(Three F-35 Lightning II static test articles. Six ground test airframes were built in total)
Before entering full scale production, an aircraft design must undergo a series of exhaustive tests, taking it from the drawing board to wind tunnel models and flying prototypes (and much more in between). If the prototypes are successful, flight testing may continue to a small batch of early production airframes, on which any bugs are hopefully ironed out, and other technologies proven. But another part of this intricate jigsaw lies with a number of unsung heroes that rarely get the recognition they deserve.
These early test airframes often disappear into the shadows once projects end or when full scale production ramps up. They are static test articles, airframes that are built to the same specifications (though often incomplete and not intended to fly) as flying examples in a bid to simulate the conditions and stresses that a plane will encounter during its lifetime. To illustrate, this article examines three F-35 Lightning II static test articles built by Lockheed Martin as part of the Joint Strike Fighter (JSF) programme – see below.)
Static test articles, which may be used for fatigue/durability testing, weapons firing and more, form a crucial part of aircraft development and may remain in use after the type has entered operational service. We’ve featured a number on this blog before, including the giant Airbus A380 and A350 “PAMELA” rigs, an F-117 Stealth Fighter (known as Airframe 777) that became a radar cross section model at Holloman AFB before disappearing (presumably into storage or scrapped), and an unusual aircraft boneyard at Paine Field, Washington, where the hulks of Boeing fatigue articles slowly rust away.
Occasionally, static test articles are refurbished to flight status. Such was the case with NASA’s ill-fated Space Shuttle Challenger, which began life as a structural test rig known as STA-099. Others are brought up to museum standard, like the B-2 Spirit Stealth Bomber static test article designated AT-1000, which is preserved at the National Museum of the USAF at Dayton, Ohio.
Often erroneously described as a “mockup”, AT-1000 is one of two Northrop B-2 structural test airframes built without engines or instruments. It was tested in a giant rig that enabled engineers to push the airframe to the limits without ever leaving the ground, subjecting AT-1000 to forces far greater than a B-2 would ever encounter in flight. After the structure fractured, it was donated to the museum, where staff spent more than a year restoring the broken Spirit.
Lockheed Martin F-35 Lightning II Static Test Articles
(Image: Vought Aircraft Industries via F-16.net; F-35C static test article CG-1)
The F-35 Lightning II (Joint Strike Fighter) is one of the most ambitious and expensive defence procurement programmes in history, and involves three different versions of the fifth generation stealth jet: F-35A, the conventional takeoff and landing version; F-35B, the short take-off and vertical landing (STOVL) version; and the carrier-based F-35C.
A total of six non-flyable ground test versions of the F-35 were built by Lockheed Martin, including three static test articles and three durability test airframes (one of each per variant). This feature examines the three F-35 static test articles, known as AG-1, BG-1 and CG-1. Their durability counterparts (AJ-1, BJ-1 and CJ-1) are not outlined, but more information can be found here (pdf).
F-35A Static Test Article ‘AG-1’
(Image: Lockheed Martin via F-16.net)
In April 2009, after a three week ocean voyage, F-35A static test article ‘AG-1′ arrived at BAE Systems’ Structural and Dynamic Test facility at Brough, East Yorkshire. AG-1 was one of six ground test versions of the Lightning II, which entered trials alongside 13 dedicated flight test development aircraft. Built in 2008 to demonstrate the strength and stability of the F-35A airframe, AG-1’s structure was verified up to 150 per cent of its design limits or 13.5 times the force of gravity, according to a press release issued by Lockheed Martin. The airframe was then returned to the USA.
(Image: Lockheed Martin via The North Spin)
Above, the F-35A static test article is pictured in a protective bag during transportation from Lockheed Martin’s plant at Fort Worth, Texas to the BAE Systems facility at Brough, UK.
All three variants of the F-35 static and durability test articles were expected to complete three full lifetimes of virtual flight testing during their development programmes. At that point, like other aircraft projects before them, the well-worn ground-based specimens will likely retreat into the murky annals of test aircraft history. Perhaps one of them, like the B-2 bomber (mentioned above), will get a cosmetic restoration and find its way into a museum. But with so many other F-35 development aircraft built for flight testing, now to mention the programme’s early technology demonstrators, that option seems unlikely.
F-35B Static Test Article ‘BG-1’
(Image: Lockheed Martin via F-16.net)
Set to replace the RAF/Fleet Air Arm’s and US Marine Corps’ iconic Harrier ‘jump jets’, the F-35B is the short take-off and vertical landing (STOVL) variant of the Lightning II Joint Strike Fighter family. Despite being at the cutting edge of aircraft design and development, the F-35B static test article pictured above, known as BG-1, has clearly seen better days. BG-1 was photographed in September 2014 following four months of live fire testing on the vast ranges at NAWS China Lake.
According to F-16.net: “The live fire testing… consisted of a series of fifteen shots directed at various positions on the airframe. The effort was led by Naval Air Systems Command’s Weapons Survivability Laboratory.” The open doors on BG-1’s spine are for the patented lift fan, a system unique to the F-35B variant which, along with deflected thrust from the engine exhaust, enables the aircraft to hover and achieve short take-offs and vertical landings. BG-1 is thought to have been built in 2008, meaning these live fire tests may have been the ground test article’s last assignment. BG-1 isn’t the only F-35 condemned to such a role. After around 100 flights, the first F-35A development aircraft (2AA-0001) was retired to live fire testing also.
F-35C Static Test Article ‘CG-1’
In 2010, Vought Aircraft Industries undertook a series of drop tests at their plant in Grand Prairie, Texas, using the F-35C static test article known as CG-1. The F-35C will form the backbone of the US Navy’s strike fighter force, and will also serve with the US Marine Corps. The trials were conducted in order to simulate aircraft carrier landings and test the structural integrity of the F-35C airframe. Carrier landings place a great deal of stress on aircraft, far beyond that experienced by land-based jets operating from conventional runways.
The above video shows CG-1 being dropped from more than 11 feet in one of 53 tests planned for the ground test article. A Lockheed press release stated: “The drop conditions included sink rates, or rates of descent, up to the maximum design value of 26.4 feet per second, as well as various angles and weight distributions. The tests were used to mimic the wide range of landing conditions expected in the fleet.”