The simple and lightweight F-5 airframe proved to be extremely adaptable to a variety of uses, some of which required extensive modifications to the airframe
The following article titled Ultimate and unusual F-5s appeared in issue 19 – Northrop F-5 of Aviation Classic.
The simple and lightweight F-5 airframe proved to be extremely adaptable to a variety of uses, some of which required extensive modifications to the airframe. The success of the fighter with international air forces also led to the development of an advanced version intended as a replacement. As well as the developments by Northrop, additional unlicensed life-extension and redesign programmes have been carried out by HESA in Iran to produce a viable fighter bomber for the Islamic Republic of Iran Air Force (IRIAF).
The Northrop F-20 Tigershark
The increasing capabilities of Soviet Bloc aircraft throughout the 1970s meant that several F-5E customers, especially Taiwan, pressed the US to provide greater capabilities for the fighter, such as an improved radar and the carriage of the semi-active radar guided AIM-7 Sparrow missile. The US Department of Defence requested that Northrop study the feasibility with which a suitable radar could be fitted to the F-5E, resulting in a design known as the F-5G. Changes in the US Government’s weapons export policy in 1977 meant that this study was never taken any further, but by the end of the decade, this policy had caused a number of US Allies to seek advanced fighter aircraft elsewhere, such as the aerospace industries of France and Britain.
On January 4 1980, a new competition for an export fighter, known as the Intermediate Export Fighter or FX, was begun by the State Department in order to recapture this market and appease America’s allies. Northrop reissued the F-5G proposal, General Dynamics responded with a lower powered, simpler version of the F-16, known as the F-16/79. With the inauguration of Ronald Reagan as US President in 1981, politics were again to interfere in the development of the F-5. Initially, the FX project was approved, then shelved, as the export of front line equipment was now allowed, then resurrected in 1982 as a fully-fledged programme. However, by 1980 Northrop realised that the new F-5 model would have to at least be on a par with the F-16 in terms of performance and capability, as the little fighter was now in direct competition with it.
To meet these performance requirements the F-5G proposal was much modified, resulting in the new designation of F-20 and the name Tigershark being applied to the project in 1983. The redesign began with basing the aircraft on a single and much more powerful engine, a concept that had been under design study since the beginning of the F-5 programme. In order to surpass the F-16’s performance, the General Electric F404 was selected, as Northrop already had some experience with the original YJ101 version of this turbojet in the YF-17 Cobra prototypes. In fact, the F-5/404 concept had been put forward as early as May 1975 as a possible F-5E upgrade.
This choice alone called for wide ranging changes to the fuselage design, which had to be 5in longer with bigger inlet ducts and redesigned ramp doors to accommodate the greater amount of air the engine required at the greater than Mach 2 projected maximum speed. The wing did not change much, except for modifications to the leading edge extensions, which improved the lift performance for a minimum of increase in wing area, and internal strengthening to take the stress of 9G loading. A 30% larger tailplane was added to improve the pitch rates the aircraft could achieve and reduce its turning radius, an improvement assisted by the addition of a new fly-by-wire control system. Improved avionics and radar would allow the carriage and delivery of a greater range of weapons than previously.
The first aircraft, initially known as the F-5G-1, was essentially a re-engined F-5E with a modified rear fuselage to accommodate the General Electric F404 of 16,000lb (7257kg) thrust. Little else was changed. It was given the serial 82-0062 and the civil registration N4416T and test pilot Russ Scott made the first flight for 40 minutes on August 30, 1982, from Edwards Air Force base in California’s Mojave desert. The performance that the 60% increase in thrust gave the fighter was astounding, enabling it to fly in excess of Mach 2 and reach an altitude of 55,000ft (16,800m). The lower fuel consumption of the F404 also gave an increase in range of 10% over the J85 engined version. The sustained turn rate was also improved to 11.5º per second at Mach 0.8, the supersonic turn rates were also improved by 47%.
The test flying proved so successful that an initial order had been placed by Bahrain in November 1982 and evaluation by 10 overseas customers had been completed by April the following year. In June 1983, the original F404 engine was replaced by an F404-100 with an additional 1000lb (454kg) of thrust which offered a further increase in performance, including the ability to reach 40,000ft (12,200m) in only 2.3 minutes from a cold start. On August 26, 1983, the second prototype, 82-0063 (N3986B) made its first flight, featuring a completely redesigned nose to accommodate the full advanced avionics suite and the General Electric G-200 radar.
The canopy was also redesigned, featuring a bulged hood to give the pilot greater visibility. Known initially as the F-5G-1, this aircraft was followed by 82-0064 (N44671) with a similar full avionics suite and modified nose which first flew on May 12, 1984. A fourth prototype was built using the intended production tooling, incorporating larger fuel tanks, improved flap and slat actuators, General Electric AN/APG-67 multi mode air-to-air and air-to-ground radar and yet another 1000lb (454kg) of thrust from an upgraded engine. The airframe was redesigned to incorporate composite panels and structure, improving still further the power to weight ratio. The F-20A had arrived, as all the aircraft were redesignated as such in the light of the extensive modifications to the original design.
Northrop was in discussion with a number of potential customers when tragedy struck. Test pilot Darrell Cornell was killed on October 10, 1984, when demonstrating the first prototype, 82-0062, in South Korea. Less than five months later in May 1985, Northrop test pilot David Barnes was killed at Goose Bay in Labrador while en route to the Paris Air Show. He was practicing his display as part of a company demonstration and crashed under similar circumstances to Darrell Cornell. In both cases, no failure was found in the aircraft or its systems, but the crashes had an effect on the programme and delays were inevitable.
Despite its high performance, the USAF and the overseas customers increasingly favoured the F-16, so interest in the F-20 dwindled. Bahrain replaced its F-20A order with one for 10 F-5Es and two F-5Fs, and in 1986 the USAF announced it would be purchasing the F-16 for both the Air National Guard and the Aggressor squadrons, both roles envisaged for the F-20. Later that year, Northrop realised the hoped-for overseas market had been won over by the widely supported F-16 and the $1.2 billion F-20 was finally cancelled. The F-20 was a jewel of a high performance aircraft, completely privately funded by Northrop. Without doubt, its cancellation was a blow to the company, but it was already busy with the development of the B-2 Spirit stealth bomber.
The X-29
Two F-5A airframes, 63-8372 and 65-10573 were converted by Grumman to become the only X-29As, 82-0003 and 82-0049 respectively. These aircraft were extensively modified with canard foreplanes and 33º forward swept main wings and were intended to act as flying testbeds to investigate the aerodynamics of such a configuration. Since forward swept wings suffer from aerodynamic twisting forces which can be sufficiently strong to destroy them, only the extensive use of modern composite materials allowed the construction of a sufficiently strong wing to fully explore the performance.
The wing made the aircraft extremely unstable, and was only flyable by way of a triple redundant computerised flight control system. On December 14, 1984, Grumman’s chief test pilot, Chuck Sewell, took the first X-29A into the air at Edwards Air Force Base, four months after which NASA began an extensive test flying programme using both aircraft. The X-29s proved extremely reliable for such an advanced concept, reaching an unheard of angle of attack of 67º and becoming the first forward swept wing aircraft to fly supersonically, the latter achievement occurring on December 13, 1985.
The first aircraft was retired in 1986 after 242 successful flights, the second aircraft continuing to fly until 1991. They are both on display today, the first at the National Museum of the USAF at Wright Patterson Air Force Base in Ohio, the second at the Dryden Flight Research Centre at Edwards Air Force Base in California.
The SSBD
Without doubt the strangest looking of the F-5 developments, the Shaped Sonic Boom Demonstrator, or SSBD, was a modified F-5E with a bulged forward fuselage. The aircraft was to demonstrate that sonic booms created by aircraft could be substantially reduced by special shaping techniques and allow unrestricted supersonic flight over land. Northrop Grumman as the firm now was, Northrop having purchased Grumman in 1994, won the Defence Advanced Research Projects Agency (DARPA) competition to produce the test aircraft in July 2001 as part of the Quiet Supersonic Platform programme.
Northrop’s F-5 was considered ideal to modify to the required smoother equivalent area distribution as it was known, as it was a simple and reliable platform. A US Navy F-5E was restructured at Northrop Grumman’s St Augustine facility in Florida, using modified components constructed at the company’s El Segundo plant in California. The first part of the flight testing was aimed at establishing the flight envelope of the modified aircraft before the SSBD flights could begin. Making its first flight on July 24, 2003, from St Augustine and Cecil Field in Florida, the envelope was expanded with a T-38 in support to include supersonic flight at Mach 1.1, before the aircraft was flown to Palmdale in California.
With the support of a NASA F-18 from the Dryden Flight Research Centre, the flight envelope tests were completed and the SSBD flights began at Edwards Air Force Base throughout August 2003, several occurring on August 27. These were conducted over an area strewn with ground sensors under the aircraft’s flight path and to either side to record sonic boom and pressure wave generated by the aircraft. On three flights a standard F-5E from VFC-13 at NAS Fallon in Nevada flew 45 seconds apart from the SSBD to provide a comparison shock wave. Both aircraft were flying at 32,000ft and Mach 1.4 in the USAF High Altitude Supersonic Corridor. Two more flights were conducted with a NASA F-15B fitted with a pressure measurement probe to assess the shockwave near the SSBD.
These flights showed the SSBD did indeed have a reduced sonic boom, and a second series, known as the Shaped Sonic Boom Experiment or SSBE, was then conducted in January 2004, again over Edwards, by NASA. Altogether 21 flights recording the sonic boom pressures in varying flight conditions and turbulence were conducted, eight with the standard F-5E and four with the F-15B. In addition, a Blanik L-23 glider from the USAF Test Pilots School was fitted with several sensors and was flown 10,000ft below the SSBD on 13 of the flights. More than 1300 sensor recordings were made and 45 probe data sets were compiled, the aim being to allow future aircraft designers to control the sonic boom produced by their aircraft.
The F-5 SSBD is now on display at the Valiant Air Command Warbird Museum at Titusville in Florida, and will eventually be placed in the National Museum of Naval Aviation at Pensacola.
Tiger 2000/2001
The Republic of China Air Force (ROCAF) had purchased the F-5A and B as early as 1964 and by 1975 the Aerospace Industrial Development Corporation of Taiwan was producing the F-5E and F locally in their hundreds. The aircraft were replaced by the Mirage 2000, F-16 and the AIDC F-CK-1 Ching-kuo, also known as the Indigenous Defence Fighter (IDF) during the 1990s, but at least 40 were still in service in training roles into the 2000s, with between 90 and 100 maintained in reserve. AIDC began the Tiger 2000/2001 programme with a view to upgrading these second line aircraft, using the GD-53 radar from the F-CK-1 fighter. The GD-53 was a development of the General Electric AN/APG-67 multi mode radar intended for the F-20, which was itself originally a development to meet Taiwanese defence needs. A single prototype flew for the first time on July 24, 2002, but official disinterest ended the programme. The prototype remains on display at AIDC in Taiwan.
Azarakhsh, Saeqeh, Simorgh and Kowsar
With the Iranian Islamic Revolution of 1979, support for the many US aircraft purchased by the deposed Shah was no longer available. Since then, the Iranian Ministry of Defence, the Islamic Republic of Iran Air Force (IRIAF), its Owj Industrial Complex (OIC) and the Iran Aircraft Manufacturing Industrial Complex (HESA) have undertaken modification and upgrades using existing equipment as a basis.
The first of these, the Azarakhsh (Thunder) was first announced in April 1997 and is believed to be a Northrop F-5E Tiger II with a Russian developed radar which extended the nose by some 6.6in (17cm). This entered series production in June 1999, with between six and nine being built by 2001 against an order for 30. A second prototype flew in February 2001 with strengthened wings, wide use of composites throughout the airframe, uprated J85 engines and the ability to carry locally manufactured weapons. Little is known about the use of these aircraft, but two single seat and a two seat aircraft based on an F-5F took part in the National Army Day flypast at Tehran on April 17, 2008. An enlarged version of the F-5E, also known as the Azarakhsh, was planned, using Russian engines and radar, but was not built as far as can be ascertained. The next version was revealed in 2008, and was a major redesign having its wing mounted in the centre of the fuselage, above the air intakes, to increase its ability to carry underwing loads. The wing was also fitted with a leading edge extension to improve manoeuvrability.
The Saeqeh (Lightning or Thunderbolt) first flew on May 30, 2004, having begun development in 1998. Confusion over the prototypes of this aircraft comes from the use of the name Azarakhsh-2 in relation to the first prototype. The Saeqeh has fins, canted outboard, similar to the F-18, and two different versions are known to exist, one with similar intakes to the F-5, others with enlarged square intakes, possibly representing a second version of the type, first seen in August 2006. The intended production version of the aircraft is believed to feature an advanced radar and a full glass cockpit. Three of the fighter bombers took part in the National Army Day flypast at Tehran in both 2007 and 2008, but it is unknown how many have been built.
The Simorgh is another Iranian F-5 development, driven by the lack of two seat training aircraft. HESA is undertaking the conversion of the remaining Northrop F-5A Freedom Fighters in storage into two seat versions, similar to the F-5B.
The Kowsar is the last Iranian F-5 development. Hailed as a new indigenous fourth generation fighter, the Kowsar is a carbon copy of the American F-5F. According to state media, the Kowsar can be used for “short aerial support missions” and is equipped with systems that “promote precision targeting.”
Photo credit: U.S. Air Force, NASA / DFRC / Larry Sammons, NASA/Carla Thomas, Shahram Sharifi via Wikipedia
Artwork courtesy of AircraftProfilePrints.com
