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Different approach


The thrust-vectoring X-31 is being used for high AoA extremely short landing trials - technology which could boost front-line performanceStewart Penney / NAS Patuxent River

While, in the world of theX-planes, the Joint Strike Fighter (X-32 and X-35), unmanned combat air vehicles (X-45 and X-47) and space vehicles (X-33, X-34, X-37 and X-43) are grabbing the headlines, the thrust-vectoring X-31 has quietly emerged from storage.

This time the highly manoeuvrable aircraft will be used for the VECTOR programme, evaluating extremely short take-off and landing (ESTOL) technology, which could improve the field performance of frontline fighters. ESTOL could significantly reduce the approach and landing requirements of combat aircraft, either aircraft carrier or land-based.

For instance landing speed, and therefore energy, is expected to be cut by30-40%, which would eliminate the wind over deck requirement - when a carrier turns into wind for landing and take-off operations. As well as reducing the wear and tear on the carrier's arrester gear and the aircraft, ESTOL could also increase the aircraft's bring-back weight allowance, permitting the return of unexpended, and expensive, precision-guided weapons. ESTOL could also be used for be usedfor unmanned air vehicle (UAV) operations.

Designed and built by Rockwell and Germany's Dasa (now Boeing and EADS respectively), the X-31 was used in the early 1990s to test post-stall manoeuvrability, one of several programmes looking at high angle-of-attack (AoA) operations. Following the 1995 Paris air show, where it demonstrated post-stall flight in a series of "J-curves" and "Herbst manoeuvres", the X-31 was stored awaiting funding for the next round of flight testing.

Last year the US Navy, German defence procurement agency BWB, Boeing and EADS provided $60 million to fund the VECTOR programme and the sole remaining X-31 was returned to airworthiness. It flew again on 24 February. After 10 flights lasting 7.2h, the aircraft was groundedat the US Navy's test centre at Patuxent River to be fitted with VECTOR-specific equipment.

The initial series of flight trials cleared the aircraft after six years on the ground and gave the two project pilots - USNCdr Vivian Ragusa and Rüdiger Knöpfel of EADS - experience with the aircraft. This included a feel for its landing qualities, says USN X-31 VECTOR programme manager Jennifer Young. Phase one trials included calibrating the thrust vectoring paddles with a new General Electric F404 engine, because the exhaust from each powerplant is slightly different. Inaddition, thrust-vectoring effectiveness at lower thrust levels was evaluated because ESTOL will be done at lower power settings than previous thrust-vectoring operations.

Air data advances

Major changes to the aircraft for the next test phase include the installation of an EADS flush Advanced Air Data System (AADS), autothrottle, commercial off-the-shelf multifunction display, downwards looking, camera and triplex avionics, needed because the aircraft will now operate close to the ground. The flight-test instrumentation boom will also be moved from below to above the nose. Although the X-31 has always had a triplex flight control system, three Honeywell-supplied GPS satellite inertial navigation systems are being installed to give precision guidance down to the runway, providing dynamic accuracy "to within centimetres."

Holgar Friehmelt, EADS chief engineer, VECTOR programme, says the AADS provides better air data than conventional pitot tubes and airflow vanes, particularly at low speeds and high AoA. It also has better low observability as it does not project from the aircraft's sides.

The system, contained in a cone mounted at the tip of the nose, measures static and dynamic air pressure as well as AoA and bank angle. Behind 12 holes in the cone are microsensors, with signal processing on the back of each. Using 12 ports provides redundancy, says Friehmelt. The system, or parts of it, are also applicable to missiles and UAVs. Use of the X-31 allows EADS to expand the envelope in which the sensor can be used. VECTOR also extends use of EADS's reconfigurable flight control system technology.

In the next phase of flight trials, due to start in the fourth quarter of this year and end next April, the aircraft will be flown at increasingly high AoA, up to 40í, to a virtual runway at 5,000ft (1,525m) altitude. The aircraft will automatically pitch up to the required AoA, fly the approach and the "de-rotate" close to the ground just before the wheels touch the runway. The pilot acts as a monitor only during the approach and does not take control until the X-31 is on the "runway". Gary Jennings, Boeing X-31 programme manager, says the maximum 40í AoA is set by redundancy requirements and is not a function of controllability. De-rotation will be at less than 2ft altitude and will pitch the aircraft forward to a standard 12í AoA.

The maximum 40í AoA will be approached in steps of 2í from 12í, a more usual landing approach attitude. The team does not have to do every 2í step, says Young. Once the trials are concluded, planned to take around 40 flights, the results will be evaluated and a new miniaturised AADS fitted - with a new sensor and, potentially, fibre-optic communications link in place of the current copper bus, says Michael Hahn, German VECTORprogramme manager. Fibre optics are being considered as the AADS cone is placed in the radome tip, and an electrical wire travelling back through the radome in an operational aircraft equipped with radar would create problems.

If funding is made available, phase three testing will begin in the third quarter of next year and will evaluate the automatic approach system down to a real runway.

VECTOR is due to end in early 2003. Spanish engine company ITP is interested in using the X-31 for trials of its thrust-vectoring nozzle, which has been designed for the Eurofighter's Eurojet EJ200 engine.

To make an approach, the pilot flies into an engagement box where a system evaluation is performed. The aircraft will be a minimum of 6,000-8,000m from the touchdown point. The evaluation includes a check to ensure that enough GPS satellites are available for the precision approach, that speed and height are adequate and that the landing gear is down. The automatic system is then engaged, the aircraft captures the "trajectory" and the AoA is automatically built to the desired level.

Pilot takes control

As AoA increases, speed decreases. When the exhaust paddles, which are closest to the ground, are at around 2ft, the aircraft (which is 13m [43ft] long) de-rotates, pitching forward to 12í AoA and reducing the rate of descent.

The pilot takes control as the main wheels touch the runway. The approach's automatic element takes around 2min, says Ragusa. The landing gear loads are expected to be "normal", he adds. Varying the AoA varies the rate of descent, and the system can fly steep or shallow approaches.

The pilot monitors the approach on the X-31's head-down display, where the image from the downward pointing camera is displayed. This information, however, could be presented on the head-up display of an operational aircraft, says Ragusa. The imagery should be easy to present as most next-generation fighters have internal electro-optical sensors. The display information needs to be different for monitoring, as opposed to flying, the approach. If unhappy with the approach, the pilot can abandon it at any time.

By combining enabling technologies such as post-stall, high-AoA flight with thrust vectoring for low-speed flight and a fully integrated flight and propulsion control system, the X-31 VECTOR team could change the face of military aviation.