Non-Linear Dynamic Loads Due to the Landing Impact of a Joined-Wing UAV 2011-01-2720
In this paper a method to evaluate ground loads due to the impact of a high aspect-ratio wing of an unmanned aircraft at landing is presented.
The aircraft for which the loads are calculated - code named HAPD (High Altitude Performance Demonstrator) - is an unmanned aircraft under development at the Italian Aerospace Research Center. It is a scaled performance demonstrator of an 80m-wing span High-Altitude & Long Endurance Unmanned Aircraft, born as a risk mitigation project of its full scale parent. Its peculiar feature is a Joined-Wing configuration.
The joined-wing configuration is a good compromise between high lift, low weight and low global flexibility when compared with a conventional configuration with the same aspect-ratio. HAPD configuration consists of a mainly straight Front Wing and a Rear Wing with negative sweep and dihedral angles. The junction location is at about 70% of the FW semi-span, which was chosen as the best location in the preliminary design phase. The wing junction is equipped with an elastic device able to absorb concentrated loads due to the ground impact during landing.
The loads assessment is performed by using a modal approach with the elastic modes calculated by applying the large mass method, a standard method for seismic analyses of civil engineering structures. The large mass is applied at the point of contact between the ground and the main landing gear.
The non-linearity of the system arises in the force-displacement response model of the impact absorber when it reaches the ground and deforms. A fourth-order Runge-Kutta method was used for the time integration of the system.
Structural loads, in terms of concentrated vertical forces, are evaluated for different absorber geometries i.e. different distances (in the undeformed configuration) of the absorber from the ground as well as for several values of its stiffness property.