In recent years, NASA, along with partners Boeing and Area-I Inc., have developed the spanwise adaptive wing (SAW). SAWs leverage a thermally triggered actuator made from a NASA-developed shape memory alloy (SMA) to allow outer portions of aircraft wings and control surfaces to be folded to achieve optimal angles during flight. For supersonic aircraft, SAWs can reduce drag and increase performance during the transition from subsonic to supersonic speeds. For subsonic aircraft, SAWs offer increased control and reduced dependency on the tail rudder and associated hydraulic systems, a particularly heavy part of the aircraft.
Engineers at NASA AFRC realized the aircraft weight savings that could be achieved by reducing or eliminating rudder dependency, since this would allow for rudder size to be reduced. They set out to develop an alternative system capable of providing the same “yaw control” functionality as traditional rudders. This work resulted in the invention of NASA’s outer aileron yaw damper system, which includes novel control algorithms that drive flight control surfaces to produce desired flight conditions.
The benefits of reduced rudder dependency led NASA to develop the outer aileron yaw damper to further decrease or eliminate rudder dependency for aircraft using SAWs. As mentioned, SAWs use shape memory alloy actuators to articulate the outer portion of the wing, effectively creating a movable wingtip. NASA’s invention uses an outer aileron located on the wingtips, which is driven (along with the inner ailerons) by a control algorithm.
The control algorithm, taking into account the wingtip positions, manipulates the outer ailerons to achieve the desired yaw rate. At the same time, it positions the inner ailerons to counter roll rate resulting from the outer aileron.
In other words, the control algorithm calculates a control surface ratio (i.e., position of inboard aileron and outboard aileron) that produces desired yaw and roll accelerations.
The system can also be used to offset the existing rudder in current or future aircraft designs. A second part of NASAs novel outer aileron control algorithm modifies the aircraft's rudder loop gain in proportion to outer aileron usage. This allows the outer ailerons and rudder to work in tandem, while at the same time reducing rudder usage.
As a result of this NASA invention, required rudder usage can be reduced or eliminated for aircraft with SAWs. Consequently, the size of rudders and vertical tail structures can be reduced, which in turn reduces weight and parasitic drag. The result is an aircraft with increased performance and fuel efficiency.