No. 94: Jul-Aug 1994
In SF#93, under the title: "Just Plane Weird," we questioned the long-term persistence of trailing vortices from aircraft wingtips. Could such vortices actually maintain their integrities for several minutes and thus produce the curious tubes of misty air and flapping sounds observed at the end of the Los Angeles airport runway? The answer seems to be YES, as confirmed below:
"Wingtip vortices have a core diameter of 20 to 40 feet and, as they trail behind each wingtip, remain approximately two wingspans apart. Why don't they enlarge in radius and dissipate? The rapid acceleration at the outer edge of the vortex produces low pressure in the core, and this pressure differential creates enough centrifugal force to hold the system tightly together for three to seven minutes. After that, friction takes its course, breaking these stubborn twisters apart into mere turbulence."
The rest of this article deals with how light aircraft can avoid these sometimes deadly horizontal "twisters."
(Manningham, Micah D.; "Wake Turbulence," Private Pilot, p. 69, June 1994. Cr. W.A. Welch)