## 3 Results and Discussion
### Propeller Slipstream Effects on Aerodynamic Performance
The pitching moment for analyzing the static longitudinal stability had a natural statically longitudinal stable region at \(\alpha\) < 0\({}^{\circ}\) and positive contribution on the longitudinal stability side as the incidence increased. The pitching moment coefficient slope was closer to linear than that for the isolated wing-fuselage model in Ref. [11]. In contrast, the pitching moment slope, C\({}_{\rm M\alpha}\)\({}_{\rm\alpha}\), was about \(-0.0033\) (for the linear section at incidence between 6\({}^{\circ}\) and 36\({}^{\circ}\)). The propeller showed a negative contribution on the statically longitudinal stability and a positive pitching moment slope can be identified in Figure 7f. For the cruise condition, the lift force at zero degree of angle of attack was equal to the MAV weight and the thrust force could overcome the drag force, shown in Figure 7b. Figure 8a shows the locations of C\({}_{\rm p}\) distribution on the Zimmerman wing at r/R = 1 (or z/b = \(\pm\) 0.25). The C\({}_{\rm p}\) distribution locations are indicated by the dash line for the up-going  
Figure 7: Averaged aerodynamic coefficients versus \(\alpha\) at J = 0.468, (**a**) lift coefficient, (**b**) drag coefficient, (**c**) side force coefficient, (**d**) rolling moment coefficient, (**e**) pitching moment coefficient, and (**f**) yawing moment coefficient. and down-going blade sides. Both \(\mathrm{C_{p}}\) with and without slipstream at \(0^{\circ}\) flight condition are presented in Figure 8b-e: the instantaneous \(\mathrm{C_{p}}\) distribution is plotted with various azimuth angles (in solid line). The \(\mathrm{C_{p}}\) plots show that the positive lift force was generated at the positive camber area around x/c = 0.25 and the negative lift force was obtained from the negative camber, which was located near the trailing edge. The negative lift was also generated at near leading-edge area on the down-going blade side. The averaged \(\mathrm{C_{p}}\) at \(2z\)/b = 0.5, in Figure 8f, shows that a similar amount of lift was produced by the isolated MAV model and the MAV propeller model. However, the negative lift generated at the reflex camber region by the MAV propeller model was quite significant, and more negative lift was produced due to the propeller slipstream effect. In comparison, the amount of negative lift almost doubled as compared with the isolated MAVs (dashed line). Therefore, less reflex camber could be used for the MAV with the propeller installed. The turning point is the point where the negative lift force started to occur, which shifted toward the leading edge slightly due to the propeller slipstream effect. Figure 9 shows the two-dimensional lift and drag coefficient distribution at various spanwise locations. The difference of the lift coefficients distribution for both wing sides is clearly shown. The drag coefficient, on the other hand, showed a higher value on the down-going blade and a slightly lowered value on the up-going blade. The possible explanation could be that in the up-going blade region, the propeller swirl counteracted the effects of the wing downwash such that the local angles of attack were increased. This effect simultaneously augmented the section lift and rotated the force vector forwards, which reduced the drag component at the section, as detailed in Figure 6. The sudden increase in lift or decrease in drag coefficient indicated the aerodynamic interaction on the wing and the fuselage from the propeller wake. In general, the major slipstream effect was working at 60% inboard wingspan, \(2z/b\leq 0.6\), and less effects were observed toward the wingtip.