Innovators at NASA’s Armstrong Flight Research Center are experimenting with a new wing design that removes adverse yaw and dramatically increases aircraft efficiency by reducing drag. The technology has the potential to significantly increase total aircraft efficiency by optimizing overall aircraft configuration through the reduction in size or removal of the vertical tail, as well as the reduction of structural weight.
Adverse yaw, present in current aircraft design, is the horizontal movement around a vertical axis of an aircraft in the direction opposite a turn. As an aircraft banks, differential drag creates adverse yaw. Pilots must employ some form of yaw control to counteract this effect. Unfortunately, this yaw control introduces another form of drag that degrades performance. However, a wing with proverse yaw (that is, force in the same direction as the turn) does not need such control and thus helps optimize aircraft efficiency.
The Armstrong team (supported by a large contingent of NASA Aeronautics Academy interns) built upon the 1912 research of the German engineer Ludwig Prandtl to design and validate a scale model of a non-elliptical wing that reduces drag and increases efficiency. Known as the PRANDTL-D wing, this design addresses integrated bending moments and lift to achieve a 12 percent drag reduction. The approach to handling adverse yaw employs fine wing adjustments rather than an aircraft’s vertical tail.
As a proof-of-concept, the PRANDTLD team demonstrated proverse yaw during a live flight test in June 2013. The remote-controlled aircraft had a bell-shaped spanload and no vertical surfaces of any kind.
The key to the innovation is reducing the drag of the wing through use of the bell-shaped spanload, as opposed to the conventional elliptical spanload. To achieve the bell spanload, designers used a twisted and sharply tapered wing, with 11 percent less wing area than the comparable elliptical spanload wing. The new wing has 22 percent more span and 11 percent less area, resulting in an immediate 12.5 percent efficiency gain. Furthermore, using twist to achieve the bell spanload produces induced thrust at the wing tips, and this forward thrust increases when lift is increased at the wingtips for roll control. The result is that the aircraft rolls and yaws in the same direction as a turn, eliminating the need for a vertical tail to provide yawing moment. When combined with a blended-wing body, this approach maximizes aerodynamic performance, minimizes weight, and optimizes flight control.
The commercial potential for this technology is strong. Adopting the bell-shaped spanload change will result in an immediate 12 percent drag reduction. In addition, optimization of the overall aircraft configuration, as well as extension of the concept to propulsion systems, is projected to result in significant overall performance increases. Applications to wind turbines and fans are also being explored.