Vortex Ring State (VRS)
The side-by-side rotor configuration of V-22 is susceptible to asymmetric onset of Vortex Ring State (VRS), brought on by descending too quickly. The one-rotor-in/one-rotor-out conditions results in large rolling moments and departure from controlled flight. Such a characteristic is fundamental and cannot be remedied by minor design changes. The only near-term solution is to restrict operations to avoid proximity to VRS region. V-22 advocates say V-22 pilots can escape vortex ring state by tilting the rotors forward to get out of helicopter mode.
Following the deployment aboard USS ESSEX, the Multi-Service Operational Test Team (MOTT) deployed to Marine Corps Air Station (MCAS) Yuma, AZ, to conduct a number of tactically representative missions. A key element of this evaluation involved the participation on April 8, 2000 of all four OPEVAL MV-22 aircraft in a simulated, non-combatant evacuation mission conducted by Marine Aviation Weapons and Tactics Squadron One (MAWTS-1).
This realistic mission ended in disaster with the crash of LRIP aircraft Number 14 and the loss of life of the four-man aircrew and the 15 Marines being transported aboard the aircraft. The mishap investigation determined that the cause of the crash was the aircraft flying outside the flight envelope identified in the Naval Aviation Training and Operating Procedures Standardization (NATOPS) Manual. During the last minutes of the flight, the aircraft exceeded 2,000 feet per minute rates of descent (250 percent greater than the NATOPS warning) at below 30 KTAS with the nacelles tilted all the way back to 95 degrees incidence. The high sink rate at low airspeed most likely caused one or both of the proprotors to enter an aerodynamic phenomenon known as Vortex Ring State (VRS), wherein a loss of lift occurs that cannot be cured by increasing power. Vortex Ring State can occur in all rotary-wing aircraft under similar conditions of low airspeed and high sink rate. No mechanical or electrical failures in the aircraft were found to contribute to the mishap.
The crash of this MV-22 during OPEVAL focused attention on the V-22's handling qualities near the region of the flight envelope subject to the phenomenon known as the vortex ring state (VRS). The VRS condition is often referred to by pilots as "settling with power" or "power settling." These terms are piloting interpretations of the usual flight characteristics of a rotorcraft operating at or near the VRS, but are not descriptors of the aerodynamic flow state at the rotor.
All rotorcraft are potentially subject to the effects of the VRS, which is nominally encountered at low airspeed and high rates of descent. The terms "low" and "high" are not absolute, but are relative to the down-wash velocity of airflow generated by the rotor. This down-wash velocity depends on the weight of the rotorcraft and the density altitude at which the rotorcraft operates. The basic VRS phenomenon manifests itself as a substantial increase in the power required to overcome the additional aerodynamic losses (induced losses) as the rotor descends into its own wake. Entry into the VRS decreases the amount of excess power available at the rotor(s). In the vortex ring state, the thrust generated by the rotor fluctuates up to +/- 30 percent even though the flight controls (throttle and collective/cyclic pitch) are held constant. In a single rotor helicopter, these fluctuations mostly manifest as high-amplitude, low frequency vertical vibrations (buffeting). A high piloting workload is also necessary to maintain equilibrium flight in or near the VRS.
The basic aerodynamic mechanisms of the VRS are common to all rotorcraft. However, the probable mechanism that initiates the sudden and potentially catastrophic departure mode in the MV-22 is unique to side-by-side rotor configured aircraft. Qualitatively, this phenomenon is understandable in terms of rotor flow field dynamics, however, other factors may be involved. For example, on the V-22 the proximity of the wing to the rotors means that the airflow state over and above the wing under steep descending flight conditions may have some impact on the rotor flow. Whether this aggravates the already adverse effects in the VRS or otherwise is not yet known because no experiments have been done to study the problem and a completely adequate aerodynamic theory is not yet available to describe the complexities associated with this type of interacting flow.
In the tiltrotor V-22, the onset of VRS can occur in the proprotor on one side without the other side losing lift. In such a case, the aircraft tends to roll sharply into the side that first loses lift, resulting in large, unexpected bank angles, followed immediately by a rapid dropping of the nose of the aircraft and a steep dive. At low altitudes, there may be no opportunity for recovery. In conventional single or tandem-rotor helicopters, the effect of VRS is generally a wings-level rapid, unexpected increase in the rate of descent. In either the tiltrotor or conventional helicopter, VRS onset at low altitude may present no opportunity for safe recovery. Recovery from VRS in conventional helicopters, altitude permitting, is generally to reduce power and lower the nose attitude of the aircraft, regaining forward airspeed and permitting reapplication of power to stop or limit the rate of descent and minimize altitude loss. In the V-22, upon recognition of entry into VRS, the pilot must also respond immediately, in this case by reducing the nacelle incidence to below approximately 80 degrees (a much more powerful anti-VRS input than lowering the nose) while at least momentarily reducing power. When forward airspeed begins increasing, reapplication of power is possible to minimize altitude loss.
The consequence of the asymmetrical AoA effect is that if the aircraft is operating near the VRS, the rotor on the side of the yaw direction may enter more deeply into the VRS. By virtue of the lower operating angle of attack on the other rotor, this other rotor would move further away from, or outside of, the VRS conditions. This asymmetrical VRS phenomenon, which is unique to side-by-side rotor configurations, would have the initial resultant effect of inducing a large rolling moment in the yaw direction. If the pilot is able to respond fast enough, the response would probably be a roll control input to counter the initial rolling moment. On the V-22, this requires lateral stick control inputs, increasing collective pitch (power) to the rotor "into the roll" - that is to the rotor operating deeper in the VRS. Because of the high losses associated with rotor operation in the VRS, the excess power available at this rotor is already marginal and so the normal pilot control inputs may be unable to effectively counter the induced rolling moments. Therefore, the roll would continue, and at low altitude the result would be catastrophic.
Furthermore, throughout the maneuver into the VRS, large thrust fluctuations are present on the rotors. Therefore, on the V-22, because of the side-by-side rotor configuration, an additional result of operating in or near to the VRS is out-of-phase fluctuations of thrust on the two rotors. Any unsynchronized rotor thrust fluctuations between the two rotors would always precipitate rolling moments and further compound any asymmetric aerodynamic conditions already induced on the aircraft.
In addition to direct yaw inputs or other perturbations that may change the rotor AoA at or near the VRS, a similar scenario (one rotor going deeper into the VRS than the other) could be produced or aggravated by a spatially confined gust of wind or by the effects of another aircraft's wake, again resulting in an asymmetrical aerodynamic effect on the two rotors and an induced rolling response. If at these asymmetrical VRS operating conditions the pilot requests either near maximum power (increase in collective pitch on both rotors) or a counter roll command (differential collective between the rotors), then the control inputs may be very ineffective and the roll would continue. The potential significance of this characteristic of the V-22 cannot be overstated given that the amphibious medium lift mission typically requires large numbers of aircraft operating in close proximity.
While the possible existence of VRS in the V-22 was known when flight limits for OPEVAL were established, the unusual attitude following entry into VRS was not expected. This condition occurs very rapidly with little to no warning to the pilots. When flown in compliance with NATOPS WARNING limits and with adequate training, susceptibility to VRS is nil. Prior to the mishap, the NATOPS limit was no greater than 800 fpm rates of descent permitted below 40 KTAS. After the cause of the mishap became more apparent, that limitation was modified to be more restrictive, becoming no greater than 800 fpm anytime the nacelles were tilted above 80 degrees incidence. The remainder of OPEVAL was flown within the latter limits, and no further incidents occurred. The OPEVAL evaluators concluded after completion that the limitations did not lessen the operational effectiveness of the V-22 in any way:
Following this mishap, Naval Air Systems Command initiated a developmental flight test program involving an extensive sequence of tests to explore the conditions, well beyond the authorized flight envelope, in which the MV-22 might be susceptible to vortex ring state or other loss-of-lift phenomenon, as well as prevention and recovery techniques.
To avoid the potential of a catastrophic VRS effect on the MV-22, the Navy imposed limits on the descent rate allowed when operating the MV-22 in helicopter mode. In addition, an important, extensive, developmental flight test program is underway to examine the flight conditions that involve a danger of encountering significant VRS conditions. This testing may lead to the development of a cockpit warning system capable of detecting aircraft proximity to the VRS domain and providing a pilot warning to that effect. An alternative may be to explore the possibility of modifying the flight control software such as to preclude dangerous flight control inputs in susceptible situations.
Of particular note, testing found no uncommanded responses within the original 800/40 NATOPS Warning, or within the revised 800/80 Limitation. Testing replicated the uncommanded roll characteristic when flying well outside of the NATOPS limits. The encounters occurred only at elevated rates of descent (approximately 2000 fpm or greater at 40 knots or less) that were well beyond the fleet restrictions. In addition, testing to date suggests that should a pilot inadvertently exceed published limitations, there may be no easily recognizable warning that the aircraft is nearing the danger zone-and some flight control inputs; e.g., a roll or yaw command, may trigger an asymmetric thrust condition. Such a situation can easily be envisioned in flight conditions that place a high workload demand on the pilots; e.g., night or low visibility, system malfunctions, hostile fire, etc., should a breakdown of crew coordination or loss of situational awareness occur. Thus, the first indication the pilot may receive that he has encountered this difficulty is when the aircraft initiates an uncommanded, uncontrollable roll. High rate-of-descent (HROD) testing continued to define the VRS phenomenon.
The V-22 has the potential to enter high rates of descent at high nacelle angles with low airspeed. This condition occurs very rapidly with little to no warning to the pilots. In simulation at 95 degrees nacelle, 39 KCAS, and 0 feet per minute rate of descent (ROD), pulling the thrust control lever (TCL) full aft caused an immediate descent exceeding the 800 feet per minute NATOPS WARNING. If forward TCL is applied at this point, an uncontrolled flight condition is possible. Within 3 seconds, the simulator exhibited in excess of 3,000 fpm ROD.
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