Figure 5-5

CHAPTER 5

HELICOPTER AERODYNAMICS WORKBOOK

The unsteadiness of the flow has been seen during wind-tunnel tests of model rotors using

smoke for flow visualization. Figure 5-5 shows a sequence of events based upon interpretation

of the smoke patterns. According to this model, the rotor is continually pumping air into a big

bubble under the rotor. This bubble fills up and bursts every second or two, causing large-scale

disturbances in the surrounding flow field. The bubble appears to erupt from one side and then

another, causing the rotor thrust to vary and the rotor to flap erratically in pitch and roll,

requiring prompt reaction. This is what causes the loss of control effectiveness. Recovery

includes lowering the collective and forward cyclic to fly out of the condition. Increasing the

collective only serves to aggravate the situation.

Figure 5-5

Figure 5-6 shows the power and pitch settings required to maintain constant rotor thrust in

vertical descent for a typical helicopter. Notice the increase in rate of descent with collective

increase during vortex ring state conditions.

Figure 5-6

After a helicopter is descending fast enough to pass through the worst of the unsteadiness in

vortex ring state, it will achieve vertical autorotation. Usually there is still a little induced

downflow through portions of the rotor disk, but most of the flow will be upwards. This mixed-

flow condition technically qualifies the rotor to be in the vortex ring state, but the difference in

collective setting differentiates the states. You can see entering unpowered descent and flight

will get one out of vortex ring state, but due to the usual proximity to the ground, combined with

the high rate of descent associated with this phenomenon, catastrophic results are likely. The

hazards of operation in the vortex ring state were first discovered in main rotor systems, but tail

rotors may encounter vortex ring state in conditions such as right hovering turns and left

sideward flight (for helicopters with main rotors which turn counterclockwise when viewed from

5-6 FLIGHT PHENOMENA