

Massachusetts Institute of Technology researchers recently released a concept aircraft design that could use 70 percent less fuel than current commercial airliners. The simple secret, they say: Optimize the aircraft for modern, efficient jet engines.

Jet engines these days burn much less fuel at slightly slower speeds than the original designs that influenced the swept-wing designs of the '50s and '60s. One of the main problems is that most airliners still use leftovers from those designs even though the older jet engines they were designed around haven't been used by most airlines for many years.

Today's modern jet engines are much bigger in size, and much more efficient and quieter, says professor Mark Drela, the lead designer on the MIT team.

"From a purely design viewpoint, they're really mismatched; the big engines want to go slower, which means the airplanes really want to have less wing sweep," Drela says. "That's one of the things we took advantage of."

The new design from MIT was presented to NASA as part of a research contract to improve the efficiency of commercial aircraft. NASA wants to see aircraft ideas that could dramatically reduce air and noise pollution and four teams from MIT, Boeing, GE Aviation and Northrop Grumman submitted designs. Despite the slightly slower flying speeds, the MIT group says the overall travel time on shorter trips would actually be less than current airliners, thanks to the innovative fuselage design that means less time waiting at the gate.

Drela says the team developed two aircraft designs for NASA. The first is aimed at the airliners the size of an Airbus A320 or Boeing 737, and a bigger aircraft that would be closer to the A340 or 777. The larger design is a similar design to the Boeing blended wing-body scale model aircraft being tested at NASA's Dryden Flight Research Center.

The smaller aircraft, simply called the "D" series, is a slightly more traditional design, though it uses a unique approach to solving the efficiency problem. Instead of having a single tube for a fuselage, the D Series uses a "double bubble" design that is essentially two partial cylinders side by side to create a wider, flatter fuselage. The wider fuselage is how the airplane can make up for lost speed during flight by decreasing the load and unload times on the ground. Of course, there would have to be a few changes made at the airports to accommodate the new design.

There are several other design aspects that add to the fuel efficiency, including mounting the engines at the rear of the fuselage to take advantage of slower-moving air. But one of the key design features behind the increased efficiency is the wings. The long, slender wings are much straighter than current airliners that still use the swept-wing design left over from those original jet airliners. At the time, airplanes like the iconic Boeing 707 were designed to fly at speeds where the early jet engines were most efficient.

"They really favored high speed, they liked to go fast," says Drela of the early jet engines. "The faster they go, the less fuel they burn to go a certain distance."

The 707 and just about every commercial jet made since the '50s uses the swept-back wing because it reduces drag at higher speeds. As an airplane approaches the speed of sound, or Mach 1, there can be small areas around the wings where some of the air is accelerated past Mach 1. These local areas of supersonic airflow dramatically increase the drag on the aircraft.

On an airplane with a swept-back wing, this drag is reduced because only a portion of the airflow travels perpendicular over the wing, lowering the effective airspeed, or Mach number over the wing. This reduction in the effective Mach number is most helpful at the higher speeds where the older jet engines were most efficient.

"The 707 look originated from the demands of the low-bypass engine," says Drela, referring to the types of engines used by airliners in the '60s and '70s. The original 707 flew for a few years with what is known as a turbojet engine that was also used on fighter jets of the time and was even less efficient.

On modern high-bypass engines we see at the airport today, those big fan blades in the front push most of the air past the engine to provide thrust. And since most of the air bypasses the engine, only a small amount is actually is being used for combustion inside the engine which means less fuel is used. These high-bypass engines are dramatically more efficient and quieter than the older turbojet and low-bypass engines that started on airplanes such as the 707 and Douglas DC-8.

"The new engines are actually much closer to propeller engines than the old jet engines, paradoxically," says Drela. "If you look at a propeller-driven airplane, the optimum wing sweep is zero."

So as some passengers lament over the fact that a modern airliner is 70 to 100 miles per hour slower than the original 707, the optimum design for fuel efficiency is to fly even a little bit slower, about another 50 miles per hour.

Drela says the fuel savings of 70 percent could would require some new technology for both the airframe and the engines, and admits the technology could be years away. But he says a simpler version of the D series that burns 50 percent less fuel than current airliners could be built with existing aluminum and jet engine technologies and be ready for service in the near-term future.

Top photo: MIT

Lower photo: K. Aainsqatsi/Wikipedia