Self-excited limit cycle yaw oscillation instability of external sling payloads with dual-point suspension during level flight

This work developed and verified a computational model to predict self-excited limit cycle yaw oscillation (SELCYO) instability of external sling payloads carried under aircraft with dual-point suspension. Inverted-V and inverted-Y slings during steady-state level flight are discussed. The primary goal was to provide a design tool for comparison of the onset of SELCYO between alternate sling geometries. The computational model incorporates steady-state aerodynamic loading during level flight based on scale-model wind tunnel testing. Scale-model sling tests of the onset of SELCYO in the same wind tunnel were used for validation. Predictions of cargo hook load for a full-size HMMWV-M1025 payload carried by inverted-V slings are compared to V-22 Osprey flight-test data. Predictions of stability indicate that stiffer slings are generally more stable, and inverted-V slings are significantly more stable than inverted-Y slings. Small differences between right and left front sling leg lengths caused by rigging error can significantly reduce stability.