"Morgans" wrote in message news:hschf.50

.....had smoke generators carefully place on the
wingtips. The trail was tight, and spun fiercely for a very long time.
( 2
or more minutes?) I was fascinated.

Anyone else remember the details?

Not that, but many times over the years I've crossed paths or trailed with
other a/c at a cruise altitude that generated contrails. Never saw the
other a/c, but could see their contrail still rotating as we passed. A
wingtip vortex starts out tighter, so may well last longer.