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F-15 Eagle

History

The Eagle's history is long and distinguished. It began as a Air Force fighter study in the early 1960s and was known as the Fighter Experimental (FX). By 1967 the Air Force began development of a new high performance fighter aircraft that would be extremely agile and would be capable of gaining and maintaining air superiority through air-to-air combat. The new design had to be optimized for combat with the power and agility to overcome any current or projected Soviet threat. The F-15 was the first air-to-air fighter requested by the Air Force since the F-86 Sabre. The resulting F-15 Eagle had an unequaled combination of performance, firepower, and avionics. It was the benchmark -- the plane to beat.

Experience in the Vietnam conflict showed the F-4 Phantom II to have maneuvering performance inferior to that of the Soviet-built MiG-21. In response to this finding, the USAF developed a set of requirements for a dedicated air-superiority fighter with a maneuvering capability greater than any existing or foreseeable-future fighter aircraft.

Using lessons learned in Vietnam, the USAF sought to develop and procure a new, dedicated air superiority fighter. Such an aircraft was desperately needed, as no USAF aircraft design solely conceived as an air superiority fighter had become reality since the F-86 Sabre. The intervening twenty years saw a number of aircraft performing the air-to-air role as a small part of their overall mission, such as the primarily air-to-ground F-4 Phantom and the F-102, F-104 and F-106 interceptor designs. The result of the FX study was a requirement for a fighter design combining unparalleled maneuverability with state-of-the-art avionics and weaponry.

McDonnell Douglas, North American Rockwell, and Fairchild-Republic submitted proposals in the ensuing design competition. Many of the basic design features of U.S. fighter aircraft have resulted from technology pioneered at NASA's Langley Research Center. In 1967, Langley disseminated the results of in-house studies of a fighter configuration known as LFAX-8, which incorporated several features that would later be evident in the F-15 aircraft. Some of these features were

  • Short propulsion package to minimize weight
  • Engines placed forward for balance
  • Horizontal ramp engine inlets for good performance at high angles of attack
  • Horizontal tails located far aft on booms for increased stability and control
  • Tailored twin-engine aft-end spacing and interfairing for efficient subsonic cruise conditions

In 1968, the Department of Defense requested that NASA respond to the F-15 request for proposals (RFP) in a manner similar to the industry contractors. The key person behind the NASA participation was Dr. John Foster, Director of the Defense Department Research and Engineering organization. He requested the participation for two reasons. First, Foster felt that NASA's aircraft designs for the F-15 mission would embody advanced technology and serve as the upper limit of technology for industry proposals. Second, NASA and its problem-solving expertise could minimize risks and problems later in the development program. Four fighter concepts were studied in great detail:

  • LFAX-4-a variable-sweep configuration
  • LFAX-8- a fixed-sweep version of LFAX-4
  • LFAX-9-wing-mounted twin-engine configuration
  • LFAX-10-similar in external shape to Soviet MiG-25 Foxbat

Industry design teams visited Langley during the efforts and were continually updated on the advantages, disadvantages, and technical maturity of the configurations. The NASA team also briefed high ranking DOD officials. The LFAX-4 and LFAX-8 embodied features that would subsequently be evident in the McDonnell Douglas F-15 and Northrop Grumman F-14 aircraft. The LFAX-8 design made an indelible impression on the McDonnell Douglas design team, which embraced the fundamental layout of the NASA configuration. The cranked-wing design of the LFAX-8 had to be modified by McDonnell Douglas as the requirements for transonic maneuvering became more important. Another modification to the LFAX-8 involved the installation of a larger radar dish in the nose than the NASA team had allowed for in their design. The installation required a larger diameter nose cone, and although the NASA researchers deplored the increased supersonic drag caused by the larger nose, the final design incorporated the larger dish.

An industry-wide competition ended on December 23, 1969 when McDonnell Douglas was awarded the contract for the F-15.

Previous experiences with the F-111 and other advanced fighter concepts indicated that an extremely large portion of the subsonic cruise drag of modern twin-engine fighters is contributed by the aft end of the configuration (approaches 50 percent for some configurations). Careful tailoring of the engine inter-fairings and tail surfaces could prevent excessive aft-end drag. Configuration changes to the initial F-15 design significantly reduced the subsonic cruise drag of the aircraft. Specifically, the ventral fins were removed and the height of the vertical tails was increased to compensate for the resulting loss of directional stability.



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