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How do you improve the fuel economy of aircraft that have been in service for years, or decades? Reengining is expensive and unlikely to pay off within the lifetime of older airframes. Modifications to fit winglets can be costly and the return on investment may not come quickly enough.
The U.S. Air Force Research Laboratory (AFRL) is looking for other, less invasive solutions to reducing the fuel burned by the service’s current and future transports and tankers. Specifically, the lab is looking for engineered surfaces, materials and coatings that can be applied to aircraft without substantive changes to the outer mold line that would require lengthy and expensive retrofit and recertification. AFRL wants a fast breakeven and warns that replacing aircraft skins will be too expensive.
The focus is on reducing skin-friction drag, which makes up half the total drag of a conventional tube-and-wing aircraft. Wave and interference drag are other targets. Friction drag can be reduced by increasing the amount of smooth laminar flow over an aircraft, but the dumpy, bumpy shapes of airlifters such as the Boeing C-17 and Lockheed C-130 are not conducive to laminarity, so the lab is looking for ways to reduce friction on surfaces where the boundary-layer flow closest to the skin is turbulent.