The interactions of wingtip vortices in close-formation flight of two wings were examined experimentally. Both wings consisting of a NACA 23012 airfoil section and a rectangular planform of an aspect ratio of 5 were set in echelon formation in a wind tunnel. The lift-to-drag ratio increased the most effectively when the two wings were overlapped with each other by 5% of the wingspan with zero vertical offset for the present wing configuration. Flow visualizations and particle image velocimetry measurements showed that the trailing vortex of the lead wing collided at a slightly inboard point of the trail wing and was divided into two streamwise vortices in the most effective overlap condition. It was also observed that the tip vortices of the lead and trail wings interacted closely with each other to form a pair of counter-rotating vortices for smaller overlap conditions less than 5 % with zero vertical offset. For such small overlap cases, circulation of the trailing vortex of the trail wing was reduced markedly by influences of the trailing vortex of the lead wing.

Detailed Observations of Interactions of Wingtip Vortices in Close-Formation Flight

This paper presents an uncertainty and disturbance estimator (UDE)-based nonlinear robust controller for close formation flight in light of the command filtered backstepping technique. To maximize the formation aerodynamic benefits at different flight maneuvers, the formation geometry is described in the wind frame of the leader aircraft. A novel nonlinear robust close formation control algorithm with a two-degree-of-freedom nature is developed. The command filtered backstepping technique is employed to design the baseline formation controller, whereas the UDE is introduced to enhance the robustness of the baseline formation control. The proposed control law can lead to desired tracking performance for the close formation flight under different maneuvers without using the acceleration information of the leader aircraft, asymptotical stability for the close formation flight at a level and straight flight with constant speeds, and enough robustness against the formation aerodynamic effects by purely observing system states and inputs. Numerical simulations are performed to show the feasibility and efficiency of the proposed controller.

UDE-Based Robust Command Filtered Backstepping Control for Close Formation Flight

A novel computationally efficient close formation aerodynamic model is presented using the lifting-line theory based on an elliptical lift distribution assumption. Formation aerodynamic effects ind...

Aerodynamics Modeling and Analysis of Close Formation Flight

The aerodynamic performance of two high aspect-ratio wings flying in formation is investigated with particular interest in the importance of considering wake rollup in making aerodynamic predictions. For this purpose, formations of various separations were investigated using a drag-free, fixed-wake model as well as a force-free, relaxed one. In all cases the aircraft were trimmed for roll and the follower aircraft pitch attitude was adjusted to match the lift coefficient of the lead aircraft. Both wake representations result in similar induced-drag minima for the follower aircraft which result when the lateral overlaps of the two aircraft are around 10% of a wingspan. Computations with the fixed-wake model, however, predict a wider region of lateral separations having the lowest induced drag than do the simulations with wake rollup. A small performance difference due to the wake model also exists when one aircraft follows directly behind the other. Independent of the wake model used, trimming the aircraft in roll and pitch can have a significant influence on their performance, although the required control-input solutions for trim are essentially not affected by the choice of wake representation.

Effects of Wake Rollup on Formation-Flight Aerodynamics

Numerical Aerodynamic Analysis of Formation Flight

Reported are the results of wind-tunnel tests conducted to evaluate aerodynamics characteristics of aircraft in formation flight. A vortex-lattice numerical scheme was used to investigate the effect of spatial offset (horizontal and vertical) between the leading and trailing wings. The wind-tunnel test configurations consisted of echelon, chevron, and in-line formations. Analysis of the data revealed that the spatial offset and the angle of attack of the leading wing had significant impact on the trailing aircraft. For some test conditions an increase in lift-to-drag ratio of the trailing aircraft was measured. Variation in C Lmax and/or α stall was observed as well. At higher angles of attack of the leading wing, the C L-α curve of the trailing aircraft was significantly altered.

Robin P. Sanchez

Papers

Aerodynamics of formation flight

Aerodynamics of formation flight