Effects of propulsive-induced flow on the aerodynamics of micro air vehicles

A propulsion system (DC electric motor and propeller) was installed on micro air vehicle wind tunnel models of 3, 6 and 9% camber. In one set of tests the motor was mounted in its typical location; in the next test set the motor was angled both toward and then away from the leading edge; and in the final set of tests the motor was extended 1/2 inch away from the leading edge from its usual location. All models had wingspans of 9" and wing areas of 60 in ². The models were tested in the Low Speed Wind Tunnel, with the propulsion system activated (motor-on testing), at angles of attack ranging from 4 to 43° or 0 to 35.1°, depending on the test set, at velocities of 5, 7.5 and 10 m/s, corresponding to mean aerodynamic chord Reynolds numbers of 5×10⁴,7.5×10⁴, and 1×10⁵, respectively. C_L, C_D, C_M-c/4 and L/D were obtained and plotted versus angle of attack at each velocity. The aerodynamic coefficients obtained from the motor-on testing were compared to those obtained from a previous study that were completed without a propulsion system installed. In general, it was found that the propulsive-induced flow had a positive effect on the aerodynamics of the micro air vehicle models at higher angles of attack, particularly for low Reynolds number tests, and motors angled away from the leading edge. It was also found that relocating the motor forward from its usual mounting position increased both the lift and drag coefficients. However, due to the way that the aerodynamic coefficients were calculated, the effects in the aerodynamic coefficients are partly a mathematical phenomenon. In other words, at low Reynolds numbers there is an increase in the lift coefficient, but it is due to the way that the lift coefficient is calculated. Because this reduction in the Reynolds number is due to a reduction in speed, the actual lift produced at these low speeds may decrease faster than the lift coefficient increases.