PAUL LEWIS / WASHINGTON DC
The Bell Boeing V-22 Osprey integrated test team is nearing the end of the initial phase of high rate of descent (HROD) testing, drawing a positive response from the US Department of Defense (DoD). The next focus is on expanding the machine's envelope before a May decision on the programme's future.
HROD testing is divided into two distinct efforts, with Phase I under way since November validating the V-22's 800ft/min (4.1m/s) rate of descent at a minimum 40kt (75km/h) forward airspeed limit. DoD acquisition undersecretary Pete Aldridge is reportedly "encouraged" after reviewing the results. The hope is to achieve at least six weeks of Phase II testing by May, steadily increasing the rate of descent and decreasing horizontal velocity until the tiltrotor enters vortex ring state (VRS).
Flight testing has already reached 2,000ft/min at 16kt, at which point the tiltrotor in a single-axis manoeuvre entered VRS and was recovered by "bumping" the nacelles forward and flying out. A critical Phase II goal will be to determine for the first time a hard boundary for the twin proprotor's asymmetrical "roll-off" into VRS with multiple pitch, vertical, roll and yaw control inputs. The hope is then to have these incorporated into a revised operating manual.
The V-22's existing limit reflects minima set for helicopters, but there is a widely held view among pilots that the Osprey's 97kg/m2 (19.8lb/ft2) disc loading, over twice that of the Sikorsky UH-60L, is capable of more. Officials hope that by "filling in the blanks on the real envelope" a repeat of the VRS-caused accident at Marana, Arizona, in August 2000 will be avoided.
VRS testing has shown the onset of asymmetrical thrust and roll-off at decent rates of 2,500-4,000ft/min between 30-40kt. Phase II VRS testing will extend to November and focus on descent rates of 800-2,400ft/min at 30kt and below, requiring a special air data system for low-speed measurements.
A favourable review in May will clear the way to lift production in 2005 from today's 11 machines a year. Increasing production is critical to lowering the V-22's projected $70 million unit cost, seen by everyone as too high.
Source: Flight International
