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Aviation History
1991
1991 - 1440.PDF
TECHNICAL: DEFENCE NASA could rescue redundant YF-23s BY GUY NORRIS Northrop is offering NASA use of its two YF-23 proto types for flight research rather than see them scrapped. The aircraft became redundant after losing the recent Advanced Tac tical Fighter (ATF) competition to the Lockheed-led F-22 (Flight International, 1-7 May). Northrop says the YF-23's large, close-coupled V-tails pro vide high pitch, roll and yaw rates at low speeds, high angle- of-attack (AOA) and would pro vide an ideal tool for high-alpha and post-stall-manoeuvre ad vanced airborne research. If NASA rejects the offer, details of which are not clear, then the outlook for the aircraft is bleak. "We can't give them to a museum because of the classi fied materials which make them up," says Paul Metz, Northrop chief test pilot for the pro gramme, "so they will probably be destroyed or end their days gathering dust on the Edwards ramp....unless our research pro posals to NASA are accepted." Speaking at the Society of Experimental Test Pilots' meet ing at Bath, England, Metz gave new details of the YF-23 test programme. Referring specifi cally to the aircraft's high-alpha capability he says: "We demon strated precise control at 25° AOA at low airspeed and very high roll and pitch performance at supercruise". Metz criticised the fly-off concept, adding: "We still don't fully understand the reasons why we failed". Speaking about the competition he said: "People need to look at this in several ways, but it is the riskiest ap- YF-23's high angle-of-attaek characteristics put at NASA's disposal proach. It's one thing to have two new aircraft, but to have two new engines as well is a formula for disaster. It didn't happen but people should ask themselves why." In all, the two YF-23s flew 65.2h during 50 flights over 14 weeks. A total of five pilots checked out on the aircraft, two US Air Force and three from the Northrop/McDonnell Douglas team. The flight envelope was expanded during the short pro gramme to Mach 1.8 and alti tudes of 50,000ft (15,150m). Metz says that engine thrust and aircraft drag estimates were "a bit pessimistic" and super- cruise speeds were higher than predicted. The Pratt & Whitney F119-powered YF-23A achieved a standard-day supercruise speed of Ml.43 while the Gen eral Electric FI20-powered vari ant achieved "...a higher speed. Both engines were stall free even under extremely adverse conditions like full afterburner lights at 50,000ft and 180kt [330km/hl calibrated air speed [CAS]". Metz adds that the air craft has "...an impressive su personic turn performance and its low wing loading gives it excellent low-speed capabili ties". Other highlights include "excellent formation and aerial refuelling characteristics", and the "...best gun tracking to 4gs I've seen", says Metz. One incident, which almost resulted in the loss of the sec ond YF-23A, prototype air vehi cle (PAV) 2, occurred on its third flight at 26,000ft when fuel "...streamed heavily" from overflow valves as the tanks over-pressurised by 80%. "For tunately the tanks didn't burst," says Metz, who explains that the cause was traced to a 3mm- diameter sensing tube linked to the pressure-regulating valve. The tube was blocked with manufacturer's material and was sensing ground pressure. Other incidents included windscreens cracking on both aircraft. The polycarbonate- composite screen cracked at Ml.6 on PAV-1 and at M1.8 on PAV-2. The nosewheel, an off- the-shelf F-15 unit, also shim mied during high-speed taxiing trials at HOkt CAS. The prob lem was solved by a 5kg "scis- sor-link" attached rigidly to the lower strut arm. • Tiger speeds t Eurocopter's Franco-German combat helicopter, Tiger, is set to reach its 160kt (295km/h) maximum design speed less than two months after its maiden flight. Tiger has increased its speed by around 40kt a week since its first flight in April (Flight Inteniational, 8-14 May). "The programme is right on schedule," says Tiger and MBB deputy chief test pilot Andrew Warner, adding: "Handling is remarkable....in a zero wind there is virtually no control input required to hold the hover". Warner says that a sim ple stability augmentation sys tem (SAS) planned to cope with expected "twitches" from the Tiger's fibre elastomeric rotor was not needed. "We've put the SAS on the back burner at mo ment; we don't think we need it at this stage," he says. The simple SAS planned for the first prototype will be suc ceeded eventually by a full du plex digital automatic flight con trol system, which will be inte grated into the second pro totype's avionics bus system when it flies next year. The only unexpected vibra tion experienced so far has been a small oscillation (3 omega/ 20Hz), which caused a slight flapping in the lightweight, 65kg, main-rotor blades on the aircraft's second flight. Tungsten weights were inserted in the fibreglass roving spar aft of the leading edge to balance the blade and cure the problem. Israelis pick Elbit ma| The Israeli Air Force has selected the Elbit map dis play generator (MDG) for in stallation in its Sikorsky CH-53 Stallions as part of CH-53-2000, an extensive upgrade of the heavy helicopter. The system will be supplied in early 1992. Designed for a two-seat cock pit, the MDG has two map- generator channels capable of producing a variety of images. To enable the storage of map databases on board the aircraft,' the MDG is equipped with an optical disk drive and remova ble optical disk. Two graphics 16 FLIGHT INTERNATIONAL 5-11 June, 1991
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