M.Sc Thesis


M.Sc StudentLeeran Yagil
SubjectElastic Deformations Control of Highly Flexible Aircraft in
Trimmed Flight and Gust Encounter
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Raveh Daniella
Professor Idan Moshe


Abstract

High-altitude, long-endurance (HALE) unmanned aerial vehicles (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 trimmed 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. An extreme case is the NASA Helios flying wing prototype, which in 2003, shortly after takeoff, encountered gusts for a prolonged period of time. These gusts induced a very high dihedral shape, which caused it to lose its longitudinal stability and crash into the ocean. Correspondingly, methods and tools used today for structural and aeroelastic analyses are not able to model very large elastic deformations well. Such large deformations require geometrically nonlinear structural models which are computationally expensive and not as readily available as linear models. This work aims to avoid 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 aircraft 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. The trim problem (objective and constraint) is formulated as a linear function of the trim variables and 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 in the flight envelope, while maintaining small structural deformations. The H-infinity controller significantly reduces the maximum wing deflection experienced by a gust disturbance, which is achieved to be bounded below a user specified value. 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.