Analysis of a Hovering Rotor in Icing Conditions

Abstract A high fidelity analysis method is proposed to evaluate the ice accumulation and the ensuing rotor performance degradation for a helicopter flying through an icing cloud. The process uses computational fluid dynamics (CFD) coupled to a rotorcraft comprehensive code to establish the aerodynamic environment of a trimmed rotor prior to icing. Based on local aerodynamic conditions along the rotor span and accounting for the azimuthal variation, an ice accumulation analysis using NASA’s Lewice3D code is made to establish the ice geometry. Degraded rotor performance is quantified by repeating the high fidelity rotor analysis with updates which account for ice shape and mass. The process is applied on a full-scale UH-1H helicopter in hover using data recorded during the Helicopter Icing Flight Test Program. the analysis approach and then offers a description of the Introduction Whether a helicopter is used for emergency rescue, military missions, or commercial business there are strong desires to operate them safely in all climate conditions. Operating helicopters in icing conditions is particularly dangerous because rotor blades are especially susceptible to ice growth due to their small chord. Quick accumulating ice on rotor systems leads to increased vibration, rapid loss of lift, and a large power increase to sustain flight. Shed ice from spinning rotors is common and creates hazardous projectiles. Ice protection systems can guard against adverse effects of ice accumulation. These systems require certification for commercial use and qualification for military applications. Both involve extensive flight tests which are expensive and time consuming. Several icing seasons are usually required to cover the boundaries of the flight envelope. Model-scale and full-scale laboratory tests assist in the certification or qualification, but scaling effects can complicate the interpretation of results. A highly accurate ice accumulation and rotor performance degradation prediction system can help in understanding rotorcraft behavior and limitations in adverse weather. Analytic results can guide flight test planning and add a layer of safety during icing trials. Furthermore the system can assist in reliably extrapolating model scale testing results to full-scale flight conditions. This could result in less flight tests for certification or qualification. The approach taken to develop an icing analysis system for rotors leverages methods developed for fixed-wing aircraft. Bidwell (Ref. 1) coupled ice accretion analysis with computational fluid dynamics (CFD) to evaluate ice on a high-lift wing configuration. He used Lewice3D (Ref. 2) to evaluate droplet trajectories, heat transfer, and ice growth while relying on OVERFLOW (Ref. 3 )or CFD++ 4 for aerodynamic analysis. OVERFLOW is well suited for aerodynamic assessments of rotorcraft and therefore Bidwell’s approach is a natural point of departure for an icing analysis system for rotors. A high-fidelity ice analysis system for helicopters is the subject of this paper.The paper begins with an overview of process to generate an aerodynamic solution a trimmed forrotor in forward flight. The process to predict ice on the rotor blades utilizes information from the rotor flow field is and described next. Analysis of the iced rotor concludes the rotor icing analysis process. The process is described for a helicopter in forward flight but is applied to a hover situation which represents the first step in validating the method. Following the results of the hover analysis are conclusions.