Which combination describes the aerodynamic factors that can lead into vortex ring state?

Which combination describes the aerodynamic factors that can lead into vortex ring state?

• A. Low forward airspeed with rates of descent into the rotor wash while some power is applied to the rotor system
• B. High forward airspeed with rates of descent into the downwash while no power is applied
• C. Zero forward airspeed with rates of descent into the wake while full power is applied
• D. Medium forward airspeed with rates of descent into the wake while some power is applied

Answer: (A)

Vortex ring state happens when a helicopter’s rotor is descending into its own turbulent downwash, and there isn’t enough forward speed to sweep that disturbed air away from the rotor. When forward airspeed is low, the rotor disc recirculates the rotor wash, which reduces the effective angle of attack and the rotor’s ability to generate lift. Keeping some power in the rotor helps maintain rotor RPM, but it can still produce a rapid loss of lift if the descending air is being re-ingested in this torus of swirling air. In short, the dangerous combination is a low forward speed with a descent into the rotor wash while some power is applied, because that’s when the rotor is fighting its own disturbed air and can suddenly descend faster with less lift. The other scenarios don’t match the typical VRS conditions: higher forward airspeed tends to carry the downwash away and reduces the recirculation that causes the state; descending with no power often leads toward autorotation behavior rather than a loss of lift from recirculation; and descent into wake with power at a nonzero forward speed isn’t the classic rotor-downwash recirculation situation that produces VRS.

Vortex ring state happens when a helicopter’s rotor is descending into its own turbulent downwash, and there isn’t enough forward speed to sweep that disturbed air away from the rotor. When forward airspeed is low, the rotor disc recirculates the rotor wash, which reduces the effective angle of attack and the rotor’s ability to generate lift. Keeping some power in the rotor helps maintain rotor RPM, but it can still produce a rapid loss of lift if the descending air is being re-ingested in this torus of swirling air. In short, the dangerous combination is a low forward speed with a descent into the rotor wash while some power is applied, because that’s when the rotor is fighting its own disturbed air and can suddenly descend faster with less lift.

The other scenarios don’t match the typical VRS conditions: higher forward airspeed tends to carry the downwash away and reduces the recirculation that causes the state; descending with no power often leads toward autorotation behavior rather than a loss of lift from recirculation; and descent into wake with power at a nonzero forward speed isn’t the classic rotor-downwash recirculation situation that produces VRS.