Urban Airflow: What Drone Pilots Need to Know


This video provides awareness to RPAS users on the complex airflows within urban environments caused by interactions between the wind and the structures.

Provided you are authorized to fly in this environment the range of flow types shown in this video may be within the flight path of your remotely piloted aircraft.

Changes in urban wind characteristics due to the presence of buildings and structures that may affect RPAS operational limits include speed, direction, shear, turbulence, vorticity and icing. S D S T V and icing.

Within the urban environment wind speed would change depending on height and proximity to structures.

Wind speed increases with height as the wind becomes less obstructed by the earth's surface roughness including trees and urban structures.

Wind speed increases with height at an exponential rate.

The rate of change in wind speed depends on terrain and is therefore different for open country found near most airports in comparison to city or urban terrain.

To estimate the wind speed your RPAS will encounter at maximum altitude the calculation can be made using reference wind speed and the exponential relationship.

For urban flight using a reference wind speed measured at takeoff location is not safe because the buildings in the city obstruct wind flow.

Therefore using a local weather station report is recommended where mean hourly wind speeds, unobstructed by buildings, are measured at 10 meters above ground.

From the weather station report we suggest calculating the speed at 122 meters above ground which is 1.5 times the weather station wind speed.

To estimate the wind speed closer to the ground the calculation using the 122 meter wind speed as a reference can be used if needed.

Alternatively for a quick guide a height to wind speed ratio table will be provided at the end of this video.

Within the urban environment wind speed is also variable.

Within the urban wind gradient speed can change quickly by up to 18 kilometers per hour.

The wind speed could be 10 kilometers per hour and then change to 28 kilometers per hour in short period of time.

So it is wise to limit your flight path to heights where wind speed is estimated to be 18 kilometers per hour below your RPAS sustained wind tolerance limit.

Due to disturbances caused by urban structures wind speed can increase locally.

If the spacing between buildings restricts the path of the wind the air flowing through the constriction may increase in speed causing a venturi effect.

The wind speed may increase by as much as double the speed of the upstream wind.

This can occur at any building height including pedestrian level.

Urban airflow between and around structures can also change direction.

Tall buildings within a surrounding low-rise urban scape redirect flow causing vertical and horizontal currents.

Vertical flow includes updraft and down draft on the windward side of a tall building.

Downdraft causes street level flow to divert horizontally away from the building.

Alternatively the low pressure zone on the leeward side draws horizontal flow towards the base of the building which converges into vigorous updraft.

These wake features which include flow reversal and updraft can persist for the entire height of a tall building and the recirculation cavity driven by this motion can extend downstream for up to twice the building height.

Near building corners wind speed changes rapidly.

This airflow feature is known as a shear layer.

A shear layer is a narrow band of flow at the outer boundary of a building wake where the outer side of the band is equal to unobstructed wind speed and the flow on the inner side is slowed by the presence of the buildings and is drawn into the highly turbulent recirculating flow.

Shear flow will develop at the windward corners of the building including the roof.

If the airflow has enough building length to reattach and form a recirculation bubble a sheer layer may also develop at the leeward corners.

Sheer layers are not a fixed feature.

They flap inward and outward as the wake shifts in time.

Passing through a sheer layer may cause RPAS instability due to a shifting and uneven distribution of wind force across the aircraft.

RPAS instability can also result from high intensity turbulence within urban airflow.

Atmospheric turbulence is a disorderly movement of eddies in the atmosphere.

Atmospheric turbulence intensity is a measure of the fluctuation in wind speed caused by turbulence in comparison to the average wind speed.

The continuous change of air flow in both direction and speed causes rapidly changing forces on RPAS which increase as the turbulence intensity increases. Within the urban environment turbulence intensity can be four times more severe than in the free atmosphere above open country.

Turbulence can also manifest in the form of building-induced vortex shedding.

The type of vortex shedding is dependent on wind direction and building orientation.

Vorticity can develop in a side-to-side vortex shedding pattern.

The wind approaching some structures at some angles may cause counter-rotating wake vortices that rotate about a vertical axis and are shed from the leeward building face in a side to side sequence.

The size of the shed vortices is similar to the width of the building.

Coordinated vortices can also appear within rooftop wakes.

If the wind approaches the building face at an angle two strong rooftop counter rotating vortices emanate from the windward corner of the roof with a horizontal trajectory.

Horizontal vortices can be a stability hazard to RPAS as they apply an overturning force to the aircraft particularly if the vortex and RPAS are similar in size.

RPAS users must also watch for icing conditions.

In addition to operational hazards due to the complex airflow within the urban environment for cold conditions instabilities caused by ice secretion impact the performance of rotor blades potentially making it impossible for the RPAS to fly.

For rotor diameters under 350 millimeters the quick buildup of ice can reduce thrust by 50 percent in less than 30 seconds.

Icing conditions are a consideration along with speed direction shear turbulence and vorticity. S D S T V.

As each adds potential weather-induced hazards within the urban environment that RPAS users should be aware of when planning or maneuvering through a flight path.

To reduce the risk of adverse wind effects on your RPAS operations flying in calm conditions is advised which occur more often in mornings and evenings.

This urban air flow awareness video has been presented to you by Transport Canada's RPAS task force with the support from the aerodynamics lab at the National Research Council of Canada and RWDI.

For related RPAS information go to the Transport Canada website at www.canada.ca/drone-safety.

Supporting research document: Urban airflow: what drone pilots need to know - NRC Publications Archive - Canada.ca