Assessment of Forward Sensing Turbulence Detection Strategies for Stratospheric Flight

One rapidly evolving technology for the commercialization of the upper atmosphere is that of small unmanned aerial vehicles (sUAS). For example, long-endurance sUAS equipped with solar cells have been proposed with the capability to remain aloft for long periods of time. Such aircraft can fill many roles: acting as communications relay; providing persistant surveillance; conducting atmospheric research; etc. However, by nature of their small size and slow flight speeds, low cost sUAS operating in the upper atmosphere necessarily fly at low Reynolds numbers, pushing the limits of low-speed aerodynamics and creating a unique opportunity for in-situ aerodynamic experimentation. As a result, their flight controllers must keep the aircraft within a very narrow operating margin. Unexpected deviations from nominal conditions, therefore, represent more significant risks for sUAS than they do for larger aircraft making stratospheric turbulence is a challenge for sUAS. Turbulence in the upper altitudes can arise from several sources including sporadically turbulent gravity waves attributed to mountainous terrain [cite Lilly et al (1974)] and a corresponding displacement of shear layer instabilities [cite Eherenberger] The challenge for commercial sUAS operators is highlighted by recent flights using the HiDRON aircraft developed by Stratodynamics Aviation Inc. The HiDRON (with a TRL of 7) is a balloon-launched unmanned stratospheric glider and recent test flights have revealed the system’s susceptibility to turbulent-gust-induced stall at high altitudes. This proposal seeks to examine potential gust and turbulence mitigation strategies developed for high altitude atmospheric studies (35 km) using light-weight, compact, low-cost instrumentation coupled with aballoon launched unmanned stratospheric glider called the HiDRON. The requested funds would support system integration, and high-altitude balloon launch flight services and data collation. The data from these flights will contribute to advancing our understanding of turbulence and in-situ resource utilization at stratospheric altitudes. Our project goal is to equip the HiDRON with a sensor package developed by the PI at the University of Kentucky for unmanned aircraft conducting atmospheric studies at low altitudes (and hence TRL of 4). This sensor package would include a multi-hole probe demonstrated to measure turbulence at O(100Hz), and an infrasonic microphone added to assess the feasibility of detecting the infrasonic signature of turbulence using a single microphone mounted on an sUAS. During flight, the multi-hole probe would simultaneously monitor the relative winds and infrasonic sensors looking for precursors which can be used for in-situ turbulence detection. The multi-hole probe flight data would be analyzed to identify suitable nonlinear time-series forecasting approaches, and the infrasonic measurements analyzed for acoustic signatures of turbulence, each of which may then be used to predict increasing turbulence intensity and identify the conditions when the turbulent-gust-induced stall is imminent, allowing automated flight controllers to perform risk mitigation maneuvers.