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Aviation History
1974
1974 - 2022.PDF
832 AMERICAN LIGHT FIGHTER FLY-OFF FLIGHT International, 12 December 1974' Fighters for Nato—a comparison POLITICS and economics will surely have the last word in the forthcoming choice of our F-104G replacement for Belgium, Netherlands, Denmark and Norway and much has already been said about the industrial offsets and political influences in the competition. Nevertheless much of the argument supposedly relies on the performance of the various aircraft, notably the two competitors in America's own Air Combat Fighter compe tition, now linked with the US Navy VFAX programme. Any estimate is to a great extent clouded by the ignorance of the European customers' exact specifications. We cam be sure only that the four-nation steering committee has said that all four admitted contenders, the YF-16, YF-17, Mirage F.1E and Viggen Eurofighter, meet the specifica tion. Much of the required virtues inevitably depend on how well the aircraft combine pure fighter and ground-attack capabilities, two roles which traditionally call for funda mentally different design criteria. The US claims that the technological standard of its two lightweight fighters is supreme. It is true that both aircraft began life as pure air-combat technology exercises and have become opera tional projects for balance-of-payments rather than mili tary reasons. Technology so single-mindedly applied to one end of the sought-for performance range ought seriously to compromise the desired dual-role performance. It should be remembered that Northrop's Cobra may be nearly twice as heavy as its YF-17. On the other hand, similar strictures might well apply to the Mirage F.1E. And where does the rather larger and exotic Viggen stand in the performance-trading game? And how do all these fighter-biased or dual-role designs com pare with Jaguar, which is designed primarily for ground attack? The accompanying table collects and compares some of the features which characterise the performance of fighters. In crudest terms, thrust: weight ratio determines acceleration, climb and speed, though in present wing forms producing tremendous drag rise at high angles of attack, thrust: weight is also an essential factor in manoeuvrability. Increasingly important today is good neighbourliness at military bases, and the ability to take off on most occasions without afterburner and without smoke is valuable. For general comparison, a loaded Boeing 747 has a thrust:weight ratio between 0-25:1 and 0-3:1 and has a take-off run lasting something over 50sec. The YF-17's maximum-effort take-off run takes about nine seconds, with an unstick speed probably approaching lSOkt. It weighs 3,0001b more than a fully loaded Hunter. The relative merits of pure turbojets and turbofans are hard to balance. The turbojet produces a high proportion of "cold" thrust at relatively poor fuel economy. Turbo- fans achieve as high as 80 per cent boost with afterburner, but with very high fuel consumption, and their "cold" thrust is a lower proportion of the total. Yet their cruise fuel economy at low altitude is valuable. Of the four con tending aircraft, two have very large turbofans with high afterburning thrust increases. The other two have rela tively simple engines, more like "leaky jets." Jaguar has a pair of small turbofans. There has been vigorous argument between P&W and GE on the cost of the large single F100 turbofan in the YF-16 versus the cost of the pair of simple YJlOls in the YF-17. If the claims are to be believed, the two smaller engines can be bought more cheaply, while the large fan will use 1,200 tons less fuel in its lifetime. Even the better survivability of twin engines is not clear-cut. Perhaps what will in the end count for more is the sim plicity and maintainability of the chosen engine. This will certainly be a factor in sales to countries outside Europe, where fuel may be in better supply than servicing skills. The rather simple Snecma M53 in the Mirage F.1E, with single shaft and relatively low pressure ratio, allowing quite a high boost from afterburning, may turn out to be a good compromise for performance and sales appeal. Complementing urge and thrust: weight ratio are wing planform and wing loadings. In broad terms, low wing loading helps low-speed performance, turn radius and altitude performance, but is a positive penalty when flying fast in turbulent air at low altitude. Not only does the structure suffer from the hammering, but crew efficiency can be reduced to virtually zero. Therefore the GD and Northrop claims of wing loadings around 601b/sq ft at com bat weight are bad news for low-level work, especially when combined with their lightly swept wings. In fact, combat weight in these aircraft is assumed some time after take off and our table shows the rather higher take«off wing loading. An almost direct measure of field performance and manoeuvrability is wing loading against thrust: weight ratio. This figure in the table shows Viggen as offering an extremely favourable value for an aircraft designed for good low-level behaviour. High values for YF-16 and -17 must be at the expense of low-level effectiveness. Wherever possible, manufacturers' figures have been quoted. Northrop and Saab are consider ably more reticent than GD, Dassault- Breguet and Sepecat. Thrust:weight ratios are reckoned at published take-off weights, as are wing loadings. The latter do not therefore apply to handling during escape from target area without external load. Viggen figures are those for the AJ37 combined with the JA37's RM8B engine; no ]A37 figures have been released. JA37 is largely synonymous with Eurofighter. The 4,4001b external load for the Mirage F.IE is that stated by the manu facturer, though the two quoted gross weights allow for a much higher load. The quoted Viggen gross weight may well represent more than a plain fighter loading. The YF-16 external load is carried with reduced fuel Wing span Wing sweep, leading edge or ic Aspect ratio Wing area Empty weight Fuel weight Fighter gross weight External load Max gross Fighter thrust :weighl ft. sq lb lb lb lb lb cold afterburning Max load thrust: weight cold after burning Fighter wing loading Max wing loading Fighter wing loading Thrust :weight ratio Powerplant Total cold thrust Ib/sq ft Ib/sq ft lb Total afterburning thrust lb Bypass ratio YF-16 n 30 40°LE 3 :1 ft 280 14,350 6,650 21,000 13,200 31,000 0-9 :1 1 -28 :1 0-61 :1 0-87 :1 75 111 55 P&W F100 19,000 27,000 — YF-17 35 20°ic 3-5 :1 320 — 8,400 24,900 17,000 — — 12:1 — — 78 — 65 2xGE YJ101 — 29,600 0 Mirage F.1E 278 50°LE — 269 17,360 7,388 24,662 4,400 33,440 0-49 : 1 0-76 :1 0-36 :1 0-56 :1 91-5 124-5 120 Snecma M53 12,100 18,700 0-4 :1 Viggen 349 — 2-45 :1 567 — — 37,400 13,440 — 0-43 :1 0-75 :1 — — 65 8 — 88 Volvo RM8B 16,190 28,085 1 :1 Jaguar 28 6 40°ic 3 12 :1 258 — — 24,000 10,000 34,000 0-42 : 1 0-6 :1 0-3 :1 0-42 :1 93 131 155 R-RTurbomeca 2 x Adour 10,230 14,608 0-8 :1
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