Icing Encounters Research Articles

Modeling of iced rotor dynamics via CFD/CSD coupling

Purpose The purpose of this paper is to advance the multiphysics analysis of helicopter rotors under icing conditions by coupling the iced rotor’s aerodynamics, analyzed by CFD, with the rotor’s structural characteristics, analyzed by CSD. Design/methodology/approach The current work introduces supercomputer-based computational approaches capable of assessing the impact of ice accretion on the aerodynamics, blade dynamics, vibrations and loading of a rotorcraft. The rigid and elastic motions of the blades are accounted for through a loose coupling of the flow solver to a multibody dynamics solver. The coupling framework allows for comprehensive aeroelastic simulations of iced rotors in hover and in forward flight. Findings The flow and structural modules were validated on a full helicopter configuration in forward flight using the ROBIN experimental model. The tip structural deflections were in very close agreement with the experimental measurements. Research limitations/implications The results of the CFD analyses are limited by the available experimental results they can be compared to. In dry air CFD, three-dimensional (3D) experiments occur first and CFD is then compared to them; in icing, the opposite is true: 3D experiments (if they are ever done, as they are very expensive) chase CFD and sometimes never occur. Practical implications This paper presents an outline of how CFD and computational stress dynamics (CSD) analyses can be linked and provides a toolbox for deeper investigation of the complex flows over helicopters operating under difficult in-flight icing conditions. Social implications More and more helicopters are designed to be able to operate in hostile environments such as rescuing and saving lives over the oceans or mountains, conditions under which icing encounters cannot be avoided. Originality/value A loosely coupled CFD/CSD framework that accounts for the rotor blades structural response to aerodynamic loading and ice accretion in hover and forward flight has been presented. This versatile and cost-effective framework provides a more accurate estimation of the helicopter rotor performance and its degradation due to icing encounters during the early design stages than traditional CFD tools.

Probability assessment of possible ice encounters during the petroleum hydrocarbons transportation in the Sea of Okhotsk

The increase in freight traffic along the Northern Sea Route necessitates the study of the ice regime of the freezing seas of Russia, including the Far Eastern ones (Bering Sea, the Sea of Okhotsk and the Sea of Japan). This paper, on the basis of the analysis of the Sea of Okhotsk’s ice cover and the location of the edges of the Sea of Okhotsk’s ice massif, evaluates the probability of an encounter with ice for the ships in transit during cargo transportation on the Northern Sea Route through the Far Eastern seas along two basic routes from the southern tip of the Kamchatka Peninsula: 1) traverse Cape Lopatka–Sea of Okhotsk–La Perouse Strait; 2) traverse Cape Lopatka–Sangarsky Strait. It was shown that at the stage of ice cover maximum development in the Sea of Okhotsk (February-March), route No. 2 was the most adequate and the safest for ships of ice class Arc4 and below. The section of route No. 1 with a 70 % ice encounter probability was 421 km long in February and 382 km long in March. That section of the route was dominated by very cohesive first-year thin ice up to 70 cm thick with inclusions of first-year ice of average thickness (up to 120 cm).

Adaptive Neural Networks-Based Dynamic Inversion Applied to Reconfigurable Flight Control and Envelope Protection Under Icing Conditions

Aircraft icing can result in degradation of the aerodynamic characteristics and reduction of the control effectiveness, which would pose serious threats to flight safety. Reconfigurable flight control and envelope protection of iced aircraft have become an effective solution to ensure flight safety in icing encounters. In this study, the dynamic model of the iced aircraft was established, and high precision numerical simulation method and wind tunnel virtual flight experiment were applied to obtain the icing aerodynamic database. Furthermore, the reconfigurable flight control law was designed by using the adaptive neural networks based dynamic inversion (ANN-DI) control method. Simulation results demonstrate that the control method behaves well tracking performance and strong robustness in the presence of modeling errors and control surface damage. After that, an icing envelope protection system was designed based on control limiting strategy and the ANN-DI control was applied to calculate the control surface deflection limits based on the limit values of the key flight safety parameters. Finally, the designed icing envelope protection system has been verified through simulation in two autopilot modes under icing conditions. The simulation results obtained here show that the system could keep the key flight safety parameters such as the flight speed, the angle of attack (AOA), the side slip angle, and bank angle within the flight safe region under icing conditions. The method proposed in this study is expected to provide flight safety measures for in-flight icing.

Testing of Elastomer Icephobic Coatings in the AIWT: Lessons Learned

<div class="section abstract"><div class="htmlview paragraph">A study has been conducted into icephobic properties of some highly durable “off-the-shelf” elastomer materials using a rotating ice adhesion test rig installed in the NRC’s Altitude Icing Wind Tunnel. This enabled the formation of ice at environmental conditions similar to those experienced during in-flight icing encounters. Initially, the tests indicated some very positive results with ice adhesion shear stress as low as 8KPa. On further examination, however, it became apparent that the test preparation process, in which the samples were cleaned with an ethanol alcohol solution, influenced the results due to absorption and prolonged retention of the cleaning fluid. The uptake of the ethanol alcohol solution by the elastomer was found to be a function of the surface temperature and remained absorbed into the coating during the ice accretion process changing the characteristics of the coating in such a way that led to a reduction in the ice/surface bond strength.</div></div>

Stall Warning Using Flap Hinge Moment Measurements

G ENERALLY, surface contamination of a wing leads to reductions in performance [1–3]. Premature stall due to contamination can also have devastating effects on the controllability of aircraft [4,5]. Currently, there is no established, effective method for monitoring aircraft performance and controllability in order to prevent stall under adverse conditions created by surface contamination. While angle-of-attack sensors are capable of providing an effective stall warning for an aircraft free of contaminants, the effectiveness of such systems greatly diminishes with the presence of contaminants on an aircraft wing. Gurbacki and Bragg [6] studied the steady and unsteady hinge moment behaviors of supercooled large-droplet ice-induced stall on a NACA 23012 airfoil model with a simple flap. In that study, it was found that the rms of the unsteady hinge moment increased 1 to 3 angles of attack before stall for the iced case. Thus, it was identified that the unsteady separated flow led to unsteady changes in the hinge moment, giving rise to the concept that unsteady hinge moment measurements could be used to predict ice-induced stall [7]. Such a method could be further applied to a system capable of monitoring aircraft aerodynamic performance and providing realtime predictions of the edge of a flight envelope. In addition to flight beyond the baseline-clean aircraft flight envelope, such a system could potentially detect a reduction in the envelope due to several inflight environmental contaminants resulting from icing encounters, heavy rain, surface contamination in the form of roughness, and structural damage, such as bird strikes or battle damage. To further this concept, additional development is needed beyond the initial work of Gurbacki and Bragg [6]. ThisNote reports the further development of such a technique. The specific focus of this Note is the development of a detection algorithm using the unsteady hinge moment from a simple flap on an airfoil with and without contaminants. The algorithm developed is used to predict stall several degrees before the event.

Aircraft flight characteristics in conditions of windshear and icing

AbstractComplex weather conditions, especially windshear and icing encounter, have severe effects on aircraft flight safety. The effect of low-altitude windshear and ice accretion on aircraft performance and control has been studied in this paper. With the employment of a windshear model and nonlinear inverse dynamics (NID) method, a low-altitude windshear penetration flight control law is designed. The effect of ice accretion was modeled on the stability and control of an aircraft. Several icing parameters are imported to the small disturbance flight dynamics model to calculate the change of performance, stability and control derivatives between clean and iced aircraft. These derivatives were used to calculate the elevator, the aileron and the rudder step responses to investigate the icing effect.The simulation results indicate that the NID control logic works effectively in the trajectory control of the aircraft during the penetration of windshear. The method used to study the effect of ice accretion on aircraft is valid and it can provide data for real-time calculation for icing encounter.

Supporting Trust Calibration and the Effective Use of Decision Aids by Presenting Dynamic System Confidence Information

To examine whether continually updated information about a system's confidence in its ability to perform assigned tasks improves operators' trust calibration in, and use of, an automated decision support system (DSS). The introduction of decision aids often leads to performance breakdowns that are related to automation bias and trust miscalibration. This can be explained, in part, by the fact that operators are informed about overall system reliability only, which makes it impossible for them to decide on a case-by-case basis whether to follow the system's advice. The application for this research was a neural net-based decision aid that assists pilots with detecting and handling in-flight icing encounters. A multifactorial experiment was carried out with two groups of 15 instructor pilots each flying a series of 28 approaches in a motion-base simulator. One group was informed about the system's overall reliability only, whereas the other group received updated system confidence information. Pilots in the updated group experienced significantly fewer icing-related stalls and were more likely to reverse their initial response to an icing condition when it did not produce desired results. Their estimate of the system's accuracy was more accurate than that of the fixed group. The presentation of continually updated system confidence information can improve trust calibration and thus lead to better performance of the human-machine team. The findings from this research can inform the design of decision support systems in a variety of event-driven high-tempo domains.

Icing Encounter Flight Simulator

The Icing Encounter Flight Simulator is one part of the Smart Icing Systems project at the University of Illinois at Urbana-Champaign. From the Smart Icing Systems project, an ice management system was designed that would sense and characterize ice, notify the pilot, and if necessary take measures to ensure the safety of the aircraft. The icing simulator was used as a platform to integrate and test different components of the Smart Icing System. To create an Icing Encounter Flight Simulator, functionality and Smart Icing System components were added to the FlightGear flight simulator. Functionality added to FlightGear include a reconfigurable aircraft model and an icing model, and Smart Icing System components added include an envelope protection system and a glass cockpit enhanced with ice management system features. To ensure a real-time simulation, computationally intensive processes were distributed over several desktop computers linked together through a local network. In order to demonstrate the effectiveness of the Smart Icing System components, two fictional but historically motivated icing scenarios were developed and tested with a simulated DeHavilland DHC-6 Twin Otter, specifically, a tailplane stall event during a steep descent and a roll upset event during an emergency approach. Language: en

Aircraft Autopilot Analysis and Envelope Protection for Operation Under Icing Conditions

The behavior of a typical autopilot structure is studied for flight under icing conditions. The study is based on aT win Otter aircraft model and focuses on the pitch attitude behavior. A quadratic stability analysis using linear matrix inequalities is conducted to show that the time-varying closed-loop system maintains quadratic stability under icing conditions. In addition, the problem of envelope protection in the presence of icing is considered; in particular, how to maintain the angle-of-attack within the stall limits which are time varying. Based on steady-state behavior, a practical envelope protection scheme is developed and tested via simulation. I. Introduction A IRCRAFT accidents continue to occur due to the formation of ice on aircraft in flight. The development of safer, more reliable and affordable aircraft must include better solutions for flight in icing and bad weather conditions. The primary cause of these accidents is the effect of ice accretion on the performance, stability, and control of the aircraft. 1 Accidents occur when aircraft are not properly protected against ice accretion either on the ground or in flight. As mentioned in Ref. 1, this situation may result from an inadequate ice protection system or aircraft operation outside of the iced aircraft flight envelope. Icing accidents can be prevented in two ways: 1) icing conditions can be avoided, and 2) the aircraft system can be designed and operated in an ice-tolerant manner. For all aircraft, ice avoidance is the desired goal for increased safety. However, for commercial aircraft, where revenues and schedules must be maintained, ice tolerance will continue to be the preferred method for all but the most severe icing conditions. Currently a project on developing smart icing systems (SIS) to deal with this problem of icing is being carried out by a group of researchers at the University of Illinois. 2 The goal of the SIS research is to improve the safety of operations in icing conditions. This paper is a result of this SIS research. It studies the effects of ice accretion on the autopilot stability and performance and also develops techniques for envelope protection (EP) with the autopilot in operation under icing conditions. Current practice recommendations dictate that the autopilot system be shut down immediately when ice accretion is reported during the flight. The research reported here aims at taking a step in the direction of developing an autopilot system that is reliable and robust to icing conditions. This autopilot system will be tested by integrating it into a high-fidelity real-time simulator platform, the Icing Encounter Flight Simulator (IEFS), 3 currently being developed at the University of Illinois.

FENSAP-ICE's Three-Dimensional In-Flight Ice Accretion Module: ICE3D

Two-dimensional and quasi-three-dimensional in-flight ice accretion simulation codes have been increasingly used by the aerospace industry in the last two decades as an aid to the certification process. Such codes predict two-dimensional sectional ice shapes, which are then manufactured from a light material and attached as disposable profiles on test aircraft to investigate them for stability under icing encounters. Although efficient for calculating ice shapes on simple geometries, current codes encounter major difficulties or simply cannot simulate ice shapes on truly three-dimensional geometries such as nonaxisymetric nacelles, high-lift wings, engine intakes, or systems that combine external and internal flows. Modern computational fluid dynamics approaches may not encounter or engender these difficulties, and FENSAP-ICE is a combination of four modules forming a complete and generic in-flight icing simulation system, built in a way to solve successively impingement, ice accretion, heat loads, and performance degradation. The set of equations describing FENSAP-ICE's airflow solver, FENSAP, its impingement module, DROP3D, and its accretion module, ICE3D, are presented

Supporting decision making and action selection under time pressure and uncertainty: the case of in-flight icing.

Operators in high-risk domains such as aviation often need to make decisions under time pressure and uncertainty. One way to support them in this task is through the introduction of decision support systems (DSSs). The present study examined the effectiveness of two different DSS implementations: status and command displays. Twenty-seven pilots (9 pilots each in a baseline, status, and command group) flew 20 simulated approaches involving icing encounters. Accuracy of the decision aid (a smart icing system), familiarity with the icing condition, timing of icing onset, and autopilot usage were varied within subjects. Accurate information from either decision aid led to improved handling of the icing encounter. However, when inaccurate information was presented, performance dropped below that of the baseline condition. The cost of inaccurate information was particularly high for command displays and in the case of unfamiliar icing conditions. Our findings suggest that unless perfect reliability of a decision aid can be assumed, status displays may be preferable to command displays in high-risk domains (e.g., space flight, medicine, and process control), as the former yield more robust performance benefits and appear less vulnerable to automation biases.

Inflighticing data gathering during routine flight operations — a case study

For the purpose of the design and certification of inflight icing protection systems for transport and general aviation aircraft, the eventual re-definition/expansion of the icing environment of FAR 25/JAR 25, Appendix C is under consideration. Such a re-definition will be aided by gathering as much inflight icing event data as reasonably possible, from widely-different geographic locations. The results of a 12-month pilot programme of icing event data gathering are presented. Using non-instrumented turboprop aircraft flying upon mid-altitude routine air transport operations, the programme has gathered observational data from across the British Isles and central France. By observing a number of metrics, notably windscreen lower-corner ice impingement limits, against an opposing corner vortex-flow, supported by wing leading edge impingement limits, the observed icing events have been classified as ‘small’, ‘medium’ or ‘large’ droplet. Using the guidance of droplet trajectory modelling, MVD values for the three droplet size bins have been conjectured to be 15, 40 and 80mm. Hence, the ‘large’ droplet category would be in exceedance of FAR/JAR 25, Appendix C. Data sets of 117 winter-season and 55 summer-season icing events have been statistically analysed. As defined above, the data sets include 11 winter and five summer large droplet icing encounters. Icing events included ‘sandpaper’ icing from short-duration ‘large’ droplets, and a singular ridge formation icing event in ‘large’ droplet. The frequency of ‘large’ droplet icing events amounted to 1 in 20 flight hours in winter and 1 in 35 flight hours in summer. These figures reflect ‘large’ droplet icing encounter probabilities perhaps substantially greater than previously considered. The ‘large’ droplet events were quite localised, mean scale-size being about 6nm.

Inflight icing data gathering during routine flight operations — a case study

AbstractFor the purpose of the design and certification of inflight icing protection systems for transport and general aviation aircraft, the eventual re-definition/expansion of the icing environment of FAR 25/JAR 25, Appendix C is under consideration. Such a re-definition will be aided by gathering as much inflight icing event data as reasonably possible, from widely-different geographic locations. The results of a 12-month pilot programme of icing event data gathering are presented. Using non-instrumented turboprop aircraft flying upon mid-altitude routine air transport operations, the programme has gathered observational data from across the British Isles and central France. By observing a number of metrics, notably windscreen lower-corner ice impingement limits, against an opposing corner vortex-flow, supported by wing leading edge impingement limits, the observed icing events have been classified as ‘small’, ‘medium’ or ‘large’ droplet. Using the guidance of droplet trajectory modelling, MVD values for the three droplet size bins have been conjectured to be 15, 40 and 80mm. Hence, the ‘large’ droplet category would be in exceedance of FAR/JAR 25, Appendix C.Data sets of 117 winter-season and 55 summer-season icing events have been statistically analysed. As defined above, the data sets include 11 winter and five summer large droplet icing encounters. Icing events included ‘sandpaper’ icing from short-duration ‘large’ droplets, and a singular ridge formation icing event in ‘large’ droplet. The frequency of ‘large’ droplet icing events amounted to 1 in 20 flight hours in winter and 1 in 35 flight hours in summer. These figures reflect ‘large’ droplet icing encounter probabilities perhaps substantially greater than previously considered. The ‘large’ droplet events were quite localised, mean scale-size being about 6nm.

Stability properties of a cometary plasma tail

The linear stability of the plasma tail of a comet is numerically investigated. The zeroth‐order model of the tail plasma configuration is based on the result of a companion paper (Malara et al., this issue), discussing the possible modeling of the data obtained during the ICE encounter with comet Giacobini‐Zinner. Effects both of finite resistivity and of velocity shear are taken into account. The tail appears to be unstable against Kelvin‐Helmholtz‐like modes in which a certain amount of reconnection occurs. Because of the velocity flow shear the perturbation extends far beyond the singular layer and affects in depth the magnetic lobes. The relative importance of the Kelvin‐Helmholtz turbulence in the lobes and the level of reconnection, the typical wavelengths, and the growth time of the mode depend on the particular cometary tail considered.

Magnetic pulses with durations near the local proton cyclotron period: Comet Giacobini‐Zinner

Magnetic pulses with durations near the local proton cyclotron period (in the spacecraft frame) have been detected during the ICE encounter with comet Giacobini‐Zinner. The pulses typically last only a single cycle (solitary wave), are transverse, noncompressional oscillations, and have peak‐to‐peak transverse amplitudes of 2‐3 nT. Occasional large pulses with amplitudes of 5‐7 nT have been detected. The waves typically propagate at angles 2°‐15° relative to B0 and are highly elliptically (linearly) polarized. The pulses have been detected when α, the angle between the ambient magnetic field and the solar wind velocity, is 90° ± 30° and when ICE was 350,000‐700,000 km from the comet. At the present time it is uncertain whether such pulses are generated exclusively under large α conditions or whether the presence of large‐amplitude heavy ion cyclotron waves during more moderate α conditions masks their presence. It is also unclear whether these pulses are superposed on top of cometary turbulence or simply a part of it. The above observations will be compared to recent theoretical predictions of cometary waves generated during large α conditions.

Atomic and molecular abundances in comet Giacobini-Zinner

Spectrophotometric observations of comet Giacobini-Zinner were obtained on 11 September 1985. C 2 and NH 2 were found to be depleted in Giacobini-Zinner relative to an average comet by factors of 10 and 5, respectively. The ratio of the production rates, Q( NH 2)/ Q( H 2 O) = 2 × 10 −4 indicates a very low NH 3/H 2O abundance ratio if NH 2 comes mostly from photodissociation of NH 3. The water production rate for Giacobini-Zinner at the time of the ICE encounter was found to be 2.4 × 10 28 molecules s −1, in good agreement with spacecraft results. Neither neutral Na nor C 2 + ion was detected in the ground-based spectrum at the time of the ICE encounter. Therefore, Na + and C 2 + can be excluded from the candidates for the ions in the mass range 23 to 24 amu detected by the ICE ion composition instrument.

Analytical determination of propeller performance degradation due toice accretion

A computer code has been developed which is capable of computing propeller performance for clean, glaze, or rime iced propeller configurations, thereby providing a mechanism for determining the degree of performance degradation which results from a given icing encounter. The inviscid, incompressible flow field at each specified propeller radial location is first computed using the Theodorsen transformation method of conformal mapping. A droplet trajectory computation then calculates droplet impingement points and airfoil collection efficiency for each radial location, at which point several user-selectable empirical correlations are available for determining the aerodynamic penalities which arise due to the ice accretion. Propeller performance is finally computed using strip analysis for either the clean or iced propeller. In the iced mode, the differential thrust and torque coefficient equations are modified by the drag and lift coefficient increments due to ice to obtain the appropriate iced values. Comparison with available experimental propeller icing data shows good agreement in several cases. The code's capability to properly predict iced thrust coefficient, power coefficient, and propeller efficiency is shown to be dependent on the choice of empirical correlation employed as well as proper specification of radial icing extent.

Comment on the Paper “On the influx of small comets into the Earth's upper atmosphere”

Frank et al. attribute the occurrence of transient dark spots in the terrestrial uv dayglow emission to absorption by water vapor from the tidal breakup and evaporation of small comets at high altitude. Such comets would be sufficiently large and numerous that they should already have been detected optically as faint slow‐moving objects. However, they have not been observed.

The Giacobini‐Zinner magnetotail: Tail configuration and current sheet

The high‐resolution plasma electron and magnetic field data sets from the ICE tail traversal of comet Giacobini‐Zinner have been combined to make a detailed study of the draped Giacobini‐Zinner magnetotail in general, and its field‐reversing current sheet in particular. The geometry of the magnetotail at the time of the ICE crossing is determined and is shown to be consistent with a circular tail cross section rotated 10.5° in the normal sense of aberration and 9.9° above the ecliptic plane, bisected by a cross‐tail current sheet which is rotated 43° out of the ecliptic about the solar wind velocity vector. MHD continuity, momentum, and energy equations are combined with the plasma and field observations to determine unmeasured plasma properties at ICE and upstream at the average point along each streamline where the cometary ions are picked up. The ion temperature, beta, and flow speed at ICE range from 1–1.5 × 106 K, 1–4, and −20 to −30 km s−1, respectively, in the draped lobes to ∼1.2 × 105 K, up to ∼40, and ∼−20 km s−1 in the current sheet. Upstream at the average pickup locations, the flow velocity, ion temperature, density, and ion source rates range from ∼−75 km s−1, ∼4 × 106 K, ∼20 cm−3, and ∼1.5 cm−3 s−1 in the regions upstream from the lobes to ∼ −12 km s−1, ∼1 × 105 K, 200–600 cm−3, and ∼3.6 cm−3 s−1 in the prime mass‐loading region upstream from the current sheet. Gradients in the plasma properties at the edges of this region are quite strong, and the diameter of the region is ∼1500 km, in good agreement with the expected size scale of the cometary ionopause; implications of our inferred plasma properties in this region are examined. The derived and measured flow velocity and measured plasma density at ICE are combined to calculate the transport rate of ions past the plane of the ICE encounter, and we calculate that ∼2.5–5 × 1028 ions per second (neutrals per second in the steady state) are produced by the comet with only ∼1% of these traveling down the magnetotail.

Linearly polarized magnetic fluctuations at comet Giacobini‐Zinner

The fully electromagnetic linear Vlasov dispersion equation for instabilities in a homogeneous, magnetized plasma is solved. Parameters appropriate to conditions observed during the ICE encounter with comet Giacobini‐Zinner are used to study the polarization of electromagnetic instabilities driven by the relative streaming of two ion components. If the ion/ion relative drift is near the Alfvén speed, the associated long‐wavelength modes may be approximately linearly polarized at relatively small angles of propagation with respect to the magnetic field. The relatively inhomogeneous plasma background of the comet may impose such oblique propagation, so that this result provides an alternate explanation to the observations of linearly polarized waves near the comet.