Measurement techniques and data evaluation for VTOL outwash maps

Abstract

When a VTOL aircraft approaches the ground, the column of downwards-accelerated air that its propulsors create is deflected outwards across the surface to form a fast-moving layer of air that spreads out in all directions below the vehicle, called its outwash. Most of our understanding of the properties of the flow in the outwash are based on a small number of measurement campaigns, undertaken some decades ago on fairly large helicopters, using experimental techniques which, given the rapid advances in sensor technology and processing power over the years, have largely now been superseded. Given the advent of eVTOL aircraft, interest within the community has also turned towards understanding the characteristics of the outwash of vehicles that have much smaller rotors, and particularly towards understanding the interactions within the flow when the outwash from vehicles with multiple rotors develops out across the ground. The database of experimental measurements of the velocities within the outwash of such aircraft is still very sparse, and the published record contains several inconsistencies that can be ascribed to a lack of agreement on how velocity data from within the outwash field should be measured, collected and, indeed, processed. Until these issues are resolved, it will be very difficult to make a rigorous safety case for the deployment of the next generation of electrically-powered vertical lift technology, particularly into the urban environment within which these vehicles are intended to operate.