This paper proposes a new approach to stabilize the spin of a suspended litter during air ambulance rescue hoist operations.  Complex forces generated by the helicopter’s downwash may cause a patient suspended in a rescue litter to violently spin.  In severe cases, the spin destabilizes the suspended load, risks injury to the patient, and jeopardizes the safety of the aircrew.  The current technique for rescue crews to halt litter spin is to emplace a medic on the ground.  The medic pulls on a tagline (rope) that is connected to one end of the hoisted litter.  In most cases, the tension on the tagline is effective at reducing the spin; however, there have been cases where the tagline breaks or becomes a danger to the helicopter.  For the military conducting medevac (medical evacuation) operations, a tagline adds additional tactical constraints.  The lowering and hoisting of a medic extends the duration of the rescue and increases the exposure of the helicopter to enemy fire.  In a hostile environment, the use of the tagline requires additional friendly forces to secure the medic on the ground.

The presented design employs an anti-torque device to arrest the spin that is safer and faster than a tagline and is without the tactical constraints of the tagline.  The device follows tailored control laws to accelerate a flywheel attached to the litter, thereby generating sufficient angular momentum to counteract the spin and stabilize the suspended litter.  An inertial measurement unit (IMU) measures the position, angular velocity, and angular acceleration of the litter and delivers this information to a microcontroller.  Custom programming runs a PID controller that signals a motor controller to direct appropriate voltage to a reversible DC motor.  The system is completely self-contained and attaches to the underside of any airworthy litter through a series of quick connect buckles.  The device has a compact profile to ensure easy maneuvering in and out of the cargo door of the H-60 Black Hawk helicopter. 

Analysis of severe cases of litter spin revealed that an average of 5 N-m of torque caused by rotor wash can last up to 10 seconds.  Design variables of torque, angular speed, mass moment of inertia, and flywheel radius define a design space to generate the anti-torque.  Presented are the design, mechanical models, control theory and necessary gains, electrical diagrams, and preliminary results of prototype testing to achieve the necessary anti-torque up to severe cases.  The research and prototype design were developed under the sponsorship of the U.S. Army 160^th^ Special Operations Aviation Regiment (SOAR).


Spin Stabilization of an Air Ambulance Litter

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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