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Aviation History
1972
1972 - 1710.PDF
FLIGHT International, 6 July 1972 11 F-15 rolls out at St Louis A "Flight" report on the emergence of the McDonnell Douglas air-superiority fighter for the 1980s THE MCDONNELL DOUGLAS F-15 (now named Eagle) was rolled out under its own power at St Louis on June 26. It is the first United States combat aircraft designed specifically for the air-superiority role since the F-86 Sabre. Clad entirely in new "air-superiority" light-blue matt camouflage and featuring twin smoke-free turbofans for added immunity from visual detection, the F-15 should prove a dog fighter second to none when it enters service in 1975. A thrust-to-weight ratio well in excess of unity gives the 40,0001b F-15 the ability to climb vertically at supersonic speed and to accelerate from subsonic cruise to mid-supersonic combat speeds in less than one minute. First flight is due at the end of July from Edwards Air Force base, California—the aircraft being air-freighted, with the wings and fins removed, by C-5A. Major General Benjamin N. Bellis, USAF F-15 systems programme direc tor, told Flight at St Louis last week that the F-15, with its low wing loading and subtle fixed leading-edge camber, could maintain a 5g manoeuvre (at a classified altitude) and still have excess thrust to accelerate. Deceleration at above lg is available from the massive upper-fuselage-mounted speed brake. The USAF therefore thinks the F-15 will be a competitive fighter well into the 1980s, and the commander of the Tactical Air Command, General William W. Momyer, describes the F-15 as having "more potential than a pilot can physiologically take." Bellis says a "dramatic" amount of work is being done on pressure and protective suits for F-15 crew. The primary combat envelope of the F-15 will be in the transonic regime at medium altitudes, based on Vietnam experience, but the Eagle will have a sustained Mach 2-3 capability with afterburning and a dash potential to Mach 2 • 5. Bellis claims that the prime potential F-15 adversary— the MiG-23 Foxbat—is only a straight-line aircraft at very high altitude (like the YF-12) when flying at Mach 3 and that manoeuvring in a high-dynamic-pressure environment at such speeds is "beyond our technical know-how" and might need thrust-vector control. The USAF estimates a Foxbat turning radius of 40-50 miles at Mach 3, requiring 5min for a 360° change of heading. It probably loses about one mile in range for every degree of azimuth it turns, says Bellis. The "fuel legs" of the F-15 will be greater than those of the F-4 Phantom (which will remain in production at McDonnell Douglas at least into 1976). The new fighter will have transcontinental United States range, using four under-wing drop tanks, and it will also have air-to-air refuelling capability. Extended loitering is not needed, however, so the complication of a variable-geometry wing can be avoided. Armament consists of four AIM-7F Sparrows for long- range interception, modified AIM-9L Sidewinders (being developed by the US Navy) for closer-range encounters and a single M61 20mm Gatling gun mounted in the star board wing fairing for "eyeball-to-eyeball" combat. The single-seat F-15 has 360° vision for the pilot, with a single- piece forward-vision section to the canopy. Dispensing with a second crew member to pare the weight of the F-15 has necessitated a very advanced radar and integrated head-up display, which allows a complete attack sequence to be flown without the pilot taking his eyes from the target. But even with such advances, the aircraft is likely to impose a heavy workload on its pilot. A high thrust-to- weight ratio, obtained through the use of advanced power- plants and a low wing loading, derived from a massive weight-reduction exercise on the airframe, are of para mount importance for the necessary high longitudinal and lateral accelerations to be attained and maintained. Thus the F-15 has such features as boron/epoxy-composite horizontal tailerons, titanium inboard wing skins and carbon brakes. A composite wing is also under develop ment, using primarily carbon fibre, but incorporating boron- and glass-reinforced areas. From the latter, Bellis anticipates "dramatic" weight reductions and a possible production order in the "not too far distant future." Several F-15s are in final assembly at St Louis—No 5 being the first with a radar-equipped nose—and major components for more than a dozen are on the line. Eventually 729 aircraft should be built under present plans. Average unit price including R&D, spares and simulators would then be $10-5 million (allowing for 422 per cent compounded annual inflation). The airframe is within cost estimates and the weight currently exceeds the 40,0001b target by only 1101b, including a 4001b con tingency—so the F-15 is below predicted weight in real terms. Pratt & Whitney F100 powerplant deliveries are a few weeks behind schedule and the engine cost has exceeded estimates because of a reduced production run on the essentially similar powerplant for the Gruman/US Navy F-14B. Ground-based, full-scale intake testing has already taken this—one of the F-15's most critical components—to Mach 2, 45,000ft and to angles of attack from -4° to + 11°. Fatigue testing in major components has been completed in the two-and-a-half years since McDonnell Douglas obtained the F-15 contract—including a simulated 16,000hr on the boron-composite tail. The same test article of the tail was then taken to 200 per cent of design ulti mate load without failure, to ensure the soundness of the new structure. P.M. The Eagle in retrospect At a time when major new military aeroplanes naturally sport swing-wings, impressive avionic installations and crews of two, the F-15 appears outwardly to be a throw back to an earlier age. But, despite its superficial simplicity, this fighter has already had a long and uncertain career, largely as a result of changing ideas and the slowing effect of other projects. Most of the uncertainties seem now in the past, and provided the financial aspect remains good there is every reason to think that the F-15 will make as big an impression as its St Louis stablemate, the Phantom. The history of the new fighter goes back to 1965 when both the USAF and the United States Navy were beginning to think about their respective replacements for the Phantom. The very earliest ideas, however, were beginning to crystallise some time before America became involved with the Vietnam war, and were greatly influenced by analysis of operations during the 1950-53 Korean conflict. Fighter development had for some years previously been mainly concerned with the increases in speed which were becoming possible with improved propulsion technology and a better understanding of aerodynamics. The view held by many people was that the new aircraft then entering service or projected, with speeds between Mach 1 and Mach 2, would drastically change the air-fighting picture. Closing speeds of half-a-mile/sec would restrict air combat simply to an interchange of missiles at long range. But real-life situations continued to demonstrate that, irrespective of successful efforts to push up the speed, dog-fight situations continued to devolve around classical tail chases, with speeds coming down to 500-600kt. The effort to substitute speed for manoeuvrability had failed and it was clear that a new approach was needed in which the emphasis would be shifted to acceleration, time-to- height and agility; design limitations were to be based more on the pilot's ability to fight at high-g levels than on structural considerations. Russian technology was known to be surging ahead, as continued on page 14
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