Slung load aerodynamics

When a helicopter carries external payloads, flight speed is sometimes limited by violent load oscillations, instead of engine/transmission load levels. The so-called "stability-limited" flight speeds are much lower than those dictated by available engine power. At the "stability-limited" flight speed, the lateral oscillation amplitudes of the load dramatically increase (termed a "jump"), and these motions are driven by bluff-body aerodynamics of the sling load. So far, analytical techniques based on linearized models have failed to predict (even qualitatively) the "jump" phenomenon. This work suggests that a limit-cycle type nonlinearity in the lateral load oscillation characteristics may be responsible for the "jump", as opposed to an instability that grows uncontrollably.

In this work, I used aerodynamic models of bluff-bodies that were obtained by Civil Engineering research. While developed for analyzing flow around buildings, they are equally applicable (with certain restrictions) to sling loads, since the cross-sections are very similar. The lateral oscillation amplitude increases sharply at a particular speed due to the nonlinearity in the aerodynamic forcing, as seen below.

Using these models with a stability-derivative representation of a helicopter shows a clear "jump" in the lateral oscillation amplitude at a particular speed, along with airspeed hysteresis. This trend is traced back to a combination of non-linearities in the side force curve, and swing damping effects due to motion of the helicopter.