FAA Underlines eVTOL Downwash Challenge For Vertiport Design

When the FAA released draft updated engineering guidelines for vertiports in September 2024, it introduced the requirement for a caution area to provide protection from the high wind velocities produced by the propellers of electric air taxis during vertical takeoffs and landings.

The introduction of the downwash caution area (DCA) was the result of flight surveys of multiple prototype electric vertical-takeoff-and-landing (eVTOL) aircraft for the FAA. On Dec. 27, the FAA quietly released the finalized Engineering Brief (EB) 105A along with a report on the testing.

For vertiport planning, the guidance requires DCAs to be established anywhere that wind velocities can meet or exceed 34.5 mph—“near gale” speeds, according to the Beaufort wind scale. The highest instantaneous wind speed measured in FAA testing was a hurricane-force 100 mph at 41 ft. from the center of the touchdown and liftoff (TLOF) area, the FAA report says. The highest 3-sec. average speed was 84 mph at 23 ft. from the center and speeds exceeding 60 mph were measured 100 ft. from the center.

Downwash is the airflow created by propulsion units producing lift in vertical flight. When the downwash hits the ground, it moves horizontally as outwash and can be dangerous to people, property, equipment and other aircraft. “The eVTOL aircraft surveyed produced high-velocity DWOW [downwash/outwash] flow fields that could easily go beyond the safety area of a vertiport,” the FAA report says.

The safety area surrounds the final approach and takeoff area (FATO) and TLOF and is intended to reduce the risk of damage to aircraft accidentally diverging during approach to hover and landing. Under EB 105A, the safety area is 2.5 times the diameter of a circle enclosing the entire eVTOL aircraft, with propellers turning. Most eVTOLs are sized to fit within a standard 50-ft. x 50-ft. helicopter TLOF.

According to the UK Civil Aviation Authority (CAA), eVTOLs may differ from helicopters in two key areas that affect downwash. First, most eVTOLs have higher disk loading—the total disc area of all the propulsors divided by the aircraft’s weight—than helicopters. This usually means higher downwash velocities.

Second, eVTOLs have multiple propulsors and CAA publication CAP 2576 says experience with military tiltrotors and VTOL aircraft has shown that multiple flows from several propellers, rotors or jets produce a downwash pattern different to that of single-main-rotor helicopters.

Instead of a fairly symmetrical, circular pattern, the downwash from multiple flows can produce a more irregular shape. When several separate flow patterns interact, they can produce spikes with a greater velocity than the overall average downwash or outwash speed.

To gather downwash/outwash data for use in developing vertiport design guidance, the FAA contracted with four eVTOL manufacturers to conduct testing. The eVTOLs are simply numbered and not identified in the report, but Aviation Week has established that the FAA contracted with Archer Aviation, Beta Technologies, Joby Aviation and Volocopter to conduct surveys.

Archer and Volocopter publicized their participation in the tests at the time. The report says eVTOL #4 was not surveyed because it was not made available by the OEM. Beta says its aircraft was committed to some military engagements during the time frame given by the FAA, so it was not able to participate.

Joby confirms that it participated in the surveys. The magnitudes of the outwash for the Joby S4 and Robinson R44 light helicopter are similar, the company said in a presentation on the testing to the Vertical Flight Society’s Forum 80 in May 2024. “The Joby S4 has a higher per-propeller disk loading than the R44 and approximately twice the overall weight,” the company noted.

While the aircraft are not identified, there are clues. The FAA says eVTOL #1 was remotely piloted, eVTOL #2 autonomous and eVTOL #3 piloted. Volocopter has said its FAA downwash survey flights were conducted in Tampa, Florida, while its piloted 2X prototype was on a U.S. tour.

Several ways of measuring downwash/outwash were used, including maximum instantaneous speed, 3-sec. moving average and 3-sec. moving 95th percentile. While the 95th percentile maximum speeds were lower, the report notes that instantaneous winds can be more upsetting for people than sustained winds due to the startle response.

For eVTOL #1, Joby’s S4, maximum instantaneous DWOW speeds measured ranged from 99.3 mph at 23 ft. from the TLOF center to 55.1 mph at 69 ft., while maximum 95th percentile speeds were 83.6 mph and 40.1 mph at the same distances.

“In August 2023, Joby and the FAA worked together to measure the propeller outwash of our aircraft,” the company says. "The resulting data 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.”

For eVTOL #2, Archer’s Midnight, maximum instantaneous DWOW speeds measured ranged from 98.5 mph at 27 ft. from the TLOF center to 69 mph at 126 ft., while maximum 95th percentile speeds were 83.6 mph and 40.1 mph at the same distances.

For eVTOL #3, likely Volocopter’s 2X, maximum instantaneous DWOW speeds measured ranged from 75.3 mph at 25 ft. from the TLOF center to 36.5 mph at 90 ft., while maximum 95th percentile speeds were 37.2 mph and 30.6 mph at the same distances.

“Regardless of how the speeds are analyzed, eVTOL aircraft have high-velocity DWOW that will impact people and property on the ground. This not only includes people and property on the vertiport but possibly people and property outside of the vertiport boundaries,” the FAA report concludes.

(Editor’s note: This story has been updated with a response from Joby Aviation.)