When the US Air Force's Lockheed Martin F-22 flies over Farnborough this year to make a second consecutive appearance at the show, spectators will see a potent aerial display of one of the world's great symbols of aerospace power.

What they will not see are the systems beneath the skin that enable the Raptor's manoeuvres by harnessing a different kind of power. A 270V DC current backed by 157kW of primary and auxiliary power supplies the electric and hydraulic force to move the flight controls. The crowd simply sees the F-22 roll into a "split s" or snap the nose upwards to perform Pugachev's "cobra" stunt.

As advanced as the F-22's miniature on-board power systems may be, they define the technology of an era swiftly passing into history.

USAF Raptor pair
  © US Air Force

Lockheed's F-35 Joint Strike Fighter, the F-22's younger stablemate, already has introduced the "more electric aircraft" concept into the US military's combat fleet, adopting key technologies pioneered by commercial airliners and repackaged for a tactical combat aircraft.

It may seem premature to begin work on the next wave of power and thermal management improvements. The F-35 has been flying for less than four years - two of the three variants still have six years of flight tests remaining - but the Air Force Research Laboratory (AFRL) is already working to define the key technologies to power a "sixth-generation" combat aircraft.

ELECTRIC AIRCRAFT

If the F-35 is the prototypical "more electric aircraft", the AFRL now wants to refine the concept by supporting concepts for an "energy-optimised aircraft".

The AFRL programme is called integrated vehicle and energy technology, or Invent. Phase 1 contracts were awarded two years ago to Northrop Grumman as prime, with Lockheed and Boeing as subcontractors.

Results of the Phase 1 simulations showed new technology can boost overall aircraft performance by up to 15%, says Steve Iven, AFRL's Invent programme manager.

In Phase 2, the AFRL wants bidders to develop a ground demonstrator by 2012 to validate the systems architecture for the energy-optimised aircraft.

While Phase 1 activity was led by the major aerospace primes, Phase 2 is likely to be championed by the companies directly involved with power subsystems, such as engine manufacturers or electrical system suppliers, Iven says. "We want to make sure the leadership is in the right place," he adds.

Invent is one of the main components of a broader AFRL strategy. In addition to optimising the energy systems, it is also working on two next-generation aircraft technology programmmes.

USAF Raptor Trio
 © Staff Sgt Taylor Worley/US Air Force

The adaptive versatile engine technology (Advent) programme would introduce a third stream of bypass airflow to achieve a dramatic increase in fuel efficiency. The highly efficient embedded engine technology (Heete) programme, meanwhile, seeks to deliver the next leap in high-pressure compressor turbines. By combining both improvements into a so-called Adheete engine, the AFRL intends to boost fuel efficiency of a next-generation combat aircraft by up to 30%.

Air force researchers are also working on a separate effort to develop new laminar flow techniques that could reduce airframe drag by up to 10%.

Bringing all the new technologies, including Invent, together into a next-generation system, the combined system could improve endurance or performance by 50% or more, Iven says.

The AFRL's goal is to make these technologies ready for insertion into the generation of aircraft that enters operational service after 2030. To achieve that target, each new leap in technology must be ready to enter a demonstration and validation phase on a flying aircraft before 2025, Iven says.

The timing of the AFRL's development strategy corresponds loosely to a long-term USAF plan to develop replacements for the current generation of strategic and tactical aircraft, including the Northrop Grumman B-2 and Lockheed F-22 and F-35.

IMPROVEMENTS

Investing in the technology also may offer some near-term improvements. One factor in the AFRL's evaluation for the Phase 2 Invent contract are proposals for near-term "off-ramps" for the technology, meaning existing programmes that could benefit from the technology.

Key improvements could be ready by 2020 for a block insertion upgrade, Iven says, although he declines to name specific programmes. Invent programme documents say the technology could be applied to a wide range of aircraft, including fighters, transports and intelligence, surveillance and reconnaissance aircraft.

Last October, a senior Rolls-Royce executive told Flight International that Advent was being secretly targeted for insertion as a block upgrade for F-35 engines. Although the US Department of Defense wants to cancel Advent contractors General Electric and Rolls-Royce's F136 engine, the government would own the design rights for the third-stream bypass duct, allowing Advent improvements to be inserted in the Pratt & Whitney F135 as well.

Iven acknowledges that an existing airframe would not receive the maximum benefit from the new technologies without great expense. Converting the F-35 thermal management system from air-cooled to vapour-cycle cooling, for example, would require extensive modifications, he says.

"It depends," he adds, "on how much you're willing to rip up."

The electrical demands of fifth-generation fighters have clearly demonstrated the importance of improved thermal management technology. According to Iven, systems on board the F-22 and F-35 generation of fighters generate three to five times more heat than previous aircraft types.

Stealth aircraft design, by its nature, complicates the problem of dissipating the higher heat loads. Simply venting the heat offboard usually is not an option if the goal is to minimise the aircraft's thermal signature. Instead, the heat is dumped into the aircraft's fuel system, which can have the bonus effect of creating hotter fuel that burns more efficiently.

On the other hand, using fuel as internal heat sinks within the aircraft has disadvantages as well. If low on fuel and flying at low altitude, there is less material to absorb the heat and ambient air provides no relief. Before take-off, temperatures can rise rapidly inside the aircraft before fuel is able to circulate throughout the system.

The latter problem caused the USAF to impose strict time limits on ground operations for F-22s. Lockheed claims the F-35 has so far avoided such measures, but has faced its own challenges with thermal management.

Beyond simply improving ways to absorb excess heat, the AFRL's goal for Invent is to make the basic architecture of the more electric aircraft generation function more efficiently.

JSF simulator cockpit
 © US Air Force

Nowhere is this more important on the aircraft than the systems that move aerodynamic control surfaces. Since the early 1980s, most new aircraft have used a combination of fly-by-wire and high-pressure hydraulic systems to translate flight-control inputs by the pilot into movements by the ailerons, flaps, rudders and other surfaces.

More recently, more electric aircraft, such as the Airbus A380, Boeing 787 and F-35, transformed the "fly-by-wire" architecture to a "power-by-wire" concept, which introduced electro-hydrostatic actuators. The new technology requires a flight-control system that demands short periods of peak electric power. Although the demand for such power loads is sporadic, the electric system is designed to provide sufficient power constantly in case there is a demand.

That is one of the key problems that Iven's concept for an energy-optimised aircraft is hoping to solve. Rather than continuously supply maximum electronic power to the flight-control actuators, one of the goals of Invent is to design a system that delivers peak power only on demand.

HYDRAULIC SUCCESS

On-demand power supply actually remains one of the advantages of flight-control systems actuated by hydraulics, which are otherwise less appealing due to their extra weight and maintenance cost. Hydraulic systems often include a device known as an accumulator, which stores up unused power capacity and delivers it at moments of peak demand.

No such device yet exists for flight controls actuated by electro-hydrostatic systems, but that could change. Iven himself is listed as a co-inventor with a Lockheed engineer on a patent for an electrical accumulator, and he believes the concept could be a key feature in an architecture for an energy-optimised aircraft.

Source: Flight International

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