High-altitude, long-endurance (HALE) UAVs are characterized by very large aspect ratio wings and low structural weight, and therefore are inherently highly flexible. As a result, these platforms are more susceptible to very large structural deformation in flight and in atmospheric turbulence. These deformations may severely affect the performance of the aircraft or its function, as in the case of a wing-embedded antenna. In addition, analysis of large elastic deformations requires the use of computationally expensive nonlinear models. This work aims to control the large wing deformations that highly flexible aircraft undergo in flight, by performing trim optimization to constrain wing deformation in steady trimmed flight, and by minimizing the dynamic response to gust encounter.
The flexible wing is trimmed and controlled via multiple control surfaces, typically located along the leading and trailing edges. Due to the use of multiple controls, trimming the aircraft is cast as a trim optimization problem with constraints to find the control surface deflections that trim the vehicle to a required maneuver while minimizing a specified objective function. In the current study, the optimization problem is to trim the aircraft to a symmetric maneuver while minimizing the control effort as an objective, and constraining wing elastic deformations to a user defined value. Trim optimization is performed using linear programming.
While at trimmed flight, gust disturbances may result in a large dynamic response. A robust control scheme is designed and applied using H-infinity loop shaping, to demonstrate that although the control surfaces are used for trimming the aircraft, they have enough authority left to effectively minimize elastic wing deformations during gust penetration.
The above methodology is demonstrated on a highly flexible flying wing, similar to the NASA Helios aircraft. It is controlled via two leading- and two trailing-edge control surfaces. Trim optimization effectively allows trimmed flight at various load factors and dynamic pressure values in the flight envelope, while maintaining small structural deformations, of about 1/40 of the wing span. The H-infinity controller significantly reduces the maximum wing deflection experienced by a gust disturbance. Thus, large structural deformations can be prevented, avoiding related performance issues and the need for complex structural geometrically nonlinear analyses for such highly flexible airframes.