Rotational Stabilization Of Cargo Container Slung Loads

The stabilization of 'difficult' loads that become aerodynamically unstable at airspeeds well below the power-limited speed of the helicopter-load configuration has been studied since the 1960's. This paper looks at the possibility of stabilizing slung loads in forward flight by imposing a steady rotation in yaw (spin stabilization). Slow rotations of 100-150 deg/sec suffice to suppress the pendulum motions of the load. A swivel is required at the hook and only a few ft-lbs of yaw moment are needed to overcome swivel friction and impose the desired yaw rate. The approach is limited to single-point suspensions. A stabilizer design consisting of a one-shaft anemometer-like device with hemispherical cups at the ends was developed in wind tunnel tests. The shaft angle can be controlled to vary the applied yaw moment and allow feedback regulation of the load yaw rate. Flight tests with two cargo containers demonstrated that a simple linear control law with proportional and integral error feedback and with fixed gains was effective in maintaining the desired yaw rate in forward flight over the range of configurations of the test loads. The stabilizer can also be used to de-spin the load during the approach to hover prior to losing aerodynamic effectiveness at low speeds. Wind tunnel data was obtained at all stages of the development and testing, and proved to be an accurate source of design data and an accurate predictor of performance in flight.