Honda Flies A Fuel-Efficient Jet Prototype
A unique shape made possible by composites helps cut fuel consumption while allowing faster speeds.
Honda has moved closer to bringing its first jet to market—one that uses 20 percent less fuel than similar-sized planes while also flying faster. A prototype of Honda’s light jet, which will seat five to six passengers and is scheduled to go on sale next year, made its first flight last month.
The airplane makes extensive use of composite materials—a combination of carbon fiber and resins that reduces a plane’s weight. So far the materials are rare in business jets, though they’ve become common in small, home-built kit planes. They are also beginning to see more use by big jet makers such as Airbus and Boeing, which are seeking ways to reduce fuel consumption.
The composites allow Honda not only to decrease the weight of its plane but also to give it a unique shape that reduces drag. The novel design of the plane also involves mounting the engines on the top of the wing, rather than underneath it or on the fuselage. This helps decrease drag at high speeds, says Michimasa Fujino, the president and CEO of Honda Aircraft Company, a subsidiary of Honda Motor Company.
The shape of the fuselage and wings allows air to move more smoothly over the skin of the plane. This smooth flow of air is called natural laminar flow, and it’s usually limited to small parts of the surface of a business jet. The air over the rest of the surface is turbulent, creating drag. Honda sought to extend how far the laminar flow extends along the fuselage and the wing. The shape of its plane features subtle bulges on the nose of the plane and on the wings that create “a very complex pressure distribution,” Fujino says. As air moves over these bulges, it first accelerates, then decelerates, then accelerates again, he says, creating areas of high and low pressure. The changes in pressure essentially “suck the laminar flow toward the end of the wing,” he says.
Composites are key for achieving laminar flow, says Mark Drela, professor of aeronautics and astronautics at MIT, because they allow for a smoother, more even surface than is possible with riveted sheets of aluminum. And Fujino points out that they’re important for creating the precise shapes needed for the design.
Fixing the engines to the top of the wing also helps reduce drag, particularly at high speeds. As planes approach the speed of sound, air moving over some parts of the wing reaches supersonic speeds, causing a big increase in drag (a phenomenon known as wave drag). The placement of the engines helps slow down the flow of air in this area of the wing, which keeps the wave drag from kicking in and let allows the plane to fly faster. Avoiding wave drag is also essential for letting the plane fly faster than other airplanes while using less fuel. The plane flies at 420 knots, or about 780 kilometers per hour—about 80 kilometers per hour faster than other planes its size.
Together with GE, Honda developed a new engine for the plane that further increases its efficiency. The ratio of air compressed in the front of the engine to air compressed in the interior turbines is unusually high (this is similar to increasing the compression ratio in car engines to improve efficiency). Fujino estimates that about half the fuel savings come from extending natural laminar flow, and most of the rest from the new engine and the placement of the engines over the wing.
The plane is about 20 decibels quieter than other jets its size, in part because the wings block the engine noise from reaching the ground. Because the plane is quieter, it could be allowed to operate at more airports in congested areas such as Los Angeles.
Fujima says that the basic design principles for this plane can be used for larger planes, although there is a limit to how large planes can be and still achieve laminar flow. Airflow is turbulent over the whole surface of large commercial jets, Drela says. Honda isn’t disclosing its plans for future, larger jets.