Aerodynamic Interaction Between Overlapping Propellers: A Numerical And Experimental Study

The demand of a new sustainable on-demand urban air transportation system moved the aerospace industry towards new and groundbreaking concepts of electrical VTOL vehicles. Various challenges are related to such type of vehicle, fromavionic and flight controls to the determination of efficient propulsive systems and their integration in the airframe. In particular, in many of these concepts a high number of propellers is present in such configurations where interactions within each other might represent a cause of concerns. This thesis work aims to understands the fundamental effects of the interaction between propellers in overlapping configuration, which is when the two propellers are placed one after another. In particular, the main focus is directed towards the effects of the front propeller slipstream on the rear propeller. For this purpose, a numerical model was developed, based upon an existing blade element theory code, properly modified with a slipstream model and a method through which the response of the blade due to the non uniform inflow. An experimental campaign took place in Delft University of Technology’s Open Jet Facility, using scale models of the F29 Fokker propeller. Measurements of the isolated propeller, of the front propeller slipstream total pressure distribution and of the rear propeller’s performance were taken and compared with the results obtained through the numerical evaluation. A sensitivity study of the interaction was performed with respect to several different parameters, such as lateral separation, axial distance between the propellers and their rotational speeds. Several conclusion can be drawn from the results of this work. Since a broad range of configurations was studied, it was possible to understand how each parameter influenced the interaction. It was found that a propeller working in a non uniforminflow, when ingesting the wake of another propeller, can lose up to 80% of his thrust when fully overlapped. The main factor that influences the magnitude of the experienced losses is the lateral separation, since it determines the area of overlap. The interaction has a strong impact in the behaviour of the propeller, and the fundamental reason below this is that , due to the increased axial velocity of the slipstream, the part of the blade affected by it develops less thrust and torque components. Once established that the propeller experiences losses due to the interaction, a desired requirement would be to know how much power is necessary to keep the same thrust as it was before the interaction. It was found that, for an high rate of overlap, it is necessary to provide up to 4 to 6 times the power with respect to its value affected by the interaction. The comparison between numerical and experimental results shown a good agreement, and it was possible to predict the effect on thrust and power of the wake impingement on the rear propeller. It should be noted how the code is both fast and adaptable, therefore every tested configuration has a correspondent solution performed inside the program. All of these results allow us to conclude that, when designing such vehicles, the integration of propellers is an aspect that must be studied in depth, focusing the attention especially on considering the decreased performance of a propeller reached by another propeller’s wake. A proper flight control system for these vehicles should be able to handle the effects of the interaction, since a decrease in the thru