# Advances in Riblets Design
## 9 Contamination and Deterioration in Riblets  
The long-term performance of riblet designs can be influenced by extended exposure to operational conditions, weather variations, and maintenance schedules. Factors such as UV radiation, temperature fluctuations, and particle interactions can contribute to the degradation of riblet surfaces over time, potentially affecting their effectiveness in drag reduction. Maintenance schedules that account for these factors, aligned with regular aircraft maintenance cycles, are crucial to maintaining riblet performance. In this issue, developing accurate predictive models for riblet degradation and maintenance intervals is a complex endeavor that requires a comprehensive understanding of material behaviors, environmental impacts, and operational requirements. By applying polymer-based micro-structure riblets on aircraft surfaces, an essential factor would be their durability under the aircraft in-service environment. As the aircraft surface is confronted with external and weather factors, the lifetime of these micro-grooves ideally should be aligned with the maintenance intervals of an aircraft, e.g., depainting and painting cycle every five to seven years. Strong radiation of UV light can potentially affect the effectiveness of riblet geometries or, in the case of adhesive-back riblet films, can strongly damage the adhesion quality [18]. This degradation would be even more important when the variation of the ambient temperature from flight conditions (around \(-\)60 \({}^{\circ}\)C) to extreme ground conditions (around 50 \({}^{\circ}\)C) is added to the problem. Figure 14: Comparison of analytical models with experimental data for a control diffusion airfoil. (**a**) [PERSON] and [PERSON]’s analogy with a flat plate, and exact [PERSON]’s functions and [PERSON]’s model; (**b**) [PERSON]’s and [PERSON]’s models [60]. On the other hand, the impact of high-velocity particles, including dust, water droplets, and ice, could potentially erode or remove riblets. In 1988 [83], Lufthansa conducted long-term studies on the triangular riblets of 3M Company by applying 15 riblet patches that were mounted over the surface of an Airbus 300-600 to analyze the effect of erosion on the geometries. After 18 months of in-service operation of the aircraft, the patches were carefully removed and tested by Airbus industry partners (Aerospatiale, British Aerospace, Deutsche Airbus, and 3M Company). Despite high-risk areas like the fuselage nose and all leading edges, the other parts did not show serious damage. Airbus, in collaboration with Lufthansa and Cathy Pacific Airways, initiated a test to monitor the degradation of the riblets film in commercial flights. For this purpose, patches of 3M riblet films were attached to the Airbus 340 aircraft. The results suggested that the riblet films need to be replaced every 2 to 3 years to maintain the drag reduction capacity [25]. [PERSON] et al. [66] conducted in-service investigations for about twelve months by mounting patches of micro-fabricated trapezoidal riblets over the body and door of the Airbus 300-600 ST BELUGA to investigate the durability of the coatings. They used the dentist's casting compound to make a mold of micro-fabricated patches for a specified period of time. The negative periodic generated mold was scaled up in the order of 100 for testing in the Berlin oil tunnel [47]. Their microscopic investigations revealed that the process of deterioration was non-homogenous. As illustrated in Figure 15, the effect of degradation on the negative mold of micro-riblets shows more degradation on the door patches than on the aircraft body patches. Figure 16 shows the oil tunnel measurements after application and cleaning of the paint bumps, as well as the measured drag after 411 flight hours.