Electric air taxis could generate downwash equivalent to a hurricane-force wind – or the same as a Bell-Boeing V-22 Osprey tiltrotor – according to research carried out by the US Federal Aviation Administration (FAA).
Released in late December, the report – Electric Vertical Takeoff and Landing (eVTOL) Downwash and Outwash Surveys – is intended to inform vertiport design as the regulator prepares for potentially widespread adoption of the novel aircraft in the coming years.
“eVTOL aircraft DWOW [downwash/outwash] can pose significant risks to people and property and must be accounted for in vertiport design,” says the FAA. There have been multiple cases of bystanders being killed or injured by rotorwash from helicopters coming in to land, it notes.
Using three unidentified eVTOL aircraft prototypes provided by their manufacturers, the FAA’s study found that the maximum instantaneous DWOW velocity recorded was almost 100mph (86kt/160km/h) at a distance of 41ft (12m) from the centre of the take-off and landing area.
All three aircraft tested generated a peak DWOW velocity in the 100mph range, the FAA study shows.
Even 100ft from the centre of the take-off area, speeds of 60mph were still recorded, the FAA adds, and the highest moving three-second 95th percentile velocity was 84mph at 23ft from the centre. The moving 95th percentile data for one of the three eVTOLs – a remotely piloted example – shows a figure of almost 64mph 126ft from the centre of the take-off area.
For comparison, a light helicopter like the Robinson R44 or R66 have a peak DWOW value of 35mph, a heavy-twin such as the Sikorsky S-92 is around 60mph and the V-22 is around 95mph.
The Beaufort scale – a means of estimating wind speed based on visual cues – says a speed of 32-38mph, or a strong gale, would set whole trees moving, and offer resistance when walking against it, while anything over 74mph is categorised as a hurricane-force wind.
FAA guidance says “most rotorwash-related mishaps can be avoided if separation distances are maintained so that impacting rotorwash-generated velocities do not exceed 30-40kt” or 34.5-46mph.
“eVTOL aircraft DWOW at the [safety area] and beyond surpass most of the air velocity safety thresholds” found in FAA guidance, the report states.
“The eVTOL aircraft surveyed produced high-velocity DWOW flow fields that could easily go beyond the safety area of a vertiport,” it adds, noting that it could “create safety risks to people, aircraft, equipment and infrastructure”, both on and off site.
“eVTOL OEMs propose high-volume, high-tempo eVTOL operations in urban areas, which have even greater potential of impacting bystanders with DWOW than traditional helicopters at heliports.”
The findings suggest that the eVTOL industry’s hope that landing areas could be established atop existing structures, parking garages for instance, are unlikely to be easily realised.
To mitigate the risk, the FAA recommends that vertiports create “downwash caution areas” which should be operational “when and wherever DWOW velocities exceed 34.5mph” which “moves with the aircraft as it maneuvers”.
The three aircraft studied, all pre-production prototypes, “varied in configuration, the number of propulsion systems, blades per propulsion unit, and maximum take-off weight, all of which were less than 6,500lb [2,950kg]”. Only one of the trio was flown by a pilot on board, with the other two operated remotely.
Performed in 2023 and 2024, the test flights were carried out in daylight, visual meteorological conditions. Each pilot was to conduct “several preset maneuvers based on their and the OEM’s capability and risk tolerances”.
However, the FAA notes: “At the time of testing, aircraft flight profiles were limited due to the experimental nature of the aircraft and their early stage of development.”
The novel multi-rotor designs and complexities of the wake-to-wake, wake-to-fuselage and wake-to-ground interactions – which also varied with speed, heading and altitude – resulted in “non-uniform and high-velocity DWOW flow fields that can easily go beyond the safety area of a vertiport”, it says, meriting further research.
In a real-world environment those flow fields could also be impacted by ground structures that were not present in the study.
