System Performance of Wing and Propellers in a Periodic Distributed Propulsion Experiment

Abstract

The design task for distributed propulsion (DP) aircraft is more complex than conventional twin-engine designs due to the pronounced propeller wing interaction. DP concepts rely on a beneficial and robust interaction of propulsion and lifting surface. Additionally, a good DP design is optimised as a system such that each element is not optimised by itself (i.e. ηprop and CL/CD), but with consideration of the close coupled interaction. The evaluation of such an interaction driven setup is scope of this work. Thrust and torque of a periodic co-rotating DP wing are measured simultaneously with airfoil coefficients. Thereby the influence of propeller on the wing and vice versa are identified. Two different sets of propeller geometries with a diameter of D = 0.6 m are studied. One propeller set is designed for minimum induced propeller loss (MIL). The second propeller set is designed to have a constant induced axial velocity over the radius (CIV). We shall compare how the different strategies perform in the DP system. The two element wing has a span of B = 2.4 m and a reference chord of c = 0.8 m, operating at Re = 2.1 × 106. For this study, the propellers are pitched to meet a constant cT, J and Matip . The results focus on the system performance for the combined setup in take-off configuration. While the isolated propeller efficiency benefits from the integration in front of the wing by > Δηprop = 12%, the system efficiency suffers from increased drag on the trailing wing that is roughly tripled over the clean wing. Depending on the propeller position relative to the wing, interaction losses can be minimised so that a system efficiency gain over the isolated wing and propeller of > Δηsys = 4% is achieved.