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X-31 completes first ESTOL testing


Achieving approach speeds as low as 105kt (195km/h), the Boeing/EADS X-31 has completed the first phase of extremely short take-off and landing (ESTOL) testing under the US/German VECTOR programme. The thrust-vectoring aircraft flew a series of automated approaches to a "virtual runway" 5,000ft (1,500m) above the US Navy's Patuxent River, Maryland, test centre.

Completion of "up-and-away" testing clears the way for the "ESTOL-to-ground" test phase, to be completed this month. Approaches were flown at angles of attack (AoA) up to 28° gear up and 24° gear down, resulting in virtual touchdown speeds of 105kt and 120kt, respectively. ESTOL-to-ground approaches will be flown at up to 18° AoA. Budget, not capability, will limit the final AoA achieved. "We have the capability to get to 24°. We think we can get to 18° in the time remaining," says German project pilot Rudiger Knoepfel.

The up-and-away phase validated the X-31's EADS-developed flush air-data system (FADS) at speeds up to Mach 1.18 and AoAs up to 70°. The nose-mounted FADS, developed as a more accurate and stealthy alternative to the air-data probes and ports used today, produced "excellent results", says Knoepfel.

Flight tests also validated the integrity beacon landing system (IBLS) used to guide the X-31 with centimetre accuracy during its automatic approach and touchdown. The IBLS combines differential-GPS satellite navigation with ground-based pseudolites. Each approach began with the X-31 overflying the pseudolites so the system could calculate and remove the positioning errors.

For the ESTOL-to-ground tests, the pilot will fly the X-31 into an engagement box then activate the autopilot/autothrottle system. The aircraft will fly the high-AoA approach automatically, derotating 2ft above the runway to avoid a tailstrike on touchdown. At AoAs above 14-16°, the pilot will use a downward-looking camera to monitor the approach.

The US Navy/German defence ministry VECTOR programme is intended to demonstrate the use of thrust vectoring to reduce carrier landing speeds. US programme manager Jennifer Young says the highly accurate IBLS has generated more interest that expected.

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