The temperature at which a fuel/air mixture burns in a combustor is a key factor in fuel efficiency, said Jim Heidmann, who manages NASA’s Transformational Tools and Technologies Project. “Basically the hotter the combustion,” he said, “the more efficiently the engine is operating.” The glaring weakness of ceramics is that they are brittle – but CMCs, infused with fibers that provide greater ductility, show promise not only for use as combustor liners to reduce air cooling requirements, but also as working parts of the engine, such as turbofan blades. These new materials could dramatically increase the efficiency of jet engines by enabling combustion temperatures up to 30 percent higher.

 

Noise Reduction

Of all the factors constraining the growth of the aviation industry’s capacity, noise is among the most difficult to model and evaluate. While the FAA, in consultation with the Environmental Protection Agency (EPA), issues clear noise standards for airport vicinities and different types of aircraft, multiple variables determine the frequency, intensity, and duration of aircraft noise, including not only distance but also vertical variations in humidity, temperature, pressure, and wind conditions.

… the Aircraft Noise Prediction Program (ANOPP) is used by both the FAA and industry to better understand the potential benefits of low-noise aircraft design concepts.

NASA researchers attack the problem of aircraft noise both at its source – the engine and airframe of aircraft – and in the communities surrounding airports. The Glenn Research Center’s Aero-Acoustic Propulsion Laboratory, a world-class facility, provides, under an insulated 65-foot-high dome, three state-of-the-art acoustic test rigs for evaluating the noise generated by engine components.

Aeroacoustic investigators at NASA have also created technological tools that use computer models, flight measurements, and wind tunnel data to predict and simulate the flyover sounds of aircraft – a process known as auralization – while they’re still in the conceptual phase. One of the most widely used NASA-developed software tools, the Aircraft Noise Prediction Program (ANOPP), is used by both the FAA and industry to better understand the potential benefits of low-noise aircraft design concepts.

 

More Efficient Vertical Lift

The need for more sophisticated modeling tools, such as CFD codes, becomes apparent when you consider the complex pattern of turbulence, or field flow, created by a helicopter. Though a small sector of the aviation industry, rotary-wing aircraft fill key roles, such as emergency medical services, search and rescue, and transportation to difficult-to-access locations such as offshore oil platforms.

Susan Gorton, who manages NASA’s Revolutionary Vertical Lift Technology (RVLT) Project, pointed out that the agency’s work in rotary-wing technology dates to 1920, when the NACA published Technical Note No. 4, “The Problem of the Helicopter.” Despite the suitability of rotary-wing flight for certain applications today, said Gorton, several technical barriers continue to hinder the widespread use of helicopters and tilt-rotor aircraft. “Their cost per seat mile is pretty high,” she said, “and they don’t go all that far. So you’ve got to extend the range, make them bigger and make them more fuel-efficient, to really make them affordable to operators.” Vertical-lift aircraft are also loud, she noted, a key factor preventing their acceptance among communities.

The RVLT Project focuses on overcoming these and other barriers to enable the production of aircraft that may someday operate scheduled air service. A quiet, efficient, safe, and affordable rotary-wing craft, capable of taking off vertically and transporting 50 passengers over a distance of 300 miles, said Gorton, offers several distinct advantages over a small regional jet taking off from a runway. “We’re really looking at technologies … to enable these kinds of vehicles to operate efficiently, quietly, and safely,” she said, “and to expand their current capabilities and develop new kinds of commercial markets.”

According to Gorton, advances in CFD codes will prove crucial to developing these future generations of vertical-lift aircraft. High-fidelity data will help to model the complex airflows, noise, and vibrations generated by spinning rotors. “We have a fairly large investment in computational fluid dynamics for those kinds of things,” she said. “And it particularly impacts the acoustics.” Understanding the aerodynamics of rotor blades and how they generate noise will be an important precursor to developing new configurations. “It’s very important for us to be able to get that right,” said Gorton.

 

New Generations of Fixed-Wing Aircraft

The potential for deriving further efficiencies from the traditional tube-and-wing design of a fixed-wing aircraft is rapidly diminishing; aeronautical engineers have, for the most part, wrung about as much as they can from this configuration. In October 2008, in recognition of this circumstance, NASA established a set of efficiency, noise, and emissions goals targeting each of the next three generations of fixed-wing aircraft – and then put out a call to the industrial and academic communities to help determine how these goals might be met.

“We said: ‘This is the kind of level of performance that we think will be expected. What is the best way to get there?’” said Dryer. “In essence, we were asking the external community to make their best guess about the type of aircraft they thought would be operating in the future. And then from there, [we asked them to] help distill where they thought the technology ‘long poles’ were that would help us get these new types of capabilities.”