# Advances in Riblets Design
## 3 Experimental Investigation of Riblets Performance
### Flight Tests and In-Service Applications of Riblets  
The previous section provided a brief overview of wind tunnel tests and testing techniques specifically focused on flat plates and airfoils. In this process, riblets are also tested during flight, aiming to analyze their performance and find out their real performance compared to the wind tunnel tests and numerical simulations. This is an important step in the process of commercialization when it comes to analyzing complex flow structures such as unsteady flows, different pressure gradients, misaligned flows, and wing-body flow interactions. It is noted that the overall positive benefit of using riblets in the profit of airlines for one year of in-service operation of a long-range aircraft would be roughly one million dollars per plane [6]. This calculation is based on 3% drag reduction by riblets, which reduces fuel consumption and consequently decreases the amount of carried fuel and allows replacing this load with extra commercial payloads. This is a good indication of riblets' performance  
Over the past few decades, several research activities have focused on commercial aircraft [6; 66; 103; 104; 105]. These studies have explored the effectiveness of riblet geometries through various approaches. Some investigations involved applying riblet patches over the aircraft body to measure total pressure, conduct drag balance measurements, and assess surface quality after flight. In contrast, other studies encompassed covering a significant portion of the aircraft with riblet films and evaluating their effectiveness by analyzing the aircraft's fuel consumption [19; 105; 106]. Table 2 provides a summary of the testing of riblets in the aviation industry. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Researcher** & **Laboratory** & **Rible Geometry** & **Material** & **Instruments** & **Description** \\ \hline \multirow{4}{*}{Walsh} & NASA Langley, close loop & Flat plate, & & & \(s^{*}=12,h^{*}=12,R_{eq}=867-3900\) \\  & low-speed wind tunnel, & triangular & Aluminum & Floating & Drag reduction: \% \\  & Test section: & & machined plate & balance, hot wire & \(s^{*}=16,h^{*}=8,R_{eq}=867-3900\) \\  & \(0.279\,\,\,0.178\,\,\times\,0.914\) m & Flat plate, semi-circular & Flat plate, semi-circular & & \(\upsilon=76-43\) m/s \\ \hline \multirow{4}{*}{[PERSON]} & Applied Fluid Mechanics & & & & \(s^{*}=16,h^{*}=8,R_{eq}=867-3900\) \\  & Division British Maritime & & & & \(\upsilon=76-43\) m/s \\  & Technology Ltd., Open loop & Flat plate, bladed & machine cut of soft steel & Surface flush hot film, flow visualization by smoke wire, microphone & \(R_{eq}=4600,h^{*}=12,s^{*}=20\), U = \(3\) m/s \\ \hline \multirow{2}{*}{[PERSON] [88]} & Royal Aerospace, UK & Flat plate, & & & \\  & Test section: \(0.6\,\,\,\times\,0.45\) m & Flat plate, & & & \\  & & & & & \\ \hline \multirow{4}{*}{[PERSON] et al. [55]} & Technical University of Delft & & & & & \\  & Test section: & & & & & \\  & \(0.73\,\,\,\times\,0.89\,\times\,5.7\) m & Flat plate, & Machined and flattened & Drag balance, pilot probe, & \(\upsilon=4\) m/s-17 m/s, \\  & & & & & & Drag reduction in AP57\% \\ \hline \multirow{4}{*}{[PERSON] et al. [80]} & University of & & & & \(s^{*}=28,h^{*}=14\) \\  & Maryland, open low-speed & Flat plate, & & & & \(\upsilon=1+5.5\,m/s\) \\  & wind tunnel, & triangular & Fiberglass & Hot wire, sensor probe & \(\upsilon=1\) m/s the \(R_{eq}=1200\) \\  & National Aerospace & Airfoil, Supercritical airfoil & & & & \(\upsilon=60,R_{eq}=3\times 10^{6}\) \\  & Laboratories, India & ADA-SI, triangular & & & & \\  & & & & & & \\ \hline \multirow{4}{*}{[PERSON] et al. [63]} & National Aerospace & & & & & \\  & Laboratories, India & Airfoil, NACA 0012 & & & & \\  & Test section: \(1.5\,\,\times\,1.5\,\,\)m & Triangular & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental studies of riblets in the wind tunnel.