  This paper investigates the robust nonlinear close formation control problem.
It aims to achieve precise position control at dynamic flight operation for a
follower aircraft under the aerodynamic impact due to the trailing vortices
generated by a leader aircraft. One crucial concern is the control robustness
that ensures the boundedness of position error subject to uncertainties and
disturbances to be regulated with accuracy. This paper develops a robust
nonlinear formation control algorithm to fulfill precise close formation
tracking control. The proposed control algorithm consists of baseline control
laws and disturbance observers. The baseline control laws are employed to
stabilize the nonlinear dynamics of close formation flight, while the
disturbance observers are introduced to compensate system uncertainties and
formation-related aerodynamic disturbances. The position control performance
can be guaranteed within the desired boundedness to harvest enough drag
reduction for a follower aircraft in close formation using the proposed design.
The efficacy of the proposed design is demonstrated via numerical simulations
of close formation flight of two aircraft.
