A shortening pendulum: Hoisting control for medical evacuation rescues

Strategies to vary the length of a pendular payload maneuvered by a helicopter in an aerial medical evacuation scenarios and similar situations are discussed. The aim is to rapidly stabilize the swinging payload to decrease effects of cable swing and decrease rescue time. A variable length procedure is proposed for small changes to the line length as the pendulum swings. The 2D nonlinear equation of motion of the pendulum is implemented in MATLAB, and for a proof of concept, manual intervention is used to identify values of the time interval and line parameters to achieve payload stabilization. Three time-varying line procedures are proposed. Conservation of energy is used to determine expressions for the initial angle and angular velocity that decrease the energy of the pendular payload when it swings through its highest and lowest points. Results show that the proposed stabilization strategies are able to control the swing angle due to variable length reel-in and reel-out at precise timing intervals. Further, validation of the variable length idea is demonstrated with energy calculations determined from Mathematica simulations in our prior work.