Aerodynamic Analysis and Optimisation of Wingtip-Mounted Pusher Propellers

Abstract

Ever since the late seventies great engineering effort has gone into increasing the fuel efficiency and reduction of the noise profile of aircraft. A concept that has been explored is the wingtip-mounted (pusher) propeller. In all wings energy is lost due to the lift-induced vortex at the wingtip. Wingtip-mounted pusher propellers can recover some of this energy if rotating opposite to the wingtip vortex rotation. The required propeller shaft power and wing induced drag could be reduced. Nevertheless, no aircraft utilise this setup because of aeroelastic problems and one-engine-out requirements. Nowadays this can be resolved by scaling down the propeller and using (distributed) electric propulsion. Recent developments in personal air transport and multi-rotor aircraft have sparked interest in wingtip-mounted propellers. The goal of this research is to obtain quantified insight into the propulsive efficiency gains and optimal geometry of a pusher propeller placed in a wingtip flow field.
In the first part of this research a CFD simulation of the wingtip flow field was implemented and validated with available experimental data. A simple Spalart-Allmaras turbulence model proved to be most suitable and accurate. The flow field of the Tecnam P2006T aircraft was modelled to provide a realistic wingtip flow field to which the propeller would be subjected.
In the second part of this research a lower-order tool called PROPR was built and proved to be a fast propeller aerodynamic analysis tool. Validation with experimental data showed a deviation of less than 15% in obtained thrust- and torque coefficients found. PROPR was integrated in an optimisation routine for fast optimisation of propeller geometry and operating conditions for non-uniform inflow. Total thrust, torque and their distributions obtained from PROPR and an implemented CFD model showed identical trends and were overestimated approximately 5% by PROPR.
In the final research part the Tecnam wing with installed propeller was investigated. A wingtip-mounted pusher propeller enables more than 12% increase in propulsive efficiency over the entire propeller thrust regime evaluated. Propeller optimisation was done for a thrust range of 50 < T_des < 350 N, wing induced drag was 240 N. Relative reductions in power requirement were constant for the thrust regime. Absolute power decrease did not decrease with increasing design thrust. No airfoil optimisation was performed to enable fast and stable optimization. From optimisation of a fictitious propeller with constant airfoil geometry it was concluded that the airfoil geometries are a limiting factor in fully capturing the benefits of the wingtip flow field. In optimised (installed) propeller geometry blade loadings shift towards the blade root. A smaller chord length and lower RPM are preferred given the used baseline propeller geometry.
A CFD simulation in which the propeller was represented as an actuator disk was constructed. The up- stream effect of an installed propeller was negligible. Thus, the incoming flow field was independent of pro- peller thrust within the considered thrust range. With this the implemented methodology was proven to be valid. Also, the overall power reduction of the combined setup is thus equal to the power reduction of the propeller. Comparison with transient CFD simulations of the wing with installed propeller showed great cor- respondence with results from PROPR.
In further research it is recommended to include optimisation of (root section) airfoil geometries in the propeller design. Evaluation of propellers at higher thrust levels would provide insight in power reduction at these higher thrust levels. Finally, investigation of the propeller at additional downstream locations, in- cidence angles and azimuthal positions would further validate the benefits of wingtip-mounted propellers suggested in this research.