Development of Hydrophobic Windscreen Coating for Next Generation Civil Tilt Rotor

Periodic Reporting for period 3 - HaSU (Development of Hydrophobic Windscreen Coating for Next Generation Civil Tilt Rotor)

Okres sprawozdawczy: 2020-06-01 do 2020-11-30

Aircraft canopies and windshields must keep pilots’ visibility at excellent levels under several operating conditions, particularly under heavy rain occurring during flight. To this purpose, the majority of modern commercial aircrafts use wipers to shed water from the windshields. Use of wipers is no longer foreseen in some future civil aircrafts such as the Next Generation Civil Tilt Rotor (NGCTR) while, for example, fighter jets do not possess windshield wipers since a long time due to their operational requirements of high speeds and for the low observable (stealth) constraints of modern aircraft.
A durable, surface coating on the windshield to shed water would be an ideal solution to maintain visibility without using wipers and will be of fundamental importance to limit the formation of rain-induced ice on windscreen. Today’s commercially available rain repellent treatments have poor durability and weather resistance, and they cannot be applied to polymeric transparent substrates, such as aircraft canopies/windshields. In fact, there are no solutions today proven and available on the market for effective hydrophobic coating on plastic materials such as polycarbonate, acrylic and polyurethane.
The HaSU proposal was about the design and development of a rain repellent substrate capable of forming highly durable hydrophobic coatings based on a particulate inorganic homogenously distributed throughout a polymeric matrix.
Specifically, the HaSU project directly addressed the challenges inherent to design, fabricate, and experimentally demonstrate a type of hydrophobic coating protection for the Next Generation Civil Tilt Rotor (NGCTR) wiperless windscreen. Specific objectives of the project were:

1. Design and development of a specimen of the NGCTR windscreen substrate having the requested hydrophobic functionalities and performance throughout the whole flight envelope
2. Manufacturing of hydrophobic windscreen specimens
3. Testing and qualification of proposed hydrophobic specimens according to RTCA DO-160 (Environmental Conditions and Test Procedures for Airborne Equipment of proposed hydrophobic specimens)

In the first phase of the project, the design requirements of the NGCTR hydrophobic windscreen specimen have been defined as a result of a preliminary feasibility based on both engineering considerations as well as on more detailed numerical simulations using CFD studies. The preliminary layout configuration has followed from a trade-off among overall, conflicting, requirements which eventually led to configurations agreed with the topic leader.
Next, hydrophobic coating specifications have been released to the topic manager along with preparation of a Qualification program plan (QPP) and Development plan (DP). Failure mode and effects analysis (FMEA have been performed for failure analysis.
Following this, a number of substrate specimens along with larger specimens including hydrophobic coating on top have been manufactured using sol-gel technology.
The hydrophobic coating has been applied on tiltrotor’s windshield representative samples that have been subsequently exposed to different environmental testing according to DO-160 “environmental conditions and test procedures for airborne equipment”: temperature and altitude, temperature variation, humidity, fluid susceptibility, sand and dust, fungus resistance, salt fog; and MIL-STD-810 “environmental engineering consideration and laboratory test”: solar radiation. After environmental testing windshield representative specimens have been tested according to the ATP and glass specimens have been tested for water contact angle and surface free energy.
Test results have shown that the hydrophobic coating is able to satisfy ATP (Acceptance Test Procedure) requirements even after environmental testing. No variations of properties such as coating thickness, light transmission, optical properties in terms of distortion and deviation or delamination of the coating have been observed after environmental testing. No fungi growth has been observed on the coating surface.
Regarding hydrophobic properties, the coating was able to satisfy ATP requirements after environmental testing. Test results have however shown a general trend of reduction of water contact angle and a slight reduction of surface free energy after environmental testing with the exception of the DO-160 fluid susceptibility test where the trend was the opposite.
The most critical environmental test was found to be the DO-160 sand and dust. This test generates a micro-abrasion of the coating surface consequently increasing the haze level of the window and reducing hydrophobic properties of the specimen. Nevertheless, the tested specimen was still able to satisfy the ATP even if the mean water contact angle was at the limit of acceptability, that is, 95°.
The coating has been proven to be airworthy for the application on the Next Generation Tilt Rotor (NGCTR) windscreen.

Progress beyond the state of the art:
Application of new sol-gel technology, investigated in HaSU has provided sharp visibility and clearness thanks to water, oil and solvent repellence. Coating also was able to withstand significant abrasion without sacrificing performance on glass.

Results achieved at the end of the project included:
• Improvement in adhesion of the so-gel technology to polymeric substrates
• Application to the sol-gel technology to bigger substrates, representative of a real windscreen
• Verification of the optimal composite layup in the 3D model

Regarding potential impact, a significant innovation potential is recognized in HaSU.
In fact, the validation of HPC on NGCTR windscreens has been the first one for a future commercial aircraft, and as such it will ensure a huge knowledge advantage for European aviation community. In fact, HaSU will act in the direction of improving aircraft reliability, performance and safety which, in turn, reduce both recurrent and non-recurrent costs of an aircraft.
The HaSU project will boost the capacity to create useful innovation by advancing state-of-the art on surface coating technology as a key enabling technology with the final aim of identifying proper coating methods to enhance airworthiness by improving pilot visibility and comfort. The project will contribute to advancement in knowledge on implementing new windshield technologies with credible and measurable benefits for the scientific and industrial community. In fact, HaSU will significantly increase the applicability and reliability of advanced coating technology in general, out of the research niche into industrial practice. The feasibility of such application contains the possibility of earlier, more reliable design decisions at system level, not only from global result numbers, but even more from the insight and comprehension provided by the detailed analysis of local phenomena. From a research perspective, the physical understanding of flow phenomena under improved wettability is most interesting and will contribute significantly to the growing knowledge of fluid physics in general. Furthermore, the possibility to test several chemical compounds in the coating development phase will make a significant contribution on the chemical engineering side.

Final hydrophobic sample, environmental tested

Schematic representation of the PECVD Technology

Schematic representation of the Sol-Gel Technology

CFD simulaton of water impingment on AW609 Windscreen

Periodic Reporting for period 3 - HaSU (Development of Hydrophobic Windscreen Coating for Next Generation Civil Tilt Rotor)

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