Inviscid Analysis of Extended-Formation Flight

Abstract

Flying airplanes in extended formations, with separation distances of tens of wingspans, significantly improves safety while maintaining most of the fuel savings achieved in close formations. The present study investigates the impact of roll trim and compressibility at fixed lift coefficient on the benefits of extended formation flight. An Euler solver with adjoint-based mesh refinement combined with a wake propagation model is used to analyze a two-body echelon formation at a separation distance of 30 spans. Two geometries are examined: a simple wing and a wing-body geometry. Energy savings, quantified by both formation drag fraction and span efficiency factor, are investigated at subsonic and transonic speeds for a matrix of vortex locations. The results show that at fixed lift and trimmed for roll, the optimal location of vortex impingement is about 10% inboard of the trailing airplane's wing-tip. Interestingly, early results show the variation in drag fraction reduction is small in the neighborhood of the optimal position. Over 90% of energy benefits can be obtained with a 5% variation in transverse and 10% variation in crossflow directions. Early results suggest control surface deflections required to achieve trim reduce the benefits of formation flight by 3-5% at subsonic speeds. The final paper will include transonic effects and trim on extended formation flight drag benefits.

Exergy-based Aircraft Aeropropulsive Performance Assessment: CFD Application to Boundary Layer Ingestion

“…As mentioned, in the introduction,Ė u is typically associated with wakes/jet exhausts and is used to explain the benefit of BLI for the airframe.Ė v is generally associated with (lift-induced) vortices, and can be valued through formation flight 15 or wingtip turbines.…”

Ride comfort in commercial aircraft during formation flight using conventional flight control

“…According to a study of extended formation flight by Ning [1] [26], for streamwise spacings of around 10 to 40 spans with low to moderately low atmospheric turbulence, a two-aircraft formation is capable of achieving a maximum drag reduction of approximately 30%, while a three-aircraft formation showed maximum reductions of around 40%. Later, in an inviscid analysis of extended formation flight, Kless et al [27] analysed a two-body echelon formation and found that simulations indicated peak induced drag savings for a trailing aircraft of up to 54% in subsonic flow and 35% in transonic flow. Bower et al [2] performed a case study in order to quantify the amount of fuel burn reductions achievable during formation flight.…”

“…However, this type of close proximity between commercial aircraft poses an unacceptably high risk of collision in many applications. Formation flight in the current study takes advantage of extended formation flying at separations of up to tens of wingspans [1][26] [27], where the persistent cruise wakes are still beneficial, but the safety is significantly improved. Wind tunnel tests for formation flight have shown that a substantial decrease in drag can be obtained in formation flight depending on the configuration.…”

Parametric Study on Formation Flying Effectiveness for a Blended-Wing UAV

“…Recently, there have been several studies published, presenting aerodynamic studies for both tight and extended aircraft formation flight [4][5][6]. An aerodynamic performance for extended formation is conducted by Ning et al [6] where 30% drag reduction is achieved with two aircraft and 40% with three aircraft.…”

“…An aerodynamic performance for extended formation is conducted by Ning et al [6] where 30% drag reduction is achieved with two aircraft and 40% with three aircraft. An inviscid CFD simulation was performed by Kless et al [5], where extended formation flight of two aircraft was studied with drag reduction of 54 and 35% for subsonic and transonic flows, respectively. A study conducted by Kaden and Luckner [7] employed a discrete vortex filament method to model wake vortex rollup for tight formation flight where it was concluded that the inclusion of fluid physics such as viscosity Reynolds number are required for verification and validation of current modelling approaches for formation flight.…”