# Advances in Riblets Design
## 4 Manufacturing and Fabrication of Riblets
The use of microscale features with indentations aligned to the flow can be an effective solution to reduce drag by up to 10% [67]. In this context, a number of techniques are available for laboratory tests. One of the main constraints on the fabrication process is the flow properties passing through the surface. This directly depends on the flight condition with the ability to make riblets in the order of microns to meet the optimum operation of riblet surfaces [18; 21]. As an example, for near transonic commercial aircraft, due to the reduction in density and other aerodynamic parameters, micro-scales of height (around 80 um) are required for optimum drag reduction [50]. By focusing on the aviation industry, microfabrication techniques were investigated during the past decades, with successful results [19; 66; 86; 120]. Industrial manufacturing of riblet films was introduced by 3M Company with the development of polyvinylidene fluoride plastic films using conventional treatments [118], such as rolling and casting, with h/s = 1 triangular riblet patterns and with the possibility of implementing curved and riveted surfaces. Several experiments were conducted for 3M products in both laboratory and flight tests (see Table 1). In another research study, Bionic Surface Technology and Joanneum Research [85] printed microstructures using a special process on a very thin adhesive film. BASF, a chemical producer company, worked in collaboration with Lufthansa to develop industrialized versions of adhesive riblet films. The films were tested as patches on the Boeing 747-400. Although adhesive films were easily applied on about 70% of aircraft surfaces, they had a few drawbacks, such as difficulties during the application on curved surfaces and the possibility of detachment during the flight [25]. In the meantime, to overcome the disadvantages of riblet adhesive films, innovative paint applications were introduced by researchers [67; 84; 104] and the Fraunhofer Institute for Manufacturing Technology (Tables 1 and 2). The process was applied by a transparent and flexible silicon endless belt. [PERSON] et al. [67] and [PERSON] et al. [66] carried out this technique with nanoparticle-reinforced painting of large parts, like aircraft and ship surfaces. The process consists of coating, embossing and partial curing in a single step. A roller-based mechanism of painting is used with continuous UV-curable urethane-acrylates that enable a microstructure surface with proper durability and reasonable costs; the mechanism is illustrated in Figure 5a. This process was applied by a robot using an automatic application of microstructures on the surface of aircraft and was launched as a collaboration between Lufthansa, Airbus, BWM GmbH, and Fraunhofer IFAM in a project named FAMOS and included a multi-functional coating [85]. The automatic application of riblet coating is depicted in Figure 5b. Lithographic methods can also be used for printing micro-grooves on the surface of flying vehicles [5, 45, 86]. [PERSON] [86] and [PERSON] et al. [45] applied a photo-curable coating method over a military aircraft. The polymer material was a combination of three important parts containing monomers, oligomers, and photoinitiator that, in exposure to UV light, makes a strong polymer. The main objective of this method was to achieve continuous and contactless production of riblets (Figure 6), which enables the change in height and shape of the riblet structures with optimum implementations over the aircraft surface. Due to the interaction of airflow over the aircraft body and wing, the optimum selection of riblet height is an important consideration. The lower cost of implementation, quality of micro-fabrication, and increased lifetime were some of the advantages of this method.