Planes under investigation by Nasa range from the extreme to the slightly more conventional. For example, Boeing’s Sugar Volt is a design that came about as part of the manufacturers Subsonic Ultra Green Aircraft Research (Sugar) project. The design – like many new concepts - is based around the idea of maximizing the plane’s lift. This reduces the amount of power needed to keep the plane in the air, as well as the amount of fuel it must burn.
The Sugar Volt does this by using very long, narrow, flexible wings. They are so long that engineers needed to brace them with under-wing truss support struts, making the aircraft resemble the Piper Cub and other light, high-winged planes. The SugarVolt’s wings are extended in order to increase lift, allowing shorter take-off distances and requiring less power in flight. They are so long, in fact, that the Boeing designers may have to fit them with hinges so that they could fit in existing airports and boarding gates.
There are, however, are other ways to improve performance than just making longer wings. A team from MIT in Cambridge, Massachusetts, for example, put forward the D8 for consideration by Nasa. This “double-bubble” aircraft design, features a double-wide fuselage composed of two standard body cylinders melded together side-by-side, as well as low-swept wings that cut drag and weight. The idea of the wider body shape is to increase lift generated by the fuselage, rather than it being mostly dead weight slung between two wings. The extra lift and reduced drag cuts back on the quantity of fuel that the engines must burn. If the jet were built today from standard aluminum alloys it could provide a 50% reduction in fuel use, according to the MIT designers; a low-mass polymer-composite version could give 70% efficiency gains. In addition, because the D8’s turbine engines sit on top of the fuselage in a box-shaped tail, they would cut the amount of engine noise broadcast to the ground.
The D8’s idea for generating greater lift is taken to an extreme in another design called the N3-X hybrid wing-body airplane, which Nasa developed in-house. At first glance, the N3-X looks a lot like a so-called flying wing design, used by planes such as the US Air Force’s B-2 stealth bomber. These comprise a single, thick triangular wing that enclose all of the plane’s contents - cockpit, stores, engines, fuel tanks and flight surfaces. But, unlike the B-2 flying wing, the N3-X hybrid wing-body also features two thin, rather conventional wings attached to the sides of its ultra-wide fuselage.
The primary advantage of the hybrid, or blended, wing-body design is better fuel efficiency, Del Rosario says. Like a flying wing, the hybrid aircraft produces lift with its entire aerodynamic airframe, thus ridding itself of the drag associated with the cylindrical fuselage and the tail surfaces of a conventional plane. As with the D8, the more lift that can be produced overall, the less effort is needed from the engines, which in turn means less fuel must be burned. Fuel efficiency could be raised further by building the airframe from lightweight polymer composite materials instead of metals, Del Rosario says.
Engineers are aware, however, that new airframe shapes will only get them part of the way to their goals. To really make a difference, particularly to fuel consumption and engine noise, planes will also need radically new propulsion systems mounted or integrated into the airframe in novel ways. And, like car designers, aircraft manufacturers makers are beginning to explore the possibilities of electric and hybrid engines.