Trans. Japan Soc. Aero. Space Sci. Vol. 64, No. 1, pp. 22–30, 2021 DOI: 10.2322/tjsass.64.22 Effects of Propeller Position and Rotation Direction on the Ishii Wing at a Low Reynolds Number* Koji FUJITA,1)† Kakeru KURANE,2) Koichi TAKAHASHI,1) and Hiroki NAGAI1) 1)Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980–8577, Japan 2)Department of Aerospace Engineering, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980–8577, Japan The aerodynamic characteristics of a wing in a propeller slipstream were investigated at a low Reynolds number. The effects of propeller position and rotation direction on the wing were clarified by aerodynamic measurements and particle image velocimetry. The propeller positions were the center and tip of the wing model, whereas the rotation directions were clockwise and counterclockwise. The center propeller configuration with a clockwise rotation showed a constant pitching moment and increased the lift-to-drag ratio. This was caused by the high-speed propeller slipstream (i.e. 12 and 10 m/son the upwash and downwash sides, respectively) and the wingtip vortex effect on the slipstream separation. The separation point at an angle-of-attack of 18° was delayed from x/c µ 0.1 to 0.3 by the wingtip vortex. Hence, the following two fac- tors must be considered to enhance the aerodynamic characteristics of a Mars airplane: (i) the ratio of the area of the up- wash and downwash sides of a wing in a propeller slipstream, and (ii) the effect of the wingtip vortex on the propeller slipstream. Key Words: Low Reynolds Number, Propeller Slipstream, Wind Tunnel Testing, Aerodynamic Characteristics, Aerodynamics Nomenclature (Fig. 1)1–8) as a new means to conduct Mars exploration. The Mars airplane can explore a wider range than rovers b: span length and obtain higher resolution data than satellites. c: chord length However, design difficulties arise because a Mars airplane 4 5 CD: drag coefficient flies in a low Reynolds number (Re ¼ 10 –10 ) condition CL: lift coefficient owing to the low atmospheric density of Mars. In such an en- CM: pitching moment coefficient vironment, the Ishii airfoil is a candidate for the airfoil of the D: drag main wing of the Mars airplane.9) The airfoil shows high per- J: advance ratio. J U1=ð2RnÞ formance even in a low Reynolds number condition owing to L: lift the flat upper surface and lower surface camber. n: propeller rotation speed The Japanese Mars airplane comprises a propeller propul- R: propeller radius sion system with a battery and motors. Large propellers are Re: Reynolds number required to obtain sufficient propulsion force for flights in U: flow velocity in main-flow direction low Reynolds number environments. Because the propellers U1: main flow velocity are large, the propeller slipstream may affect a large area of x: chordwise coordinate the wing. y: spanwise coordinate z: thicknesswise coordinate ¡: angle-of-attack 1. Introduction Planetary exploration has been actively conducted in re- cent years. In particular, Mars has attracted attention as a po- tential planet for human migration. In Japan, the Japan Aero- space Exploration Agency (JAXA) and Japanese universities have been developing a twin-propeller Mars airplane © 2021 The Japan Society for Aeronautical and Space Sciences +Presented at the 2016 Asia-Pacific International Symposium on Aerospace Technology, 25–27 October 2016, Toyama, Japan. Received 3 December 2019; final revision received 15 June 2020; accepted for publication 28 July 2020. †Corresponding author, [email protected] Fig. 1. Mars airplane © JAXA. 22 Trans. Japan Soc. Aero. Space Sci., Vol. 64, No. 1, 2021 The effect of the propeller slipstream on the wing has been number as shown above, the effect is still unclear. There investigated for many years. Witkowski et al.10,11) investi- are many variations of the condition, such as an airfoil, Rey- gated the effect of the propeller slipstream on the nolds number, propeller geometry, propeller position, pro- NACA0012 wing at a wing-chord-based Reynolds number peller rotation direction, and propeller advance ratio. These of 47 Â 104. They showed that there are three effects of the differences sometimes generate conflicting results. There- propeller slipstream on wing performance: (i) Change in fore, this study has two objectives. The first objective of this the local effective angle-of-attack by a swirl, (ii) Inclination study is to obtain the effect of the propeller slipstream on the of the lift force, thereby decreasing the induced drag as the wing used for Mars airplanes. Especially, this study aims to result of changing the inlet flow angle, and (iii) Change in lo- clarify the effects of propeller position and rotation direction cal velocity due to the induced axial velocity. Ananda et al.12) on the aerodynamic characteristics of the Ishii wing at a low investigated the effect of the propeller slipstream on the Reynolds number. The second objective is to offer one of the Wortmann FX63-137 wing at wing-chord-based Reynolds instances about the effect of the propeller slipstream on the numbers from 6 Â 104 to 9 Â 104. They observed an addi- wing under low Reynolds number conditions with different tional effect in this Reynolds number region. The propeller experimental conditions from previous literature. slipstream induces an early transition to turbulent flow in the regions within the slipstream and the premature forma- 2. Test Equipment tion of a separation bubble in the regions outside of the slip- stream; therefore, the lift force increased and the drag force The small low-turbulence wind tunnel at the Institute decreased. Deters et al.13) measured a velocity field of the of Fluid Science, Tohoku University19) was used. It is a propeller slipstream with a flat plate wing when static (with- circulation-type wind tunnel with a regular octagonal meas- out main flow). They observed that the upper and lower slip- urement section. The opposite side distance of the octagonal streams split by the flat plate wing moved away from each cross-section was 0.293 m. The measurement section can ei- other in the direction of their respective swirl velocities. ther be opened or closed; however, the open-section type was However, the results when in an advancing flow are not re- used in this experiment. ported. The Ishii wing model is shown in Fig. 2. The wing models A Japanese research group for the Mars airplane also in- were created using a three-dimensional (3D) printer vestigated the effect of the propeller slipstream on the wing (KEYENCE Co., AGILISTA-3100). The z-axis resolution using an even lower Reynolds number. Makino et al. re- of the 3D printer was 15 Lm. The aspect ratio of the model ported that the aerodynamic performance of a NACA0012 was 1.5 (i.e., the chord length was 100 mm and the span wing affected the propeller slipstream at a Reynolds number length was 150 mm). of 4 Â 104 based on force measurements and flow visualiza- The APC propeller 5 Â 5E was used.20) The diameter and tions.14) In this study, the propeller was fixed at the center of pitch were both 127 mm. the wingspan. Previous studies regarding propeller slip- A brushless DC motor (Maxon Motor, EC-45-flat) was streams by Ushiyama et al. indicated that the effects of the used for the propeller rotation. The number of rotations propeller slipstream on a wing differed depending on the pro- was controlled using a controller (Maxon Motor, peller spanwise and chordwise position and rotation direc- ESCON50/5). Because this study focused on the interaction tion.15,16) In these studies, the propeller at the wingtip dis- between the propeller slipstream and the wing, the test under turbed the wingtip vortex and decreased the induced drag; each condition was performed based on the same advance however, the NACA4406 wing was used for the test con- ratio J. Therefore, we controlled the propeller rotation to ducted. Additionally, we intended to investigate the effects fix the number of rotations, not the input power to the motor. on the Ishii wing, which is a candidate for the main wing of the Mars airplane. Matsumoto et al.17) performed CFD analysis for a tractor-type propeller and a flat plate wing con- figuration with several spanwise propeller positions at a 4 wing-chord-based Reynolds number of 5 Â 10 . The propel- (a) Cross-section of Ishii airfoil ler rotation direction was opposite of the wingtip vortex. The lift-to-drag ratio of the wingtip propeller configuration was higher than the middle spanwise position propeller configu- ration. Recently, Furusawa et al.18) investigated the effect of the chordwise direction position of the propeller, namely, the tractor and pusher configurations. Even though the pitching moment characteristics of the wing are one of the important points for longitudinal stability and control, the effect of the propeller slipstream on the pitching moment of the wing has 10–18) not been discussed in some studies. (b) Picture Although there is some literature that discusses the effect of the propeller slipstream on the wing at a low Reynolds Fig. 2. Wing model. ©2021 JSASS 23 Trans. Japan Soc. Aero. Space Sci., Vol. 64, No. 1, 2021 (a) center_ccw (b) center_cw (a) Picture (c)tip_ccw (d)tip_cw Fig. 4. Propeller positions and rotation directions. rotation stages coincided with the wing and propeller. The propeller angle-of-attack was changed to match that of the wing angle-of-attack. Hence, the relational position between the wing and propeller was always fixed. (b) Center position of wingspan The classification of propeller conditions is shown in Fig. 4. Five conditions for propeller positions and rotation directions were used: no propeller, center_cw, center_ccw, tip_cw, and tip_ccw.