The NASA Preliminary Research Aerodynamic Design To Lower Drag (PRANDTL-D) wing has been developed by Al Bowers at the NASA Armstrong Flight Research Center. Bowers was inspired by the work of Ludwig Prandtl and, like Dunne, by watching bird flight. As with the Dunne design, it has a wing twist sufficient to set the wing tips at a negative angle and create the same positive roll-yaw coupling.[8][9][10] Bowers developed a quantitative analysis of the lifting characteristics, leading to his more general discovery of a bell-shaped lift distribution which minimises induced drag for the aircraft weight.