V-22 HIGH RATE OF DESCENT (HROD) TEST PROCEDURES AND LONG RECORD ANALYSIS

In August 2003, the V–22 Integrated Test Team completed a thorough investigation of the V–22’s low-speed / HROD flight characteristics. Testing defined an operational envelope for the aircraft and demonstrated flight regimes free from vortex ring state (VRS). Tests also probed deeply into fully developed VRS to determine precise flight conditions where it may be encountered, and to confirm the ability of the V–22 to recover from the condition. The ability to tilt the nacelles proved to be a powerful and (in every case) reliable means for rapidly regaining aerodynamic function of the rotors, even when operating in VRS beyond the point of having sufficient controllability. Results for steady-state HROD conditions are presented and the methods for testing are described. In an operational sense, the test results show that the V–22 has a significantly higher rate of descent margin for avoiding VRS with respect to the published NATOPS limitation than conventional low disk-loading helicopters. Furthermore, dynamic maneuver testing of the V–22 showed that VRS cannot be initiated outside the steady- state VRS boundary. Simple engineering analysis is used to show that the V–22’s steady-state VRS boundary is predictable by simple methods that work for conventional helicopters. High blade twist, and the side-by-side rotor configuration of the V–22 do not play a significant role in defining the VRS boundary.