the ice forms depends primarily on free stream velocity (V∞), atmospheric temperature (T∞), Liquid Water Content (LWC), the droplet Median Volume Diameter (MVD), angle of attack (α), Reynolds number (Re), and the geometry of the structure (airfoil, chord, mean effective camber and material). In this section, an attempt is made to review the studies by various researchers to understand the influence of these parameters on the ice accretion behavior of UAVs at low Reynolds number values. The results presented in this section are mainly based on numerical studies using various ice accretion simulation codes.

The effect of atmospheric temperature (T∞) on ice accretion of a UAV was studied by Koenig et al. in 2001 [63] and Szilder in 2011 [25,64]. The nature of the ice changed from glaze to rime with a mixed ice behavior in between with a decrease in T∞. In both these studies, the influence of T∞ is studied independently by keeping the MVD and LWC value constant. However, the parameters are no longer independent when the studies are based on the FAR 25 Appendix C icing envelope. Such studies were done by Szilder in 2015 [65], 2017 [66], Krøgenes and Hann in 2017 [57,67], and Fajt in 2019 [68] on different airfoil geometries. The observations were qualitatively similar to Szilder’s initial studies that the low temperatures favor rime ice formation and glaze formed at a temperature near the freezing point with mixed ice formed in between. For temperatures very close to the freezing point like T∞=−2 ∘C the heat transfer process is not high enough to freeze all the droplets upon impingement, therefore a part of the impinging droplets runback along the airfoil surface leading to the glaze ice formation. Whereas at very low temperatures of T∞=−10 ∘C and above, most droplets freeze upon impingement leading to rime ice formation. Additionally, it was observed that the rime ice formed at lower temperatures (−30 ∘C) is just a scaled-up version of the same at higher temperatures (−10 ∘C), this is due to the low value of the LWC at higher negative temperatures as defined in the FAR 25 Appendix C icing envelope. Fajt also pointed out that the ice mass increases with temperature.

The combined effect of LWC and MVD on ice accretion of UAV was studied by Koenig et al. [63] in 2002. The ice shape changes from smooth rime ice with no oblique protrusion or horns to glaze ice with two enormous horns when the LWC and MVD values increase. Additionally, a considerable rise in the amount of ice deposited is observed. In 2011, Szilder studied the influence of LWC independently on the ice accretion behavior by keeping the MVD or LWC and T∞ constant [25,64]. For temperature greater than −8 ∘C, the ice shape changes from rime to mixed (glaze ice formed at the stagnation leading edge and rime ice formed in the downstream region) and then to glaze ice with water run back along the surface with an increase in LWC. Szilder [64] also pointed out that an increase in the size of the droplet tends to diminish the parametric space for rime but increases the space for the glaze. When the investigations are done based on FAR 25 Appendix C icing envelope [55,64,65,66,67], the ice accreted is a function of T∞, MVD and LWC values. Smaller MVD values correspond to large LWC values and favored glaze ice formation. When the LWC is large, the latent heat that must be removed in order for the impinging droplets to completely freeze is also large, thus favoring glaze ice. However, T∞ also plays an important role here as the potential to remove the latent heat increases at lower temperatures. Fajt [68] observed that when the LWC is less than 0.2 g/m3, the ice formed is always streamline shaped (rime) irrespective of MVD and T∞. When Li [69] studied the effect of LWC and MVD based on the FAR 25 icing envelope in 2020, it was observed that as the value of MVD increases, the size of the ice horns decreases because of the reduction in the LWC. With further increase in MVD, the presence of a horn has completely vanished, and ice accreted in a streamlined fashion, increasing the effective camber and enhancing the aerodynamic performance. From the analysis of droplet trajectories, Szilder inferred that the larger droplets have higher inertia and have a slower response to the spatial variations in the flow. It increases the collision efficiency and leads to a greater impingement extent. At temperatures close to freezing point, the heat transfer rate is not high enough to freeze all the water droplets leading to a glaze ice formation with water runback. Additionally, the smallest droplets result in the greatest local maximum ice thickness (especially at the leading edge) due to the large value of LWC as per the Appendix C icing envelope. Similar observations were also made by Avery [26]. However, a contradicting observation is made by Cistriani in 2007 [70] as a part of his studies on the design of Low Re

airfoil for the Falco UAV. The formation of the worst ice shape at Continuous Maximum (CM) conditions with a low value of LWC compared to Intermittent Maximum (IM) conditions with higher LWC values is mentioned. Since this study was not dedicated to icing issues, much information regarding the software used and ice shapes formed at the remaining conditions is unavailable. Koenig [10] in 2003 pointed out that the clustering of the LWC value also has an effect on the ice accretion based on his icing tunnel test with constant and variable rate LWC.

The ice accretion behavior at high Re has been widely studied in the literature but a direct transformation of these results to low Re is not possible due to the problems discussed in Section 1. Therefore, Szilder in 2011 [64] studied the effect of Re on the ice accretion behavior by conducting numerical icing studies at two different Re of 5×106 and 5×104 [25,64]. The Reynolds number is varied by changing the values of V∞ and C. The nature of the ice changes from rime to mixed and then to glaze with an increase in LWC for low Re. Whereas the nature of the ice remains glaze for all the values of LWC at high Re

with only the extent of ice increasing with LWC. Further the thickness of the ice is considerably smaller when compared to the low Re number cases. The reduction in the thickness and decrease in the rate of icing can be attributed to the high values of aerodynamic heating at higher velocities. Low Reynolds favors the formation of rime ice which results in less aerodynamic penalties, but at the same time increases the relative ice thickness making the total ice mass high.

In 2015, Szilder et al. compared the UAV ice accretion behavior at cruise conditions with α=3∘ 

Anti-icing and de-icing systems are the two types of Ice Protection Systems (IPS). The anti-icing mode heats the structure surface continually to prevent ice formation and can be done in two ways: fully evaporative or running wet. There can be no water run back in the first mode because the heat supplied entirely evaporates the droplet at the impingement location. However, it requires considerable heat flux and results in high surface temperatures. In the latter option, the heat is only used to keep the water from freezing. This mode requires less heat flux and lower surface temperature, but the surface must be heated extensively to prevent droplet freezing during water runback. In de-icing, ice is allowed to accumulate on the surface, and heat is used to melt it. The ice-surface interface can be melted or shed by aerodynamic forces. Dei-icing uses less power, but the accumulated ice might cause drag and structural damage during ice shedding. The problem of ice accretion on manned aircraft is extensively investigated, and suitable ice mitigation techniques are developed [19,93]. Traditional aircraft anti-icing methods use heat from engine bleed air, electro-thermal systems, or freezing point depressant chemicals to prevent ice buildup. Electromechanical systems like pneumatic de-icing boots, electro-expulsive systems, and electro-impulsive systems are typically used as de-icing systems. These systems are sophisticated, power-hungry, and require regular maintenance, limiting their use in smaller aircraft or UAVs.

Proper ice detection techniques need to be developed for the successful operation of an IPS. A brief overview of such systems designed for UAVs is done here before going to the review of an IPS. Botura [94] developed an Ice/No-Ice Sensor System (INISS) based on impedance measurement technique for in-flight ice detection on UAVs in 2003. The sensors are capable of detecting even thin coatings of ice. Cristofarao et al. (2015) proposed a Multiple Model Adaptive Estimation (MMAE)-based ice detection system for UAVs [95]. Unknown Input Observer (UIO) methods detect and estimate ice using changes in equilibrium caused by ice accumulation. A UIO-based ice detection method for UAVs with linearized longitudinal motion is proposed by Cristofaro et al. in 2015 [96]. This method is extended to the longitudinal nonlinear aircraft dynamics using Linear Parameter Varying (LPV) methods by Rotondo in 2015 [97]. Seron in 2015 [98] coupled the UIO and MMAE methods to develop an ice detection system with prespecified accuracy. An icing detection method based on the diagnosis of lift and drag changes on a UAV wing was proposed by Sørensen et al. in 2015 [99] and was modified by Wenz et al. in 2016 [100]. Rotondo developed an icing detection system that provides information about the icing location in 2017 [101]. The author published an extension of this work in 2019 [102]. A Fault Detection and Isolation (FDI) framework that uses model-based estimators of the various faults, implemented with multiple Kalman and Bayes filters, was proposed by Haaland et al. in 2021 [103].

Electromechanical de-icing systems shed the ice by applying impulse force on the wing surface with the help of some actuators. In 1988, Leonard Haslim invented the Electro Expulsive Separation System (EESS) consisting of two conductors embedded in a flexible material glued to the wing’s leading edge. The magnetic fields of the conductors repel each other when current flows through them and provide enough impact to melt the ice [104]. Bhakta proposed a magnetostrictive de-icing method for UAVs in 2005 [105]. When the magnetostrictive materials are magnetized, they exhibit a change in length and create an impulse action. The ability of piezoelectric crystals to strain under an applied electric field is called the piezoelectric effect. The waves, generated through piezoelectric patches bonded onto the inner surface of the wing skin, cause a shear action at the ice/skin interface [106]. The magnitude of this shear action is amplified at specific excitation frequencies (Lamb waves), corresponding to wave lengths commensurable with the skin thickness. In 2014, Ameduri et al. [107] investigated the use of lamb waves as dei-icing systems for UAVs. The ultrasonic sound waves propagating at low frequency into a material can cause vibrations and debonding [108]. Shape Memory Alloy (SMA) on heating can change shape and create force through a martensitic phase shift [109].

Carbon Nano Tubes (CNT) have excellent electrical and thermal conductivity, making them ideal for de-icing UAVs using the Joule heating property of conductors [110,111,112,113,114,115]. Buschhorn et al. [110] in 2013 developed a CNT-based IPS, which can prevent icing with a power supply of only 1 kw/m2

during less harsh icing conditions. In 2015, Sørensen et al. developed an electrically conductive paint based on graphene and carbon black nanoparticles that heat up when electricity passes through it [116]. A disadvantage of this system is the power consumption and increase in the weight of the UAV because of the coating, battery, and other components. Thermodynamic analysis [117] and flight testing [118] of the IPS are performed in successive years. The icing conditions at the high airspeed and low ambient temperature necessitate significant power consumption. In 2020, Idris et al. [115] used extrusion printing to fabricate electrical contacts on carbon fibers. The carbon fiber composites are then integrated into UAV wings to create a self-heating de-icing mechanism. The carbon-based materials can shield the radio frequency signals, so an RF transmitting heat was proposed by Hwang et al. in 2020 [119].

Hann et al. [88] evaluated LEWICE with FENSAP ICE for simulating and estimating anti-icing loads in 2019. A sudden increase in the heat flux is observed at the transition region as the rate of convective heat transfer increases due to higher turbulence. The studies reveal the necessity of proper transition modeling for the icing studies at low Reynolds numbers. Hann [120] then tested Sørensen’s IPS mounted on a wing with RG-15 airfoil for ice detection, anti-icing, and completely autonomous capabilities. The stagnation point at the leading edge has the highest power requirements. The author also concluded that the average heat flux numbers might not be a fair estimate of overall necessary heat loads, but the peak values around the leading edge are more significant. Hann et al. [120] tested the anti-icing, de-icing, and Parting Strip (PS) approach for ice mitigation on the same IPS in 2021. In the PS method, the stagnation zone at the leading edge is continuously heated to avoid ice formation, separating the ice on the upper and lower part of the airfoil. The ice split increases the aerodynamic force on the ice and allows for better shedding, as shown in Figure 6. The parting strip model requires substantially less shedding time and heat flux than conventional techniques. Thus, the author considers the parting strip mode as the most energy-efficient ice mitigation mode.

Superhydrophobic coatings can reduce the surface free energy and can generate hierarchical micro/nano-structured roughness. This can reduce the ice adhesion strength on the surface and thus prevent ice accumulation. Numerous studies related to different superhydrophobic coatings and the applicability of the same as an aircraft anti-icing material are done by Bhushan et al. [121,122,123,124] and Farzaneh et al. [23,125,126,127,128,129]. Ice accretion behavior of three different aluminum surfaces (hydrophilic, hydrophobic, and superhydrophobic) were compared by Wang in 2010 [130]. The research focuses on transmission line anti-icing; however, the findings can also be used for UAV ice mitigation. Initially, just a few portions of the superhydrophobic sample were coated with water droplets, whereas the hydrophobic was partially and the hydrophilic was entirely covered. Water droplets turned into the ice with increasing spraying time; however, no new ice crystals developed on superhydrophobic surfaces. Ice covered more of the hydrophobic surface and the whole of the hydrophilic surface with time. Thus, super hydrophobicity reduces surface wettability by increasing the average water contact angle to over 150∘. The same for hydrophobic surfaces of more than 90∘ and for hydrophilic surfaces, it is less than 70∘. Piscitelli [131] in 2020 proposed a superhydrophobic coating for small aircraft that can reduce the surface free energy by 99% with respect to the reference considered in the study. Additionally, the proposed coating was effective at temperatures as low as −27 ∘C.