## 2 Flight Test Bed, Measurement System and Equipment, and Test Conditions  
### Flight Test Bed and Measurement System Onboard  
Skin friction is one of the most difficult flow properties in aerodynamics to measure. Among various methodologies that can be applicable to measure the skin friction force, the pitot-rake measurement was employed in this study for the following reason. First, the direct estimation method from the change of the fuel consumption rate was not feasible as the riblet surface was only implemented on a very small part of the whole aircraft, and thus evaluating the fuel consumption rate with and without the riblet was extremely difficult over the measurement error caused by different flights [1]. A cantilever measurement system, which has been widely used to measure the skin friction force in wind tunnel experiments [30; 31; 32], is very sensitive to error caused by the configuration, such as a very small gap or misalignment. Optical measurement methods such as using oil flow measurement are also very difficult because the riblet width is very tiny [33; 34; 35]. Conversely, direct measurement of the velocity profile in the boundary layer by the pitot-rake measurement is reliable to measure the velocity profile with highly accurate pressure transducers [33; 36]. The measured velocity profile is a time-averaged value as the pitot-rake measurement involves pneumatic probe systems. Figure 1a shows the flight test bed and pitot-rakes with static pressure and temperature ports implemented on the airframe surface, and just in front of the pitot-rake. The pitot-rake was designed to have a rigid structure made of stainless steel and was constructed on the metal window surface, the original window of which was replaced with the metal surface so as to install the pitot-rake. The riblet portion is also shown in the figure. It should be noted that the riblet size is in the order of 100 \(\upmu\)m; therefore, the measurable range would be from the log-law region to the outer region of the boundary layer. The details of the riblet will be described briefly in a later section. Two pitot-rakes were located behind the riblet sheet, which had a cord length of 1306 mm, an effective span length of 574 mm, and a thickness of approximately 100 \(\upmu\)m.  
Figure 1b shows a schematic of the pitot-rake, which consists of 45 single-hole probes for measuring pitot pressure to identify the boundary layer profile on the aircraft fuselage. The minimum probe diameter employed in the present study was 0.5 mm. The direction where the pitot-rakes were aligned was determined by assuming that the pitot-rakes were located on the streamline along the fuselage which was estimated by numerical simulation [17]. Note that an actual streamline can vary depending on flight conditions and, hence, the location of the pitot-rake may not be aligned with the flow. However, since the measurement in this study was conducted at steady cruise conditions where the angle of attack was near zero, the actual streamline was assumed to be close to that simulated by the computational analysis. Additionally, the pitot-rake was considered to measure the streamwise velocity  
of the flow accounting for past studies [37] that showed that the measurement accuracy was ensured up to the angle of attack of 20\({}^{\circ}\) at flight Mach number of 0.64 [37]. Furthermore, a calibration wind tunnel experiment, which was conducted prior to the flight experiment [23], showed that the uncertainty in measuring the pitot pressure for the designed pitot-rake can be negligible in the range of the angles of attack of less than \(\pm 5^{\circ}\)[23]. Thus, the pitot-rake was considered to measure the streamwise velocity profile of the flow on the aircraft fuselage at targeted steady cruise flight conditions. The aft-part of the pitot-rake has a rhombus cross-section to avoid generating extra lift and to avoid vibration introduced by the unstable flow separation.