Supercritical Wing Design Cuts Billions in Fuel Costs

Langley Research Center aeronautics engineer Richard T. Whitcomb was 34 when he did something no other single person could do. Whitcomb overcame the aviation challenge of the day — the so-called sound barrier. However, he was still working to improve flight efficiency at speeds approaching that barrier, now with a seemingly counterintuitive wing design, almost the inverse of what were then conventional wings. He called it the “supercritical” airfoil.

The supercritical wing was one of three major contributions Richard Whitcomb made to aeronautics.

Now known as the “area rule,” this is the idea that a fuselage that tapers where the wings are attached can pass the speed of sound more easily than the traditional bullet shape. In the early 1960s, after several years of work designing a Mach 2-rated jet, he became frustrated and returned to the more familiar field of transonics — speeds at or around the speed of sound.

As an object moves through air, it collides with the air molecules, creating a disturbance that propagates away from the object by means of weak pressure waves; essentially sound waves. As the object moves faster, approaching the speed of sound, these disturbances that travel at the speed of sound cannot work their way forward, and instead coalesce to form a shock wave. That shock wave tends to stand on the aircraft’s wing, creating drag as the air has to flow over it. This is the sound barrier that aeronautical engineers had struggled to breach. Flying near the speed of sound remained highly inefficient because of the drag caused by these standing shock waves.

Whitcomb’s first insight into a possible supercritical wing design came when observing an airfoil meant for a verticaltakeoff jet. Air passages between the wing and its flaps appeared to delay the formation of the troublesome shock wave, but Whitcomb decided this slotted design ultimately wouldn’t work. With this curtailed shock wave in mind, he returned to the wind tunnel. He used auto body putty to add bulk to some areas while filing away others, testing and re-testing his models in Langley’s high-speed wind tunnel.

NASA-developed supercritical wings save the airline industry billions of dollars’ worth of fuel every year, which also means significant reductions to greenhouse gas emissions.

The initial design for a supercritical wing was produced in 1964, and Whitcomb and his colleagues spent the next five years working through different models and concepts. What they ended up with almost looked upsidedown compared with standard wings of the day, because it was nearly flat on top and rounded on the bottom. It was also thicker than the norm, especially on its blunt leading edge. Most commercial companies decided that, rather than use the new wing design to achieve transonic cruising speeds, they would use it to save fuel while continuing to cruise at around Mach 0.8. The Boeing 787, for example, was originally planned to cruise at Mach 0.9, but the company decided to drop that to Mach .85 and take a 20 percent fuel savings over its other two dominant twinengine models. It had turned out that the wings were more efficient at subsonic speeds as well.

Because a thicker wing forms a sturdier attachment to the fuselage, it requires less reinforcing structure. The resulting weight savings allows more weight to be spent on either widening wingspan or reducing wing sweep, and a wider span brings greater fuel efficiency. Today, supercritical wings are used in commercial, business, and military aircraft all over the world, including all Boeing commercial and military transports.

In the 1970s, Whitcomb was inspired by an article on birds to refine what he called “winglets” — the small, vertical wings now seen on the ends of nearly all airliner wings. This innovation saves airlines 4 to 6 percent in fuel, with comparable reductions in emissions.

Visit http://spinoff.nasa.gov/Spinoff2015/t_2.html .