Formation Flight

Abstract

A research simulation model was created to perform extended research into formation flight dynamic behaviour. The principle of aircraft flying in formation is to lower the induced drag and fuel flow. A trail aircraft that flies within the wake vortex field of a lead aircraft encounters an increased effective angle of attack, reducing the induced drag component. This benefit is accompanied by induced interference effects, most pronounced in roll and pitch. Control deflections are required to retain a predefined flight path within the formation flight (trim). These deflections during cruise flight diminish the benefit of drag reduction. This research focuses on quantifying this reduction in benefit. The test case investigated was an Airbus A330-300 in an assumed cruise flight at Mach 0.6 at an altitude of 11,000 meters with a lift coefficient of 0.623. An extended vortex lattice method combined with a simulation model of non-linear equations of motion based on rigid multi body dynamics was used. The aerodynamic results showed a margin of error of 10% around the vortex core. The position for highest induced drag reduction, the sweet spot, was located at -0.15 y/b and 0.1z/b, where the lead aircraft vortex is located at -0.1 y/b and 0 z/b. In close proximity of the vortex, variations in loads with position remained small compared to the deeper wakefield. The untrimmed aircraft had a reduction in induced drag of 52.6%, where the trimmed aircraft had a reduction of 47.5%. The benefit is effectively lowered by 5.1%, and possibly more of the benefit is lost by compressibility and viscous effects that are not within the scope of this thesis. The aileron deflection at the sweet spot is 2.32 degrees. The trail aircraft at the sweet spot showed unstable behaviour in pitch and roll. The aileron hinge moments caused by deflection during cruise formation flight were estimated. When compared to the design limit in solo flight condition, loads were shown to exceed the limit by 6.8% for deep wake field positions. At the sweet spot, the hinge moments were well within the limit. A study on the design of the aileron control surfaces revealed that an increase in area, by using both inboard and outboard ailerons, negatively affects the stability of the formation. Nevertheless the hinge moments are reduced by using both ailerons. A positive effect on trim in formation flight was identified by deflecting the ailerons non-differential, through application of a predefined deflection on the in-vortex aileron to remove the rolling moment. The total benefits for the trail aircraft in formation flight at the sweet spot are 16.59% in total drag and 18.27% in fuel flow. Important to note is the absence of pressure drag for the total drag determination.