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Development of SmarT Eco-friendly anticontamination technologies for LAminaR wings

Periodic Reporting for period 2 - STELLAR (Development of SmarT Eco-friendly anticontamination technologies for LAminaR wings)

Reporting period: 2021-04-01 to 2022-09-30

The aircraft industry is facing issues with the increase of drag directly impacting the fuel consumption of the fleet. Achieving natural laminar flow requires high surface quality. Tiny air flow disturbances at the surface can indeed cause an early transition from laminar to turbulent flow. The accumulation of insect debris on the leading edge of laminar wings has been recognized as one of the most significant operational concerns associated with laminar flow.

The main objective of STELLAR is to develop efficient and durable anticontamination coating and cleaning solutions designed as a result of a deep understanding of the insect residues properties. Hence, STELLAR project seeks to gain insight on the understanding of the biochemical transformation of hemolymph during flight phases and the consequent physico-chemical properties of the surface with a special focus on the coating adhesion. While it has been confirmed that surface wettability properties (hydrophobicity) and texture play a key role in the adhesion of the insect residues on a surface, STELLAR also highlighted the role of the chemical composition of the coating which can affect the hemolymph coagulation process. Coagulation of hemolymph leads to the chemical transformation of proteins leading to a stronger adhesion on surfaces. This transformation is strongly impacted by the aircraft environment as UV exposure.

The project consortium has gathered cutting edge multidisciplinary knowledge and the needed facilities to provide a deep understanding of the contamination issues. This approach has the potential to significantly enhance the current understanding of the key issues and highlight which surface characteristics have the greatest influence on insect residue adhesion. From this approach, new coating and cleaning solutions are developed. Special focus is made on the durability in aircraft environment for the development of anticontamination coatings while the cleaning solution formulation is based on an eco-design approach, using low amount of solvent combined with specific enzymes.

The knowledge acquired and the solutions developed are evaluated through large scale tests : tests in wind tunnel which allow simulating extreme conditions occurring during flights, on a test aircraft (short flight tests) equipped with cameras and sensors (T°, RH, pressure) to monitor all the parameters responsible for real condition contamination. These tests mainly revealed the easy-to-clean properties of the coating and allow the selection of a promising candidate that will be applied on the slat of a commercial aircraft which will operate for 6 months at higher altitudes and allow a full validation of the newly developed solution.

As a summary, the project STELLAR aims at producing sustainable solutions to reduce drag on aircrafts and enable laminar flow over time with direct impact on fuel consumption and thus contributes to aviation’s environmental footprint reduction.
1. Establishing the link between hemolymph modification (under atmospheric conditions) and interaction with the surface
A comprehenseive study was carried out by ESPCI and UMONS on the biochemical modification of hemolymph which allowed to determine the main proteins undergoing chemical transformation and the study of physico-chemical properties of hemolymph under coagulation. During the second period, the laboratory experiments focused on the evaluation of impact velocity and UV exposure on the biochemical modifciation of hemolymph and the consequent interaction with the surface. This was done on model coated surfaces and on stainless steel. The main outcomes from this part of the project is the definition of critical parameters for the design of anticontamination of coating and cleaning solutions; the unexpected and crucial role of chemical composition on coagulation kinetics and the influence of exposure to aircraft environment on the transformation of hemolymph and the consequent increased adhesion on the surface (especially under UV exposure). At least two scientific publications are foreseen : one under submission by ESPCI and CNRS; one in preparation by UMONS. CIDETEC is also considering patent application on the coating and the link with hemolymph coagulation kinetics.
2. Formulation of smart eco-friendly anticontamination coatings
Coatings were developed from a predefined palette of technologies designed on the basis of the knowledge acquired on hemolymph modification and interactions with surfaces. The coatings are based on several chemistries and include slippery effect and self-healing properties. The formulations were optimized and their durability under aeronautic requirements evaluated. This led to the selection of 3 candidates which were tested following a set of aeronautic requirements (impact and erosion resistance, UV, etc). Several communications (conferences, general publications) were carried out to communicate on anticontamination coatings.
3. Development of eco-friendly cleaning solution
SOPURA developed solvent and enzyme-based formulas providing very good detergency effect. The application process was evaluated at laboratory scale and the best solution was tested in real conditions on the test aicraft of SONAIR. Life cycle assessment was carried out by Materia Nova on the developed solution and allows to reveal hot spots which might be used for further optimization.
4. Development of a test device for wind tunnel and short flight tests
Fuel tank of the S200 Sonaca Aircraft has been selected to be used for testing the 3 coatings and has been equipped with sensors and cameras (captiflex). The test device was evaluted in wind tunnel, than revised and adapted to satisfy secure flight tests with S200 aircraft.
5. Wind tunnel and short flight tests on selected coatings
The 3 selected coatings were tested in wind tunnel (combined or not with water injection to evaluate self-cleaning under rain) and further tested on the S200 aircraft. Many communication (general press, etc) were carried out during the short flight test campaign.
- Formulation of coatings which have direct impact on insect coagulation kinetics which is an unexpected results which was never reported in the literature; Cidetec is considering to patent the coating formulation.
- Formulation of self-healing polyurethane coating : replacement of a fraction of the binder by self-repairable (under RH trigger) was done with success and provide coatings with promising properties (further explored in the second period of the project).
- Formulation of innovative cleaning solutions showing synergistic effect with anticontamination coating leading to 100% removal of highly sticky insect debris.
- Design of a test device that can be used in wind tunnel and for short flight test
- Test protocols in wind tunnel to evaluate anticontamination and self-cleaning properties of coatings
By the end of the project, new coating and cleaning formulations will be available that fully or partially meet aviation requirements and provide an anti-contamination effect for leading edge applications. This type of coating may have a wider scope of application (including trains, automotive, wind turbine) and the knowledge acquired will be useful in understanding and minimizing contamination in general. Creation of new jobs can be expected at the end of the project in order to reinforce commercial, technical assistance and the industrialization of the new solution
Test aircraft used for validation of anticontamination coating (SONAIR)
Shop trials at Sonaca
Microrheology measurement on hemolymph (ESPCI)
Sliding angle measurement on coating (MANO)
S200 fuel tank as test device for wind tunnel tests
Hemolymph extraction from insect (UMONS)