Aeroservoelastic design for closed-loop flight dynamics of a MAV

Some fixed-wing micro air vehicles (MAVs) have high levels of structural flexibility, a property which can change the flight dynamics and control characteristics of the vehicle. However, the exact level of flexibility is typically the result of a trial-and-error approach instead of being part of a rigorous design framework and may result in unknown aeroelastic effects on the flight dynamics. The current research investigates the nature of these aeroservoelastic effects by using a generic MAV configuration. The main parameter of interest is the stiffness of the wing.;Bending and torsional stiffness of the wing are independently varied from 1.0 Nm2 to 0.07 Nm2 while the trim conditions, flight dynamics, and structural dynamics are analyzed. Large changes in both the frequencies and damping ratios of the oscillatory flight modes are seen. The bending stiffness mainly affects the lateral-directional flight modes through an increase in the effective dihedral angle due to increased wing tip deflection. The direction and magnitude of the effect varies greatly between modes. Non-traditional mode shapes resulting from decreased bending stiffness are observed in the dutch roll mode and phugoid mode.;The effects of torsional stiffness depend on the relative positioning of the elastic axis and center of pressure. When the elastic axis is near the center of pressure, changing torsional stiffness has only minor effects on the flight dynamics. Elastic axis locations which are further away from the center of pressure result in stronger effects from changes in torsional stiffness. In general, the torsional stiffness affects the longitudinal modes more than the lateral directional modes because of the changing angle of attack and pitching moment.;Aeroservoelastic effects of wing stiffness on the tracking performance of the aircraft are investigated. For an LQR controller with fixed weightings, the tracking performance decreases as stiffness decreases. Changes in the phugoid mode damping and shape at low bending stiffness are found to have a very strong effect on the longitudinal tracking performance.;The possibility of virtually changing the stiffness of the wing by using a model-following control scheme is investigated. It is observed that the stiff aircraft can approximate the response of the flexible but the flexible aircraft is unable to adequately approximate the performance of the stiff aircraft.;An important consideration for micro air vehicles is their response to a wind gust. A frequency-domain approach is used to evaluate the aircraft's longitudinal gust response in the presence of aeroservoelastic effects. The level of wing bending stiffness is found to have an important effect on the gust sensitivity and gust rejection properties of the aircraft. The direction and frequency of the gust can drastically change the gust sensitivity of the aircraft. Lowering wing stiffness can reduce the gust sensitivity at low gust frequencies but can increase it at high frequencies. Changes in modal damping and shape due to decreasing wing stiffness have a strong influence on the gust sensitivity. For a basic LQR controller with fixed weighting matrices, the gust rejection properties are very good across the range of stiffness values.