HELICOPTER AERODYNAMICS WORKBOOK
During transition to forward flight the pilot will encounter several phenomena exclusive to
helicopter flight which, although encountered almost simultaneously, are discussed separately in
the following sections. The first is ground vortex.
As the helicopter accelerates from a hover in ground effect to forward flight, benefits of
ground effect can be lost at an altitude of less than rotor diameter and airspeed between 5 and
20 knots. This is called ground vortex. As the helicopter moves forward, the rotor downwash
mixes with increased relative wind to create a rotating vortex, which eventually causes an
increased downwash through the rotor system. This simulates a climbing situation, increasing
power required. Eventually this vortex is overrun at a higher speed. These flow patterns are
better described in figure 3-15.
HELICOPTER POWERED FLIGHT ANALYSIS 3-13