Understanding The Aerodynamics Of Evtol Outwash

Abstract

The flows out across the surface that are produced by eVTOL aircraft when operating close to the ground are very different in character to those produced by the conventional helicopters on which much of our current understanding of rotorcraft outwash is based, yet accurate quantification of the velocities within these flows is seen as an important part of ensuring the safety of this new form of aerial transport. An analysis of the fundamental fluid mechanics that governs the formation of the flows in the outwash below these aircraft shows the importance of the inherent instability of the vortical structure of the wakes produced by their propulsors in governing both the form and strength of the flow out across the ground below the aircraft. Numerical experiments show the outwash to be particularly susceptible, through the long-term action of this vortical instability, to losing the symmetries that might have been imparted to it by the geometry of the aircraft. This results in flows that in many cases are far more spatially inhomogeneous than might be expected, where the overall characteristics of the flow can vary considerably over time, and, indeed, where the character of the flow might be dependent on the way in which it was formed in the first place. These observations suggest that we might need to know a great deal more about the physics that governs the aerodynamic behaviour of these systems before we place too much faith in our ability to quantify the risks posed by these aircraft to bystanders and infrastructure that might become exposed to their outwash.