A Boeing 727 jetliner is shown in this 1974 NASA Flight Research Center photograph taking part in a wake vortex study. Smoke from “smoke generators” installed on the tips of the aircraft’s wings is shown being entrained by the wing tip vortices. A Piper PA-30 Twin Comanche and/or an F-104 or Lear Jet model 23 would fly behind the 727 to measure the effects of the airliner's wake vortices during landing approaches.