FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1987
1987 - 1204.PDF
DEFENCE X-29 to test extreme agility WASHINGTON D.C. The US Air Force and Nasa are set to move into extreme- agility testing with the Grumman X-29 forward- swept-wing (FSW) aircraft, reports Julian Moxon. A year-long programme beginning next October will concentrate on very-high- angle-of-attack flight as part of a drive to improve the tactical agility of future fight ers. A $4-65 million contract to begin designing a system that will give the X-29 a 70° angle-of-attack (AoA) capa bility has been awarded to Grumman, builder of the two X-29 prototypes, by the US Air Force's Systems Division at Wright-Patterson AFB, Ohio. "We've demonstrated 70° AoA in windtunnel tests at Nasa Langley," says X-29 programme manager Dr Thomas Weeks. "The aircraft remained stable, with good roll authority, and we antici pate we'll get the same results with the real thing—there's been quite good correlation with the windtunnel so far". 70° AoA tests The X-29 is currently limited to 21° AoA by its flight control computer software. The control laws will be modified by Nasa Dryden, with support from Grumman and Nasa Langley. A spin parachute—not covered in the original contract—will also be added. "We're trying to prove that the close-coupled can- ard/forward-swept-wing con figuration makes sense for military applications." "We want to understand what X-29 technologies look like at high AoA, where it's been long known that a forward swept wing has bene fits, in terms of reverse stall progression. When a canard is put directly in front of an FSW it does a better job of delaying stall, since it is directly ahead of the wing root. The combination of FSW and canard further increases the stall penetration capability to a higher AoA. "We're looking at control lable performance at 70°," says Weeks. "Like other aircraft, we begin to run out of rudder authority at about 40°. F~^ ~""~rT"*"°!m^mJ JXZMpi i^U ftumn itemffi, «s»' ^t£-% '*wmm? iffffilMfFlf^ - '* f.., -t/nA. V K •*& • .,' , •- ' ' ^Z& •*». W ,-\<^ •- •*•/< Grumman's X-29 A shows off its unique planform over Edwards AFB Without any kind of thrust- vectoring augmentation, or other devices, we'll be limited to 40° AoA in all three axes. The modifications mean [that] above that we'll be able to pitch and roll the aircraft, but not yaw. There'll be a limit on rudder authority until yaw augmentation is added." Weeks says that the US Air Force is now starting to look seriously at "something called tactical agility," the object being to develop a configur ation that can quickly change its manoeuvre state with precision and control. "It is apparent that this will be an emerging requirement for the next weapons system we develop," he adds. There is no direct rela tionship between the X-29 programme and the USAF's Advanced Tactical Fighter (ATF), claims Weeks, adding that it is "probably true" by implication that neither of the ATF designs are FSW. "If there's going to be a payoff, it will be in the areas of digitally controlled relaxed static stability and close-coupled canards. We've briefed the Air Force on both these subjects. The major challenge has been to make it all work together, the digital flight control system, aeroelastic tailoring, relaxed static stability, and so on." It is only in the last few months, says Weeks, that confidence in the FSW programme has grown to the point at which new ideas can be considered. "We now recognise that the aircraft is very reliable. It doesn't sit on the ground a long time. So there's a good reason to go ahead with more trials into extreme manoeuvrability." The Air Force will decide in the autumn which prototype it will use for the high-AoA tests. The second prototype —built as a backup—has never flown. It is presently at Grumman, where it is being used as a mockup for installa tion of the spin parachute and instrumentation. 110 flights The $300 million FSW programme was launched in 1981, principally under Defence Advanced Research Projects Agency funding. The first flight was made in the summer of 1984, since when the aircraft has accumulated more than 110 flights at Edwards AFB, with flight durations averaging one hour. The aircraft has recently taken to the air again after being grounded early this year for the installation of a cali brated General Electric F404 engine. Findings so far have been that the X-29 can fly at 35 per cent relaxed static stability I 10 within the state-of-the-art in digital Flight Controls. The transonic benefits of a forward swept wing have also been demonstrated, the X-29 performing consistently well at its Mach 0-9/30,000ft design point. The maximum speed and altitude flown to date are Mach 1 • 47 and 50,200ft. The X-29 programme has also shown that the aero- elastically tailored wings prevent a secondary, and little known, divergence effect called body freedom flutter. This would occur if the wing first bending mode coupled with the fuselage oscillation mode, resulting in a runaway flutter condition. "We demon strated that the body freedom flutter condition is well outside the envelope of the aircraft," says Weeks. Weeks says there is co ordination with the joint US European X-31 high agility programme, "although not very aggressively at this point. We co-ordinate with our super manoeuvre steering group that includes some European counterparts, and there may be some further inter relationship as we find out what we can really do with the airplane. There are some things we can do on the area of lateral (roll) agility. We'd like to see where that may fit in with what the Europeans are doing." FLIGHT INTERNATIONAL, 18 July 1987
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
