Figure 3-15

CHAPTER 3

HELICOPTER AERODYNAMICS WORKBOOK

Figure 3-15

TRANSLATIONAL LIFT

About the same time ground vortex is overrun, the helicopter encounters another beneficial

aerodynamic effect called translational lift. This phenomenon occurs due to a decrease in

induced velocity. How is induced velocity reduced? Recall that during a hover we have a nearly

vertical mass airflow through the rotor disk and the continuous recirculation of our own wingtip

vortices, both of which contribute to a high induced flow (see figure 3-16). When transitioning

to forward flight the rotor outruns this continuous recirculation of old wingtip vortices and

begins to work in relatively undisturbed air. Moreover, the mass airflow through the rotors

becomes more horizontal as airspeed increases (see figure 3-17). Both effects combine to cause

a sharp decrease in induced flow, induced drag and, therefore, power required. Depending on

wind conditions, the onset of translational lift and ground vortex may or may not be noticed or

encountered during transition to forward flight.

3-14 HELICOPTER POWERED FLIGHT ANALYSIS