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Aviation History
1990
1990 - 1341.PDF
NORMAL LIMITS wing agility demonstrator, which was eventually awarded to Grumman. After MBB withdrew from the Brit ish Aerospace Agile Combat Air craft—later renamed the Experi mental Aircraft Programme— the two companies met in October 1982 and decided to pursue jointly the concept of supermanceuvrability. The idea had already attracted some atten tion among the US and European fighter communities, and MBB had joined a pro gramme with Sweden's SAAB to modify a J37 Viggen to demonstrate fuselage pointing through flightpath decoupling. The Luftwaffe decided not to pursue the concept as part of its requirement for the European Fighter Programme (EFA) because of its immaturity, but in 1984 Rockwell and MBB persuaded the US Defense Advanced Research Projects Agency (DARPA) to fund a joint study, with additional money from the West German ministry of defence. Rockwell's own independent research pro gramme, nicknamed SNAKE (super-natural kinetic enhancement), had established that thrust vectoring was indeed a requirement for enhanced fighter manoeuvrability (EFM) in the post-stall regime, and included thrust- vectoring paddles developed by the US Navy fa- on an F-14 spin demonstrator. A large-scale model of the purely subsonic aircraft was flown successfully in a windtunnel at the NASA Langley Research Center. In December 1985 the results of the Rock well and MBB design studies, and of exten sive manned simulations testing, were pre sented to DARPA and the German defence ministry which decided to pursue the devel opment of two test vehicles. Coincidentally the US Senate, through the Nunn/Quayle initiative, had decided to promote co-opera tion on defence programmes with NATO allies. In May 1986 a memorandum was signed between West Germany and the USA under which the two countries shared the $270 million cost of developing and testing two X-31 aircraft, with the US Navy appoint ed to manage the programme. Although the X-31 would be designed only for high subsonic performance, the pro gramme goals specified that it should have the inherent potential for "superior super sonic manoeuvre performance" to Mach 1.3, with its ability to conduct beyond-visual- range air combat not significantly degraded by the EFM features in the design. POSING CHALLENGES Development of the X-31 configuration posed some interesting challenges for the MBB/Rockwell team, which worked separate ly in West Germany and California, transfer ring data across the Atlantic through comput er modem links. The contract specified that the two test vehicles should be built as cheaply and quickly as possible, using "off- the-shelf items wherever possible. Eventu ally 43% of X-31's empty weight comprised parts cannibalised from production aircraft such as the F-16 and F-18. Rockwell took overall system responsibil ity for the X-31, including final assembly and supervision of the early stages of the flight test programme at Palmdale, California. In addition, Rockwell is responsible for the aerodynamics, for design and fabrication of the fuselage foreplanes and vertical tail and for implementing the flight-control laws and flight-control software. MBB designed and built the wings and thrust-vectoring paddles, and developed the flight-control laws. " The aerodynamic design of the X-31 is based on the configuration proposed by MBB for EFA, with a clipped, double-delta wing planform with straight trailing edge and long- coupled foreplanes. The original design was a compromise solution to the competing re quirements of low drag at supersonic speeds, maximum lift, minimum induced drag, and a balance between relaxed stability at low angles of attack (alpha) and a pitch-down recovery moment at high alpha. TONAt9-15 May 1990 55
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