## 3 Results and Discussion  
### Propeller Slipstream Effects on Aerodynamic Performance  
From previous investigations on both pusher and tractor propeller configurations, the latter was generally found to have higher aerodynamic efficiency and produce better control surface's effectiveness at landing and take-off. The MAV model considered here had a tractor configuration, and this was mainly to ensure the safety of the hand-throwing technique used on MAV take-off. Figure 6 illustrates the tractor propeller configuration effect on the left and right side of the Zimmerman wing. The propeller wake flow generated areas of upwash and downwash on the wing. Figure 5(a) shows the wing-airfoil aerodynamic loading in the upwash case where the propeller tangential velocity was greater than the downwash. The resultant force was tilted forward, and a localized wing thrust was created at this section. Figure 5(b) shows the aerodynamics of wing section under the propeller downwash. The wing downwash and the propeller tangential velocity acted together, resulting in a further reduction in the local angle of attack. The resultant force, F, shifted further backwards and resulted in an increased sectional drag. Figure 6: Propeller wing in tractor configuration, (a,b) wing local effective angle of attack due to propeller slipstream effect. Figure 7(a)–f shows the aerodynamic coefficient of the propeller at a rotational speed of 555 rad/s. The forces and moments for different components clearly showed in the plots individually (propeller, wing-fuselage, and the vertical stabilizer). The lift coefficient in Figure 7(a) showed a linear variation for \(\alpha<6^{\circ}\) and became nonlinear as the angle of attack was further increased. The maximum lift coefficient, \(\text{C}_{\text{L,max}}\), was about 0.81 at the incidence of \(12^{\circ}\). The propeller had negligible lift contribution at low angles of attack. The vertical stabilizer also showed a near zero contribution on the lift coefficient. At high angles of attack, the propeller had a significant contribution on the lift, which improved the maximum lift coefficient. The drag coefficient, in Figure 7b, increased continuously with the angle of attack when the rotational speed was fixed at 555 rad/s. A nearly constant thrust was produced by the propeller with various angles of attack (C\({}_{\rm T}\) is roughly around \(-0.09\)). The main drag force was formed from the wing-fuselage. The vertical stabilizer, however, showed a negligible drag contribution for the entire range of incidences. Figure 7c shows the side force generated by the MAV model. In general, the MAV side force increased continuously as the angle of attack was increased. At 0\({}^{\circ}\), the main side force coefficient was generated by the wing-fuselage, approximately 0.0037 (or 0.0143N). However, the propeller generated a negative side force coefficient of \(-0.0016\) (or \(-0.0062\)N), and the side force from the vertical stabilizer was nearly zero at a 0\({}^{\circ}\) angle of attack. In general, the side force from the MAV had a small effect on the MAV as compared with the total weight. The rolling, yawing, and pitching moments are shown in Figure 7d-f, respectively. A continuous increase of the rolling moment coefficient is shown in Figure 7d. At 0\({}^{\circ}\), the overall rolling moment coefficient was 0.0014 (or 0.0024 Nm). A peak value was found at 18\({}^{\circ}\) mainly from the wing-fuselage part. The yawing moment coefficient, C\({}_{\rm M}\)\({}_{\rm y}\), indicated that the MAV with a propeller spun clockwise as viewed from the rear and the moments caused a left yaw. However, it was negligible at low incidences around \(\alpha\) = 0\({}^{\circ}\) with a value of approximately 9e\({}^{-5}\). A linear increase in the propeller yawing moment can be found from the plot.