Reduction of Fuselage Form Drag by Vortex Flows

Abstract

Wind-tunnel tests were performed in the California Institute of Technology’ s 10-ft-diam tunnel in a parametric study of 1:17 scale models of fuselages of Boeing 747 and Lockheed C-5 aircraft. The purpose was to validate the theoretical model advanced in 1981, that pairs of vortex generators would reduce the form drag of transport aircraft. The longitudinal vortices generated by e at-plate lifting planforms in the lower aft regions of the fuselage prevent, or delay, separation by energizing the boundary layer and also by creating a strong transverse oute ow from the plane of symmetry. The search for the minimum drag involved three vortex generator cone gurations, with three sizes of each in six locations, clustered in the lower aft regions of the fuselages at the beginning of the tail upsweep. The local Reynolds number that referred to the surface distance from the nose was about » 10 7 . Fully developed turbulent e ow was cone rmed by noting insignie cant drag differences between test runs with and withoutupstream boundary-layertrippingstrips.Vortex generatorplanformsrangedfromswepttapered,through swept straight, to swept reverse tapered wings, whose semispans ranged from » 50 to 125% of the estimated local boundary-layer thickness. Inboard actuators under the control of an externally mounted proportional digital radio controller varied the pitch angles of the vortex generators. While certain combinations of vortex generator parameters increased drag, other cone gurations, locations, and pitch angles of vortex generators reduced drag by 3% for the 747 and » 6% for the C-5, thus cone rming the arguments that effectiveness increases with the angle of upsweep of the tail. Therefore, the greatest gains in performance are expected on aft-loading military transports. Incremental changes in overall aircraft drag and fuselage drag are approximately in the proportion 1:3. Overall drag reductions are expected to be » 1% for the 747 and 2% for the C-5 aircraft.