Aviation is confronted with the requirement to reduce emissions. To achieve this, future aircraft designs can take advantage of a stronger integration of the propulsion system and the aircraft components. Against this background, this dissertation deals with the question of the application of distributed propeller propulsion in cruise flight conditions. The focus is on a design of a hypothetical short-haul aircraft whose classic reference configuration is characterized by two propeller propulsion units. For this aircraft design, the question of what a configuration with multiple propellers (distributed propulsion) should look like in order to achieve the best possible, i.e. lowest, required power of the propellers is investigated. The work uses the CFD solver TAU developed by DLR with the Actuator Disk based on the blade element theory. Fully resolved simulations are used for validation purposes.
After a literature review and an overview of the basics of propeller aerodynamics, in which the method used for the propeller design is also shown, three chapters of results follow. The first of these three results chapters shows the parameter studies with the same thrust of the propeller. Nine different positions of the propeller around the wing are used. In addition, the installation angle is varied as the angle between the chord of the wing and the axis of rotation of the propeller, as is the rotational speed of the propeller. For selected positions, the distance between the propellers is reduced. All parameter studies carried out in this section aim to identify and explain sensitivities and mechanisms.
Based on the results of this first section, follow-up investigations are carried out for positions above the wing. These investigations are carried out under identical cruise conditions (constant lift coefficient and drag coefficient), so that the power of the propellers remains as the evaluating parameter. A regression model is then adapted using a test plan from the central composite design. This allows the five-dimensional result space to be evaluated and shows that the lowest power is achieved above the trailing edge of the wing. A propeller is then designed for this purpose and its spanwise distance to the neighboring propeller is varied. The resulting configuration with a spanwise distance of 1.5 propeller diameters is then validated using a transient, fully resolved simulation.
In the third results chapter, the previously developed configuration with a constant chord length is transferred to the three-dimensional wing. This results in a configuration with five propellers per half-span for the reference aircraft, which was calculated using the actuator disk and also transiently with fully resolved propellers. The results show significant savings in the drag of the wing of up to 13.1% compared to the conventional reference. At the same time, up to 3.9% of the power can be saved by adjusting the rotational speed of the propellers. For scaling purposes, the configuration developed is then transferred to three other aircraft of different sizes and compared with their conventional references. All configurations with distributed propulsion achieve reduced power. However, the power saving decreases with increasing flight Mach number and aircraft size due to the increasing efficiency of the reference propellers. In addition, the installation situation of the propellers above the wing with higher inflow speeds counteracts lower power.