Due to their unique structural configuration, nonlinear aerodynamics, and rigid-body dynamics, supersonic cruise aircraft present highly complex Aero/Propulso/Servo/Elastic (APSE) flight dynamic phenomena. Such phenomena have the potential to affect flutter, gust loads, ride quality, flight dynamics and control, and engine performance. The design of a supersonic vehicle that is safe, comfortable, and easy to fly requires the accurate modeling and effective control of APSE phenomena.

Testing of the SemiSpan SuperSonic Transport (S4T) model in the Transonic Dynamics Tunnel at Langley Research Center has focused on a set of active controls to mitigate APSE effects, including ride quality, gust load alleviation, and flutter.

Flutter is a dangerous phenomenon encountered by aircraft as a result of interactions between aerodynamic, elastic, and inertial forces. As wind speed increases, structural damping may become insufficient to handle increasing motion due to aerodynamic energy being added to the structure. This vibration can cause structural failure.

The S4T model employs a ride-control vane, an active wing aileron, and an all-moveable horizontal tail, plus a flexible fuselage beam and engine nacelles with flexible attachments. The first round of tests, one in 2007 and another in 2008, determined the open-loop characteristics of the S4T model to enable the design of control laws for use to improve ride quality, alleviate gust loads, and suppress flutter.  The third test of the model, taking place in the summer of 2009, is a closed-loop test to validate various control law designs.

The knowledge gained from testing of the S4T model will be combine with developments in computational tools, rapid modeling techniques, and sophisticated control system design to enable the integrated design of supersonic aircraft that use the synergy of aerodynamics, structures, and control systems to achieve unprecedented levels of performance, ride comfort, and safety.

Features
http://www.nasa.gov/vision/earth/improvingflight/s4t.html