Concurrent aerodynamic design of the wing and the turbines of airborne wind energy systems

Abstract

The aerodynamic design of the fly-gen airborne wind energy system aircraft, referred to as a windplane here, is a largely unexplored yet crucial problem for improving power production. To this end, an engineering model for the aerodynamics of the onboard turbines, the aerodynamics of the wing, and their interactional aerodynamics is developed and coupled to a steady-state windplane model and a far-wake model. This novel comprehensive model is then used to design the windplane aerodynamics for a given wingspan. Initially, a design space exploration study reveals that placing the turbines at the wing tips and rotating them inboard down increases the power production compared to other locations and rotation directions. This improvement arises because the turbines' wake swirl reduces the wing's induced drag, which increases flight speed and, consequently, the generated power. Moreover, airfoils with a high lift-to-drag ratio are found to be optimal for windplanes. As a consequence, NACA4421 airfoils are used for the design of the wing and the tip-mounted turbines. The trapezoidal wing with constant twist which maximizes power production has an aspect ratio of 5.1 and a taper ratio of 0.60. By design, the onboard turbines operate at a low tip speed ratio of 1.9 to increase the wake swirl. The results from the vortex models of the wing, the turbines, and their interaction show very good agreement with the lifting line, the vortex lattice method, and the vortex particle method implemented in the well-established code DUST. Finally, the windplane is studied with DUST at different wing angles of attack and at different turbine tip speed ratios to characterize its behavior away from the design point. The maximum in power production is observed near to the design point, well separated from the stall regions of both the wing and the turbines.