Protecting the Future: Trials and Simulation of Downwash and Outwash for Helicopters and Powered Lift Aircraft.
UK Civil Aviation Authority CAP3075, April 2025
Introduction
Downwash and outwash are two different, but related phenomena associated with the aerodynamics of helicopter rotors. Downwash is a consequence of Newton's third law of motion - effectively the result of the downwards acceleration of air through the rotor in the process of generating the lift required to keep the aircraft in the air. The outwash is, as the name suggests, the outwards flow of air that takes place if the downwash hits the ground and spreads laterally outwards. An outwash is formed only when the aircraft is relatively close to the ground (and is moving relatively slowly), whereas the downwash through the rotor always exists - at least as long as the rotor is producing an upwards component of force.
A very common misconception regarding the outwash below a helicopter hovering above the ground is that it is a relatively constant outflow of air, the velocity of which decreases quite rapidly with distance away from the helicopter. This view is commonly adopted in establishing the size of the operational "safety zones" around helicopters and is reinforced by the depiction of the outwash in several influential publications that have underpinned the safety case.
One of the aims of CAA CAP3075 was to understand better the characteristics of the flow in the outwash below hovering helicopters, and in particular to explore the physical origin of observations that the variation of the velocity, both spatially and in time, is far more complex than portrayed by these simple conceptualisations of the flow.
The movies below supplement the images presented in CAP3075, and show results from simulations conducted by Sophrodyne Aerospace of the flow that is generated in the air surrounding a helicopter when hovering at relatively low height above the ground.
The movie below (Fig. 18 in CAP3075) shows the flow in the outwash from a point of reference located above the helicopter. The velocity just above the ground is represented by the colours: air moving
at high speed is represented in red; spots where the flow is relatively quiescent are in blue. The simulation shows clearly that the flow out across the ground is highly unsteady - gusty bursts of
high-velocity air spread out across the ground in wave-like patterns that originate below the rotor of the hovering helicopter.
The movie below (Figs. 12 and 13 in CAP3075) shows a side-view of the flow in the outwash from the helicopter when hovering quite low above the ground.
The speed of the flow at various heights above the ground at the location of an observer standing out in front of the helicopter (at the point defined in CAP3075)
is represented using a set of arrows attached to a vertical stake.
These instantaneous speeds are compared on the scale at the bottom of the frame to the mean
speeds encountered in winds of varying strength as defined on the Beaufort Scale - a measure commonly used in meteorology to quantify the strength of winds in the atmosphere.
The simulation shows clearly how variable the velocities at any location within the flow within the outwash below a helicopter actually are. The velocities at the observation point vary considerably over time, and, for a helicopter of the size and weight that was simulated, velocities as high as 50-60 knots might even be transiently encountered.
CAP3075 discusses some of the physiological and cognitive aspects of being immersed in a flow that has this intense, gust-like character.
The simulation shows clearly how variable the velocities at any location within the flow within the outwash below a helicopter actually are. The velocities at the observation point vary considerably over time, and, for a helicopter of the size and weight that was simulated, velocities as high as 50-60 knots might even be transiently encountered.
CAP3075 discusses some of the physiological and cognitive aspects of being immersed in a flow that has this intense, gust-like character.
The movie below (Figs. 14 and 15 in CAP3075) shows the same information as in the two movies above, but this time from a 3D perspective that allows the relationship
between the velocities as experienced at any point in the outwash and the cellular, gust-like structure of the overall flow to be appreciated more directly.
The movie below shows the flow in the outwash below the helicopter, again when hovering quite close to the ground (condition beta in CAP3075), from another 3D perspective that allows
the relationship between the velocities as experienced at any point in the outwash and the cellular, gust-like structure of the overall flow to be discerned quite clearly.
The velocity just above the ground is represented by the colours: air moving at high speed is represented in red; spots where the flow is relatively quiescent are in blue.
The simulation shows clearly that the flow out across the ground is highly unsteady - gusty bursts of high-velocity air spread out across the ground in wave-like patterns
that originate below the rotor of the hovering helicopter.
The movie below shows the flow in the outwash below the same helicopter, but when hovering somewhat higher above the ground than in the movie above (condition alpha in CAP3075). The 3D perspective
allows the relationship between the velocities as experienced at any point in the outwash and the cellular, gust-like structure of the overall flow to be seen quite clearly. In this case the flow out across
the ground is even more unsteady than when the helicopter is hovering low above the ground - gusty bursts of high-velocity air still spread out across the ground, but the wake descending from the main rotor
also has time and space to meander, on occasion, off to the port side of the aircraft. (This meandering is caused principally be the interaction of the wake of the main rotor with that of the tail rotor).
When this happens, the flow out across the ground becomes quite directional. The effects of this meandering are quite clearly apparent in the time-histories of the velocity that are measured on the ground
below the aircraft (see Figs. A1 and A2 in CAP3075).
The movie below shows the flow in the outwash below the same helicopter and under the same conditions as in the movie above, but from a lower perspective that allows the viewer to experience the outwash
from the perspective of the CAA observers standing ready to measure the velocities in the outwash with their hand-held equipment. Hopefully the movie gives some appreciation of the strength and intensity of the
flow past these observers, as well as an appreciation of how much they may have been buffetted about as they tried to record accurate data.
CAP3075 describes in extensive detail the physiological and psychological experience of being immersed in the outwash, and suggests that our conceptualisation of how humans are affected by the outwash may need to be updated based on a more extensive appreciation of these effects.
CAP3075 describes in extensive detail the physiological and psychological experience of being immersed in the outwash, and suggests that our conceptualisation of how humans are affected by the outwash may need to be updated based on a more extensive appreciation of these effects.
What about eVTOL aircraft?
Another of the aims of CAA CAP3075 was to lend some credence to the predictions made in the earlier CAA CAP2576 regarding the character of the outwash that might be generated by multi-rotor eVTOL-type aircraft when operated close to the ground. CAP 2576 had suggested that the outwash below an eVTOL aircraft would be proportionately stronger, depending on the disc loading of the vehicle's rotors, but also would have a directionality that would be dependent on the configuration of the vehicle itself (for example the number, size and layout of the rotors).
As a taster of how we intend to extend the work that was published in CAP 3075, the videos below show an example calculation of the outwash below a generic twelve-rotor eVTOL aircraft, comparing the flows that are generated when the aircraft is operated at two different heights above the ground. The unsteadiness within the outwash is clearly apparent, but if you look closely you can also discern considerable order within the swirling patterns as they form and dissipate on the ground. Indeed, the directionality of the flow in the outwash of this particular vehicle becomes clearly apparent once the velocities in the flow are averaged over several hundred rotations of its rotors.
The movie below shows the flow in the outwash below the 12-rotor eVTOL aircraft when in a relatively high hover above the ground. When the aircraft is operated at this height, the inherent
instability of the wakes of the individual rotors has time to erode much of the orderly structure of the flow before it reaches the ground. As a result, the outwash, especially at larger distances from the
aircraft, is quite patchy and cellular, and the gusts to which a bystander might be exposed, although frequent, are short-lived and relatively moderate in strength.
The movie below shows the flow in the outwash below the 12-rotor eVTOL aircraft while hovering at somewhat lower height above the ground than in the movie above. At this height, the inherent instability of
the wakes of the individual rotors has less time to erode the structure of the flow before it reaches the ground than in the case shown above. As a result, the outwash is considerably more intense,
and, instead of being cellular in nature, the flow out across the ground consists of large, intermittent but regular 'flares' of coherent, high-velocity air that are able to travel quite considerable
distances out into the surroundings of the aircraft before dissipating.
Summary
One of the aims of CAA CAP3075 was to understand better the characteristics of the flow in the outwash below hovering helicopters, and in particular to explore the physical origin of observations that suggest that the variation of the velocity, both spatially and in time, might far more complex than is widely appreciated - particularly within the safety community.
Most present methods for quantifying the risks that are posed to bystanders and nearby infrastructure by the velocities in the outwash that is generated in the flow below a hovering rotorcraft are based on a conceptualisation of the flow in terms of time-averaged quantities. The tendency is thus to ignore or downplay the effects of unsteadiness in the flow, especially on the physiological and cognitive response of individuals that happen to be immersed in the outwash.
CAP3075 provides a first-hand account of being exposed to helicopter outwash, and shows how quality numerical simulations can be used in conjunction with simple measurements to understand the fundamental physics that was responsible for the observer's subjective experiences.
It is hoped that CAP3075 will contribute to a more nuanced understanding of the effects of rotorcraft outwash (including that of conventional helicopters as well as of eVTOL aircraft) on the humans that might interact with these machines, and thus to improving our understanding of the risks that might be posed when these aircraft are operated close to bystanders and to infrastructure on the ground.
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