A groovy solution for engineers who find turbulence a real drag
San Francisco — Jerry Hefner would like to ''pin stripe'' America's fleet of jetliners. No, he's not a preppie Wall Streeter with delusions of grandeur: Dr. Hefner is an aerodynamicist with the National Aeronautics and Space Administration. He works at NASA's Langley Research Center on a program aimed at improving the fuel efficiency of aircraft. And one of the discoveries his group has made is that etching the skin of an airplane fuselage with a parallel pattern of barely visible grooves may allow it to slip through the air more easily.
This work has caught the eye of the air transport industry because of the potential for significant fuel savings, as much as $200 million per year for the commercial airline fleet, Mr. Hefner estimates. (Currently, about half the direct cost of flying a commercial airliner is the cost of fuel.) Also, this approach to ''turbulent drag reduction,'' as it is called, could be employed on existing, as well as new, aircraft, unlike most other methods for increasing aircraft fuel efficiency now being pursued.
''It will be two or three years yet before this is ready for application,'' says Hefner. ''So far all the work has been done on small-scale wind-tunnel models. But this summer we will be testing it on one of our Lear jets.''
Research on the concept began in the mid-1970s after the Arab oil embargo. Langley aerodynamicists decided to concentrate their efforts on what is called ''skin friction.'' This is the energy consumed by turbulence created at the surface of an object traveling through the air. It accounts for about half an aircraft's total drag, or resistance to movement.
Searching the literature, they found some work done on air flow in triangular-shaped air ducts which suggested that such a shape could reduce skin friction. ''We thought: 'Why can't we do a similar thing on an open surface like the skin of an airplane,' '' Hefner recalls.
So a fellow researcher, Michael Walsh, began looking at various designs - 69 of them, all told. The geometry that looked most promising was a series of abutting triangular grooves a few thousandths of an inch wide and deep, which form rows of tiny ''riblets.''
In wind-tunnel tests, surfaces etched with an almost-microscopic saw-tooth pattern of this type, with the grooves running parallel to the direction of air flow, appear to slip through the air with less effort than a smooth, polished surface.
According to NASA calculations such a surface should use about 8 percent less energy than current aircraft surfaces at jetliner cruising speeds.
Despite all their work, the researchers aren't totally sure why this is so. The grooving doesn't appear to have a direct effect on the energy-eating turbulent air flow created by the aircraft skin. Instead, the grooves seem to hold the turbulence at bay. The current theory, as Hefner explains it, seems almost perverse. The grooves, the researchers now think, actually make the layer of air next to the skin stickier, and this more viscous air somehow moderates the turbulence.
It turns out that nature may already have discovered this trick: The skin of a shark is covered with similar riblets. They are much smaller than the pattern proposed for aircraft, but, when the differences between air and water are taken into account, the shark's riblets are the right size to reduce its skin friction in similar fashion, the aerodynamicist reports.
If full-scale tests bear out this work, the airlines may not need to scratch up the paint on all their jumbo jets to get the benefits. The 3M Company has produced plastic film with the pattern extruded on its surface. And NASA wind-tunnel tests have found that this works as well as or better than mechanical grooving. Still to be determined is whether air leaks from a pressurized aircraft cabin at altitude could unstick such a film.
Ultimately, the groove effect may find application in more areas than just aircraft. Techniques of this sort may someday reduce the amount of energy required to pump water, natural gas, and other fluids around the world, not just help keep down the price of airline tickets.