Aircraft of the future may not look significantly different from today’s aircraft, but a peek “under the hood” will reveal technologies that are vastly different. Commercial aviation giants such as Boeing and Airbus — in addition to NASA and academia — are developing breakthrough airframe, propulsion, materials, and cabin designs that will help aircraft of the future fly quieter, cleaner, and more fuel-efficiently, with enhanced passenger comfort.

NASA Visualizes the Next Passenger Aircraft

Figure 1. GE Aviation's 20-passenger concept aircraft. (NASA/GE Aviation)

An 18-month NASA research effort called the NASA Fundamental Aeronautics Program was launched in April 2010 (www.nasa.gov/topics/aeronautics/features/future_airplanes.html ) to visualize the passenger airplanes of the future. Four industry teams submitted designs for airplanes that may enter service 20 to 25 years from now. Just beneath the skin of these concepts lie breakthrough technologies, including ultramodern shape memory alloys, ceramic or fiber composites, carbon nanotube or fiber-optic cabling, self-healing skin, hybrid electric engines, folding wings, double fuselages, and virtual reality windows. The teams were led by General Electric, Massachusetts Institute of Technology (MIT), Northrop Grumman, and The Boeing Company.

“Standing next to the airplane, you may not be able to tell the difference, but the improvements will be revolutionary,” said Richard Wahls, project scientist for the Fundamental Aeronautics Program’s Subsonic Fixed Wing Project at NASA’s Langley Research Center in Hampton, VA.

The GE Aviation team conceptualized a 20-passenger aircraft (Figure 1) that could reduce congestion at major metropolitan hubs by using community airports for point-to-point travel. Features include an aircraft shape that smoothes the flow of air over all surfaces, and electricity-generating fuel cells to power advanced electrical systems. The aircraft’s advanced turboprop engines sport low-noise propellers and further mitigate noise by providing thrust sufficient for short takeoffs and quick climbs.

With its 180-passenger D8 “double bubble” configuration, the MIT team strays farthest from the familiar, fusing two aircraft bodies together lengthwise and mounting three turbofan jet engines on the tail. Important components of the MIT concept are the use of composite materials for lower weight and turbofan engines with an ultra-high-bypass ratio for more efficient thrust.

The Northrop Grumman team foresees the greatest need for a smaller 120-passenger aircraft that is tailored for shorter runways in order to help expand capacity and reduce delays. The team’s Silent Efficient Low Emissions Commercial Transport (SELECT) concept features ceramic composites, nanotechnology, and shape memory alloys in the airframe, and ultra-high-bypass-ratio propulsion system construction. The aircraft would use smaller airports, with runways as short as 5,000 feet, for a wider geographic distribution of air traffic.

The Boeing Company’s Subsonic Ultra Green Aircraft Research (SUGAR) team examined five concepts. The team’s preferred concept, the SUGAR Volt (Figure 2), is a twin-engine aircraft with hybrid propulsion technology, a tube-shaped body, and a truss-braced wing mounted to the top. Compared to the typical wing used today, the SUGAR Volt wing is longer from tip to tip, shorter from leading edge to trailing edge, and has less sweep. It also may include hinges to fold the wings while parked close together at airport gates.

MIT Flies the Eco-Friendly Skies

MIT’s green airplane designs — which were submitted to NASA’s above-mentioned study of future aircraft designs — are estimated to use 70 percent less fuel than current planes while also reducing noise and emission of nitrogen oxides (NOx).

Figure 2. Boeing's SUGAR Volt is a twin-engine aircraft with hybrid propulsion technology, a tube-shaped body, and a truss-braced wing mounted to the top. (NASA/The Boeing Co.)

The engineers conceived of the 180-passenger D “double bubble” series (Figure 3) by reconfiguring the tube-and-wing structure. Instead of using a single fuselage cylinder, they used two partial cylinders placed side-by-side to create a wider structure whose cross-section resembles two soap bubbles joined together. They also moved the engines from the usual wing-mounted locations to the rear of the fuselage. Unlike the engines on most transport aircraft that take in the high-speed, undisturbed airflow, the D-series engines take in slower-moving air that is present in the wake of the fuselage. Known as Boundary Layer Ingestion (BLI), this technique allows the engines to use less fuel for the same amount of thrust, although the design has several practical drawbacks, such as creating more engine stress.

The D Series travels about 10 percent slower than a 737. To further reduce the drag and amount of fuel that the plane burns, the D series features longer, skinnier wings and a smaller tail. (http://web.mit.edu/newsoffice/2010/nplus3-0517.html )

Boeing Drives New Horizons of Form and Function

Since the beginning of the jet age nearly 40 years ago, Boeing has succeeded in reducing the noise impact of airplanes. As Boeing continues development of the 787 Dreamliner, the plane’s noise improvement comes from a new generation of engines that have a very high bypass ratio, which allows more air to go through the engine. Boeing engineers also wrap the engines with special linings and other acoustic improvements. (www.newairplane.com/environment/ )

Early data show the 787’s noise footprint will be as much as 60% smaller than today’s comparable airplanes, thanks to a host of design improvements, including advanced acoustic linings, new engine inlets and nozzles, lightweight composite materials, and a new, more aerodynamic wing.

Boeing has chosen to increase the use of composites in the design of the 787. It is, in fact, 50 percent composite by weight. Carbon Sandwich is a class of composites made by attaching two thin skins to a lightweight, thick core, similar to a honeycomb. The core material is usually a low-strength material, but its thickness provides the sandwich composite with high bending stiffness. Carbon Laminate is composed of layers of carbon fiber impregnated with a polymer. These structures on the 787 are composed of strands of carbon formed into a tape infused with resin. The layers are laminated to create a desired thickness and shape, and are cured through heat and pressure.

Engine enhancements include a more electric architecture. Today’s planes use pneumatic systems powered by high-pressure air diverted from the engines. The system requires manifolds, valves, and ducts to power other systems in the aircraft. The design of the 787 eliminates the pneumatic system. The electric system extracts only the power needed during each phase of flight.

Airbus Develops a Concept for the Future

Figure 3. MIT's D

Airbus is looking more than 40 years in the future to anticipate what the future of aviation will look like. One key part of the company’s research and technology efforts is to investigate, test, validate, and optimize the most advanced technologies, design features, configurations, and architectures.

The A380 is the first commercial aircraft to incorporate as much as 25% composites. The carbon-fiber-reinforced plastic composite center wing box has saved up to 1.5 tons. Airbus is also focusing on low-noise nacelle designs, acoustic treatments, and low engine noise technologies, including the “zero-splice” inlet technology for engine nacelles to reduce fan noise. It also contributes to the quiet flight of the A380, which satisfies the noise requirements of international airports.

Airbus has developed an aircraft design that illustrates what air transport could look like in 2050. The Airbus Concept Plane (www.airbus.com/innovation/future-by-airbus/concept-planes ) features ultra-long and slim wings, semi-embedded engines, a U-shaped tail, and a lightweight, intelligent body. The result is lower fuel burn, lower emissions, less noise, and greater comfort.

Biofuels Could Power Future Aviation

NASA recently performed emissions testing on alternative, renewable fuels for a greener and less petroleum-dependent future. Renewable means that the fuel source isn’t some form of fossil fuel. The source could be algae, a plant such as jatropha, or even rendered animal fat. In late March and early April, a team at NASA’s Dryden Flight Research Center in California tested renewable biofuel made from chicken and beef fat in one of the four engines of a DC-8 airplane. (www.nasa.gov/topics/aeronautics/features/aafex_biofuels.html )

The experiment’s chief scientist, Bruce Anderson of NASA’s Langley Research Center in Virginia, said that in the engine that burned the biofuel, black carbon emissions were 90 percent less at idle and almost 60 percent less at takeoff thrust. Anderson added that the biofuel also produced much lower sulfate, organic aerosol, and hazardous emissions than the standard jet fuel.

Boeing is leading a process to gain approval for synthetic paraffinic kerosene (Bio-SPK) jet fuel, a drop-in biofuel that has an energy density equal to or greater than conventional jet fuel. The biofuel has to be able to function in very high and very low temperatures. Airbus is also investigating the use of alternative energy sources such as biofuels, hydrogen, and solar power.


NASA Tech Briefs Magazine

This article first appeared in the June, 2011 issue of NASA Tech Briefs Magazine.

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