Icing has been and continues to be an enemy of aviation. Current ice protection systems are a heavy penalty to engine and electrical power. Ultrasonic vibration has shown the potential to provide safe flying icing conditions while reducing the power drainage related to currently used de-icing systems. When aircraft vehicles enter an icing environment, super-cooled water droplets impinge on the leading edge of the wings or rotor blades. The water droplets freeze on impact with the lifting surfaces.
Ultrasonic De-icing: Wind Tunnel Testing Demonstrates Instantaneous Ice Shedding
Ice accretion degrades the vehicle performance and handling qualities, and can be extremely dangerous. The noise and vibration levels of the vehicle may also be adversely affected. De-icing and anti-icing systems used to protect against ice accretion are typically based on electrothermal energy or bleed air heating, with the exception of pneumatic boots used in some fixed-wing configurations. To overcome drawbacks of current ice protection systems (high power consumption, thermal approaches not compatible with composite structures), ultrasonic vibration has been proposed by the AERTS laboratory as a non-thermal, low-power de-icing solution. The ultrasonic de-icing system creates transverse shear stresses at the ice/airfoil interface exceeding the adhesion strength of ice and thus promoting ice delamination (<2 mm ice thickness). The power consumption of ultrasonic de-icing has also shown to be a fraction of that required by electro-thermal de-icing, making the concept appealing for its implementation in small and medium size aircraft. The thin layers of ice that ultrasonic de-icing has proven to shed during wind tunnel testing and rotor testing would not present ballistic concerns to the vehicle. Our research focuses on the design and testing of the ultrasonic de-icing concept. Information regarding actuator bonding design and driving conditions to maximize ice interface transverse shear stresses created can be found in our publications. Experimental results including wind-tunnel and rotor icing test results, as well as a consumption power comparison to electrothermal de-icing have also been reported. (Sponsors: ARMY, National Rotorcraft Technology Center, Vertical Lift Consortium)
A novel pneumatic approach to protect helicopter rotor blades from ice accretion is currently under research. The system relies on centrifugally generated pressures to deform a 0.508 mm (0.02 in.) thick titanium leading edge cap. The leading edge cap is protected by a 10 µm (390 microinch) thick Ti-Al-N erosion resistant coating. Beneath the titanium leading edge, six (6) pneumatic diaphragms were installed. The diaphragms are normally deflated under vacuum against the surface of the blade, and are inflated when ice accretion thickness reaches a critical value. The deformation of the leading edge introduces transverse shear stresses at the interface of the ice layer that exceed the ice adhesion strength of the material (868 KPa, 126 psi), promoting instantaneous ice debonding. The applied input pressures to the system (+/- 25.5 KPa, 3.7 psi) were representative of the pressures generated centrifugally by a medium helicopter size rotor system. With these pressures, the maximum deformation of the leading edge was quantified to be 5 mm (0.2 in). The aerodynamic performance degradation effects related to the leading edge deformation were quantified during low speed (1 M Re) wind tunnel testing. Results were compared to the aerodynamic performance degradation due to ice accretion. It was measured that the penalties related to the deployment of the pneumatic diaphragms was 35% lower than the aerodynamic drag penalty due to ice accretion. The lower aerodynamic penalty of deploying the proposed deicing concept with respect to that of ice accretion case indicates that the system would not introduce any aerodynamic penalty while removing accreted ice. The system was tested under representative rotor icing conditions and at centrifugal loads that ranged from 110g to 514g. The deicing successfully promoted instantaneous shedding of ice layers ranging from 1.5 to 5 mm (0.06 in. to 0.1 in.) in thickness for varying icing conditions within FAR Part 25/29 Appendix C Icing Envelope. (Sponsor: Leading Edge Aeronautic Research for NASA)
Heating of Graphite Heaters on AERTS Hover Stand
In addition to novel de-icing concepts, state-of-the-art electrothermal de-icing systems are tested. Graphite heater configurations are being evaluated to determine most effective configurations to promote ice debonding for a minimum amount of power. These results are the benchmark comparison point to any other de-icing technology tested in the laboratory. (Sponsor: Vertical Lift Consortium)
Active Twist De-icing
Active twist blades designed by DLR have integrated piezoelectric actuators to twist the blade. The blades are designed to introduce individual blade control (IBC) for helicopter noise and vibration reduction. The AERTS lab has conducted research to investigate the capability of active twist for blade de-icing. Two active twist blades were tested at different icing conditions and at representative CF forces. The electrical input power to the actuators was about 200 Watts for one blade (1.5 m radius blade). At temperatures warmer that -17°C, the active twist rotor blade, excited at its first torsional resonance, removed accreted ice of varying stagnation thickness, ranging from 3 mm to 8 mm. These cases were simulated with finite element methods to provide insight into the ice interface stresses and their origin. Based on the finite element simulation a new design for an active twist blade was proposed. The modeled configuration could increase the ice interface transverse shear stresses, predicting to shed accreted ice layers with a thickness of less than 2 mm. This rotor de-icing approach remains in its conceptual stage. (Sponsors: DLR - German Aerospace Center)
At the AERTS laboratory centrifugal testing of active rotor actuation systems is conducted regularly. Trailing edge flaps and Miniature Trailing Edge Effectors (MiTEs) actuation schemes have been evaluated. MiTEs are segmented Gurney flaps that can deploy to achieve multi-purpose functions such as performance enhancement, noise/vibration control, and/or load control on rotor blades. The unsteady aerodynamics of MiTEs and a deployable plain flap (with an equivalent lift gain) are quantified experimentally at a reduced frequency of 0.21 and a Reynolds number of 1M.
Video of MiTEs being Deployed in the AERTS Hover Stand. Testing conducted at 95% CF Force of a Representative Rotor Blade
These experiments are also simulated using computational fluid dynamics (CFD). The combination of the wind tunnel experiments and CFD are used to quantify the aerodynamic effects of MiTE deployment, to compare their unsteady aerodynamics to plain flaps, and to evaluate the fluid dynamics of MiTEs against experimental data. The current experiments display unsteady aerodynamics that corroborate previous CFD findings that indicate that MiTEs shed on-surface vortices during deployment, affecting the unsteady aerodynamics of the system. CFD also predicted that MiTEs require 1/55 power to deploy as compared to a plain-flap configuration. Power reduction is a key attractor for the integration of devices on smart rotors. This work is concluded with an effort that displays that the low power requirement of MiTEs enable simple deployment methods, such as the use of pressure differentials inherent to the rotor blades. The proposed pneumatic MiTE configuration was tested at centrifugal forces representative to helicopter rotor blades. (Sponsors: Vertical Lift Consortium)
Ice Protective Coatings
The physical mechanism responsible for ice adhesion variations for different coatings is not well understood. Jose is seeking to understand the parameters that dictate the ice adhesion strength to different materials. The AERTS lab has the capability to measure ice adhesion strength by analyzing natural ice shedding under centrifugal loads.Using these techniques many ice protective coatings have been evaluated for Boeing, NASA, Nusil, GE and other companies. Recent research effort examines the effects of surface characteristics of ice adhesion strength for three erosion resistant materials. The materials tested were titanium grade 2 (Ti 2), titanium aluminum nitride (TiAlN) coated on Ti 2, and titanium nitride (TiN) coated on Ti 2. The surface roughness of the material contributed to the ice adhesion strength but did not explain the variation in ice adhesion strength between the materials. When the surface roughness of the Ti 2 substrate increased from 26.4 µin Ra to 86.1 µin Ra the ice adhesion strength increased 29%. At the lowest surface roughness tested, the ice adhesion strength of TiN was 31% higher that the uncoated Ti 2 substrate, and the TiAlN was 62% higher than the uncoated Ti 2. The average ice adhesion strength of the coatings was twice that of the uncoated substrate. Mechanical clamping was determined to be the primary mechanism for ice adhesion strength. Macro structures with “cliff” like structures, observed on the coatings, increased the ice adhesion strength over what would be predicted from a surface roughness characterization. These structures also explained the 40% increase in ice adhesion strength of a material subjected to oxidation. (Sponsors: Boeing, Vertical Lift Consortium, NASA Langley)
A physics-based empirical correlation between icing conditions and the corresponding drag coefficient was developed for NACA 0012 airfoils, and compared to other three existing prediction methods. The correlation was developed based on experimental aerodynamic databases of iced airfoils, and derived using statistical methods. The correlation model also provides drag coefficients for varying angles of attack for a given icing condition. The calculated drag coefficients resulted in 33.40% mean absolute deviation with respect to reference data from three different experimental databases. To validate the proposed degradation model and to further extend the database for helicopter-rotor performance degradation, rotating ice-accretion experiments were conducted. Four ice shapes obtained at the NASAIcing Research Tunnel were reproduced on a 53.34-cm-chord, 1.37-m-radius NACA0012 rotor blade at the Adverse Environment Rotor Test Stand facility. Ice-shape molding and casting techniques were introduced to capture delicate ice features, such as ice feathers. The iced-airfoil castings were tested in a dry-air wind tunnel. The drag-coefficient comparison between the proposed analytical determination method and the experimental results from both rotor ice testing and icing-wind-tunnel testing showed to be satisfactory, ranging from to 25%depending on the icing condition. The effect of ice feathers on drag degradation was investigated. Ice-feather formation can account for up to 25% of the drag introduced by ice accretion before stall.
In addition, Jose is investigating the effects of ice surface roughness on heat transfer. This information is critical for accurate model of ice accretion. Surface roughness effects on heat transfer of ice roughened airfoil still need further analytical and experimental evaluation. The objectives of current efforts are: 1. to obtain representative natural ice surface roughness on an airfoil; 2. to fabricate detailed ice surface roughness casting model which roughness can be quantified by 3D digitization; 3. to conduct wind tunnel heat transfer testing based on transient surface temperature variation techniques; 4. to correlate the local heat transfer coefficient to surface roughness height and physical icing condition parameters. (Sponsor: National Rotorcraft Technology Center, Army: Vertical Lift Research Center of Excellence)
The change of the rotor broadband noise due to different surface roughness thickness is also being investigated. The goal of the present work is to quantify the surface roughness at the early stage of the helicopter rotor ice accretion through acoustic measurement. Significant changes are observed between blades with tripped boundary layers, which are essentially the same as blades with surface roughness, and clean blades. Then preliminary measurements performed at the Adverse Environment Rotor Test Stand (AERTS) facility at the Pennsylvania State University are used to explore the effect of different surface roughness heights due to ice accretion on rotor broadband noise. The results of the broadband noise change due to different surface roughness show a clear trend: the sound pressure level from each roughness height is separated by about 2 dB in the high frequency region (frequency larger than 12 kHz). This demonstrates the feasibility of quantifying the surface roughness through acoustic measurement.
Photos of 1/5th Scale LAMA RC Helicopter with and without Representative Ice Surface Roughness on Blades
Flight testing was also conducted on a 1/5th scale turbine engine RC helicopter. Rotor blade surface roughness could also be detected. Flight testing efforts are on-going. (Sponsor: National Rotorcraft Technology Center, Army: Vertical Lift Research Center of Excellence)