Search Results

Gel residues occur as the result of repeated anti-icing fluid application that leaves a powdery film upon dryout that, when rehydrated, can swell up to over 600 times its weight. When these gels collect on aircraft flight control surfaces in aerodynamically quiet areas and freeze, they give rise to reduced performance, increased stick force, slowed rotation and have caused jammed flight controls. Laboratory tests have been developed to simulate the gel formation by drying out fluids and rehydrating them. However, by their complex nature, much variation is seen between test results from different laboratories and the results are not yet considered by fluid users. Testing carried out at AMIL on different fluids with different test methods has led to a more reproducible results and a potential classification of fluids based on their gel formation potential (GFP).

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 FAA's request, a study was undertaken to see if aerodynamic certification testing should be different for Type IV fluids as opposed to Type II.

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.

This paper reviews the current knowledge on super-hydrophobic coatings (SHC). Using an ideal super-hydrophobic surface patterned with identical cylindrical flathead posts forming a square network with constant periodicity, models are proposed to explain SHC, wear and ice adherence on SHC. The models demonstrate that SHC based on Cassie-Baxter state improve the bead mobility compared to SHC based on Wenzel state and more suitable for aircraft application. Their erosion resistance can be improved by increasing the post height and the hydrophobic material thickness. Their ice adhesion reduction factor (IARF) is better but SHC based on Cassie-Baxter state have a limitation to reduce ice adherence dependence on the surface pattern and IARF of the hydrophobic material. The bead mobility is calculated from advancing and receding water contact angles (WCA).

Two commercial hydrophobic coatings: StaClean™ with a water droplet contact angle of 101° and Wearlon® Super F1-Ice with a contact angle of 115° and one superhydrophobic coating: HIREC 1450 with a contact angle of 152° were studied combined with a thermoelectric anti-icing system under icing conditions. All coatings and the reference surface were tested under glaze and rime ice. The deicing tests were conducted in the Anti-icing Materials International Laboratory's (AMIL) low speed closed loop Icing Wind Tunnel with 0.4 g/m₃ liquid water content, a 26.7 ± 2.6 μm water droplet median volumetric diameter, 21 ± 0.5 m/s air speed and temperatures of -5 and -20 ± 0.5°C. For these tests, a 4\mi chord NACA 63-415 airfoil 2D blade box-section of 10\mi covered with a thin aluminum sheet protected at the leading edge and on the bottom by a thermoelectric anti-icing system composed of two 1/2\mi x 10\mi heating elements with a power density of 40 W/in₂, was used.

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.

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.

Ice adhesion on critical aircraft surfaces is a serious potential hazard that runs the risk of causing accidents. For this reason aircraft are equipped with active ice protection systems (AIPS). AIPS increase fuel consumption and add complexity to the aircraft systems. Reducing energy consumption of the AIPS or replacing the AIPS by a Passive Ice Protection System (PIPS), could significantly reduce aircraft fuel consumption. New coatings with superhydrophobic properties have been developed to reduce water adherence to surfaces. Superhydrophobic coatings can also reduce ice adhesion on surfaces and are used as icephobic coatings. The question is whether superhydrophobic or icephobic coatings would be able to reduce the cost associated with AIPS.

This paper depicts icephobic coating performances of 274 different coatings, including 11 grease-type coatings, which were tested over the past 10 years in various research projects at the Anti-Icing Materials International Laboratory (AMIL). Icephobic performance is evaluated using two comparative test methods. The first method, the ice Centrifuge Adhesion Test (CAT), measures the force required to separate the accreted ice from the coating (e.g. adhesive failure). The test involves simultaneously icing, under supercooled precipitation, the extremity of bare reference and freshly coated aluminum samples. The ice adhesion shear stress is calculated from the ice detachment rotation speed. The results are reported as Adhesion Reduction Factor (ARF), which is the ice adhesion stress on the bare aluminum reference samples divided by the ice adhesion stress on the coated samples.

Many uncertainties in an in-flight ice shape prediction are related to convection heat transfer coefficient, which in turn depends on the flow, turbulence and laminar/turbulent transition models. The height of ice roughness element used to calculate the Equivalent Sand Grain Roughness height (ESGR) is a very important input of the turbulence model as it strongly influences the shape of the accreted ice. Unfortunately, for in-flight icing, the ESGR is unknown and generally calculated using semi-empirical models or empirical correlations based on a particular ice shape prediction code. Each ice shape prediction code is unique due to the models and correlations used and the numerical implementation. Ice roughness correlations do not have the same effect in each ice shape prediction code. A new approach to calculate the ESGR correlation taking into consideration the particularities of the ice shape prediction code is developed, calibrated and validated.

Simulating freezing and frozen precipitation in an indoor laboratory setting can permit year round evaluation of de/anti-icing systems and fluids. At AMIL, freezing rain, freezing drizzle, icing fog and in-cloud icing as well as frost, snow, ice pellets and icing clouds can be simulated in a variety of cold chambers of different heights and with different wind conditions using specialized spraying systems and temperature set-ups. Freezing rain is simulated using a 9 m high vertical chamber capable of supercooling water droplets from 100 to 1000 μm, so they freeze not long after impact. The freezing drizzle is simulated in a 4 m high chamber where supercooled droplets from 50 to 250 μm freeze on impact. Icing fog and in-cloud icing are simulated with the help of a pneumatic spray nozzle system which allows for a finer water spray, in the 20 μm diameter range. The frost is simulated by saturating a cold room with humidity generated from a heated, temperature controlled water bath.