Researchers Stabilize Active Drag Reduction for Swept Airfoils; 10-15% Gains in Fuel Economy Possible
by Rafael Seidl
German news magazine Der Spiegel reports that researchers at the University of Stuttgart (Germany) have succeeded—at least in computer simulations—in stabilizing active drag reduction for the airfoils of commercial airliners. They claim their approach could lead to fuel economy gains of 10-15%. For a major carrier such as Lufthansa, that would translate to savings of hundreds of millions of Euros per year and a corresponding reduction in CO2 emissions.
The concept of active drag reduction has been around for a while: the surface of an aircraft wing moving through the air at high speed generates a boundary layer that increases in thickness toward the trailing edge. Along the way, the layer becomes increasingly turbulent and eventually sheds eddies. Both the layer’s thickness and the eddies it sheds generate significant drag.
If, however, the airfoil is perforated with a very large number of tightly spaced microscopic perforations (~50 microns diameter), a vacuum can be applied from the inside to manipulate the shape and internal structure of the boundary layer. The manufacture of the perforation patterns adds significant cost (and affects mechanical stresses)—one reason why it is only considered worth pursuing if jet fuel are expected to remain high.
The other reason is technical: traditional hole patterns work fine for straight wings, such as those found on ridge soarers. Applied to the swept wings typical of commercial jets, Boeing discovered they can actually produce chaotic vortices that interfere with stable airflow over the wing. The crux of the present breakthrough (patent pending) is the discovery of subtly different hole patterns that keep the boundary layer both thin and the vortices inside it from shedding. Stable vortices can also be achieved with passive surface dimples [cp. golf balls] and artificially increased surface roughness, but only the vacuum technique also keeps the thickness of the boundary layer small.
The team leaders, Markus Kloker and Ralf Messing, are careful to stress that they have hitherto limited their analyses to wing sections, due to the massive compute power required for whole-wing CFD simulations of the internal structure of the boundary layer. Also, the scope for savings still has to be confirmed in real-world experiments. The researchers have entered into talks with Airbus (whose A380 and A350 models must compete against Boeing’s lightweight 7E7 with regard to fuel economy).
If successful, the technique could be valuable for other airfoil applications, such wind turbine blades (and helicopter rotors, race car spoilers, compressor vanes etc.)