An Initial Flight Investigation of Formation Flight for Drag Reduction on the C-17 Aircraft

Abstract

Many theoretical and experimental studies have shown that aircraft flying in formation could experience significant reductions in fuel use compared to solo flight. To date, formation flight for aerodynamic benefit has not been thoroughly explored in flight for large transport-class vehicles. This paper summarizes flight data gathered during several two-ship, C-17 formation flights at a single flight condition of 275 knots, at 25,000 ft MSL. Stabilized test points were flown with the trail aircraft at 1,000 and 3,000 ft aft of the lead aircraft at selected cross track and vertical offset locations within the estimated area of influence of the vortex generated by the lead aircraft. Flight data recorded at test points within the vortex from the lead aircraft are compared to data recorded at tare flight-test points outside of the influence of the vortex. Since drag was not measured directly, reductions in fuel flow and thrust for level flight are used as a proxy for drag reduction. Estimated thrust and measured fuel flow reductions were documented at several trail test point locations within the area of influence of the lead's vortex. The maximum average fuel flow reduction was approximately 7-8%, compared to the tare points flown before and after the test points. Although incomplete, the data suggests that regions with fuel flow and thrust reduction greater than 10% compared to the tare test points exist within the vortex area of influence. Nomenclature AFF = autonomous formation flight AWODS = advanced wireless open data system F = engine thrust, lbf FFS = formation flight system HUD = heads up display KCAS = knots calibrated airspeed NASA = National Aeronautics and Space Administration N2 = engine core rotor speed, percent RPM SKE = station keeping equipment USAF = United States Air Force V= velocity w f = fuel flow, pounds per hour Y offset = cross-track distance between vehicle fuselage centerlines, ft Z offset = vertical distance between vehicle fuselage waterlines, ft δ a = aileron deflection, deg δ s = spoiler deflection, deg ____________________________

Cruise Range in Formation Flight

“…In the past years, the Air Force Research laboratory has been investigating the potential benefits for C-17 aircraft flying in formation, in the Surfing Aircraft Vortices for Energy ($AVE) project. Average fuel burn savings for the trailing aircraft in the order of 7-9% are reported in the project [5]- [7]. To analyze the fuel burn saving for various operational conditions, mission simulations were conducted in the SAVE project.…”

Experimental Measurements of Passenger Ride Quality During Aircraft Wake Surfing

“…[4,5]), and large military transport aircraft (Refs. [6,7]). The recent NASA flight research measured the effects of wake surfing on passenger ride quality and correlated those effects with performance benefits, specifically reduction in fuel flow.…”

Dynamic Load Alleviation in Wake Vortex Encounters

“…The potential benefits for transport aircraft have been demonstrated in recent flight tests 5,6 and numerical studies. 7 In either case, advanced flight control techniques are required to ensure optimal trajectories of the vortex-generating and the follower aircraft, 8 particularly with adverse atmospheric conditions.…”