The starting vortex

The wing-bound vortex, together with the trailing vortices, form a kind of horseshoe shape, and this is sometimes called the horseshoe vortex system. The horseshoe system forms three sides of the predicted closed ring. The circuit is completed, as shown in Fig. 2.3, by the starting vortex. In the next chapter, we describe how this starting vortex is formed.

A strong starting vortex is formed and left behind just above the runway when the aircraft rotates at take-off. More starting vorticity is produced and left behind whenever the aircraft produces an increase in wing circulation. An additional starting vortex is thus formed, when an aircraft starts to pull out of a dive.

The counterpart of starting vorticity is stopping vorticity, which rotates in the opposite sense, and is shed every time the circulation is reduced, as on landing.

As we mentioned in Chapter 1, in level flight, the amount of circulation required reduces as the speed increases, so stopping vorticity is shed when an aircraft accelerates in level flight.

Starting and stopping vorticity is left behind the aircraft, and eventually damps out due to the effects of viscosity. It may, however, persist for several minutes, and the rotating masses of air left behind represent a considerable hazard to any following aircraft. It is necessary to leave a safe distance between two aircraft, particularly when landing. This may be several miles if the following aircraft is much smaller than the leading one.

Strong starting and stopping vortices can be generated during violent manoeuvres, and may significantly affect the handling. The formation of starting and stopping vortices is described further in the next chapter.