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Wake Vortex Generation by Helicopters

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Category: Wake Vortex Turbulence Wake Vortex Turbulence
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Description

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).

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.

Typical helicopter wake vortices Source: AAIB Bulletin: 7/2010 EW/C2009/09/07

Separation Requirements

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.

Accidents and Incidents

  • P28A / S76, Humberside UK 2009 (WAKE): On 26 September 2009, a privately operated Piper PA28-140 with only the pilot on board was about to touch down on Runway 26 at Humberside Airport, after a day VMC approach when the aircraft rolled uncontrollably to the right in the flare and struck the ground. The aircraft came to rest inverted beside the runway and suffered significant damage but there was no fire. The pilot sustained serious injuries. The Investigation concluded that:
“The uncontrollable right roll experienced by the pilot of (the accident aircraft) was probably the result of the aircraft flying through the wake turbulence generated by the preceding Sikorsky S76.”

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