Load Alleviation on a Joined-Wing Unmanned Aircraft

In this paper a method of alleviating wing structural load of a flexible aircraft during a symmetric balanced maneuver is presented. An application on the unmanned aircraft in development at the Italian Aerospace Research Center, high altitude performance demonstrator, characterized by a joined-wing configuration, is illustrated. This load alleviation technique enables a desired value of the bending moment on a fixed wing control station to be obtained. The load reduction is achieved by deflecting a suitable set of flight-control surfaces, by always keeping the vertical load factor constant to preserve the maneuvering performance. The main hypotheses are: significant aeroelastic effects, linear behavior of aerodynamics and structure, and unvarying tensor of inertia under structural deflections. High-altitude performance demonstrator is a scaled performance demonstrator of an 80m-wing span high-altitude and long endurance unmanned aircraft in a joined-wing configuration. The advantages in terms of performance, fatigue life extension, and weight reduction can be achieved from the integration of an onboard load alleviation system. The results show that the attainable value of load alleviation in terms of bending moment reduction at the wing root is 37%. Moreover, the test-case analyses show that the maximum value of the alleviation increases with respect to the dynamic pressure although the load distribution varies because of significant aeroelastic effects.