Slung Load Amplification Detector

Abstract

Externally slung loads and their mission applications are becoming more common on human and autonomously piloted air vehicles. Flight speed is often limited not by the performance envelope but by the danger of divergent load oscillations. Certifying this limiting speed for every load-vehicle combination, is a huge barrier to operations. The conservatism dictated by this uncertainty may itself be life-threatening in critical applications. Computing the dynamics of slung loads for a specific load/vehicle combination has been hindered by lack of knowledge on bluffbody aeromechanics. The prevailing top-down approach is to incorporate slung load aeromechanics calculations into large comprehensive aeromechanics codes for rotorcraft. We argue for a bottom-up approach. This allows on-the-fly system identification and dynamics simulation. The Slung Load Amplification Detector (SLAD) concept provides an on-board safety system to predict, detect, avoid and alleviate divergent oscillations. SLAD is based on a knowledge base derived from wind tunnel data and simulation results including canonical geometries, as well as practical shapes. Validation of simulation results against two practical test cases lends confidence. SLAD allows reliable distinction between pseudo and absolute divergence, permitting an increase of as much as 50% speed in safe flight speed, and guidance on active alleviation of oscillations.

Testing-Based Approach to Determining the Divergence Speed of Slung Loads

“…Some of the models on which aerodynamic load maps have been generated in the wind tunnel using the continuous rotation technique. From [100] by permission of authors.…”

“…One approach is to use a full-physics simulation including coupled fluid-structure interactions, attempting to model all aspects with fine resolution. This is illustrated on the left side of Figure 2 by Hiremath et al [100] . From the author's perspecive, this is a long way and time from yielding useful results.…”

“…Comparison of approaches to certify the maximum safe speed of a vehicle with slung load. From[100] by permission of authors.…”

“…Maximum roll amplitude versus flight speed for an empty engine container, predicted by dynamic simulation with model-scale wind-tunnel aerodynamic data, compared to flight test points. From[100] by permission of authors.…”