Vision for the Aircraft of the 21st Century

S. Venneri, M. Hirschbein, M. Dastoor, National Aeronautics and Space Administration

The airplane will soon be 100 years old. Over that period of time, it has evolved from the cloth and wood biplanes of the 1920s to the first all-metal single-wing aircraft of the 1930s, to the 100-passenger commercial transports of the 1950s, to the modern jet aircraft capable of reaching any point in the world in a single day. Nevertheless, the design of the modern airplane really has not changed much in the last fifty years. The grandfather of the Boeing 777 was the Boeing B-47 bomber designed in the late 1940s. It had a sleek, tubular aluminum fuselage, multiple engines slung under swept wings, a vertical tail, and horizontal stabilizers. Today, the fuselage is lighter and stronger, the wings more aerodynamic, and the engines much more efficient, but the design is a recognizable descendent of the earlier bomber.

The aircraft of the 21st century may look fundamentally different (Figure D.3). NASA is beginning to look to birds as an inspiration for the next generation of aircraft — not as a "blueprint," but as a biomimetic mode (Figure D.4). Birds have evolved over the ages to be totally at home in the air. Consider our national bird, the eagle. The eagle has fully integrated aerodynamic and propulsion systems. It can morph and rotate its wings in three dimensions and has the ability to control the air flow over its wings by moving the feathers on its wingtips. Its wings and body are integrated for exceptional strength and light weight. And the wings, body, and tail work in perfect harmony to control aerodynamic lift and thrust and balance it against the force of gravity. The eagle can instantly adapt to variable loads and can see forward and downward without parallax. It has learned to anticipate the sudden drag force on its claws as it skims the water to grab a fish and how to stall its flight at just the right moment to delicately settle into a nest on the side of a cliff. The eagle is made from self-sensing and self-healing materials. Its skin, muscle, and organs have a nervous system that detects fatigue, injury, or damage, and signals the brain. The eagle will instantly adapt to avoid further trauma, and tissues immediately begin to self-repair. The eagle is designed to survive.

Figure D.3. Towards advanced aerospace vehicles: "Nature's Way."
Figure D.4. Inspiration for the next generation of aircraft.

NASA is pursuing technology today that is intended to lead toward just such a biomimetically inspired aircraft (Figure D.5). Advanced materials will make them lighter and more efficient to build. Advanced engines will make them fast and efficient. The airframe, engine, and cockpit will be "smarter." For decades, aircraft builders have worked to build wings that are stronger and stiffer. However, the wing that is needed for take-off and landing is not the wing needed for cruising. During take-off and landing, the wing needs to be highly curved from leading edge to trailing edge to produce enough lift at low speed. But this also produces a lot of drag. Once airborne, the wing needs to be flat for minimal drag during cruise. To change the wing shape, NASA has employed leading-edge slats — an articulated "nose" that runs along the length of the wing — and multipiece flaps that can drop the trailing edge of the wing by 60 degrees. All of this requires gear, motors, and hydraulic pumps.

Figure D.5. NASA's dream of a future flight vehicle.

Imagine a bird-like wing of the future. It is not built from multiple, mechanically connected parts. It is made from new smart materials that have imbedded sensors and actuators — like nerves and sinew. The sensors measure the pressure over the entire surface of the wing and signal the actuators how to respond. But even the sensors are smart. Tiny computing elements detect how the aircraft responds to sensor signals. They eventually learn how to change the shape of the wing for optimal flying conditions. They also detect when there is damage to a wing and relay the extent and location to the pilot. And, like an injured bird, the wing adjusts its response to avoid further damage. This will not only be a very efficient and maneuverable airplane, but a very safe one.

Like the wings, the engines of this plane have integral health-management systems. Temperatures, pressures, and vibrations are all continuously monitored and analyzed. Unique performance characteristics are automatically developed for each engine, which then continually operates as efficiently as possible, and very safely. Long before a part fails, damage is detected and protective maintenance scheduled.

Inside the cockpit compartment, the pilot sees everything on a 3-D display that shows local weather, accentuates obstacles, all near-by aircraft, and the safest flight path. The on-board clear air turbulence sensor uses lasers to detect unsteady air well ahead of the aircraft to assure a smooth ride. When approaching a major airport, the lingering vortices that were shed from the wingtips of larger aircraft and that can upset a smaller one, can be easily avoided. This is a long-term vision, but emerging technology can make it real.

A key to achieving this vision is a fusion of nanoscale technology with biology and information technology (Figure D.6). An example is intelligent multifunctional material systems consisting of a number of layers, each used for a different purpose. The outer layer would be selected to be tough and durable to withstand the harsh space environment, with an embedded network of sensors, electrical carriers, and actuators to measure temperature, pressure, and radiation, and to trigger a response whenever needed. The network would be intelligent. It would automatically reconfigure itself to bypass damaged components and compensate for any loss of capability. The next layer could be an electrostrictive or piezoelectric membrane that works like muscle tissue with a network of nerves to stimulate the appropriate strands and provide power to them. The base layer might be made of biomolecular material that senses penetrations and tears and flows into any gaps. It would trigger a reaction in the damaged layers and initiate a self-healing process.

Carbon nanotube-based materials are an example of one emerging technology with the potential to help make this a reality. They are about a hundred times stronger than steel but one-sixth the weight of steel. They can have thermal conductivities seven times higher than the thermal conductivity of copper with 10,000 times greater electrical conductivity. Carbon nanotube materials may also have piezoelectrical properties suitable for very high-force activators. Preliminary NASA studies indicate that the dry weight of a large commercial transport could be reduced by about half compared to the best composite materials available today. The application of high-temperature nanoscale materials to aircraft engines may be equally dramatic. Through successful application of these advanced lightweight materials in combination with intelligent flow control and active cooling, thrust-to-weight ratio increases of up to 50 percent and fuel savings of 25 percent may be possible for conventional engines. Even greater improvement can be achieved by developing vehicle designs that fully exploit these materials. This could enable vehicles to smoothly change their aerodynamic shape without hinges or joints. Wings and fuselages could optimize their shape for their specific flight conditions (take-off, cruise, landing, transonic, and high-altitude).

In the long-term, the ability to create materials and structures that are biologically inspired provides a unique opportunity to produce new classes of self-assembling material systems without the need to machine or process materials. Some unique characteristics anticipated from biomimetics are hierarchical organization, adaptability, self healing/self-repair, and durability. In the very long term, comparable advances in electrical energy storage and generation technology, such as fuel cells, could completely change the manner in which we propel aircraft. Future aircraft might be powered entirely electrically. In one concept, thrust may be produced by a fan driven by highly efficient, compact electric motors powered by advanced hydrogen-oxygen fuel cells. However, several significant technological issues must still be resolved in order to use hydrogen as a fuel, such as efficient generation and storage of hydrogen fuel and an adequate infrastructure necessary for delivering the fuel to vehicles (Figure D.7).

Figure D.6. Revolutionary technology vision as applied to future aircraft.

Information Technology

Figure D.6. Revolutionary technology vision as applied to future aircraft.

Figure D.7. Attributes of a future flight vehicle.

None of this is expected to happen quickly. Over the next decade we will likely see rapid development of advanced multifunctional, nanotechnology-based structural materials, such as carbon nanotube composites. Integrated health monitoring systems — for airframe and engine — may be developed, and deformable wings with imbedded actuators may also be developed. The cockpit will likely begin to become more of an extension of the pilot with greater use of senses other than sight to provide "situational awareness" of the aircraft and its operating environment. In two to three decades, we may see the first "bio/nano/thinking/sensing" vehicles with significant use of nanotechnology-based materials, fully integrated exterior-interior flow control, and continuously deformable wings. By then, the aircraft may also have a distributed control/information system — like a nervous system — for health monitoring, some level of self-repair, and cockpits that create a full sensory, immersive environment for the pilot.

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