Extended formation flight at transonic speeds

Abstract

Aircraft flown in formation can realize significant reductions in drag by flying in regions of wake upwash. However, most transports fly at transonic speeds where the impact of compressibility on formation flight is 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 two-aircraft formation. Formations with large streamwise separation distances (10-50 wingspans) are considered. This work indicates that compressibility-related drag penalties in formation flight may be eliminated by slowing 2-3%below the nominal out-of-formation cruise Machnumber, either 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. Although optimal in-formation lift coefficients are not as high as those estimated by incompressible analyses, modest increases in altitude can yield further improvements in aerodynamic efficiency. Increasing the lateral separation of the aircraft allows 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 (vertical spacing aligned with the vortex) achieves a total formation drag savings of about 10%.