Arlene Beisswenger

Prediction of Ice Shapes on NACA0012 2D Airfoil

The objective of this communication is to present the new capability at AMIL in ice accretion simulation on 2D Airfoils at low speed. AMIL, in a joint project with CIRA (Italian Aerospace Research Center), has developed a numerical model called CIRAMIL. This model is able to predict ice shapes in wet and dry regimes. The thermodynamic model used is similar to existing ones. The major difference is in the approach of calculating the surface roughness and the residual, runback and shedding liquid water masses on an airfoil surface. The numerical ice shapes are compared to rime and glaze shapes obtained experimentally in wind tunnel for a NACA0012 wing profile. The new roughness computation method generates the complex ice shapes observed experimentally in wet and dry regimes and the results agree well with icing profiles obtained in wind tunnel experiments and in many cases are better than those predicted by the models available.

Issues and Testing of Non-Glycol Aircraft Ground Deicing Fluids

Deicing fluids are used to remove and prevent ice formation on aircraft before takeoff. These fluids are essentially composed of water, a freeze point depressant (FPD) usually glycol, a surfactant or wetting agent and a corrosion inhibitor. All commercial fluids are qualified to SAE (Society of Automotive Engineers) specifications, which test for aerodynamic acceptance, anti-icing endurance, corrosion inhibition, material compatibility, fluid stability and environment. However, these tests have been built around a fluid with a glycol FPD. More recently, with environmental pressure, fluids with other FPDs have been developed and qualified. The other FPDs include: acetates and formate salts, sorbitol, and other undisclosed FPDs. The acetates and formates, which came out in the early 1990s led to suspected corrosion problems. This led to the additional requirement for corrosion tests for non-glycol deicing fluids in paragraph 3.1.1 of AMS1424. This is essentially only a relevant for such a salt based non-glycol fluid. Next, came a sorbitol, or sugar, based fluid in the early 2000s. As with the formate and acetate salts, it passed all the required tests of AMS1424 including the additional corrosion test. But then in field tests, where the fluid was heated as per usual use, there were problems with foam, sticky and slippery residues. All standard specification laboratory tests are conducted on cold fluids, since this is the worst case for glycol-based fluids, where they are most viscous. However, other FPDs may have the fluid increase in viscosity with heating and evaporation. Following the failed field tests, tests were conducted in the laboratory which showed that when the fluid was heated to high levels of evaporation, the aerodynamic acceptance test was not met and with further evaporation, the fluid solidified. This does not occur with glycol-based fluids since glycol is a liquid. Furthermore, in the lab, mold developed on some exposed fluid left out in a Petri dish. The FAA has since removed this fluid from their list of qualified fluids in the official FAA Holdover Time Tables [1]. More recently, there have been newer fluids that are non-glycol-based, or have another FPD along with the glycol (low-glycol). These fluids all are qualified at least for aerodynamic performance and anti-icing endurance and two are on the current FAA list of qualified fluids. However, the specification has no tests to address stickiness, solidification or tendency for mold to form. For the foam, a test was added to the specification since this issue was arising equally with glycol-based fluids. As part of a grant from the FAA, AMIL is developing test protocols to be added to the test specifications to address the new potential issues that may be required of non-glycol fluids before their use on aircraft. Beyond the corrosion and foam issues for which tests currently exist in the specification, test for aerodynamic acceptance on evaporated heated and sheared fluids, tendency to from mold and slipperiness are proposed.

Experimental Study of Snow Precipitation Over a Generic Deicing Fluid without Fluid Flow

Deicing and anti-icing fluids are used to remove and prevent ice formation on aircraft before takeoff. Holdover times (HOT) published by the FAA are used by pilots as guidelines indicating the amount of effective time of a fluid under certain freezing precipitation types. However, the times on these tables are based on endurance time tests involving a visual estimate of failure on a flat plate [1]: when 30% of the fluid is covered with white snow under snow precipitation, although the times have been correlated to aircraft wing tests [2] they do not address the mechanism of fluid failure. To measure and understand the fluid mechanisms conducting to failure, the Anti-icing Materials International Laboratory (AMIL) developed a simplified test with a generic deicing propylene glycol-based fluid. The test consisted of pouring 400 mL of the generic deicing fluid on a 5 dm by 3 dm level flat plate where the plate edges were rimmed with insolated walls to make a waterproof open box. The flat plate covered with deicing fluid was submitted to snow precipitation in the form of regular snow and simulated snow pellets. The snow precipitation intensities and temperatures tested were based on ARP5485. The standard test method for testing snow indoors includes using a heating pad to compensate for the lack of thermal equilibration which occurs in outside with wind and the larger air mass. It was the energy solicited by the fluid melting the snow from the heat pad that was used to develop a model for fluid failure. Three precipitation intensities and eight temperatures with the generic deicing fluid and commercial Type I and Type IV fluids were studied. At fluid failure, snow mass, and energy provided to melt the snow were independent of snow intensity and type of snow, however endurance time and supplied power were dependent on snow intensity but independent of snow type. Visual observations showed that only a fraction of the falling snow in contact with the fluid covering the aluminium plate melts, the unmelted snowflakes descend by gravity into the fluid and accumulate on the aluminium plate surface. The energy to melt the snow was provided by a heating system. The proposed model assumes that enough energy is available to melt all the snow that a deicing fluid is able to absorb under water form and that the water can diffuse rapidly into the deicing fluid due to multiple diffusion sites resulting in an uniform propylene glycol concentration and predicted well the endurance time and the snow mass at fluid failure for a generic deicing and Type I fluid with an error of 7%. A semi-empirical relation was used in the model to evaluate the melting fraction; this relation, based on a phase diagram, can be used to estimate the propylene glycol concentration, the power and the energy as a function of time at the fluid failure. However, the model could not predict the endurance time of Type IV anti-icing fluids.

Aerodynamic Flow-off of Type II and Type IV Aircraft Ground Anti-icing Fluids

The certification process for aircraft ground anti-icing fluids involves flat plate wind tunnel aerodynamic flow-off tests. This test method was developed in 1990 from flight and wind tunnel tests results on full scale and model airfoils, and flat plates; the resulting lift losses were then correlated to the Boundary Layer Displacement Thickness (BLDT) on a flat plate. This correlation was made for Type II fluids existing at the time. Since the introduction of Type IV fluids in 1994, with significantly longer anti-icing endurance times, the same test procedure was applied. However, Type IV fluids are generally more viscous than Type II fluids of the same concentration. At the FAAs request, a study was undertaken to see if aerodynamic certification testing should be different for Type IV fluids as opposed to Type II. After a comparison of existing certification BLDT data which showed no significant differences between Type II and Type IV fluids, aerodynamic tests were performed on five commercial fluids, two typical Type II and three typical Type IV fluids. Tests with different initial thickness showed that the thickness had little effect on BLDT and fluid elimination data, with the exception of one fluid, near the temperature at which it is acceptable. Examination of energy data showed that more energy was required by the wind tunnel to move the Type II fluids as opposed the Type IV fluids. When these fluids were tested using an identical fixed fan speed profile at 0, -10 and -25°C, little difference was seen in the BLDTs as compared to those generated by a certification profile adjusted to obtain the same acceleration. These preliminary tests suggest that the aerodynamic certification method developed for Type II fluids is adequate for assessing Type IV fluids.

Endurance and Aerodynamic Performance Certification of Aircraft De/Anti-icing Fluids

Three performance certification tests are required for the assessment of aircraft de/anti-icing fluids. The two first, measuring anti-icing endurance, consists of the Water Spray (WSET) and the High Humidity Endurance Tests (HHET). The third, for aerodynamic performance, consists of the Flat Plate Elimination Test (FPET). The three performance tests, used for both deicing and anti-icing fluids, are described in the annexes of AMS1424 and AMS1428. Since February, 2003 they are covered by aerospace standards AS5900 for aerodynamic, and AS5901 for anti-icing endurance performance. The WSET anti-icing endurance test measures the time that a fluid protects a plate, inclined at a 10° angle, subjected to freezing precipitation, from a specified amount of icing. This WSET time then defines the fluid type: it must exceed 3 minutes for a Type I, 30 minutes for a Type II and 80 minutes for a Type IV fluid. The aerodynamic FPET performance test involves measuring the Boundary Layer Displacement Thickness (BLDT) during fluid elimination on a flat plate in a wind tunnel at temperatures from 0 °C down to a possible –45 °C. Two take-off acceleration profiles can be simulated, one for large transport type aircraft, and the other for commuter-type aircraft. A fluid is considered acceptable at a temperature interval if its BLDT is below an acceptance criteria defined by BLDT measurements of a reference fluid tested simultaneously and the fluid elimination is below a given percentage depending on fluid type.