Lockheed YF-22

The Lockheed/ Boeing/ General Dynamics YF-22 is an American single-seat, twin-engine fighter aircraft technology demonstrator designed for the United States Air Force (USAF). The design team was a finalist in the USAF's Advanced Tactical Fighter (ATF) competition, and two prototypes were built for the demonstration/validation phase of the competition. The YF-22 team won the contest against the YF-23 team for full-scale development, and the design was developed into the Lockheed Martin F-22. The YF-22 has a similar aerodynamic layout and configuration as the F-22, but with notable differences in the overall shaping such as the position and design of the cockpit, tail fins and wings, and in internal structural layout.

In the 1980s, the USAF began looking for a replacement for its fighter aircraft, especially to counter the advanced Su-27 and MiG-29. A number of companies, divided into two teams, submitted their proposals. Northrop and McDonnell Douglas submitted the YF-23. Lockheed, Boeing and General Dynamics proposed and built the YF-22, which, although marginally slower and having a larger radar cross-section, was more agile than the YF-23. The Lockheed team was picked by the Air Force as the winner of the ATF in April 1991. Following the selection, the first prototype was retired to a museum, while the second continued flying until an accident relegated it to the role of an antenna test vehicle and was later stored.

Concept definition
In 1981, the U.S. Air Force began developing requirements for an Advanced Tactical Fighter (ATF) that would eventually become a new air superiority fighter to replace the F-15 Eagle and F-16 Fighting Falcon. This was made more crucial by the emerging worldwide threats, including development and proliferation of Soviet MiG-29 "Fulcrum" and Su-27 "Flanker"-class fighter aircraft, A-50 "Mainstay" airborne warning and control system (AWACS), and more advanced surface-to-air missile systems. It would take advantage of the new technologies in fighter design on the horizon including composite materials, lightweight alloys, advanced flight-control systems, more powerful propulsion systems and stealth technology.

The ATF request for information (RFI) was sent out to the aerospace industry in May 1981 to explore what the future fighter aircraft could potentially look like, and a subsequent Concept Development Team was established to analyze the results. Initially code-named "Senior Sky", the ATF at this time was still in the midst of requirements definition, and consequently there was substantial variety in the responses from the industry. Lockheed's initial concept was a particularly large aircraft called CL-2016, nicknamed "battlecruiser" for its size, that resembled the SR-71/YF-12 with large delta wings and engines mounted in nacelles spaced away from the fuselage and would have had similarly high operating speed and altitude.

In 1983, the ATF Concept Development Team became the System Program Office (SPO) at Wright-Patterson Air Force Base. After discussions with aerospace companies and Tactical Air Command (TAC), the SPO narrowed the requirements to an air superiority fighter with outstanding kinematic performance in speed and maneuverability to replace the F-15. Additionally, the SPO began to increasingly emphasize stealth for increasing survivability due to the Air Force's experience from "black world" projects such as the Have Blue/F-117 and the Advanced Technology Bomber (ATB) program (which would result in the B-2). With stealth becoming a core requirement, Lockheed design team, under its Skunk Works division at Burbank, California, migrated away from its initial SR-71-like "battlecruiser" concept and instead began drafting a design that resembled the F-117. However, the faceted shape, resulting from Lockheed using the same "Echo" computer program that it had used to design the F-117, gave the design very poor aerodynamic performance that would be unsuitable for a fighter. Lockheed would perform poorly throughout the concept exploration phase, placing amongst the bottom of the competing contractors.

Demonstration and validation
By November 1984, the SPO had further narrowed the requirements and released the Statement of Operational Need (SON), with requirements calling for a 50000 lb takeoff weight fighter that places strong emphasis on stealth and supersonic cruise without afterburners, or supercruise. In September 1985, the Air Force sent out technical request for proposals (RFP) to a number of aircraft manufacturing teams for demonstration and validation (Dem/Val). In addition to the ATF's demanding technical requirements, Dem/Val also placed a great deal of importance on systems engineering, technology development plans, and risk mitigation. The top four proposals, later reduced to two, would proceed with Dem/Val. At the time, the Air Force anticipated procuring 750 ATFs at a unit flyaway cost of $35 million in FY 1985 dollars. Furthermore, the U.S. Navy under Congressional pressure eventually announced that it would use a derivative of the ATF winner to replace its F-14 Tomcat as the Navy Advanced Tactical Fighter (NATF) and called for the procurement of 546 aircraft.

Having performed poorly during ATF concept exploration while also losing the ATB to Northrop who had a curved surface design, Lockheed abandoned faceting in 1984 and began incorporating curved shapes and surfaces. Although its analytical tools were initially not able to calculate for such shapes, good empirical results from radar range testing at Helendale, California gave Lockheed confidence in designing a stealthy aircraft with smooth, curved surfaces, thus greatly improving its aerodynamic characteristics. As Lockheed gradually became able to analyze curved shapes, the final design submitted for Dem/Val, designated Configuration 090P, would have an arrowhead-like forward fuselage shape, swept trapezoidal wings, four empennage surfaces, S-shaped inlet ducts obscuring the engine face, and an internal rotary missile launcher. In addition to the change in aircraft design, Lockheed also shifted much more engineering talent and manpower to its ATF effort, appointing Sherman Mullin as the program manager, and had its draft proposals aggressively red-teamed by a group led by retired Air Force general Alton D. Slay. The resulting proposal improvements were substantial, particularly the systems engineering volume.

The ATF RFP would see some alterations after its first release; stealth requirements were drastically increased in December 1985 after discussions with Lockheed and Northrop regarding their experiences with the Have Blue/F-117 and ATB/B-2, and the requirement for flying technology demonstrator prototypes was added in May 1986 due to recommendations from the Packard Commission. The seven bids were submitted in July 1986. Owing to the immense investments companies were expected to make on their own, teaming was encouraged by the SPO. Following proposal submissions, Lockheed, Boeing, and General Dynamics formed a team to develop whichever of their proposed designs was selected, if any. Northrop and McDonnell Douglas formed a team with a similar agreement.

On 31 October 1986, Lockheed and Northrop, the two industry leaders in stealth aircraft, were selected as the first and second place respectively; Sherman Mullin would credit the Lockheed proposal's system engineering volume for the top rank. The two teams, Lockheed/Boeing/General Dynamics and Northrop/McDonnell Douglas, were awarded $691 million contracts in FY 1985 dollars and would undertake a 50-month demonstration phase, culminating in the flight test of the two teams' prototypes, the YF-22 and the YF-23; Pratt & Whitney and General Electric had earlier been awarded contracts to develop the propulsion systems with the designations YF119 and YF120. Because the requirement for flying prototypes was a late addition due to political pressure, the prototypes were to be "best-effort" vehicles not meant to perform a competitive flyoff or represent a production aircraft, but to demonstrate the viability of its concept and to mitigate risk.

Design evolution
Work would be divided roughly equally amongst the team. Because Lockheed's submission was selected as one of the winners, the company assumed leadership of the program partners. It would be responsible for the forward cockpit and fuselage, as well as final assembly at Palmdale, California. Meanwhile, the wings and aft fuselage would be built by Boeing, with the center fuselage, weapons bays, tail and landing gear built by General Dynamics. The team would also invest $675 million combined into their ATF effort during Dem/Val in addition to the government contract awards. The partners brought their design experience and proposals with them. Boeing's design was large and long with a chin-mounted inlet, trapezoid wings, V-tail empennage surfaces (deemed sufficient due to the high operating speed), and palletized internal weapons. General Dynamics' design was smaller with fuselage and delta wings optimized for maneuver and supercruise, shoulder-mounted inlets, a large single vertical tail as the only empennage surface (which compromised all-aspect stealth), and weapon bays in the center fuselage. However, much of the scrutiny fell on Lockheed's Configuration 090P, which was problematic due to being highly immature as a result of Lockheed's greater focus on systems engineering rather than a point design. Nevertheless, 090P was the initial starting point that the team worked to refine.

Throughout Dem/Val, the SPO held System Requirement Reviews (SRR) with contractor teams and used the results of their performance and cost trade studies to adjust ATF requirements and delete ones that were significant weight and cost drivers while having marginal value. The requirement for eight internal missiles (represented by the baseline AIM-120A) was reduced to six. The team continually refined the design, making extensive use of analytical and empirical methods such as wind tunnel testing (18,000 hours by the end of Dem/Val), pole testing at radar ranges, and computational fluid dynamics (CFD) and computer-aided design (CAD) software. By early 1987, the design had evolved into Configuration 095, which replaced the rotary launcher with a flatter weapons bay to reduce volume and drag, and the shapes of the forward fuselage and leading-edge root extensions were recontoured to prevent uncontrollable pitch-up moments. Around this time, the design had split into two families, the 500 prefix that represents the full system design to be carried forward for full-scale development and the 1000 prefix that represents the same external shape but designed to be built as prototype air vehicles; Configuration 095 thus became 595 and 1095 respectively.

By mid-1987, detailed weight analysis of Configuration 595/1095 revealed that it was overweight by 9000 lb even if it could still nominally meet maneuver parameters. With weight likely to increase and compromises not forthcoming, the team chose to completely start over with a new design in July 1987, with Lockheed bringing a new director of design engineering, Dick Cantrell. Various different layouts were explored and after an intensive three-month effort, the team chose a new design, Configuration 614/1114, as the starting point with shoulder-mounted inlets and diamond-like delta wings similar to General Dynamics' design, and four empennage surfaces; notably, the diamond-like delta's aerodynamic characteristics approached the original swept trapezoidal profile's while offering much lower weight due to the longer root chord. The design evolved through the rest of 1987 and into May 1988, when Configuration 632/1132 was frozen as the YF-22. Changes include the shapes of the empennage surfaces to diamond-like and recontouring of the fore and aft fuselage to reduce wave drag following the deletion of the thrust reverser requirement after another SRR; the prototype thrust vectoring nozzles still retained some thrust reversing hardware provisions however, resulting in the prototype aft fuselage being bulkier than needed. Ultimately, the 50,000-lb takeoff weight still proved to be unachievable for both teams and was adjusted to 60000 lb, resulting in engine thrust increasing from 30000 lbf to 35000 lbf class. While the YF-22 configuration was frozen to begin construction of the prototypes, the team continued evolving the design into the F-22 for full-scale development.

In addition to the advanced air vehicle and propulsion design, the ATF required an integrated avionics system for sensor fusion to increase the pilot's situational awareness and decrease workload; this demanded a leap in sensor and avionics capability. Avionics development was marked by extensive testing and prototyping and supported by ground and flying laboratories, with Boeing being responsible for avionics integration. As the YF-22 was a technology demonstrator for the airframe and engines, it would not have any of the mission systems avionics. Boeing would build the Avionics Ground Prototype (AGP) and also provide a Boeing 757 modified with the mission systems as a flying laboratory for avionics development; this aircraft would later be named the Flying Test Bed. The SPO would similarly adjust avionics requirements as a result of SRRs with contractors. Side-looking radar and infrared search and track (IRST) were deleted from the baseline requirement and became provisions for potential future addition, and a $9 million cap for avionics per aircraft in FY 1985 dollars was placed by the SPO in 1989 on the baseline proposal for full-scale development.

Naval variant
Due to Congressional pressure, the U.S. Navy joined the ATF program initially as an observer and in 1988 announced that it would procure a variant/derivative of the winning design as the NATF to replace the F-14. Because the NATF would have lower landing speed than the ATF while still attaining Mach 2-class speeds, the Lockheed team's NATF design would incorporate variable-sweep wings, although the resulting aircraft would have been heavier, more complex, and more expensive than the ATF counterpart. Lockheed would submit its NATF design along with its ATF full-scale development proposal in December 1990, although the Navy would withdraw from the program shortly afterwards.

Design


The YF-22 (internally designated Configuration 1132) is a prototype air vehicle intended to demonstrate the viability of the ATF air vehicle and propulsion design, which was ultimately meant to meet USAF requirements for survivability, supercruise, stealth, and ease of maintenance. The airframe has large diamond-like delta wings, shoulder-mounted inlets, three internal weapons bays, and four empennage surfaces: canted vertical tails with rudders and all moving horizontal stabilizers. It has a tricycle landing gear, an aerial refueling receptacle centered on its spine, and an airbrake between the vertical tails. The cockpit has a throttle and sidestick arrangement. Compared with its Northrop/McDonnell Douglas counterpart, the YF-22 has a more conventional design – its wings have larger control surfaces, such as full-span leading edge, and, whereas the YF-23 had two tail surfaces, the YF-22 had four, which made it more maneuverable than its counterpart.

The YF-22 was powered by two engines, with the General Electric YF120 mounted on the first aircraft and the second with Pratt & Whitney YF119. The caret inlets are spaced away from the forward fuselage to divert the boundary layer and generate oblique shocks with the upper inboard corner; the serpentine inlet ducts fully shield the engine faces from any exterior view. Chines run from the nose along the sides of the forward fuselage where they eventually meet the upper edge of the inlets; those then transition to sharp leading edge root extensions further aft. The aircraft had relaxed stability and was controlled via fly-by-wire. To reduce supersonic drag for supercruise, area rule is applied to the airframe shape and most of the fuselage volume lies ahead of the wing's trailing edge, although the late configuration redesign meant that the prototype shaping was not quite refined. The two-dimensional thrust vectoring nozzles help reduce the infrared signature by flattening the exhaust plume and facilitating its mixing with ambient air.

Formally designated as the YF-22A, the aircraft was given the unofficial name "Lightning II" after Lockheed's World War II-era fighter, the P-38 Lightning, which persisted until the mid-1990s when the USAF officially named the production F-22 "Raptor". The F-35 later received the "Lightning II" name in 2006.

NATF-22
The Lockheed team's design for the Navy Advanced Tactical Fighter (NATF), sometimes referred to as "NATF-22" or "F-22N" (the design was never formally designated), would have differed from the Air Force version in many ways. Because the NATF needed lower landing speeds than the F-22 for aircraft carrier operations while still attaining Mach 2-class speeds, the design would have incorporated variable-sweep wings. The fuselage shaping was similar, while the landing gears and arresting hook were strengthened for aircraft carrier landings; all of these changes would have resulted in a heavier, more complex, and more expensive aircraft. It retained four empennage surfaces and thrust vectoring nozzles, and the avionics would initially have been largely common with the F-22, although additional sensors and mission avionics had also been planned for maritime missions. The design would have had a similar weapons bay arrangement but with expanded weapons carriage, including the AIM-152 AAAM, AGM-88 HARM, and AGM-84 Harpoon.

While the Lockheed team would submit the NATF-22 design with its F-22 full-scale development proposal in December 1990, the Navy began backing out of the NATF program in late 1990 to early 1991 and fully abandoned NATF by FY 1992 due to escalating cost and the design never progressed beyond Dem/Val for full-scale development, or engineering and manufacturing development (EMD). Lockheed would leverage aspects of the design, such as the variable-sweep wings, for several concepts for the Navy's Advanced-Attack (A-X) program, which later became the Advanced Attack/Fighter (A/F-X) program with added fighter capability, the successor to the canceled A-12 Avenger II; however, A/F-X would also be canceled as a result of the 1993 Bottom-Up Review due to post-Cold War budget pressure.

Evaluation
The first aircraft (PAV-1, serial number 87-0700, N22YF), with the GE YF120, was rolled out on 29 August 1990 and first flew on 29 September 1990, taking off from Palmdale piloted by David L. Ferguson. During the 18-minute flight, PAV-1 reached a maximum speed of 250 kn and a height of 12500 ft, before landing at Edwards AFB. Following the flight, Ferguson said that the remainder of the YF-22 test program would be concentrated on "the manoeuvrability of the aeroplane, both supersonic and subsonic". The second YF-22A (PAV-2, s/n 87-0701, N22YX) with the P&W YF119 made its maiden flight on 30 October at the hands of Tom Morgenfeld.

During the flight test program, unlike the YF-23, weapon firings and high (60°) angle of attack (AoA, or high-alpha) flights were carried out on the YF-22. Though not a requirement, the aircraft fired AIM-9 Sidewinder and AIM-120 AMRAAM missiles from internal weapon bays. Flight testing also demonstrated that the YF-22 with its thrust vectoring nozzles achieved pitch rates more than double that of the F-16 at low-speed maneuvering as well as having excellent high angle-of-attack characteristics, with trimmed alpha of over 60° flown. The first prototype, PAV-1 with the General Electric engines, achieved Mach 1.58 in supercruise, while PAV-2 with the Pratt & Whitney engines reached a maximum supercruise speed of Mach 1.43; maximum speed was in excess of Mach 2.0. Flight testing continued until 28 December 1990, by which time 74 flights were completed and 91.6 airborne hours were accumulated. Following flight testing, the contractor teams submitted proposals for ATF full-scale development, with the Lockheed team design evolving to Configuration 638 for its submission.

On 23 April 1991, the Lockheed team was announced by Secretary of the Air Force Donald Rice as the winner of the ATF competition. The Lockheed team was rated higher on technical aspects, was considered lower risk (the YF-22 flew more hours and sorties than its counterpart), and was considered to have more effective program management. Both designs met or exceeded all performance requirements; the YF-23 was stealthier and faster, but the YF-22 was more agile. It was speculated in the aviation press that the Lockheed design was also seen as more adaptable to the Navy's Navalized Advanced Tactical Fighter (NATF), but the US Navy abandoned NATF by 1992. Instead of being retired, as with the case of PAV-1, PAV-2 subsequently flew sorties following the competition – it amassed another 61.6 flying hours during 39 flights. On 25 April 1992, the aircraft sustained serious damage during a landing attempt as a result of pilot-induced oscillations. It was repaired but never flew again, and instead served as a static test vehicle thereafter. In 1991, it was anticipated that 650 production F-22s would be procured.

F-22 production


As the Lockheed team won the ATF competition, it was awarded the full-scale development, or engineering, manufacturing and development (EMD) contract in August 1991 initially worth about $11 billion, which would ultimately allow it to proceed with production of operational aircraft. The EMD/production design would evolve into Configuration 645. The EMD initially called for seven single-seat F-22A and two twin-seat F-22Bs, although the latter was eventually canceled to save on development costs and the orders were converted to single-seaters. On 9 April 1997, the first of these, Spirit of America, was rolled out. During the ceremony, the F-22 was officially named "Raptor". Due to limited funding, the first flight, which had previously been scheduled for mid-1996, occurred on 7 September 1997. Flight testing for the F-22 continued until 2005, and on 15 December 2005 the USAF announced that the Raptor had reached its initial operational capability (IOC); with the collapse of the Soviet Union and the Department of Defense focused on counterinsurgency at that time, F-22 production only reached 195 aircraft and ended in 2011.

In many respects, the YF-22s were different from EMD/production F-22s as the design progressed from relatively immature Configuration 632/1132 to the final Configuration 645. Contrary to the F-117 Nighthawk, which was initially difficult to control because of small vertical stabilizers, the YF-22 had its fin area over-specified by Lockheed. Therefore, the company reduced the size of those on F-22s by 20–30 percent. Lockheed and its partners recontoured the shape of the wing and stabilator trailing edges to improve aerodynamics, strength, and stealth characteristics; the wing and stabilitor sweep was reduced by 6° from 48°. The shapes of the radome and fuselage were changed to improve radar performance and aerodynamics. The dedicated airbrake was eliminated in favor of feathering control surfaces using the control laws. Finally, to improve pilot visibility, the canopy was moved forward 178 mm, and the engine intakes were moved rearward 356 mm.

Accidents
In April 1992, the second YF-22 crashed while landing at Edwards AFB. The test pilot, Tom Morgenfeld, escaped without injury. The cause of the crash was found to be a flight control software error that failed to prevent a pilot-induced oscillation.

Surviving aircraft



 * YF-22A PAV-1, S/N 87-0700 – Air Force Flight Test Center Museum, Edwards Air Force Base, California
 * YF-22A PAV-2, S/N 87-0701 – Rome Laboratory, Rome, New York.