PATUXENT RIVER, Md. (NNS) -- The X-31 VECTOR (Vectoring Extremely Short Takeoff and Landing Control Tailless Operation Research) test team reached a major milestone recently, when Maj. Cody Allee engaged the aircraft's extremely short takeoff and landing (ESTOL) mode and performed the project's first two ESTOL landings to a virtual runway 5,000 feet in the air. Five more ESTOL approaches were later performed at the Naval Air Systems Command (NAVAIR) Patuxent River complex.
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. Through the VECTOR program, the Navy is exploring the applications of thrust vectoring technology for ESTOL, with a particular eye toward the carrier landing environment.
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. Besides reducing fatigue and increasing system life, such reduced-energy landings would permit aircraft to land heavier, bringing weapons and fuel back to the ship that might otherwise need to be jettisoned, she explained. ESTOL may also have applications for unmanned aerial vehicles.
Monday's flight was an important step toward proving the viability of thrust-vectored ESTOL landings, said Doug Wilkin, lead flight test engineer for VECTOR. A later flight test phase, scheduled for early in 2003, will culminate in ESTOL-type landings all the way to the runway, he added.
Coupled with the right flight control software, the thrust vectoring system on the X-31 permits it to fly at up to 70 degrees angle of attack, meaning that its nose can be pointed 70 degrees above the aircraft's actual direction of flight. The X-31 performed such feats in the early '90s under the enhanced fighter maneuverability program, in which the Navy was exploring the combat applications of thrust vectoring.
For VECTOR, the Navy hopes to demonstrate ESTOL landing approaches to the virtual runway at up to 45 degrees angle of attack. Allee flew approaches at 12 and 14 degrees angle of attack, though a subsequent flight Went up to 24 degrees.
"We're getting excellent data," Young said. "A year ago, we were talking about the theoretical; now we're proving things. These are not just ideas anymore, they are products."
Because of the complex approach profile, the landing is performed automatically, culminating in a derotation maneuver that drops the X-31 onto its landing gear when its tail is just two feet above the runway. The aircraft is guided throughout the approach by an integrity beacon landing system, which uses differential global positioning system data along with ground-based beacons to pinpoint the aircraft's location.
"The landing went exactly as expected," said Allee. "If everything works as advertised, it is a fairly uneventful flight. It's a testament to all the hard work of the engineers, the programmers and the designers who have spent years getting us to this point."
After touchdown on the virtual runway, the X-31 transfers control back to the pilot, who has 5,000 feet of cushion beneath him. When the team begins "ESTOL-to-ground" in a few months, there will be no such safety net. For that reason, ESTOL landings to the real runway will be limited to 24 degrees angle of attack, Wilkin said. At 24 degrees, the X-31 still has sufficient aerodynamic control power to complete the landing maneuver were there to be a failure of the thrust vectoring system during landing.
The key, Wilkin said, is not in proving the viability of a given angle and landing speed as much as in proving the viability of the complex ESTOL maneuver, itself.
ESTOL is not the only goal for VECTOR, which is a joint venture among the Navy, Germany's federal defense procurement agency (BWB), European Aeronautic Defence and Space Company (EADS), and Boeing Aerospace. The X-31 is being used as a test bed for EADS new Flush Air Data System (FADS), a nose-mounted sensor that measures local pressures that are used to calculate airspeed, angle of attack, sideslip and altitude information.
To gather data on FADS performance, the team plans to fly the X-31 all the way up to 70 degrees angle of attack, a feat which it has not done since the conclusion of the EFM (Enhanced Fighter Maneuverability) program in 1995, when the aircraft was configured differently. "We want to see how FADS performs at this high [angle of attack]," Wilkin said. "We're also interested in supersonic performance. We'll go as high as 1.2 Mach, and see how FADS performs in that environment, as well."
Reflecting the international nature of the program, roughly half the X-31 missions are flown by German Naval Reserve pilot Cmdr. Rdiger Knpfel. Allee is the primary American test pilot and is backed up by Lt. J.R. Hansen, who made his first test flight in the X-31 Nov. 19. Allee and Hansen are both test pilots with NAVAIR Patuxent River's Air Test and Evaluation Squadron 23 (VX-23), which has custody of the X-31 during VECTOR.
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