Hardware-in-the-loop dynamic wind tunnel investigation of slung loads dynamics with application to active cargo hook stabilization of an M119 howitzer

A new hardware-in-the-loop (HIL) dynamic wind tunnel setup is used to study the behavior of a slung load at high speeds and methods of stabilizing problematic loads. The main element of the setup is a movable cargo hook. In addition the cable angles, model spatial attitude, and hook force are measured continuously. All the measurements are fed into a computer that calculates the cargo hook resultant motion in real-time by summing the rotorcraft angular motion effects (not used in the current study) and the hook motion relative to the rotorcraft fuselage. The computer output includes motion commands to the hook. The slung loads are two configurations of an M119 howitzer: folded and ready for firing. Initial wind tunnel studies showed that these loads exhibit significant LCO (Limit Cycle Oscillations) and severe instabilities at high speeds. Frequency sweep tests are used to derive dynamic models of the slung loads. These models are used to develop two controllers based on an Active Cargo Hook (ACH) approach. These controllers were implemented, tested, and studied. It was shown that both were able to suppress LCO and stabilize the slung loads along the entire airspeed range.