| Abstract | For the second year of the RPAS operator guidance and safety assurance tools for the urban environment project, between TC and NRC, two sub-projects were carried out. The first sub-project included wind tunnel testing to evaluate the flow-fields of four urban models to determine the magnitude and locations of extreme airflow features identified as potentially hazardous to RPAS operations. The characteristics of interest measured for the four urban models representing Canadian cities were speed, direction, shear, turbulence and vorticity (S₁D S₂T V). The scope of this current phase of work included analysis of the results of two out of the five urban airflow features, specifically speed and turbulence (S₁T).
Key findings from the urban airflow results included:
• For locations where airflow was not within tall building wakes, the upstream wind speed vertical gradient persisted from outside of the urban site into the urban environment at the small RPAS maximum altitude limit of 122m (400 ft);
• For locations within building wakes or wake interaction, the upstream wind speed vertical gradient was not evident within the urban environment as results showed high variability with minimal height-to-speed correlation;
• For all city models, locations were found where lateral wind speed tripled over a full scale distance of 18 m, where measurement locations crossed building wakes; and,
• High wind speeds and extreme turbulence intensities between 26% and 33% were found in the wakes of tall high-rise buildings.
The urban airflow test turbulence intensity results were used to determine full scale flow conditions for the second sub-project, a wind tunnel test where five off-the-shelf small RPAS were evaluated for flight stability for a range of turbulence. The wind speed limit for stable flying for each RPAS was found for a range in wind -speed and -turbulence intensity determined by the ability of the auto-pilot to maintain hover position. Results indicated that:
• The wind speed limit was sensitive to a range of turbulence intensity that is typical for urban airflow;
• All of the models tested were affected by turbulence intensity to a similar degree; and,
• The internal log file results recorded by the RPAS sensors during each test suggest an increase in airflow turbulence intensity causes an increase in variation of RPAS control moment, decreasing attitude controllability, thereby reducing the wind speed limit at which the RPAS could maintain position.
When the results are graphed for wind speed versus turbulence the trendline of the RPAS wind speed limit can be compared to the urban airflow results. The relationship was used to develop a method for determining an estimated wind speed limit at which the tested RPAS could avoid urban airflow instability for the four representative Canadian cities. The estimated safe wind speed limits suggest that conditions within Canadian urban environments will exist that the tested RPAS manufacturer-specified sustained wind speed tolerance does not take into account. Specifically, the intensity of turbulence for high winds in the wake of tall buildings is expected to cause a loss of attitude controllability, lowering the wind tolerance to below that of the specified value. |
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