In forward flight, the
downwash from the main rotor of a helicopter is transformed into a pair of
trailing vortices comparable to the wing tip vortices generated by a fixed wing
aircraft and there is some evidence that these vortices are more intense than
those from a similar-weight fixed-wing aircraft. It is therefore recommended
that helicopters should be operated well clear of light aircraft when hovering
or whilst air taxiing.
Two blade main rotor
systems, typical of lighter helicopters, produce stronger wake than rotor
systems with more blades.
The strength of the
vortex depends not only on the blade geometry and loading, but also on the
aircraft’s operational state (i.e., hovering, climbing, descending, or maneuvering)
but is more pronounced in forward flight, especially when pitch is being
applied, such as at takeoff or landing
When the descent velocity of a rotor approximately matches its wake’s velocity, the helical wake tends to roll up into a thick vortex ring that remains near the rotor plane and interferes with the rotor’s inflow. Vortex Ring may be considered as the helicopter equivalent of aerodynamic stall and is generally of greater danger to the helicopter than to other aircraft in the vicinity because other aircraft are unlikely to fly a profile in proximity to such wake turbulence of vertical, rather than horizontal, extent.
Note that downwash,
although not a vortex issue, must always be considered if planning to pass
above a light aircraft.
Typical helicopter wake vortices
Helicopters are also susceptible to the effects of wake turbulence from large fixed wing aircraft, and ICAO separation requirements apply equally to helicopters as well as fixed wing aircraft.
No specific separation
minima are considered necessary for helicopters because of the way they operate
in relation to streams of in trail fixed wing aircraft.
Separation can be
applied as a minimum distance or time, dependent on criteria such as aircraft
weight and weather conditions.
Separation minima are
not usually applied to helicopters unless they are operating IFR and in streams
of fixed wing aircraft.
A hovering helicopter
generates a downwash from its main rotor(s). Pilots should avoid taxiing or
flying within a distance of three rotor diameters of a helicopter hovering or
in a slow hover taxi, as the downwash can contain high wind speeds.
In forward flight, the wake energy is transformed into a pair of strong,
high-speed, trailing vortices similar to wing-tip vortices of larger fixed-wing
aircraft. Pilots should avoid helicopter vortices since helicopter forward
flight airspeeds are often very low, which generate strong wake turbulence.