Electric vertical take-off and landing aircraft present some obvious safety challenges with respect to their flammable lithium-ion batteries and their novel configurations, which, unlike an airplane or helicopter, may not be able to glide to the ground in the event of a total power loss. Now, a new paper suggests they may also face special challenges in avoiding a hazardous aerodynamic condition: vortex ring state (VRS).
To the extent that most people who aren’t vertical-lift pilots have heard of VRS, it’s probably in the context of the Bell-Boeing V-22 Osprey tiltrotor. In April 2000, one of the first Ospreys delivered to the U.S. Marine Corps crashed while on approach to land near Marana, Arizona, killing all 19 people on board. The investigation determined that the aircraft encountered a VRS condition in which one of its two proprotors lost the ability to generate sufficient thrust; the resulting asymmetry flipped the Osprey upside down before it impacted the ground.
VRS occurs when the velocity of air flowing up through one or more rotors of an aircraft approaches the velocity of the downward airflow created by the rotor itself. That usually happens when an aircraft is descending at slower forward airspeeds on approach, and can be aggravated by sudden tailwinds and updrafts, such as occur in the mountains and around tall buildings. The rotor wake collapses into a chaotic vortex that displaces the clean air a rotor needs to create the necessary thrust.
SOURCE : THE AIR CURRENT