NASA revives X-plane hopes with environmental goals

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NASA's aeronautics branch aims to ride a new wave of research on the commercial aviation sector's ambitious environmental goals to drive the agency back into the business of building and testing full-scale X-planes by the end of the decade.

A full-scale flight research aircraft - provisionally dubbed the "experimental vehicle testbed" (XVT) - is being promoted within the agency to succeed the NASA/Boeing X-48B/C, an unmanned, 8.5%-scale representation of a hybrid wing body airframe.

"While we don't have an X-plane beyond X-48 in the budget profile yet, we're building the case for why that would make sense," says Fay Collier, project manager for the environmentally responsible aircraft (ERA) project.

Collier's project is mid-way through a process that is designed to support a follow-on X-plane programme, but taking the next step is not guaranteed. Arguably the toughest barrier is the question of how to finance such a project.

NASA spends about $65 million annually on ERA, plus about $75 million a year on the subsonic fixed-wing programme's more ambitious - and futuristic - N+3 vision.

Even if NASA could extend ERA beyond its scheduled closure in 2014 or 2015, an X-plane design and flight-test programme would require significantly more funding.

Collier says NASA is exploring cost-sharing partnerships with academia and industry to support a possible X-plane programme. As the US Air Force considers options for replacing the Lockheed Martin C-130, military funding could also be an option as Collier notes that "multi-agency partnerships" are possible.

Although the funding question remains unresolved, the possibility of an environmentally driven X-plane programme would be a stark break from the recent past. NASA has not funded a full-scale, manned X-plane in more than a decade. Moreover, since the early 1980s, X-planes have been focused on evaluating new concepts in high-performance military aircraft, such as fighters, or spacecraft.

A notable exception is the Lockheed Martin X-55 all composite cargo aircraft, a project sponsored by the Air Force Research Laboratory to investigate a new method for mass-producing advanced materials for aircraft structures.

 
© NASA

But NASA's interest in the XVT concept (above) also speaks to the difficulty of reaching the environmental goals set by the commercial aviation industry with conventional aircraft. The so-called "tube-and-wing" approach to large, modern airliners is reaching its potential, with propulsion and systems improvements as the main drivers for possible efficiency improvements.

Two NASA projects - ERA and subsonic fixed wing (SFW) - are researching the feasibility of breaking from the tube-and-wing configuration for the next generation of large airliners.

The hybrid wing body approach pioneered by the flight tests of the X-48B and the X-48C have been the basis of NASA's plans for the XVT concept. However, Collier says, NASA has other configurations under review as well. But the hybrid wing body approach appears to have the "best chance of meeting the fuel and noise goals simultaneously", he adds.

The Cranfield Aerospace-built, 226kg (500lb) X-48B (below) has tested the low-speed handling of the hybrid wing body design.

 
© Boeing

The X-48C, meanwhile, will perform flight tests over the next two years to investigate a low-noise configuration, with additional shielding added around three aft-mounted JetCat turbine engines.

Meanwhile, NASA has reached out to industry and academia over the last four years to propose additional design concepts, even as the agency itself investigates more alternatives. The so-called N+2 and N+3 studies have generated research on technologies that can reach prototype stage by 2020 and 2025, respectively.

The N+2 studies have been performed within Collier's ERA project, while the more ambitious N+3 effort remains funded inside the SFW programme, says NASA programme manager Ruben Del Rosario.

The concepts "expand the intellectual input" as NASA also pursues its own approaches to meeting a challenging list of environmental goals, Rosario says.

As a hybrid wing body, the improved aerodynamics of the airframe design alone deliver the most improvement in fuel burn, according to NASA's analysis. Compared with a "777-200ER-like" tube-and-wing aircraft, a hybrid wing body generates greater lift. But NASA's N+2 concept seeks to introduce three other major technologies by 2020.

To achieve the N+2 goals, NASA will not only need to introduce a radically different airframe design than tube and wing, it must also shepherd through a new way of designing and producing composite structures.

Composite structures, both lighter and stronger than metal, have evolved significantly over the past 20 years. After being used mostly as flight-control surfaces and non-load-bearing structures, Boeing introduced the 787 with an all-composite fuselage skin.

But composite technology remains costly and difficult to manufacture. To cure resins strong enough to meet US Federal Aviation Administration certification requirements, large autoclaves are necessary to bake the carbonfibres into the plastic matrix. The composite skins also do not save any time in final assembly, with thousands of fasteners required to attach the skins to supporting structure.

To solve these issues, NASA is proposing an emerging concept called pultruded rod stitched efficient unitised structure (PRSEUS). It is one of the key efforts in the N+2 project.

Instead of laying up composite material on mandrils, the basic shape of PRSEUS structure is stitched like fabric, Collier says. The stitched fabric matrix is then infused with resin using pressure applied by a vacuum bag. The stitching process promises potential benefits, such as eliminating autoclave curing, reducing fabrication time and improving damage tolerance.

Meanwhile, a pultruded rod attached to the skin adds a stiffening element to the structure, reducing the need for potentially thousands of fasteners over an entire aircraft. Collier says the PRSEUS concept can reduce structural weight on an aircraft by 10%.

In the 1990s, NASA and McDonnell Douglas experimented with a two-sided advanced machine. PRSEUS, however, is based on a one-sided stitching machine discovered in East Germany after the Berlin Wall fell, he adds.

"I believe it might be one of those very significant finds," Collier says. "I don't know how it was discovered."

The PRSEUS machine is now installed inside Boeing Research and Technology's Marvin Dow Stitched Composites Development Center in Huntington Beach, California, replacing the AST programme's two-sided machine.

Boeing has already put the PRSEUS technology to use. To solve a problem with a cargo door on the C-17, Boeing replaced the metallic structure with a door fabricated by the PRSEUS stitching machine, with the resin cured in low-temperature vacuum bags. "That was sort of a proof of concept," Collier says.

The next step is to expand the application of PRSEUS-based technology to a major load-bearing structure. In 2012, NASA plans to test a 9.14m (30ft)-wide by 3.7m-tall section that would represent a mid-fuselage region of a hybrid wing body aircraft. If the demonstration is successful, NASA could use PRSEUS structure in an X-plane.

Taking the next leap in subsonic aircraft efficiency will not be easy. Part of NASA's vision for a next-generation airliner that meets the N+2 environmental goals requires applying laminar flow control techniques, an elusive but highly promising area of aerodynamics.

Collier acknowledges the challenge is significant. "For aerodynamicists this is the 'Holy Grail'," he says. But emerging methods of laminar flow control offer the "best shot at reducing the drag significantly compared to a turbulent designed airplane".

There are various approaches to ensuring air flows smoothly across the high-drag areas of an aircraft, such as the nacelles, and eliminating pockets of turbulent flows, such as vortices that tumble span-wise across the wing.

A natural control technique is to simply slow down the aircraft to Mach 0.7 and straighten the wing, Collier says. New tube-and-wing designs employing open rotor propulsion systems can exploit this technique.

A hybrid wing body envisaged by the N+2 studies, however, does not have that option, yet still needs to find new ways to reduce drag to meet efficiency targets.

NASA is experimenting with two techniques. One is the hybrid laminar flow control technique - pioneered in previous decades by the Northrop X-21 and a NASA-owned Boeing 757. It uses suction systems in the wing leading edge to reduce turbulent flow.

More ambitious is a demonstration of a relatively new technique in which an appliqué covered with "discrete roughness elements" is applied to a wing skin. "Think of it as a dot that might stick on the surface," Collier says.

A flight test planned at NASA's Dryden Research Center in 2012 or 2013 on a Gulfstream GIII will seek to prove that such an approach works on large subsonic aircraft with higher Reynolds numbers, Collier says.

"We're trying to understand the barriers to application," Collier says. After those issues are understood, NASA will "try to put plans in place and execute work plans that eliminate the barriers", he adds.

The next step-change in fuel-burn reductions will not come only by improving the efficiency of the aerodynamics and structures. Increasing the efficiency of the jet engine is still critical for achieving NASA's goals.

Modern high-bypass turbofans have steadily improved since they were introduced in the late 1960s. But NASA's ERA project is helping manufacturers to fund the introduction of ultra-high bypass engines over the next decade. The ultra-high bypass engine requires improvements to its combustor and propulsor components. Within the combustor, the challenge is to increase overall pressure ratios while dramatically reducing the "hot spots" that generate nitrous oxide emissions, particularly in the landing and take-off stages of flight.

NASA is jointly investing with industry to improve on current methods, such as ceramic matrix composite liners and multipoint fuel injection. Improvements to the propulsor, however, have generated the most public interest, with the launch of the Pratt & Whitney PW1000G geared turbofan and revival of General Electric's open rotor concept. The first generation of the PW100G introduces a gear that slows the fan stage, allowing both the fan and the turbine stages to spin at their most efficient speeds. A second generation of the PW1000G technology is being developed under the ERA programme, with a contract awarded under the US FAA continuous low energy, emissions and noise (CLEEN) contract.

Collier notes that the second-generation improvements do not include significant changes to the gear itself. The P&W engine has already been launched with the Bombardier CSeries, Irkut MS-21 and Mitsubishi MRJ.

The CLEEN goal is to prepare P&W's technology for integration on larger, narrowbody aircraft, he says. The challenge is to overcome the impact of adding the gear, which increases engine diameter. "There's installation impacts as the engine gets a little larger in diameter," Collier says. "Propulsion-airframe integration is one of the things we're working on."

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