The Experimental Investigation of a Rotor Hover Icing Model with Shedding

Icing environments have long been an issue for rotorcraft. Flight in icing conditions is fraught with operational hazards, including reduced vehicle performance and large torque rises. Shedding of ice from blades due to centrifugal forces poses a ballistics danger to the aircraft and creates large vibrations due to imbalanced rotors. Modeling the effects of accreted ice on rotorcraft flight performance has been a challenge due to the complexities of periodically changing conditions as well as spanwise variations of angle of attack, velocities and surface temperatures. A new icing model has been developed to model the ice accretions across a rotor in hover. A shedding module is used to predict the shedding events and station. This model was correlated with published ice shapes for both small and full scale ice accretion results. The shedding module was evaluated based upon experimental results in the newly developed Adverse Environment Rotor Test Stand (AERTS). Favorable comparisons have been made between ice thickness, impingement limits and ice shape, especially at inboard stations. Shedding behavior was also evaluated, but required correction factors to improve test data correlation. Further investigations are required, but the icing model has demonstrated the ability to predict rotor icing trends. INTRODUCTION A critical operational problem for rotorcraft is flight in icing conditions. Ice accretes on critical components of rotorcraft, such as rotor blades, engine inlets, windscreens, and empennage surfaces, when combinations of temperatures close to freezing and high supercooled water concentrations are encountered (Ref. 1). Accreted ice can severely reduce aircraft performance, rendering safe flight impossible. The effects of a highly modified flowfield resulting from accreted ice include slightly reduced sectional lift coefficient and modified pitching moment characteristics, as described in References (2) (4). The most significant effect of icing, however, is the increase in blade profile drag (Ref. 5). Ice does not typically accrete in a uniform coating on the rotor; rather a rough and sometimes jagged structure is formed at the leading edge, causing premature flow separation and a considerable reduction in rotor lift/drag ratio. The torque can increase rapidly as described in Reference (6), potentially reaching the transmission or engine limits (Ref. 1). This trend has been observed both in flight and in experiments in icing wind tunnels (Ref 7, 8). Another problem of rotorcraft icing is the due to the high rotational speed of the rotor. The system generates high centrifugal forces on accreted ice, especially near the tips. This provides a natural de-icing capability, which is theoretically beneficial for helicopter. The problem arises when the ice does not shed in a symmetric fashion from all blades. Uneven ice accretion causes rotor imbalance and subsequent severe vibrations. The transmission or engine torque limits can be exceeded due to the fluctuating drag, making maintaining a given flying condition impossible for the pilot (Ref. 1). Furthermore, researchers have identified problems with damage to the fuselage, engines, empennage and tail rotors from high velocity shed particles. Larger, multiengine vehicles, Presented at the American Helicopter Society 66th Annual Forum, Phoenix, AZ, May 11-13, 2010. Copyright © 2010 by the American Helicopter Society International, Inc. All rights reserved. especially those of tandem rotor configuration, are more susceptible to this hazard (Ref. 9). The icing problem is compounded by the fact that helicopters are more susceptible to icing than a fixed wing aircraft of similar gross weight. Due to performance and mission limitations, operations occur almost exclusively at low altitudes, where the atmosphere may contain supercooled water droplets (Ref. 10). Furthermore, rotorcraft operations are typically time sensitive. The hovering capability of the vehicle allows it to participate in search and rescue, medical evacuation and critical supply missions, in which the urgency requires rotorcraft to fly in Instrument Meteorological Conditions (IMC). This proximity of rotorcraft and high concentration of liquid water generate a high probability of icing, as described in Reference (11). Significant effort has been invested in understanding ice accretion phenomena to rotor systems to improve rotorcraft flight safety. In recent years, effort has been focused on simulations, so that the expensive icing flight trial duration can be reduced (Ref. 12). Ice accretion on rotors and its subsequent effect on helicopter performance cannot be analyzed in the same manner used to explain ice accretion on the leading edges of a fixed wing aircraft (Ref. 9). Rotor icing predictions are made more complex by periodic changing of angle of attack and velocity. In addition, spanwise surface temperature differences and thinner airfoils make all rotor blade icing analysis challenging. If these items are properly accounted for, the resulting model can provide insight into the effects of various conditions and designs on the icing performance on rotor configurations. Correct shape prediction allows for the development of icing flight envelopes and for the proper sizing and implementation of ice protection systems. Finally, the model can be directly applied to the certification process, for interpolation or extrapolation of icing flight trial results (Ref. 12). Current efforts to model rotor ice accretion are limited either by significant code complexity or more importantly by a lack of available validation data. The lack of data may be a result of the existence of few icing facilities focusing on rotorcraft research. With the closing of the Canadian National Research Council (NRC) Helicopter Spray Rig, only two primary facilities remain. Ice protection system and other component level testing can be accomplished at a number of icing wind tunnels, but these tests are often limited by tunnel velocity and the fact that the centrifugal forces inherent to rotor rotation are not represented. Complete rotor icing studies can be undertaken in NASA Glenn’s Icing Research Tunnel (IRT), but model rotor diameters are limited to 6 ft by the test section (Ref. 13). Full scale studies may be accomplished with the Helicopter In-flight Spray System (HISS), but detailed icing shapes are difficult to acquire. Test aircraft need to descend through layers of warm air and may shed ice prior to landing. This often causes ice shedding, which aside from being inherently dangerous, eliminates the possibility of ice shape data collection. RESEARCH OBJECTIVE The primary objective of this research was to develop a rotorcraft icing model that includes the effects of shedding for the ultimate application to ice protection system sizing studies. The second objective was to validate the model based upon published icing results, including both full and small scale. Finally, the third objective was to correlate the icing model accretions and shedding behavior to experiments performed in a new icing facility. The current research is the first attempt to correlate predicted and experimental ice shapes in the new facility. ARISP MODEL The Vertical Lift Research Center of Excellence at the Pennsylvania State University has developed a new icing facility for rotorcraft icing research. Achieving an initial operational capability in November 2009, the Adverse Environment Rotor Test Stand (AERTS) is designed to generate an accurate icing cloud around test rotor. The primary mission of the facility is to evaluate new ice protection system concepts and test new “ice phobic” materials. Efforts are currently underway to fully calibrate the lab for ice accretion model validation. The AERTS Facility required an ice accretion model to assist in calibration of the icing cloud. The primary objective of the model is to support basic rotor icing research. If the model can accurately predict impingement limits and shedding location and frequency, it will be useful for ice protection system sizing studies by identifying the required location and relative power density of such systems. As both the facility and model grew in complexity, it became the AERTS Rotor Icing, Shedding and Performance Model (ARISP). In addition to calculating ice accretion on a test rotor, the model accounts for ice shedding and torque rises. The AERTS specific corrections in the code can be disabled and the model can be used as a simple supplement to more complex icing codes such as those currently under development under the current “High Resolution CFD Analysis of Rotorcraft Rotor Icing” NRA program. These models require a full flow field solution and rotor dynamics analysis as well as an ice accretion calculation. The model is based around NASA’s LEWis ICE Accretion program (LEWICE). This industry standard program has been extensively validated in the Icing Research Tunnel (IRT) at NASA Glenn Research Center and is described in Reference (14). The overall process is similar to the model developed by Britton (Ref. 15), but performance degradation is modeled empirically using coefficients derived by Flemming after extensive wind tunnel analyses (Ref. 16). Like most rotorcraft icing models, the ARISP code differs from a standard fixed wing icing analyses in the fact that conditions are changing as time passes. Sectional angles of attack may change due to ice accretion and/or ice may shed from the rotor. Spanwise variations kinetic heating, numbers of particles encountered and centrifugal loads are also present. As such, the icing analysis must be broken up into a number of steps, with the ice profile and performance constantly being updated. A sectional approach is taken, with rotor performance and ice accretions calculated at various rotor stations. Rotor performance is determined with a Blade Element Momentum Theory (BEMT) routine, chosen for its speed of calculation a