Liberi, B., Kijjakarn, P., and Komerath, N., "Slung Load Divergence Speed Predictions for Vehicle Shapes," SAE Technical Paper 2015-01-2570, 2015, doi:10.4271/2015-01-2570.
Loads slung under aircraft can go into divergent oscillations coupling multiple degrees of freedom. Predicting the highest safe flight speed for a vehicle-load combination is a critical challenge, both for military missions over hostile areas, and for evacuation/rescue operations. The primary difficulty was that of obtaining well-resolved airload maps covering the arbitrary attitudes that a slung load may take. High speed rotorcraft using tilting rotors and co-axial rotors can fly at speeds that imply high dynamic pressure, making aerodynamic loads significant even on very dense loads such as armored vehicles, artillery weapons, and ammunition. The Continuous Rotation method demonstrated in our prior work enables routine prediction of divergence speeds. We build on prior work to explore the prediction of divergence speed for practical configurations such as military vehicles, which often have complex bluff body shapes. Results from simulations are presented for 3 vehicle shapes: one resembling a military truck, a Humvee, and a Sentinel Tactical Response Vehicle. Generic comparisons are shown where mass and sling length are held constant, across a variety of shapes. As expected, lower density causes a lower divergence speed. Between the vehicles, the flatter Humvee and the Sentinel show lower divergence speeds compared to the truck model. The results indicate that with specified inertia distributions, Froude scaling can be used to predict divergence speed for arbitrary configurations.