But the X-31 was not yet done with flight testing. The performance of the X-31 at the Paris Air Show was so impressive that it also attracted the attention of senior engineering and operations planners in the U.S. Navy. Moreover, the sev-eral Rockwell and MBB X-31 personnel had discussed taking advantage of the high-AOA control to significantly reduce landing speeds—a keen interest of the Navy, whose pilots operated from the decks of ships. Thus, both the Navy and industry were interested in determining whether thrust vectoring might enable very short landing distances, which can result from the very slow touchdown speeds that are made possible by approaching at very high angles of attack. The original Rockwell-MBB concept for a demonstrator was called the “Giraffe”
owing to an extremely long (foldable) nose gear that allowed the plane to be landed at high angles of attack. But it soon became clear that such a concept was not very realistic for an operational system. In response, Steve Holowell and Mike Robinson had developed a totally different approach to both the landing and takeoff potential that was afforded by integrated thrust-vector control. This concept was called extremely short take off and landing (ESTOL), and it uti-lized thrust vectoring to rotate the aircraft early in takeoff and to “derotate” the
Desert Disaster, Triumph in Paris, and a New VECTOR
aircraft from a high-AOA approach just before touchdown. Their study results showed that there were significant pos-sible benefits from ESTOL, and they subsequently patented the concept.41
A new demonstration program sub-sequently emerged from this concept with keen Navy interest. Unlike the Enhanced Fighter Maneuverability pro-gram, this program did not include the participation of DARPA or Dryden. It was solely a NAVAIR-managed program
and was flown at the Naval Air Test Center at NAS Patuxent River. The contrac-tors were Boeing (Rockwell) and the European Aeronautic Defense and Space Company (EADS), which now included the former MBB.42 German govern-ment participation once again included the WTD-61 test center, BWB, and the Deutsches Zentrum für Luft und Raumfahrt (the German Aerospace Center).
The program was named Vectoring ESTOL Control Tailless Operations Research (VECTOR), and it explored the use of thrust vectoring to allow slower final landing approaches at higher-than-normal angles of attack.43 Note that while the ESTOL patent addresses both the takeoff and landing phases of opera-tion, the VECTOR program only addressed the most challenging segment: the landing. The advantages of ESTOL landings on production jet fighters, par-ticularly carrier-based fighters, were many. There would be greater operational flexibility and lower life cycle costs because aircraft would not be punished by the extremely hard landings of current carrier fighters, and the “bring back”
weight could be increased enough to eliminate the need for pilots to jettison ordnance or fuel in order to reduce the airplane’s landing gross weight enough to be accepted by the carrier’s arresting system. Aircraft could also land with less wind over the deck, thus providing the carrier air wing’s commander with more flexibility in employing aircraft. Wear and tear on the carrier’s arresting gear and deck would be less. Since ESTOL would provide lower structural weights, it would be easier for designers to provide lighter structures, thus providing more commonality for jointly developed airplanes for both the Navy and Air Force.
The Marine Corps would benefit by having aircraft that could use very short runways or even highways, befitting its expeditionary nature.44
Obviously, the thrust vectoring that was demonstrated during the EFM pro-gram would be a major enabler in demonstrating these high-AOA, slow-speed landing approaches, but some other technologies also required development.
The aircraft would have to be flown very close to the runway at a high angle of attack and “de-rotated” to a lower angle of attack just prior to touchdown so
X-31 VECTOR team logo. (USN)
Flying Beyond the Stall
that the tail would not strike the runway. This required centimeter-level accu-racy in aircraft position determination, including altitude. The IntegriNautics Company in Menlo Park, CA, had recently developed a specialized differential-GPS-based navigation system, the Integrity Beacon Landing System (IBLS).
Ironically, it was developed and demonstrated in conjunction with United Airlines so that commercial aircraft could utilize the existing onboard GPS for precision landings in lieu of adding a new microwave landing system (MLS).
Mike Robinson uncovered the concept through professional acquaintance with United’s engineering director, Gordon McKenzie. Then Boeing’s engineering staff, working with NAVAIR engineers and IntegriNautics engineers, adapted the concept to function with an autothrottle system from an F/A-18 and an autopilot developed by the VECTOR team. The landing was a fully coupled autoland during which the pilot remained hands off until the airplane was on the ground. Of course, the pilot could command a wave-off or go-around at any time, but early studies showed that the timing was so critical that pilots were not capable of reliably performing the derotation maneuver, so the team went to a fully automated landing system early in the program. One of the problems was that the new navigation system was more accurate than available ground-based measurement systems, such as ground-based lasers. This led to the approach of testing at altitude by simulating a landing on a virtual runway prior to attempting an actual landing.45
Another technology was also demonstrated on VECTOR: the advanced Flush Air Data System (FADS) that was developed by EADS Military Aircraft and Nord-Micro. This system involved flush pressure ports located around the tip of the nose cone that provided more accurate airspeed, angle-of-attack, and angle-of-sideslip measurements at high angles of attack. Initially, the array was known as the Advanced Air Data System (AADS) and was flown on the X-31 because of its high AOA capabilities; it had now evolved into the Flush-mounted Air Data System (FADS), a separate EADS experiment flown on the X-31 in parallel with VECTOR.46 Interestingly, for initial flight-test data the original Kiel probe was installed—and this time it was heated. A smaller Rosemount Pitot probe was also mounted under the radome to provide an alternate source of air data, which was routed to an additional air data computer that monitored pitot data and would instruct the pilot to revert to fixed gains via the R-3 rever-sionary mode if a substantial error was detected in the primary air data from the Kiel probe. This Rosemount probe was also heated! Finally, redundancy in air data sensing was built into the X-31.47
In testing the FADS, the X-31 was flown through airshow-type maneu-vers up to 70° angle of attack and a Mach number of 1.18 at 39,000 feet MSL. While supersonic, the test pilot—German Naval Reserve Cmdr. Rüdiger
“Rudy” Knöpfel—induced combinations of angle of attack and sideslip to tax
Desert Disaster, Triumph in Paris, and a New VECTOR
the FADS. After processing the data, engineers were able to declare that the FADS was performing as desired throughout the flight regime.48 There were actually two versions of the FADS tested; one was a breadboard system tested for functionality and the other was a miniaturized version that was more representa-tive of production. The Boeing VECTOR program manager, Gary Jennings, said of the FADS, “The advantage of the nose mounted FADS is to provide a full envelope from 70° angle of attack all the way out to supersonic speed. Almost all existing air data systems using probe sensors cannot be relied on above 30°
angle of attack so inertial derived data must be used instead.” The Navy program manager, Jennifer Young, commented, “This is a whole new way of collecting the data, from probes on the aircraft. This system is better in two ways. First it is miniaturized and doesn’t interfere with the radome. Second, we’ve never gotten the algorithm right on a flush data system before and I have been very pleased with the result of this one.” Jennings summed up the success of FADS by saying, “While others have achieved some of the same results within fairly narrow flight envelopes and at relatively moderate AOAs, the German FADS was extremely successful up to Mach 1.2 at 39,000 ft. The other significant part is we did air-show-type maneuvers, up to 70° angle of attack. So they have a device that has been demonstrated throughout and envelope most airplanes can’t even fly in, to replace conventional systems. This is a single, solid-state, small device with a far more functional software system running behind it. This one little cone at the front end of the radome did it all.”49
The VECTOR program was conducted in three phases. Phase I involved a functional checkout of the airplane, pilot familiarization, and thrust-vector calibration. Phase II evaluated ESTOL avionics, navigation performance of the IBLS, autopilot functionality, and the first version of the FADS. Phase III evaluated the ESTOL landings and tested the miniaturized version of the FADS. Originally, the VECTOR program was also to include tailless research, during which the X-31’s tail would actually be removed altogether, thus greatly expanding the quasi-tailless work done in the EFM program. Funding limita-tions restricted this part of VECTOR to a paper study only. Initial VECTOR planning included the replacement of the thrust-vectoring paddles with an axisymmetric vectoring exhaust nozzle (AVEN) in cooperation with the Swedish and Spanish governments.50
Phase I of the VECTOR program was known as the Program and Requirements Definition Phase. The Navy signed a contract for Phase I with Boeing on February 18, 1998, with planned completion of the phase on August 14, 1998. This phase, in which Dryden participated, included multinational team negotiations for a Memorandum of Agreement, an X-31 aircraft parts count, a fit-check of a Saab JAS-39 Gripen fighter RM-12 engine (which was a derivative of the GE 404 engine used in the X-31 during the EFM program), and painting of the airplane.
Flying Beyond the Stall
The X-31 VECTOR over the southern Maryland country-side. (USN)
The RM-12 engine was to be used in what was envi-sioned as the demonstra-tion of GE’s axisymmetric nozzle—a U.S.-Swedish part of the program that never materialized. Phase I work began at Dryden on March 2, 1998, and included the engine fit-check and aircraft parts count by VECTOR gram partners. As the pro-gram evolved, participation by the Swedes, Spanish (who collaborated with GE on the nozzle), and Dryden ended and the program became a joint venture between the U.S. Navy and Germany’s BWB, managed on the U.S. side entirely by NAVAIR. The contractors on the program were the European Aeronautic Defense and Space Corporation and Boeing Aerospace. Flight testing was con-ducted under the auspices of the Navy’s VX-23 test squadron at Patuxent River.
Modifications of the X-31 began in Palmdale by Boeing, and the airplane was moved to Patuxent River in April 2000 to undergo a major overhaul effort that took over 10 months to complete.
On February 24, 2001, X-31 Ship 2 flew for the first time since travel-ing back to Manchtravel-ing followtravel-ing the Paris Air Show. This flight was flown by Navy Cmdr. Vivan “Noodles” Ragusa.51 Since this was a joint program with Germany, subsequent flights were also flown by Germany’s Rudy Knöpfel. The X-31 flew 10 functional check flights in this phase of the program and achieved an altitude of 24,000 feet MSL and a speed of Mach 0.8. On April 6, 2001, Knöpfel engaged the X-31’s thrust vectoring for the first time since the airplane had flown in the Paris Air Show. This was accomplished at 30° angle of attack, which is just below what is considered to be post-stall maneuvering. Knöpfel reported, “It was a very stable, smooth flight.” He took the airplane to 5,000 feet and performed a series of pitch, roll, and yaw maneuvers while the thrust-vectoring vanes provided directional control in all three axes. In the following year, the X-31 was reconfigured for up-and-away ESTOL flight, and the FADS was also installed.52
Phase II started on May 17, 2002, with a functional check flight by Rudy Knöpfel. The X-31 had received a number of upgrades and modifications, including new flight control software, the F/A-18 autothrottle system, the IBLS, and the FADS. Technicians also installed a belly-mounted video camera to allow the pilot to view the runway for obstructions at the very-high-AOA approaches
Desert Disaster, Triumph in Paris, and a New VECTOR
that were anticipated. “The airplane flew nicely and as predicted. I’m very con-fident for the future of the program,” said Knöpfel following the flight. This phase focused on evaluating the ESTOL flight control software, the precision position measurement performance of the IBLS, the avionics integration of the triplex INS/GPS and triplex air data computers (a redundancy improvement over the EFM program), the new autopilot, and the ESTOL head-up display functionality. Test pilots and engineers also tested the advanced FADS air data system.53 Following checkout of all the new systems, the culmination of Phase II was to conduct ESTOL landings on a “virtual runway” 5,000 feet in the air.
This was first accomplished on November 18, 2002, by Maj. Cody Allee, USMC, who engaged the X-31’s ESTOL mode and made the project’s first two ESTOL landings onto a virtual runway. These approaches were flown at angles of attack of 12°and 14°. Allee reported, “The landing went exactly as expected.
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.”54 Allee had replaced Ragusa as the primary American test pilot on the program, and he was joined by Lt. Gerald Hansen, USN, a backup pilot whose only program flight occurred on November 19, 2002. Subsequently, five more ESTOL approaches were performed at alti-tude, reaching an angle of attack of 24°. Approaches were to be limited to 24°
angle of attack for the following phase of testing, during which “ESTOL-to-the-ground” landings would be accomplished. The rationale for this was that at 24° angle of attack, the X-31 still had sufficient aerodynamic control power to complete the landing maneuver if there was a failure of the thrust-vectoring system during the landing.
The approach profile was complex. Final approach was flown at a higher-than-normal glide path and, of course, at angles of attack much larger than conventional aircraft. A derotation maneuver prevented the tail from hitting the ground prior to the main landing gear by dropping the aircraft onto its land-ing gear when its tail was just 2 feet above the runway. Due to this complexity, the landing was flown completely on autopilot. Due to the extreme preci-sion required to accomplish this approach and landing, the X-31 was guided throughout the approach by the IBLS, which uses differential global positioning system data along with ground-based beacons to very accurately track the air-craft’s position and altitude. Jennifer Young said, “We’re getting excellent data. A year ago, we were talking about the theoretical; now we’re proving things. These are not just ideas any more, they are products.”55 The final flights of Phase II were flown on March 22, 2003, by Knöpfel. He flew two supersonic flights to Mach 1.06 and 1.18, respectively, in full afterburner at 39,000 feet MSL. These final flights were to assess FADS performance at supersonic speeds.
Flying Beyond the Stall
Phase III began with preparation of the X-31 for the final “ESTOL-to-the-ground” test phase. The airplane received a new software load for its flight control computers. This load included the control laws for an actual ground landing. The VECTOR team was also making minor airframe modifi-cations. The expectation was to accomplish all the VECTOR goals within
14 flights, with the first actual ground landing occurring around the eighth flight. The first VECTOR flight to an actual ground touchdown occurred on April 22, 2003, and was flown by Rudy Knöpfel. He flew the airplane to an invisible engagement box in the sky, at which point the autoland system was engaged and Knöpfel monitored as the X-31 flew to touchdown, after which he took over control and lifted off again. On this first attempt, the thrust vectoring was engaged but the angle of attack was limited to 12°, which was the airplane’s normal landing attitude. Following the flight, Knöpfel reported, “Everything worked perfectly and was just as we had done it in the simulator. There was a very smooth flare and touchdown. I must admit that it was a smoother landing than I can sometimes do.”56
Follow-on landings increased the final approach angle of attack one degree at a time, up to a maximum of 24°. Of the higher-AOA approaches at a steeper glide path than conventional aircraft, Knöpfel commented, “[This is] a view that we have to get accustomed to.”57 This comment was made in reference to the fact that, at higher angles of attack above 15°, the pilot loses sight of the runway and must rely on a video camera in the belly of the aircraft to verify that the runway is free of obstructions. The final flight in Phase III was flown by Cody Allee to touchdown at 24° angle of attack (twice the normal 12°) at only 121 knots (31 percent slower than the normal 175-knot landing speed). Following touchdown, Allee needed only 1,700 feet of runway to slow the X-31 down sufficiently to make a turn-around in the middle of the runway and then taxi in a complete circle! This provided a significant contrast to the normal X-31 landing distance of 8,000 feet for a conventional landing. The resulting energy savings was over 50 percent, a factor of huge importance for the Navy. Energy is the key parameter for evaluating payoff to the Navy because it has a major X-31 VECTOR landing just prior to derotation. Note the nozzle paddles far below the main landing gear. (USN)
Desert Disaster, Triumph in Paris, and a New VECTOR
impact on the design and construction of arresting gear on carriers, as well as impacts on the design and operations and support (O&S) of the aircraft.
This final landing was greeted by cheers in both English and German by the many VECTOR team members watching from the side of the runway.
Commenting on the feel of landing at 24°angle of attack, Allee said that the world scrolled slowly by at a pace that was “almost sedate. From the start of the approach, it is very obvious that the aircraft is sitting at a pretty extreme angle. You are still at one g, but you’re leaning way back in the seat with the nose pointed way up at the sky.”58 The team then had to finish celebrating and move into a data-analysis and reporting phase, creating what was essentially a how-to manual for thrust-vectored ESTOL and the other technology demon-strated in the X-31.
Commenting on the feel of landing at 24°angle of attack, Allee said that the world scrolled slowly by at a pace that was “almost sedate. From the start of the approach, it is very obvious that the aircraft is sitting at a pretty extreme angle. You are still at one g, but you’re leaning way back in the seat with the nose pointed way up at the sky.”58 The team then had to finish celebrating and move into a data-analysis and reporting phase, creating what was essentially a how-to manual for thrust-vectored ESTOL and the other technology demon-strated in the X-31.