Full-scale flight tests of active flow control to reduce Tiltrotor aircraft download

The vertical force, or download, acting on the airframe of current tiltrotor configurations during hover amounts to approximately 10% of rotor thrust, or about 6000 lb for the V-22. Various mechanical means have been experimentally tried to reduce this penalty, but none has been implemented, largely because of mechanical complexity. This paper describes the research conducted on the application of active flow control to the problem, since this technique may offer a solution without large weight penalties and unacceptable complexity. The research was conducted as part of the Defense Advanced Research Projects Agency Micro Adaptive Flow Control program. The work culminated in June 2003, when the NASA/U.S. Army/Bell XV-15 tiltrotor aircraft was used to successfully demonstrate the effectiveness of active flow control in reducing airframe download during hover. The wing flaps were fitted with zero-mass-flow actuators that periodically injected/removed air in the flap upper surface boundary layer through slots from the interior of the flap. The active flow control was effective in delaying flow separation from the flap, which reduced the download on the wings. The flight tests were the culmination of extensive laboratory experiments on two-dimensional models and on a powered full-span 16%-scale model of the XV-15 aircraft. The XV-15 flight tests confirmed the laboratory findings by successfully reducing the download measured in hover by as much as 14%, demonstrating that the aerodynamic principles of active flow control can be applied to full-scale air vehicles.