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Aviation History
1990
1990 - 1345.PDF
X-31's agility will be tested against conventional fighters and in mock combat between the two experimental aircraft built jointly by Rockwell and MBB triplex-redundant computer system, with a fourth computer used as a tie-breaker and for additional redundancy. The control system will be adapted to include a flightpath decou pling mode, in which the pilot could point the aircraft's nose independently of its trajec tory, with the aircraft responding adversely in pitch and yaw to stick inputs. Special flight-attitude symbology will be included on the headup display (HUD), to prevent the pilot from becoming disorien tated during unusual manoeuvres. The con trol program has been written in the JOVIAL computer language, rather than the Depart ment of Defense's required Ada, in order to use as much of the existing software as possible. The system is similar to the one used on Lockheed's modified L-100 high- technology testbed (HTTB), with the same flight-control computers, The integration of existing systems and components is typical of the X-31's design. The landing gear, for example is taken from a Cessna Citation III, with main wheels from the A-7 and the nosewheel from an F-16. Other elements cannibalised from produc tion aircraft include the cockpit canopy, ejection seat and most of the displays and controls from the F-18, which also supplied the airframe-mounted accessory drive, pri mary electrical generator and the leading- edge flap actuators. Fuel pumps, leading- edge flap power drives and rudder pedals were taken from the F-16, with other parts lifted fom the V-22, F-5, F-20 and T-2C. Development costs were also kept down by using the major fuselage bulkheads as tem plates and tools for the installation of longe rons, fittings and eventually fuselage skins. The fuselage was built up from 11 principal bulkheads joined by four longerons, with intermediate frames made of sheet metal added between the major frames. The carbonfibre material system developed for the B-1B was used extensively on the X- 31, providing skins for the forward fuselage around the cockpit area, and for the vertical tail. The rudder is also made of carbonfibre over a honeycomb core. The centre-fuselage is made of titanium skins over titanium frames to withstand any unknown heat loads which might accumulate through the use of the thrust-vectoring system. MBB designed and built the wing spars and bulkheads from aluminium, with carbonfibre material for the wing skins and the leading and trailing-edge flaps. When the wings were taken to Palmdale, mating with the fuselage was perfect, despite that fact that MBB had done all of its computer-aided design work in metric, while Rockwell had used imperial measurements throughout. The end result is two experimental aircraft which have been built for around the same cost as two production F-16s, which obviou sly achieve substantial discounts on materials through bulk buying. The tooling costs for the X-31 were one-third of those for an equivalent new aircraft. To avoid the need to build a third airframe for structural tests, DARPA, the German defence ministry and the US Navy agreed to the proof-load concept, which involved test ing all major components on the flight vehi cles to 110% of their design load limits. No problems were met which would result in flight restrictions, and the aircraft should be certificated by two authorities—the West German certification agency ML, which will ensure the airworthiness of the wings and thrust-vectoring system, and the US Navy, which will clear the overall aircraft structure, including all systems. ENSURING SAFETY The first aircraft rolled out in Palmdale on 1 March, and is due to fly in the middle of June, slightly behind the original schedule because of "safety of flight issues", according to Rockwell, which will not elaborate. High speed taxi tests are due to start in the middle of this month, and the maiden flight will be handled by Rockwell chief test pilot Ken Dyson. Rockwell will supervise all of the early envelope-expansion flight tests at Palmdale, taking responsibility for conventional perfor mance and agility, while MBB pilots will demonstrate its post-stall capabilities. The test schedule calls for 18 initial airworthiness flights, with a total of 80 flights planned to validate the conventional envelope. Post-stall testing will begin in September at Palmdale, but will then transfer to the Navy's flight-test centre at Patuxent River, Mary land, where Navy pilots will join the pro gramme. The X-31's post-stall capabilities will be proved in 200 flights, after which the aircraft's combat effectiveness will be tested in 120 sorties flown at Patuxent River and on instrumented ranges, against conventional aircraft, and between the two X-31 vehicles. It is expected that further "fine-tuning" of the flight-control system and the aircraft's configuration will be necessary to exploit •fully its remarkable capabilities. When the flight-test programme ends in early 1992, however, the X-31 should have proved that the' "Pougachev's Cobra" manoeuvre with which the Sukhoi S-27 startled the aviation world at Paris last year was a mere appetiser for the levels of agility which could soon be seen in Western fighter aircraft. Q FLIGHT INTERNATIONAL 9-15 May 1990 59
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