Hurricane-force winds registered in FAA eVTOL downwash study

A Federal Aviation Administration (FAA) study, published in late December 2024, has highlighted the risks of downwash and outwash (DWOW) generated by eVTOLs at vertiports – revealing that all three pre-production prototypes tested generated hurricane-force winds at 82ft from the takeoff and landing centre.

Aerial Vertiport

As several eVTOL OEMs push forward towards certification of their craft, so too is consideration of their ultimate operating environment evolving, with particular attention paid to the design and development of their proposed ‘vertiport’ take-off and landing sites. However, with eVTOL aircraft different in form and function to traditional rotorcraft, additional provision must be made for their safe integration into service.

Noting that current research into the effects and potential mitigation of the downwash and outwash – defined by the FAA as “the airflow generated from the [eVTOL] aircraft’s rotors/propellers during takeoff and landing” – is limited, a new FAA study aims to shed light on this as-yet little-understood area. In the first experiment of its kind, aircraft movements of three pre-production eVTOL aircraft under 6,500 lbs (two remotely-piloted) were studied, with custom ultrasonic instrumentation documenting air disturbances. Tests were carried out in daytime VMC conditions.

Perhaps alarmingly, the highest instantaneous maximum velocity was approximately 100mph at 41ft from the takeoff and landing centre, with all three aircraft registering hurricane-force winds at 82ft. (Under the Beaufort Wind Scale of measurement, 64-73 mph is classified as a ‘violent storm’, with anything over 74mph classed as hurricane strength). Additionally, although the highest DWOW velocity was generally registered closer to the takeoff and landing centre, this “does not slow below dangerous thresholds at the sensors farther away”.

As such, “the eVTOL aircraft survey produced high-velocity DWOW flow fields that could easily go beyond the safety area of a vertiport,” concluded the study; recommending that a downwash caution area (DCA) should be operational “when and wherever DWOW velocities exceed 34.5 mph”. This figure – the highest number at which no mitigating measures are required – was set out in FAA engineering brief 105 (entitled ‘Vertiport Design’).

The requirement for a DCA is in line with a FAA study already required for most new vertiports, stating that “DWOW must be mitigated through either vertiport design features or operational procedures”. It also builds on the FAA’s conventional rotary-wing Rotorwash Analysis Handbook, indicating that “most rotorwash-related mishaps can be avoided if separation distances are maintained so that impacting rotorwash-generated velocities do not exceed 30-40 knots (34.5 – 46mph) across the ground”.

However, given “the variances in design and configuration types, and the complexity of emerging eVTOL aircraft,” the study noted it is “difficult to develop a universal model for the prediction of DWOW flow fields from these aircraft” – adding that eVTOLs also differ significantly to traditional rotary craft. “The proximity of propellers can play an important tole in the prediction of the DWOW profile. More specifically, the propellers operating near one another can induce strong aerodynamic interactions, which inherently affect the DWOW profile,” continued the study.

With the need to fully understand and mitigate any potential dangers caused by DWOW particularly important given the “high-volume, high-tempo eVTOL operations in urban areas, which have an even greater potential of impacting bystanders with DWOW than traditional helicopters at heliports,” further research is now required. Additionally, with the study in part limited due to each OEM’s risk tolerance and the pre-production nature of the three aircraft involved, vertiport infrastructure and protocol will doubtless continue to develop hand-in-hand with the aircraft themselves.

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