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.
