# Advances in Riblets Design
## 3 Experimental Investigation of Riblets Performance
### Flight Tests and In-Service Applications of Riblets
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Total pressure measurements have been implemented mostly in research activities by use of boundary layer pitot rakes [76; 96; 103]. [PERSON] et al. [103] conducted flight tests with 3M plastic films, which were mounted over the wing surface of a T-33 aircraft. The effect of riblet films was investigated using pitot rakes positioned at 83% of the chord. Results indicated a 6% reduction in drag in the optimum condition, based on cruise Reynolds number and non-dimensional spacing between 10% and 15%. [PERSON] et al. [76] tested riblet geometries on the Learjet 28/29 twin-engine business jet. The instrumentation was installed in the baggage compartment area, and tests were carried out by the 3M Company, using a \(30\times 90\) cm patch of two different sizes of triangular microgrooves. The test areas were equipped with two boundary layer-type total pressure rakes and two drag balance mechanisms to evaluate the drag force in both smooth and ribbed surfaces. To ensure the effectiveness of the process, two test panels were considered, i.e., a smooth panel for reference and another for measuring the flow properties after the riblet patch. Measurements from the boundary layer pitot rakes indicated a reduction in drag of about 6%, while drag balance required some correlations due to the crossflow angles resulting in degradation of the drag value. [PERSON] et al. [96] and [PERSON] et al. [104] investigated the drag reduction of trapezoidal riblets by use of two pitot pressure rakes on the fuselage of a JAXA research aircraft. The position of the riblet surface and pitot rakes are illustrated in Figure 4. During flight tests, the window was replaced with a flush-mounted metal plate aiming to embed the two pressure rakes. Mean velocity measurements in the \(y^{+}\) between 100-10,000 show the increase of the U-velocity based on the direction of pitot rakes. In-service tests indicate the effectiveness of covered-by-riblet surfaces in reducing fuel consumption. In the [PERSON] [105] research study, 70% of the area, or 600 square meters of Airbus 320 aircraft surface, was covered by triangular riblet films with about 80 kg of additional weight. Based on the fuel consumption, the results show a 2% reduction in drag. During the past years, Lufthansa has produced comprehensive research studies on the effectiveness of riblets besides the possibility of applying them on the aircraft surface [85]. Lufthansa, Bionic Surface Technologies, and Joanneum Research have developed and tested aviation-size trapezoidal riblet films. In 2019, tests were performed in a Boing 747-400 aircraft, with 500 square meters of surface covered by riblets. Lufthansa, Technique and BASF [19] made a successful collaboration for the application of riblet films in the 800 square meters of Cargo Boeing 777 F surface by using trapezoidal riblets with a size of about 0.05 mm in height and spacing. Flight showed an annual saving of around 3700 tons of kerosene for the fleet of ten aircraft, with a reduction of about 11,700 tons of carbon dioxide when using the riblets. The result was more than 1% of drag reduction. After the successful application of riblets on a Boing 777 F, Swiss International Airlines (SWISS), in collaboration with Lufthansa and BASF, implemented riblet film technologies on  
Figure 4: Riblet surface and position of two pitot rakes [107]. the twelve Boing 777-300 ER aircraft by using 950 square meters of riblets-covered surface, which lead to 4800 tons of kerosene annual savings and approximately 1.1% reduction in drag. This reduction in fuel consumption corresponds to a decrease of 15,200 tons of carbon dioxide. In 1988, Airbus, Lufthansa, and 3M Company conducted tests with 15 samples of riblet films mounted on the body and wing of an Airbus 300-600 [105]. After 18 months and checking the durability of riblet patches, no serious damages to the riblet geometries and no degradation in the properties of the riblet films were identified. [PERSON] et al.