Testing-Based Approach to Determining the Divergence Speed of Slung Loads

An approach is presented, to the long-term problem of certifying the safe limits for helicopter operations with slung loads of arbitrary geometry. Recent work showed that at least two combinations of roll and yaw oscillations would amplify in free flight without rotor wake swirl. The acquisition of detailed airload maps for arbitrary shapes, with sufficient resolution in attitude, became the primary aeromechanical obstacle to predicting divergence speed. The conventional approach is to use wind tunnel testing at a few sample conditions and computational predictions to fill in the detailed parameter space. However, the technique of swing tests and continuous-rotation (STCR) presented in this paper, opens the way to direct calculation of the divergence speeds for known configurations, and a large enough empirical knowledge base to predict divergence speeds for entirely new configurations. The STCR and example test cases are described. A low-inertia cylinder, a loaded flat plate, and a porous box are used as test cases. Assumptions of symmetry are removed, and uniformly high resolution is available through regions of high gradients, to generate closed-form periodic representations of air loads for simulation. Wind tunnel video and encoder information improve efficiency in finding likely modes of amplification, and to validate simulations.