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Aviation History
1994
1994 - 1304.PDF
lishment (founded in 1936). Today's DLR has some 4,400 employees — including about 700 based at Braunschweig. The DLR says that its annual budget amounts to some DM700 million — of which 60% comes from the Federal Ministry of Research and Technology, 15% from the Ministry of Defence, 10% from the regional governments and "a small amount" from the Federal Ministry of Transport. The remain der comes from com mercial contracts such as the FASA work or the institute's research into a future ultra-high capacity aircraft for Airbus Industrie. The multi-national Airbus consortium has contracted the DLR to look at potential flying- qualities problems faced in an airliner with a capacity of up to 800 passengers — a project which Airbus now refers to as A3XX. "Controllability will be a problem," says Hamel. "You need huge control surfaces and then, of course, you have a lot of dynamic problems. It might be you need different kinds of control sur faces than on a conventional aircraft." A specific problem would be met on take-off. If the aircraft took off convention ally, then the force on the tailplane as the elevators deflect to rotate the aircraft would amount to "tens of tonnes" of negative lift — holding the aircraft on the ground and lengthening an already long take-off run. An alternative approach would be direct-lift control, using the landing flaps as an active control surface and providing extra lift right at the aircraft's centre of gravity. This could mean that the aircraft need not rotate at all, but also that the wing's centre of pressure would have to be close to the aircraft centre of gravity — cre ating an unstable balance which would have to be stabilised by the digital flight- control system. The Institute has already done some work on direct-lift control on its MBB-built HFB 320 Hansa Jet research aircraft and the ATTAS itself. The ATTAS' original VFW 614 wing flaps have been replaced with new trailing- edge flaps consisting of a one-piece forward segment and three independent rear seg ments, controlled by six actuators. The flaps have a high (1007s) rate of movement and a range of ±30°. "It is easier 'on a fly-by-wire aircraft' to have large wings with many split surfaces," says Hamel. "Imagine you have one surface which damps the elastic mode of the wing, one only used for manceuvre-load control, a different one for gust loads. This makes the 'flight-control' system more modular and simple," he adds. If all these oft-conflicting requirements had to be tackled by one control surface, the pilot could end up demanding more in a manoeuvre than the surface is capable of. The DLR's VFW 614 has simulated the Hermes spaceplane and lPTN's N250 regional turboprop Citing the example of the Eurofighter, Hamel says that the trailing edges of the aircraft's delta wing are dedicated to stabil ity only because the aircraft is naturally highly unstable. If the aircraft's quadruplex digital FCS were to fail, he adds, the results would be explosive — with the time to double the amplitude of a disturbance being "something like one second". HANDLING QUALITIES As a member of the four-nation Eurofighter management agency, NEFMA, the DLR was assigned in 1988 to prepare the handling- qualities definition document for the pro gramme. This document details handling requirements beyond the already-specified performance criteria of the aircraft, and was the basis for the design of the aircraft's con trol software. One of the most critical points of the document is to ensure that pilot-induced oscillations (PIOs) are avoided by optimis ing the "man-machine interface" — the interaction between the pilot and aircraft. PIOs have caused losses in the Saab JAS 39 Gripen and Lockheed F-22 programmes and are difficult to predict using ground simulation only. This is where in-flight simulation comes into its own. According to Hamel, "...thousands of ground-based simulations" had been car ried out in the Gripen and F-22 pro grammes, yet PIOs showed up only in the real environment, where the pilot was car rying out aggressive, "high-gain", manoeu vres with large amplitude and high-fre quency displacements. The DLR has now developed a testing method which attempts to identify the defi ciencies which cause PIOs — the system is known as ground-attack test equipment and IT has now been adopted by NASA. The test requires a pilof to attempt to "shoot" a series of halo gen lamps distributed on the ground. The lights are activated in an unpredictable sequence, requiring the pilot to manoeuvre violently to fulfil his task, which Hamel acknowledges is "very demanding". It can be made even more so by programming an un known time delay into the aircraft's controls, resulting in "a typical PlO-inducing situation". Hamel says that in flight simulation still has many other advantages over ground-based simu lation. Ground-based simulators are still unable to reproduce accurately the "patchy" quality of real tur bulence, or realistic sensations of motion. Nor is a computer-generated view from a simulator cockpit sufficiently realistic. Hamel sees in-flight simulation as an essential step between ground-based simu lation and the flight of an aircraft prototype and hopes to establish Braunschweig as a European centre of excellence for the tech nique, as the seven main European aero space research bodies move towards a more co-ordinated research policy (Flight International, 11-17 May). Future co-operation is by no means restricted to Western Europe, as the DLR has been pushing to establish closer ties with the Russian Gromov Flight Research Institute (LII), based near Moscow. In October 1993, the LII presented its Tupolev Tu-154 flight simulator to the West for the first time at Hanover Airport. This aircraft had been used previously to simulate the Russian Buran space vehicle during a landing approach. Hamel says that in-flight use of thrust reversers on the Tu- 154's outer two engines was needed to make the aerodynamics sufficiently dirty for an accurate simulation. "They were very convincing during their visit," says Hamel. "We are interested in co-operation in the field of very-large- capacity aircraft." n 50 FLIGHT INTERNATIONAL 25 - 31 May, 1994
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