Vectored-Thrust Landing Was 'Hands-Off'

The VECTOR test team completed the world's first fully automated, thrust vectored landing at Patuxent River (MD) on April 22, and plans subsequent landings this week at increasing angles of attack and decreasing speeds. At about 2:30 p.m. in the afternoon last Tuesday, German Naval Reserve Cmdr. Ruediger Knoepfel flew the X-31 into an invisible engagement box in the sky, then watched as the aircraft controlled itself to touchdown before he took over control and lifted off again.

"Today we saw what we have worked three years for," Knoepfel said. "We have proved the basic functionality of our system."

Since the X-31's arrival at 'Pax River' in April 2000, the VECTOR test team has been exploring the mechanics of automated, thrust vectored, extremely short takeoff and landing, with the focus on landing as the more difficult portion of ESTOL.

The X-31 uses thrust vectoring -- controlling the direction of its engine exhaust with paddle-like vanes -- to maintain control at high angles of attack and reduced speeds, even well below the typical landing speed for an aircraft of its type.

Thrust vectoring was engaged for last week's ESTOL touch-and-go, said Navy deputy program manager Bill Voorhees, but for the first attempt the angle of attack was limited to 12 degrees, which is the aircraft's normal landing attitude. Future attempts will increase the angle of attack one degree at a time, up to a maximum of 24 degrees before the program's conclusion on April 29. (At 24 degrees angle of attack, landing speeds are reduced 30 percent.)

The automated ESTOL landing maneuver has already been demonstrated at altitude, with Knoepfel and fellow project test pilot Marine Corps Maj. Cody Allee having flown approaches in past months up to 28 degrees angle of attack to a "virtual runway" at 5,000 feet.

How can they do that?

The ESTOL approach is complex relative to a conventional landing, but is managed by redundant flight control computers, which actually fly the landing. At higher angles of attack, with the aircraft's nose pointed well above the horizon, the first part of the X-31 to touch the ground would be the engine nozzle, not the landing gear. To prevent a tail strike, the X-31 performs a derotation maneuver when the tail is just two feet above the runway, dropping onto its main landing gear.

The X-31 derives its location relative to the runway from an Integrity Beacon Landing System, which uses differential global positioning system data combined with links to beacons on the ground to pinpoint the aircraft's position in space. The system is accurate to within a couple centimeters, said Doug Wilkin, lead flight test engineer.

"Everything worked perfectly and was just as we had done it in the simulator," Knoepfel said of the successful flight. "There was a very smooth flare and touchdown. I must admit that it was a smoother landing than I can sometimes do."

"That was the highlight in my flying career," he added, walking back to his office after the flight. That is no small boast from Knoepfel, who was selected as Germany's chief test pilot this year.

'I can't see.'

The X-31 flies a steeper glide slope for the ESTOL approaches than for a conventional landing, providing "a view that we have to get accustomed to," said Knoepfel. At higher angles of attack, the view will get even more interesting; above 15 degrees, the pilot loses sight of the runway and must rely on a video camera in the belly of the aircraft to verify the runway is free of obstructions.

Despite the potential for white knuckles, Knoepfel said "there wasn't time to be nervous." When he touched down, he realized the dream of more than 50 test team members waiting on the ground, who have been working together for three years to get to that moment. The VECTOR project has been a partnership between the Navy, Germany's Federal Office of Defense Technology and Procurement, European Aeronautic Defence and Space Company and Boeing Aerospace.

"This was really a team success," Knoepfel said. "I was just the man in the cockpit."

For the Navy, demonstrating the feasibility of thrust-vectored ESTOL landings could give planners an important set of options as they shape acquisition decisions over the coming decades, program officials believe. In the fleet, landings at reduced speeds and subsequently higher angles of attack would impart significantly reduced forces to both the aircraft and, in the case of carrier landings, the arresting gear on the ship, said VECTOR program manager Jennifer Young.

In the short term, however, the VECTOR remains a research project based around the only manned X-plane currently in flight. The X-31 is in that sense a "true" X-plane, being solely a tool to explore concepts and technologies. "Our main product," Young said, "is knowledge."

[Thanks to James Darcy, NAS Patuxent River Public Affairs --ed.]

FMI: www.navy.mil