• Determine the performance, stability and control characteristics of an iced aircraft
• Determine steady state inflight icing effect characterization methods
• Define envelope protection strategies
• Support other groups in Smart Icing Systems Project

• Prof. Michael Bragg
• Prof. Eric Loth
• Andy Broeren

• Sam Lee
• Jason Merret
• Devesh Pokhariyal
• Kishwar Hossain
• Ed Whalen
• Leia Blumenthal
• Tim Hutchison
• Ryan Oltman

• Kishwar N. Hossain, Vikrant Sharma, Michael B. Bragg, and Petros G. Voulgaris

Abstract: The goal of this research was to imporve the envelope protection capabilities of an aircraft in icing conditions.  To accomplish this goal, open and closed loop envelope protection algorithms were developed to ensure the safe operation of an iced aircraft during both manual and autopilot modes of flight.  The Iced Aircraft Envelope Protection system (IAEP), develooped as a part of the Smart Icing Systems (SIS) research project at the University of Illinois, was based on data from wind tunnel tests, flight tests and iced aircraft simulations obtained from a six-degree-of-freedom computational flight dynamics model.  The sustem consisted of estimative and predictive methods for apporximating, and avoiding the envelop boundaries.  Simulation results demonstrated that IAEP was capable of successfully avoiding incidents and accidents during flight in icing conditions.  This paper includes a summary of the basic sscheme of the longitudinal iced aircraft envelope protection system and a discussion of results obtained through simulation.

• Jason Merret, Kishwar N. Hossain, and Michael B. Bragg

Abstract: Research is reported on aircraft performance and control in icing, related to the development of Smart Icing    Systems for improved flight safety.  Microburst and gravity wave atmospheric disturbances were modeled,   and their effects on the aircraft performance and control were compared to that of an icing encounter.  Simulations were run using a six degree-of-freedom computational flight dynamics model.  The study showed that microbursts could easily be differentiated from icing encounters.  On the other hand gravity waves are more difficult to differentiate.  A plan was formulated for developing an envelope protection system effective in icing conditions.  Two dimensional airfoil data were analyzed and showed promising results for prediction of envelope limit exceedence.  Changes in unsteady hinge moments were especially effective in predicting stall.            

• James W. Melody, Devesh Pokhariyal, Jason Merret, Tamer Basar, William R. Perkins, Michael B. Bragg
• Abstract:  This work advances a neural network that characterizes aircraft ice accretion in order to improve flight performance and safety. Neural networks have been developed previously for use within an ice management system that monitors inflight aircraft icing and its effects upon performance, stability, and control. The previous work has applied these networks to stability and control derivative estimates provided by an H°° parameter identification algorithm during a longitudinal maneuver. This paper extends those results by addressing ice characterization in the absence of pilot input when poor excitation of the flight dynamics limits the accuracy of parameter estimates. To compensate for this shortcoming inherent to steady-level flight scenarios, the neural network presented in this paper integrates steady-state characterization and hinge moment sensing with parameter estimates. The neural network provides icing characterization in terms of an estimate of the previously developed icing severity factor, rj. Extensive simulation results are presented that indicate the accuracy of neural network characterization during steadylevel flight in the presence of sensor noise and turbulence over a broad range of flight trim conditions and turbulence levels. Furthermore, the relative utiltiy of each information source is investigated via consideration of network accuracy of networks trained only on that information source.
  
  

• D. Pokhariyal, M.B. Bragg, T. Hutchison, and J. Merret
• Abstract:  The effect of ice accretion on aircraft performance and control during trim conditions was modeled and analyzed. A six degree-of-freedom computational flight dynamics model was used to study the effect of ice accretion on the aircraft dynamics. The effects of turbulence and sensor noise were modeled and filters were developed to remove unwanted noisy data without affecting the short period and phugoid modes. This study is part of a larger research program to develop smart icing system technology. The goal of the study reported here was to develop techniques to sense the effect and location of ice accretion on aircraft performance and control during trimmed flight. Control surface steady and unsteady hinge-moments were modeled as a potential aerodynamic performance sensor. Microburst and gravity wave atmospheric disturbances were modeled and their effects on the aircraft performance and control were compared to that of an icing encounter. The simulations showed that atmospheric disturbances could be differentiated from icing encounters. The hinge-moment sensors proved very useful in identifying the wing versus tail location of aircraft icing.

• M.B. Bragg, T. Hutchison, J. Merret, R. Oltman, and D. Pokhariyal
• Abstract: The effect of ice accretion was modeled on the performance and control of an aircraft.  A simple method was presented to alter the aircraft stability and control parameters to model the effect of ice accretion.  A six degree-of-freedom computational flight dynamics model was used to study the effect of the ice accretion on the aircraft dynamics including the effect of atmospheric turbulence and sensor noise.  This study is part of a larger research program to develop smart icing system technology.  The goal of the study reported here was to develop techniques to sense the effect of ice accretion on the aircraft performance and control during quasi-steady-state flight.  A simple model to relate ice accretion effects to icing and flight parameters is proposed.  The computational model showed large changes in V, a, and d _e as the ice accretes for a constant power and altitude case.  Atmospheric turbulence and sensor noise are modeled and a filter is shown to remove most of these effects.  Aircraft operated at constant velocity show smaller effects and aerodynamic sensors are proposed to aid in the characterization of these cases.

• M.B. Bragg
• Paper No. 96-0932, AIAA 34th Aerospace Sciences Meeting, Reno, NV, January 15-18, 1996.
• Abstract: The effect of large-droplet ice accretion on aircraft control and in particular lateral control is examined.  Supercooled large droplet icing conditions can result in the formation of a ridge of ice aft of the upper surface boot.  By comparing this ice shape to data acquired with a spanwise protuberance on a different airfoil, it is clear that a ridge of ice aft of the boot can lead to large losses in lift, increases in drag and changes in the pitching moment.  This effect is most likely due to the formation of a large separation bubble aft of the ice accretion which grows with angle of attack and eventually fails to reattach, leading to premature airfoil stall.  The bubble alters the pressure distribution about the airfoil resulting in a more trailing edge up (negative) hinge moment on the aileron and the resulting change in aileron stick force.  This can lead to aileron hinge moment reversal and aileron snatch.  In aileron snatch the hinge moments are altered to the extent that the aileron is pulled up by the low pressure over the upper surface of the aileron with sufficient force to induce a rapid roll if a large stick force is not immediately exerted to oppose it.  There is evidence in the literature which shows that similar lateral control problems are possible with other types of ice accretions and airfoil types.

• M.B. Bragg
• Proceedings of the FAA International Conference on Aircraft Inflight Icing, Springfield, VA, Report No. DOT/FAA/AR-96/81,II, Vol. 2, Aug. 1996, pp. 387-400. 
• Abstract:  The effect of large-droplet ice accretion on aircraft control is examined.  Supercooled-large-droplet icing conditions can result in the formation of a ridge of ice aft of the upper surface boot.  By comparing this ice shape to data acquired with a spanwise protuberance on an airfoil, it is clear that a ridge of ice aft of the boot can lead to large losses in lift, increases in drag and changes in the pitching moment.  This effect is most likely due to the formation of a large separation bubble aft of the ice accretion which grows with angle of attack and eventually fails to reattach, leading to premature airfoil stall.  The bubble alters the pressure distribution about the airfoil resulting in a more trailing edge up (negative) hinge moment on the aileron and the resulting change in aileron stick force.  This can lead to aileron hinge moment reversal and aileron snatch.  The fundamental aerodynamic cause of this lateral control problem is the same as that experienced when elevator control is lost due to horizontal tail stall.