The Contribution of Swirl Recovery to the Induced Drag of Propeller-Wing Systems

Abstract

From the desire to reduce the impact of traveling on the environment and the increase in fuel prices over the past years, the propeller has regained interest as a propulsion mechanism in the aviation industry. Two main reasons can be found in their high propulsive efficiency and ability to combine with electric motors. The scaling of the motors is rather insensitive to the efficiency, allowing the propeller to be placed around the aircraft, resulting in new design freedoms.

The influence of the propeller slipstream can increase the aerodynamic performance of a propeller-wing system. Propellers with inboard-up rotational configuration are found to reduce the induced drag of the wing in propeller-wing systems compared to isolated wings. To increase the aerodynamic performance of propeller-wing systems and certify design choices, a detailed understanding of the relative contribution of the lift-induced drag and swirl recovery to the induced drag of propeller-wing systems is desired. A low-order numerical model has been used for a parametric study on propeller-wing systems. From the results follow that the gradient of the spanwise wing lift distribution is of influence on the amount of net swirl recovery that the wing can obtain. It is shown that when a propeller is placed in front of a wing section where the gradient of the isolated spanwise wing lift distribution is positive from root to tip, more swirl recovery can be obtained by the inboard-down rotating configuration. In this case, the difference in lift between the wing sections behind the two propeller sides is increased. The increase of the difference in the lift also increases the lift-induced drag of the wing. With low disk loadings, the contribution of the swirl recovery is found to be higher than the increase in lift-induced drag. Increasing the disk loading results in more significant differences in lift between the wing sections behind the propeller sides, such that the gradient of the spanwise wing lift distribution increases. From this increase, the contribution of the lift-induced drag can become more significant than the contribution of the swirl recovery of the wing, increasing the induced drag of the system.