Advanced Machines for Aerospace Manufacturing

The last time BAE Systems designed and flew a U.K.-funded combat aircraft demonstrator, things were different. The company was called British Aerospace, the aircraft was manned, and aerodynamic performance was king.

First flown in 1986, the Experimental Aircraft Program (EAP) demonstrator was the scion of a line of supersonic combat aircraft reaching back to the English Electric Lightning prototype in 1954 and including the BAC TSR2 in 1964 and multinational Panavia Tornado in 1974. EAP was the precursor to the four-nation Eurofighter Typhoon, which first flew in 1994.

The Taranis unmanned combat air vehicle (UCAV) demonstrator is a distinctly different beast. Where EAP was optimized around the supersonic agility possible with advanced aerodynamics and flight controls, Taranis is designed for low observability (LO) and radar cross-section (RCS) is king.

As France and the U.K. begin to jointly study the feasibility of a Future Combat Air System, Taranis is proof BAE still has the strength in aerodynamics to secure the U.K. a central role in collaborative development of a future unmanned combat aircraft, says chief aerodynamicist Chris Lee, giving the Royal Aeronautical Society’s Lanchester Lecture at Bristol University on July 22.

“EAP’s aerodynamics were developed by the U.K. and built on BAe’s flight control system capability,” he says. “[Typhoon] is a direct descendant of EAP. . . . It is easy to lose sight of the role the U.K. played in Eurofighter aerodynamics.”   

While Typhoon helped sustain BAE's supersonic aerodynamics capability, a new challenge emerged in the early 2000s when the U.K. began looking toward a stealthy unmanned combat aircraft. The result was two small U.K.-funded demonstrator UAVs, Raven and Corax in 2003-05, that gave engineers their first taste of designing for low observability. “The U.K. put in place plans to mitigate the risks and collect data. With the flights of Raven and Corax, a large-scale mission-representative demonstrator became feasible,” Lee says.

Up through Typhoon, aerodynamicists enjoyed almost unlimited control over external shape, Lee says. With the advent of stealth, “the radar range equation has come to dominate aircraft design,” he notes. “Low-observability requirements continue to be the dominant influence on aerodynamics.” 

LO design means a tailless aircraft (see photo) that is inherently unstable longitudinally and directionally, with non-linear aerodynamics and severely constrained effectors for stability and control, compromised air supply to the engine and aerodynamic effects from LO treatments. Much about Taranis is still classified, and Lee says only that BAE took an “innovative approach”   to addressing the stability and control characteristics caused by adverse aerodynamics. These include rapid non-linear changes in pitch and yaw with incidence that arise from initial flow breakdown over the stealthy shape.

A serpentine inlet and exhaust hide the Rolls-Royce Adour engine from radar at all lines of sight. Taranis “transgresses all good air intake design paradigms,” says Lee. “We let RCS tell us what shape it had to be and use aerodynamics to mitigate the result,” which includes unsteady, swirling, separated flow at the engine fan face. A full-scale inlet and engine were static-tested at Rolls. The UCAV's stealthy exhaust posed a further challenge. The high-aspect-ratio rectangular nozzle interacts with the wing control surfaces, Lee says. A dedicated afterbody wind tunnel model was tested to determine the throttle-dependent effects that had to be factored into the flight control laws.

“Aerodynamic performance was not a primary requirement for demonstration. Performance had to be adequate to demonstrate the mission,” Lee explains. Flight results were in good agreement with modeling, although drag was less than estimated. Initially Taranis flew with an air data probe, but for later flights this was replaced by a “novel” low-observable conformal air data system. “Results were almost indistinguishable from the boom-on flights,”  he notes.

Beyond Taranis, the challenge is how to turn what has been demonstrated into an operational military capability with demanding payload and range requirements and a wider envelope in terms of speed and maneuverability, Lee says. He calls for greater investment in sustaining the U.K.’s combat-aircraft aerodynamics capability, with closer collaboration among government, industry and academia.

Lee cites as an example the Flaviir program funded by BAE and government, managed by Cranfield University and involving nine other U.K. universities in developing technologies for a low-cost UAV with no conventional control surfaces. Supersonics is one area of potential collaboration with academia. “Typhoon was 25 years ago. There have to be better ways to do it,” he adds.