Keywords

aircraft formation flight, compressibility, Euler CFD, high-speed formation flight, extended formation flight

Abstract

Aircraft flown in formations may realize significant reductions in induced drag by flying in regions of wake upwash. However, most transports fly at transonic speeds and compressibility effects in formation flight are not well understood. This study uses an Euler solver to analyze the inviscid aerodynamic forces and moments of transonic wing/body configurations flying in a 2-aircraft formation. We consider formations with large streamwise separation distances (10-50 wingspans) in an arrangement we term extended formation flight. Compressibility-related drag penalties in formation flight may be eliminated by slowing 2-3% below the nominal out-of-formation drag divergence Mach number, at fixed lift coefficient or fixed altitude. The latter option has the additional benefit that the aerodynamic performance of the formation improves slightly at higher lift coefficients. Optimal in-formation lift coefficients are not nearly as high as those estimated by incompressible analyses, but if not limited by engine performance, modest increases in altitude can yield further improvements in aerodynamic efficiency. Increasing the lateral separation of the aircraft can allow for slightly higher cruise speeds in exchange for higher induced drag. For the configurations examined here, a 1-2% reduction in Mach number combined with a lateral spacing increase of 5% span achieves a total formation drag savings of about 10%.

Original Publication Citation

Ning, A., and Kroo, I., “Compressibility Effects of Extended Formation Flight,” 29th AIAA Applied Aerodynamics Conference, Honolulu, Hawaii, Jun. 2011. doi:10.2514/6.2011-3812