Modern-Aviation

In the late 1970's, concern arose among military planners about the aging design (first flight in 1972) of the F-15 and the possible loss of future air superiority of the fighter. Soviet fighters such as the MiG 29 and Su-27 had demonstrated remarkable maneuverability and performance. In addition, fighter technology had taken enormous strides forward with the introduction of stealth, or low observable, technology. There was also growing concern over the increased effectiveness of the Soviet air defense system that posed a highly lethal environment for the F-15. Therefore, the Air Force initiated an Advanced Tactical Fighter (ATF) Program to develop a replacement for the F-15.

In 1981, the Air Force developed a requirement for an Advanced Tactical Fighter as a new air superiority fighter. It would take advantage of the new technologies in fighter design on the horizon including composite materials, lightweight alloys, advanced flight control systems, higher power propulsion systems and stealth technology. Air Force leaders believed these new technologies would make aircraft like the F-15 and F-16 obsolete by the early 21st century. A Mission Element Need Statement for an Advanced Tactical Fighter (ATF) was released in October 1981.

In September 1985 the Air Force sent out technical requests for proposals to a number of aircraft manufacturing teams. The October 1986 Milestone I review directed a DEM/VAL phase prior to entry into EMD. On 31 October 1986, the Air Force awarded each team a $691-million fixed-price contract to build two prototypes: Northrop-McDonnell Douglas' YF-23, and the Lockheed-Boeing-General Dynamics YF-22. In contrast to the F-117A and the B-2, both of which had been point designed for stealth, these two prototypes were the first airplanes ever to blend stealth with agility and high-speed, supersonic cruise capability.

A naval variant of the ATF that could operate from aircraft carriers (the NATF) was expected to replace the Navy's F-14 fighter; however, funding for the NATF was not requested by the Defense Department after 1990.

Two YF-23 prototypes were designed and built by the contractor team of Northrop and McDonnell Douglas as part of the demonstration and evaluation phase of the US Air Force's Advanced Tactical Fighter selection program, which concluded in 1990. During the ATF program, one YF-23 was powered by twin Pratt and Whitney YF119 turbofan engines, while two General Electric YF120 turbofan engines were installed in the other prototype. Featuring a diamond-shaped planform, two large, sharply-canted ruddervators, and a serrated aft profile, the high performance aircraft was larger than the F-15 it was designed to replace. The YF-23 prototypes are 67.4 feet in length and have wingspans of 43.6 ft. The YF-23 employed stealth characteristics and was capable of supersonic cruise flight without afterburner. The aircraft achieved a speed of Mach 1.8 during the program.

There was no official USAF "nickname" for the YF-23A. The Northrop YF-23A team personnel chose the name "Black Widow II" -- commemorating the Northrop P-61 Black Widow, the first American aircraft specifically designed as a night-fighter.

The YF-22 and YF-23 were different in many ways. The YF-23 was designed for speed and maneuverability. The YF-22, however, was designed more for maneuverability. Both aircraft, were designed for a type of flight called supercruise. Supercruise is when an aircraft is designed to be flown at Mach 1 or above in cruise, that is without afterburners. For this reason, both aircraft had to be more aerodynamic. The YF-23 was slightly more aerodynamic as it's cruising speed was Mach 1.25, Mach 0.08 faster than the YF-22 (roughly). Both of their speeds varied however. That is because the two prototypes were produced and tested. Each one was fitted with a different engine configuration. One prototype would be fitted with the Pratt & Whitney F119 powerplant and the other with the General Electric F120 powerplant. Both powerplants were roughly equal but the F120 was better suited to supercruise.

Lockheed chose a somewhat conservative hybrid planform airframe layout for the YF-22, reminiscent of the F-15 and F/A-18, with closely spaced engines, long inlet tunnels, outward canted vertical tails and rudimentary strakes over the inlet boxes to promote vortex lift over the outboard wing sections. Northrop chose a far more radical airframe layout, driven by the objectives of stealthiness and supercruise. The extensively blended fuselage has rudimentary chines which smoothly blend into the wing leading edge, the blending allowing good area ruling and low supersonic drag. The low wing aspect ratio is used to optimise supercruise performance.

The exhausts of the two aircraft differed radically. Lockheed chose a layout aimed at maximising lower speed manoeuvrability via the use of thrust vectoring, even though this was not a mandatory USAF requirement. Two dimensional thrust vectoring nozzles provide vectoring to enhance response in pitch. Northrop on the other hand rated stealth and drag so important they employed a serrated planform beavertail with B-2-like submerged ventral exhaust troughs. This approach reduced both depressed tail aspect infrared emissions and tail aspect radar cross-section, but precludes any vectoring.

The YF-23 took a very raidcal departure from the conventional design of aircraft. By using a very unusual shape the aircraft became very maneuverable and had a high top speed. By using the same angle on all flying surfaces (i.e. the nose, wing fronts, wing backs, ruddervator {rudder/elevator} fronts, ruddervator backs, and engine exhausts), the stealthiness is increased. Another advantage of using such unusualy shaped flying surfaces, is that the uncontrolability of the aircraft is increased so that when fly-by-wire is used, the manueverablity of the aircraft is increased greatly.

The Northrop/MDC YF-23 employed planform shaping with extensive blending, the latter technique used to advantage with the large B-2A. Blending has the major strength of not compromising high speed aerodynamics, the blended airframe offering very low drag by avoiding vortices which may be produced by a faceted geometry. In addition to RCS reduction through shaping, the YF-23 also employed carefully shaped exhausts to conceal the engine hot end, yet another technique developed during the B-2A program. The unusual 'diamond' planform of the YF-23 is a 2 major lobe design, as all major edges fall into groups of two parallels. The result of the low observables techniques was a major reduction in aircraft detectability by radar, and in the YF-23, also detectability by Infra-Red Search & Track (IRS&T) systems. This will radically shrink the usable envelope of hostile radar guided weapons and in the instance of the YF-23, also heatseeking weapons.

In April 1991, the Air Force selected Lockheed's YF-22 design for full-scale development, now termed "Engineering & Manufacturing Development" (EMD). According to the Air Force, factors in the selection for production of the F-22 were a better designed for maintainability, greater potential for future development, and slightly lower cost. Secretary of the Air Force Donald B. Rice stated that the choice was based on confidence in the ability of the Lockheed team and Pratt & Whitney to produce the aircraft and its engine at projected costs. Emphasizing the importance of the Lockheed team's management and production plans, he denied that either prototype was significantly more maneuverable or stealthy. A popular view is that the decision reflected a preference for maneuverability over stealth.

It is universally held that the YF-23 was by far the better looking aircraft, and supporters offer a number of other reasons why the YF-23 should have won the ATF competition.

The YF-23 was a very agile aircraft. The YF-23 is a very unstable aircraft; however, when this instability is coupled with a fly-by-wire control system, this results in a very agile aircraft. Another attribute that lends itself to high mobility is the uniqueness of the tail. On the YF-23, instead of using two rudders and two elevators, it uses a ruddervator, a combined rudder/elevator. This ruddervator is angled at a roughly 45° angle to horizontal.

The YF-23 was stealthier than the F-22 Raptor. The two ruddervators reduce the Radar Cross Signature of the YF-23 significantly. This is beacuse instead of having four extremly large control surfaces on the tail, there are only two. The F-22 Raptor design utilizes the traditional configuration of two rudders, which are canted outward, and two elevators. This make the RCS larger. Another RCS reducing feature is the engines. These are mounted in nacelles in the wing that blend gracefully into the wing on the top, and form an extension of the fueslage on the bottom. The larger bottom fuselage lets it pack more missiles and other expendable weapons. The intake duct is angled up and inward to reflect radar beams and keep them from hitting the fast moving compressor face. The intake duct starts on the lower edge of the wing and moves through it onto the top of the wing. This feature can also reduce the RCS signature from a look down-shoot down radar from an aircraft flying overhead. Also reducing the RCS, is the way the leading and trailing edge of all surfaces are angled. All of the leading and trailing edges are angled the same. Therefore, the front of the right wing is parallel to the left wing's trailing edge, and the left section of the nose. For example, the leding edge of the wing is parallel to the trailing edge of the wing on the other side.

The YF-23 had a greater fuel capacity and therefor a longer range. By having a larger wing area and greater fuselage volume, the fuel storage of the YF-23 is increased over the F-22 Raptor. As a result of this, the YF-23 can fly longer CAP (Combat Air Patrol) missions. It can also fly deeper into enemy territory without the need to be refueled by tankers. As a result of the first one, fewer aircraft must be purchased. As a result of the second, less money needs to be spent on refueling tankers.

Both YF-23 prototypes have been transferred from Northrop to the NASA Dryden facility at Edwards. Only one was used in NASA calibration techniques study. The other remained in storage at NASA Dryden. There are no engines in the two aircraft, and NASA had no plans to fly the YF-23s in any research program. Transfer of the aircraft from Northrop to NASA was at no cost to the government. Both aircraft remained in storage until the summer of 1996 when the aircraft were transferred to museums. The YF-23A "Black Widow II" PAV-2 (S/N 87-801) is on display at the Western Museum of Flight in Hawthorne, California, on long term loan from NASA. YF-23A "Black Widow II" PAV-1 (S/N 87-800) is currently at the USAF Test Center Museum at Edwards Air Force Base, California.