UK researchers believe they have discovered a “powerful new approach” to demonstrate safety risks posed by hovering helicopters and other vertical take-off vehicles by using video simulations to “capture the true physical form” of the gusting and swirling winds generated.
In addition, the study – Protecting the Future: Trials and Simulation of Downwash and Outwash for Helicopters and Powered Lift Aircraft – highlights the impact to cognitive function from exposure to this highly turbulent airflow, a “surprising” effect experienced by the researchers.
The Civil Aviation Authority (CAA)-led project had been attempting to better understand the airflow dynamics beneath rotorcraft but concluded that “tables of data” fail to accurately convey the “visceral experience” of the downwash and outwash flows.
Instead, researchers found that by using data collected to feed digital simulations and visualisations it was possible to link real-world experiences with the complex airflows produced.
“This ability to show the physics that drives human responses is a potentially powerful new approach to understanding the safety risks of downwash and outwash, as well as in identifying new ways to address them,” the CAA says in its report.
Work on the topic was spurred by a safety recommendation from the Air Accidents Investigation Branch in its final report into a fatal accident at Derriford hospital in Devon in which a bystander was blown over and killed by the downwash from an approaching Sikorsky S-92.
The CAA’s report notes the commonly held misconception that downwash and outwash are a “form of wind, relatively steady in form”, a view which “fails to show the transitory aspects of downwash” and “the very different physics of outwash, which is not simply downwash that has ‘turned a corner’.”
It also sought to validate simulations showing “vortex-dominated, transitory flows” that formed part of the CAA’s 2003 publication, Understanding the Downwash/Outwash Characteristics of eVTOL Aircraft, also known as CAP 2576, which examined the physics of the complex airflows caused by powered-lift aircraft.
Such studies have gained additional importance given the potential introduction into built-up areas in the coming years of electric vertical take-off and landing (eVTOL) aircraft performing urban air mobility missions.
A study by the Federal Aviation Administration published in December showed high peak levels of downwash and outwash generated by such aircraft.
For the CAA project, an initial data-gathering trial was conducted at the heliport at the Silverstone motor racing circuit in July 2024 using hand-held sensors rather than remote sensor arrays.
This meant researchers “experienced downwash and outwash as a person on the ground does”, the report says.
“We found outwash has both a physical effect and a cognitive one. Simple tasks like reading out numbers become much harder when you are immersed in high-speed air.
“The CAA test team had long-term aviation personnel in it who had experience of jet blast, yet outwash was clearly a different phenomenon, particularly the turbulence of the airflow.”
While the data collected showed certain trends – notably greater air flow velocities along the nose-to-tail axis of a helicopter – the report says the “physiological experience of the tests” was “of much greater interest”.
“Many of the team experienced being both physically and cognitively overwhelmed by the outwash, making the [data recording] task much harder to perform than expected.”
Researchers experienced “constant buffeting” from the outwash, were peppered by “small stones and grit” and faced communication difficulties thanks to the ear and eye protection required.
“More noticeable was a very odd effect where the person reading the numbers would turn to shout them to their colleague, and in that moment would forget them,” it says. “This cognitive difficulty was surprising and hindered the entire trial.”
A 3D simulation shows airflow velocities and patches of vorticity during a flow reversal.
A second trial was carried out using an S-92 – supplied by Bristow Helicopters – at Caernarfon airfield in North Wales in September that year.
Recordings were taken with the heavy-twin hovering at heights of 65ft and 6.5ft with the highest outwash levels again found on the nose-to-tail axis.
But the report adds: “Once again, the turbulence of the outwash field led to both physical and cognitive disturbance.”
Information relating to the test conditions – such as wind direction, sensor and helicopter location, but no test data – were then passed to contractor Sophrodyne Aerospace for simulation. Sophrodryne had carried out the modelling work in CAP 2576.
This allowed the company to model the downwash and outwash of two notional aircraft of the same take-off weight and disc loading – one based on the S-92 and the other a “simple helicopter with an eVTOL-like rigid rotor”.
The simulations showed the two aircraft produced different downwash and outwash patterns – with the S-92’s “more structured” pattern likely due to its fully articulated main rotor.
Initial attempts to validate the simulations by simply comparing the numbers found “good agreement” between the simulations and data and the real-world test results, but this approach “did not capture the fundamental physical properties of the vortex-dominated flow that lies at the heart of Sophrodyne’s [Vorticity Transport Model], and as experienced during the tests.”
Instead, to gain a “clearer visualisation of the flow physics” several videos were produces showing “the dynamic nature of the airflow and the resulting velocities,” the report states.
“The striking thing about the videos was that they captured not only the physics of the flow but also the physical experience of being immersed in an outwash flow.
“The visualisations allow direct apprehension of the visceral experience of outwash as well as matching data points.”
These visualisations “may help prepare and train people on the ground to respond safely to outwash and downwash”, aiding the widespread uptake of eVTOLs, the report says.
“By understanding the physics and seeing the airflows, it is hoped that the potential risks presented by the previous, simplified, view of outwash and downwash as a ‘linear wind’ can be avoided.
“The complex, transient and vortex-dominated flow needs to be properly understood to ensure safe flight operations.”
Additionally, the “identification of a previously unreported cognitive effect of downwash and outwash, and a method by which to gauge it, may also allow future risks to be avoided well before flight operations begin,” it states.
