Transportation of external loads by helicopters is very common and is used for a fast transfer of large loads to places that are hardly accessible by other means. In these cases, the load is connected to the bottom of the helicopter by slings. During flight, the load is subjected to aerodynamic forces and moments that cause it to oscillate and to rotate about its vertical axis.
Due to the rotation, the slings twist and wind together to form a helical ply structure. The winding of the slings creates a torque that opposes the rotation; as a result, the rotation slows down, stops and then a rotation in the opposite direction develops (unwinding). This periodic motion is difficult to model, not only because of the complex aerodynamics of the torsional moment (autorotation), but also because of the difficulty in modeling the cables periodic winding and unwinding and predicting the torque that the slings apply on the rotating load. The present research deals with the latter torque, which is a superposition of the torque induced by the torsional rigidity of each of the slings, and the torque induced by changes of the slings spatial geometry as a result of the rotation of the load.
Ropes are complex structures and their mechanical properties are affected by many factors. An empirical model for the torsional rigidity of the ropes is presented. This model includes the influence of tension, number of rotations in each direction, and hysteresis. The empirical model is used to describe four different kinds of ropes and is verified by various tests that include different combinations of tensions and torsion histories.
The model of the torque induced by changes of the slings spatial geometry as a result of the rotation of the load is derived using energy methods. The solution includes two different regions: cases where the ropes are not in contact with each other and cases where the ropes are in contact and wind together to form a ply structure.
The coupled model is verified by comparisons of its results to detailed tests that include different magnitudes of the load rotation angle and different load weights. The verified model is used for parametric studies. Finally, the dynamic rotation of the load is modeled and solved. The derivation includes the effects of friction between the ropes and damping caused by each rope. The solution is verified for the four types of ropes and for different load weights and moments of inertia.
