The knowns and unknowns of eVTOL downwash

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In the final days of 2024, the U.S. Federal Aviation Administration quietly released a technical report on its surveys of the downwash and outwash created by electric vertical take-off and landing aircraft. The report did not make an immediate splash, but by mid-January, trade publications and eVTOL detractors had picked up on one of its key findings: that hovering eVTOLs can generate “hurricane-force” levels of outwash beyond the distances that would be expected for comparably sized helicopters.

This was not wholly surprising: an influential 2023 report from the U.K. Civil Aviation Authority had predicted the same thing. But the experimental data reignited a conversation that had previously been confined to mostly academic circles. Framing of the FAA’s findings varied widely — from the restrained observation that eVTOL downwash and outwash velocities may pose a challenge for vertiport design, to the confident assertion that dangerous downwash will torpedo the urban air mobility business model. Confronted with the perception that its five-seat, roughly 5,000-pound eVTOL will generate the same damaging winds as a 50,000-pound Bell Boeing V-22 tiltrotor, Joby Aviation issued a statement to the effect that “no, it won’t.”

Related: Momentum builds against standalone eVTOL vertiport standards

These questions around downwash and outwash are consequential ones for the eVTOL industry, because they directly affect how and where these aircraft will be able to operate. Yet, like so much else about this emerging industry, the evidence available thus far is too thin to support many sweeping conclusions. Although it is unlikely that downwash alone will be a deal-breaker for the industry, it is probable that eVTOL operations will require some special precautions to protect bystanders from hazardous outflows, and the specific precautions will vary by aircraft type.

In this report, The Air Current explains the surprising complexity of downwash and outwash, the helicopter industry’s mixed record in managing the related hazards, and why the downwash from some but not all eVTOLs may pose a particular risk. A primary takeaway is that the present moment offers an opportunity to improve the safety of the entire vertical-lift industry by focusing attention on a problem that has frequently been underestimated or overlooked.

Complex flows

Downwash is the column of air accelerated downward by a propulsor — such as a rotor, propeller or ducted fan — in the process of supporting the weight of an aircraft. It is the manifestation for vertical-lift aircraft of Newton’s third law of motion: for every action, there is an equal and opposite reaction. When the aircraft is operating close to the ground, the downwash that meets the ground and spreads outwards is called outwash or sidewash.

The potential for downwash to cause damage is related to both the size of the downwash column and the speed of the air within it, but these factors have different drivers. The diameter of the downwash column is determined by the diameter of the propulsor: a larger helicopter main rotor creates a broader downwash column than does a smaller main rotor. However, the mean velocity of the air in that column is a function of a mathematical property called disc loading, which is the weight of the aircraft divided by the total area covered by the blades of its propulsors during rotation. The greater the disc loading, the higher the mean velocity of the downwash. If two helicopters have the same main rotor diameter but one of them is significantly heavier, the downwash columns will be the same size, but the heavier helicopter will have a higher downwash velocity.

The concepts of downwash and outwash are often illustrated with simple diagrams that depict lines of air flowing through a rotor and then spreading out parallel to the ground. However, according to the aerodynamicist Richard Brown of Sophrodyne Aerospace, a noted expert on the subject, downwash in real life is not a steady stream of air, but a collection of dynamic vortices that arise at the blades. As a result, the instantaneous velocities within the downwash column vary widely from place to place and moment to moment.

Rather than extending indefinitely below the aircraft, these vortices eventually decay into diffuse clouds of eddies and disperse into the surrounding air. As with the size of a downwash column, how far it extends below the aircraft is related to the diameter of the propulsor — the downwash column will extend a greater distance below a helicopter with a large main rotor than it will below a helicopter with a smaller one.

The complex structure of the downwash means that the outwash structure is complex, too, and this is central to understanding the potential threat it poses. Even under a helicopter with a single main rotor, the outwash field is not a steady flow evenly distributed around the aircraft (which is how it is usually approximated). Instead, vortices can bunch up into larger structures that create intermittent pulses of air with a much higher velocity than in the surrounding outwash.

Research has shown that this type of unsteady flow can be much more difficult to walk through or work in at lower velocities than more consistent winds. One foundational study from the mid-1970s put around 40 volunteers in a wind tunnel where they were asked to walk and perform a variety of everyday tasks, such as putting on a raincoat, in a range of steady and gusty winds. The researchers concluded that most people could walk safely in steady winds of up to 40 miles per hour, but with sudden changes in wind velocity this threshold drops to 30 mph, and may be even lower for people who have difficulty maintaining balance.

A legitimate problem

The hazards that downwash and outwash can pose to people and property in the vicinity are well documented. Downwash from low-flying helicopters and tiltrotors routinely breaks branches off of trees and kicks up pebbles and clouds of dust. Just in the past few years, helicopter downwash has lifted an inflatable bounce house into the air with children still inside and scattered relief supplies intended for victims of Hurricane Helene. Meanwhile, fast-moving outwash from landing or departing rotorcraft can pack enough force to topple bystanders.

Despite a widespread acknowledgement of the hazards posed by rotor wash — an all-encompassing term for rotorcraft downwash and outwash — the helicopter industry has not addressed them in a particularly comprehensive or systematic way. It is generally up to an individual pilot, operator or heliport owner to decide how to manage these hazards, and guidance on the subject is unevenly distributed throughout the industry.

In sectors such as the military and offshore oil-and-gas, where personnel frequently operate in close proximity to running helicopters, downwash and outwash hazards are typically well understood and addressed through standard operating procedures and personal protective equipment. However, these precautions don’t readily translate to less controlled environments, as evidenced by numerous videos of military rotorcraft wreaking havoc with their downwash. The FAA’s Helicopter Flying Handbook for pilots and flight instructors mentions downwash hazards only in passing, and many members of the general public do not recognize the danger posed by downwash and outwash at all.

Recent incidents at hospital helipads have brought this state of affairs into stark relief. A 2023 report from the Australian Transport Safety Bureau (ATSB) describes nine downwash-related incidents involving medium twin-engine Leonardo AW139 medical transport helicopters within the preceding five-year timeframe. Six of these resulted in injuries to pedestrians who were within around 100 feet of a helicopter landing site, and some of these injuries were serious, including head injuries and broken bones.

More tragically, in March 2022, 87-year-old Jean Langan was killed when she was blown over by a landing search-and-rescue helicopter, a heavy twin-engine Sikorsky S-92, at Derriford Hospital in Plymouth, England. The helicopter landing site was located in a public car park, and pedestrians were not prevented from walking through the car park when helicopters were landing or taking off. An investigation by the United Kingdom’s Air Accidents Investigation Branch determined that the hospital staff who were responsible for managing the helicopter landing site “had insufficient knowledge about helicopter operations to safely manage the downwash risk around the site.”

Enter eVTOLs

When Uber’s Elevate initiative began building hype for the eVTOL industry in 2017, most of these aircraft existed strictly as artist’s renderings, untethered from the laws of physics. Any suggestion of downwash or outwash tended to be missing from these placid scenes, which typically depicted well-coiffed urban professionals walking to their waiting air taxi as others buzzed about in close proximity. Carefree passengers wandered cluelessly across vertiport ramps, placed there by artists for whom illustrating safe operations was not a top priority.

The eVTOL industry did not begin seriously grappling with the problem of downwash and outwash until 2022, when the U.K. CAA commissioned a study on the downwash and outwash characteristics of eVTOL aircraft. The selected contractor was Sophrodyne’s Richard Brown, who had already gained attention for his research suggesting that eVTOLs could be more susceptible than conventional helicopters to vortex ring state, a hazardous aerodynamic condition typically encountered during descents. The CAA began sharing preliminary results with the eVTOL community in mid-2023 and its full report was published in October of that year.

Related: An old aerodynamic problem could pose a new challenge for eVTOL aircraft

Unlike most conventional helicopters, which have a similar single main rotor configuration, most of the eVTOLs now in development have multiple smaller propellers and widely varying designs. The study used computational modeling to understand the downwash and outwash patterns of a variety of generic eVTOL configurations, with the goal of identifying general characteristics that could be used to guide future research. Brown leveraged a specialized numerical tool he called the Vorticity Transport Model, which he developed in the context of his previous work involving downwash and vortex ring state. Compared to other computational modeling techniques, he said, the tool more accurately tracks the distribution of the vorticity within the flow, especially over longer time periods.

Most eVTOLs now in development have significantly higher disc loadings than comparably sized helicopters because their multiple small propellers cover less area than the large area swept by a single main rotor. One would expect an eVTOL to have higher downwash velocities on that basis alone, and Brown confirmed that is indeed the case. However, he also found that the interactions between their many distributed propellers create outwash patterns that are more complex — and potentially more hazardous — than the flows beneath conventional helicopters. Especially notable was the discovery that vortices from two or more rotors can concentrate to form directional jet-like structures that can shoot out what he described as “vortex bullets” at much higher velocities than the surrounding flow.

Outwash patterns vary significantly between eVTOL designs, and some aircraft are small and light enough that their downwash is unlikely to be especially problematic. However, Brown concluded that for some eVTOLs that are similar in size and weight to popular helicopters, the velocities in these turbulent jets of air could be two to three times the outwash velocity of a comparable helicopter.

“Indications are that the unsteadiness of the flow within these jets may pose a particular risk to personnel on the ground, exposing them to sudden, unexpected buffets of wind that, even when experienced at quite low intensities, their physiology is not particularly well-suited to countering,” Brown wrote in his report. He added: “Small changes to procedures, together with an improved understanding and situational awareness of what is happening in the air surrounding the vehicle at any point along its trajectory, may have large positive effects on the safety of operations.”

Diverging methodologies

Brown’s report for the CAA was not a condemnation of eVTOLs, but rather a call to action. In a separate paper presented to the European Rotorcraft Forum in September 2023, Brown urged the eVTOL community to continue researching downwash and outwash by combining theoretical analysis and numerical simulation with carefully structured real-world measurement and laboratory-based experimentation.

He also had some advice as to how to go about this, warning against reliance on computational fluid dynamic methods that are not able to conserve the vorticity of the flow and are thus likely to mischaracterize the magnitude, variation and distribution of the velocities in the outwash. He also cautioned against conducting real-world experiments without a sufficiently dense array of test sensors to capture the complexity of the downwash. “Too sparse a matrix will simply allow the more compact and directional structures within the flow to slip between the fingers of the measurement process, with obvious implications for the accuracy of any quantification of the velocities in the outwash field,” he wrote.

Brown additionally emphasized the importance of the human factor, noting that existing models for predicting the effect of rotorcraft outwash on people focus on the overturning moments induced by constant winds, without consideration for the startle effect of being hit by an unexpected gust. “The situation becomes even more complex when very human characteristics such as frailty or infirmity are taken into account — yet our existing armamentarium of techniques applies most readily to military personnel, and largely ignores the diversity of human condition that any public transport system should be designed to accommodate,” he pointed out.

Even before Brown’s final report was published in October 2023, the FAA had already commenced its own efforts to conduct preliminary measurements of the downwash and outwash of eVTOL aircraft. From August to December 2023, the FAA worked with Joby, Volocopter and Archer Aviation to collect this data for their prototype vehicles. These measurements provided the basis for the FAA’s technical report on the subject, and Joby used the data for its aircraft as the foundation for a separate paper presented at the Vertical Flight Society’s Forum 80 in May 2024.

Although both papers cite Brown’s research, neither takes his advice to heart. For the Joby aircraft, for example, measurements were recorded with a relatively sparse array of 14 ultrasonic anemometers arranged in the shape of a cross, reflecting the experimenters’ best guesses as to where the maximum wind velocities would be seen. These guesses were informed by computational modeling techniques — Reynolds-Averaged Navier Stokes and Viscous Vortex Particle Method (VVPM) — that Brown specifically identified as inappropriate for the purpose. The authors of the FAA report acknowledged that their VVPM models did not accurately predict outwash airspeed for sensors at greater distances from the aircraft, but blamed this shortcoming on a lack of computational memory.

Nevertheless, the sensors did capture some of the very high outwash velocities that Brown had predicted. The orientation and distances of the sensors varied for each of the three eVTOLs measured, making direct comparisons difficult, but one unidentified eVTOL’s outwash was clocked at more than 60 mph at 100 feet from the center of the aircraft’s touchdown and liftoff area (TLOF). The FAA researchers concluded that eVTOLs’ “novel designs and complex wake-to-wake interaction, wake-to-fuselage interaction, and wake-to-ground interaction — all changing depending on aircraft speed, heading, and altitude — result in non-uniform and high-velocity [downwash/outwash] flow fields that can easily go beyond the safety area of a vertiport.”

Joby, which recorded a maximum instantaneous outwash velocity of 55 mph at around 70 feet from the TLOF center, had a different interpretation of its data, concluding in its Forum 80 paper that “the outwash of the Joby S4 aircraft does not present an increased risk over traditional light helicopters.” To arrive at this conclusion, Joby used the mean outwash value over 15 seconds at each test point, an approach that smooths out the contribution of any intermittent, high-velocity pulses of air. Joby also evaluated the impact of its outwash on humans using the PAXman model, which was created to compute overturning forces on military personnel. Although Joby added a 20% safety factor to its calculations for conservatism and possible weight growth with future aircraft, its paper does not address whether the model is appropriate for the full range of urban air mobility passengers, not limited to burly young soldiers.

When asked for comment, Joby did not directly address these criticisms. “We participated in the FAA’s outwash survey as part of our continued support for industry-wide efforts to characterize aspects of operational safety for novel vertical-lift aircraft. The resulting data, which we published in the paper submitted to VFS’ Forum 80, shows that the Joby aircraft’s outwash does not present an increased risk relative to traditional light helicopters and is acceptable within established safety areas for heliports of all sizes,” a Joby spokesperson told TAC via email. “We intend to continue engaging with the FAA and our wider industry to promote safe and efficient operations.”

Proactive safety

The high-velocity outwash generated by certain eVTOLs does not mean that their developers’ business models are necessarily doomed, as the industry’s most gleeful detractors have suggested. But it does mean that additional precautions may need to be taken to ensure that people on the ground are not harmed by it.

The FAA is taking steps to encourage these precautions through the development of new guidance for vertiport design, contained in Engineering Brief (EB) 105A, which will eventually be rolled into a planned revision of its existing advisory circular (AC) for heliport design. (Although the standards in the AC are only mandatory for public use heliports that receive funding through the Airport Improvement Program, states and municipalities may choose to require compliance for other heliports in their local codes.)

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EB 105A, also released at the end of December, introduces the concept of a downwash/outwash caution area (DCA), defined as “an operational area that is identified to protect persons and property from downwash and outwash (including jet blast or propwash) that may meet or exceed 34.5 mph.” This is not a permanently defined area of a vertiport, but rather an operationally enforced perimeter where the movement of people and vehicles is controlled while an aircraft is operating. The size of the DCA may vary depending on the type of aircraft that is in operation, as different aircraft have very different outwash patterns.

Notably, EB 105A does not specify whether 34.5 mph is a maximum instantaneous or average velocity value — a distinction that, as the Joby paper illustrates, can have a significant impact on the size of the DCA for both eVTOLs and helicopters.

The approach of establishing an operationally enforced caution area is similar to some of the procedures that have been adopted locally in the wake of helicopter rotor wash incidents, with the difference that the FAA is attempting to act proactively rather than reactively. For example, the ATSB report on safety risks from rotor wash at hospital helicopter landing sites notes that the number of these incidents has increased since the larger, heavier AW139 helicopter was widely adopted for medical transport operations (MTO) in Australia. In the aftermath of incidents, some hospitals have added marshallers to keep pedestrians well clear when a helicopter is landing or departing, in addition to measures such as placing restrictions on the flight paths used for approach and departure.

“The recent increase in rotor wash incidents does not suggest that hospital [helicopter landing sites] are more dangerous now than they were previously, rather, the risk mitigation measures employed for previous generations of MTO helicopters are likely inadequate in managing the rotor wash hazard produced by the current generation of heavier MTO helicopters,” the report states.

Meanwhile, the FAA has made it clear that its initial technical report on eVTOL downwash and outwash is not its last word on the subject. In a virtual industry day to discuss the release of EB 105A, Robert Bassey of the FAA’s Office of Airport Engineering acknowledged that the measurements the agency has collected thus far are limited, and that it expects to engage with original equipment manufacturers over the next several months to collect additional data to build “a more robust framework around design infrastructure.”

As previously reported by TAC, the FAA also plans to begin engaging with helicopter OEMs over the next year to commence testing for downwash/outwash profiles, climb/descent gradients and landing precision as it works toward unified, performance-based guidance for the design of heliports and vertiports. Bassey said the FAA aims to issue its updated heliport design AC by June 30, 2027, and to that end is aiming to collect as much data as possible by the end of 2025.

“The data that we collect will then be used by our FAA research team to fill in aircraft information gaps, and this will require also continuing coordination within the FAA across various lines of businesses, as well as external collaboration with manufacturers — VTOL aircraft manufacturers, helicopter OEMs and other stakeholders,” Bassey said. “Overall we have and continue to use as an agency a data-driven approach, because we want to ultimately ensure that [advanced air mobility] infrastructure or vertical-lift infrastructure standards allow for safe operations.”

Write to Elan Head at elan@theaircurrent.com

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