Unconventional Propeller−Airframe Integration for Transport Aircraft Configurations

Abstract

It is envisioned that the future generations of regional and short to medium-range aircraft employ a high level of propeller integration to achieve low-emission flight. The objectives of unconventional propeller installations include the enhancement of the airframe aerodynamic efficiency, increasing the propeller efficiency, improving cabin comfort, and improving the overall aircraft design by lower operating empty weight. Furthermore, by employing the aerodynamic interaction in specific phases of the flight, beneficial propulsion integration can also enable the use of alternative energy sources and increase the electrification level of the propulsion system.

The closer proximity of the propeller and airframe requires a more dedicated integral design (approach) of both the airframe and propulsion unit. The objective of this dissertation is: to characterize the role of the aerodynamic interaction between the propeller and the airframe on the performance and static stability characteristics for selected aircraft configurations which aim for a beneficial propeller-airframe interaction. To this end, three different types of analyses are performed. First, fundamental phenomena are investigated which provide insight for related configurations and derivatives thereof. Second, a configuration study indicates the expected trends on various performance indicators. Finally, two detailed studies on aircraft level demonstrate the relative importance and the coupling between aerodynamic interactions.

The first configuration features propellers that are mounted to the horizontal tailplane. This is an example where there is a strong interaction between the propeller and airframe that affects performance, stability, and control, and contains various interaction mechanisms that are of interest for other configurations as well. A second specific case is the a distributed propulsion configuration with propellers mounted to the inboard part of the wing (in front of the high lift devices), together with a propeller mounted to the tip of the wing.

One of the focal points of the current study is extending the understanding of nonuniform inflow effects on propeller performance and its role in aircraft stability and trim. Compared to the conventional configuration, for various unconventional propeller installations, the nonuniform inflow to the propeller differs both in type and magnitude, and varies with flight condition. The slipstream shape and consequently its interaction with lifting surfaces are affected as well. These factors directly affect the gradients and offsets of the propeller force curves and therefore the aircraft stability and trim, respectively.

By employing CFD results, a study has been performed on the {sensitivity} of the radial load distribution to a change in inflow condition that is expressed as a change in local advance ratio. The constructed distributions provide insight into what region of the disk is responsible for the largest changes of the propeller forces. This has been demonstrated to be the region of highest loading. It is also shown that for a given propeller design, nonuniform inflow can be represented by an `installation coefficient' kappa such that the efficiency curve of this uninstalled propeller is scaled along the advance ratio and efficiency directions by kappa to obtain the installed propeller efficiency. Using the data of the isolated propeller for an arbitrary blade angle, the advance ratio at which the installed case has highest efficiency, as well as the value of the maximum efficiency, can be quantified immediately. The computational intensive analyses to find the optimum blade angle for the installed cases are therefore redundant and the formulation of the installation coefficient is therefore highly valuable to the aircraft designer. The installation coefficient also gives insight in what regime of the efficiency-advance-ratio curve the largest changes occur due to nonuniform inflow...