Icing Detection for Small Fixed Wing UAVs using Inflight Aerodynamic Coefficient Estimation

In cold and humid environments, airfoil icing is a major hindrance to UAV operations. Airfoil icing increases the aerodynamic drag coefficient, while reducing the maximum lift coefficient and the stall angle. This results in degraded endurance and safety of an UAV. Recently de-icing solutions for fixed-wing UAVs have been developed. These solutions use resistive heating in order to melt the ice on the wings. However, since this requires a high amount of energy it is desirable to only heat the wings if significant icing occurs. In this paper, a method for automatic icing detection is presented. A moving horizon estimator (MHE) is used, which combines aerodynamic, kinematic and stochastic wind models with data from a typical autopilot sensor suite to estimate angle of attack and lift coefficients. The sensor suite consists of an inertial measurement unit (IMU), a global navigation satellite system (GNSS) receiver, a heading reference and a pitot-static tube. Within the MHE an Unscented Kalman Filter (UKF) is used for arrival cost approximation. FENSAP icing simulations show that in severe icing conditions, both the offset and the gradient of the lift coefficient change. Based on these icing simulations an UAV flight simulator that can simulate icing has been used. Simulation results show that the MHE is capable of monitoring changes in offset and gradient of the lift coefficient due to icing. A faster convergence to the estimated coefficient values could be achieved when using an external trigger signal, i.e. from a temperature and humidity sensor, to reset the covariance matrix of the arrival cost. We also investigate the effect on convergence speed resulting from an altitude change giving additional excitation. The estimation results show angle of attack estimation errors below 1 degree. These estimates can be used to limit the angle of attack and adjust the commanded airspeed in the autopilot in order to avoid stall

Ice Accretion on Fixed-Wing Unmanned Aerial Vehicle—A Review Study

Ice accretion on commercial aircraft operating at high Reynolds numbers has been extensively studied in the literature, but a direct transformation of these results to an Unmanned Aerial Vehicle (UAV) operating at low Reynolds numbers is not straightforward. Changes in Reynolds number have a significant impact on the ice accretion physics. Previously, only a few researchers worked in this area, but it is now gaining more attention due to the increasing applications of UAVs in the modern world. As a result, an attempt is made to review existing scientific knowledge and identify the knowledge gaps in this field of research. Ice accretion can deteriorate the aerodynamic performance, structural integrity, and aircraft stability, necessitating optimal ice mitigation techniques. This paper provides a comprehensive review of ice accretion on fixed-wing UAVs. It includes various methodologies for studying and comprehending the physics of ice accretion on UAVs. The impact of various environmental and geometric factors on ice accretion physics is reviewed, and knowledge gaps are identified. The pros and cons of various ice detection and mitigation techniques developed for UAVs are also discussed