An aircraft departing or arriving too close behind a large aircraft can be subjected to rolling moments exceeding the roll-control authority of the encountering aircraft, which cannot be compensated by any flight maneuver. Counter control is usually effective and induced roll minimal in cases where the wingspan and ailerons are outside the rotational flow of the vortex. You should fly at or above the preceding aircraft's flight path, altering course laterally as necessary to avoid the area below and behind the generating aircraft. This is especially critical at low altitude where a momentary encounter could be deadly. Wake turbulence is not restricted to fixed-wing aircraft.
Helicopters can cause significant downwash and vortices.
Various small aircraft were selected to penetrate the wakes. NASA was attempting to make changes to large aircraft that would reduce the severity of the vortices they shed. In one instance the 747's wake caused the T-37 to perform two unplanned snap rolls and develop a roll rate of 200 degrees per second, despite trailing the jetliner by more than three miles. Dryden's B-747 wake turbulence studies clearly indicated that the use of spoilers could reduce the severity of wake turbulence. Unfortunately, the attenuation of vortices was not effective when you put the gear down, and as such, interest waned and it was not studied.