Predicting and controlling helicopter rotor wash

Figure 1: Rotor wash from a helicopter landing in an urban environment. Trees and buildings contain the effects, however high speed regions between buildings require controls.

Rotorcraft often operate in close proximity to pedestrians and cars. At low altitude the rotor downwash and outwash from a helicopter can exceed safety criteria. This can result in wind speeds that may knock some pedestrians off their feet. Establishing a pedestrian-free  Downwash Protection Zone (DPZ) around the heliport is the most common way to control such effects. Fixed barriers to deflect or dampen the rotor wash are another potential solution. This is addressed by Section 6 of the Manual of Standards for Airports which places limits on jet blast, propeller wash and rotor wash velocities in regions where people or equipment are likely to be found. Depending on the location, additional measures may be appropriate, for example children and older adults are likely to be more vulnerable to strong winds than airport ground staff.

The complexity of the airflow patterns involved is one of the key challenges in modelling rotor wash. The spinning rotor generates a downward flow of air that generates turbulence and vortices. This can cause significant changes in both air pressure and velocity, which in turn impact nearby objects and people. Understanding the impact of the rotor wash on anemometers and wind vanes is also critical in some locations. With interest in establishing of vertiports in urban locations the separation between pedestrians and rotorcraft is likely further diminish.

Synergetics used CFD to estimate the rotor wash from a helicopter approaching and landing on a helipad to identify regions where air speeds may be sufficiently high to require controls. These targeted locations were then addressed through a combination of physical barriers and access restrictions. This targeted solution provided confidence in the controls whilst also minimising the costs.

Figure 2: Contours of velocity 1.5 m above ground level, immediately before landing, with the helicopter 5 m above ground level.