This article describes an approach to close-formation flight of autonomous aircraft. A standard LQ-based structure was synthesized for each vehicle and for formation position error control using linearized equations of motion and a lifting line model of the aircraft wake. We also consider the definition of a formation management structure, capable of dealing with a variety of generic transmission and communication failures among aircraft. The procedure was developed using a decentralized approach and relies on the Dijkstra algorithm. The algorithm provides optimal path information sequencing in the nominal case, as well as the redundancy needed to accommodate failures in data transmission and reception. Several simulations were carried out, and some of the results are presented. The overall scheme appears to be a valuable starting point for further research, especially specialization to situations representing more detailed and operational failures.

/pdf/autonomous-formation-flight-a6ahs73dgx.pdf

Autonomous formation flight

https://people.bath.ac.uk/jldb20/pubs/Thomas.etal2014Advancesinair.pdf

Advances in air to air refuelling

Results from a wind-tunnel test of two delta-wing aircraft in close proximity are presented and compared with predictions from a vortex lattice method. Large changes in lift, pitching moment, and rolling moment are found on the trail aircraft as it moves laterally relative to the lead aircraft. The magnitude of these changes is reduced as the trail aircraft moves vertically with respect to the lead aircraft. Lift-to-drag ratio of the trail aircraft is increased when the wing tips are slightly overlapped. Wake-induced lift is overpredicted slightly when the aircraft overlap in the spanwise direction. Wake-induced pitching and rolling moments are well predicted. A maximum induced drag reduction of 25% is measured on the trail aircraft, compared with a 40% predicted reduction. Three positional stability derivatives, change in lift and pitching moment with vertical position and change in rolling moment with lateral position, are studied. Predicted boundaries between stable and unstable regions were generally in good agreement with experimentally derived boundaries.

Comparison of Predicted and Measured Formation Flight Interference Effects

This paper addresses the problem of controlling the receiver aircraft to achieve a successful aerial refueling. For the performance veriflcation of the controller, a new set of nonlinear, 6-DOF, rigid body equations of motion for the receiver aircraft has been derived. The equations are developed using the reference frame as one that is attached to, translates and rotates with the tanker aircraft. Furthermore, the nonlinear equations contain the wind efiect terms and their time derivatives to represent the aerodynamic coupling involved between the two aircraft. These wind terms are obtained using an averaging technique that computes the efiective induced wind components and wind gradients in the receiver aircraft’s body frame. Dynamics of the engine and the actuators are also included in the study. A linear position-tracking controller has been designed using a combination of integral control and optimal LQR design. The controller does not use the information of the tanker’s vortex induced wind efiects acting on the receiver aircraft as well as the mass change that occurs during refueling. The performance of the controller is evaluated in the high fldelity simulation environment employing the new sets of equations of motion. The simulation and control design are applied to a tailless flghter aircraft with innovative control efiectors and thrust vectoring capability. Various allocation schemes for redundant control variables are analyzed in a realistic approach maneuver to the refueling contact position behind the tanker aircraft. In this paper, the performance evaluation is presented only during the initial phase of aerial refueling maneuver when the receiver aircraft maneuvers to reach the refueling contact position.

Flight Control and Simulation for Aerial Refueling

This paper presents an analysis of data obtained in an automated aerial refueling test flight conducted with a KC135 as the tanker and a Learjet 25 as the surrogate receiver unmanned aerial vehicle. The purpose is to identify the wind induced by the tanker wake and its effect on the receiver aircraft. From the available flight data, a direct computation of the winds experienced by the tanker and receiver is carried out. The mean variation of the receiver wind is compared with the tanker wind when the receiver is at the observation and contact positions. This results in the identification of the wake-induced wind. A spectrum analysis is conducted to characterize the turbulence and to identify the pilot effects. The paper also presents methods used to model 1) prevailing wind, 2) wake-vortex-induced wind, and 3) turbulence as the three sources of wind that the aircraft are exposed to and the approach used for incorporating the wind effect into the dynamic simulation of the aircraft. The test flight is simulated in various caseswithdifferentturbulencemodelsand flightcontrollers.Thesimulationresultsareanalyzedandcomparedwith the flight data in terms of the power spectral densities and mean variations to validate the wind and turbulence modeling techniques.

Flight Data Analysis and Simulation of Wind Effects During Aerial Refueling

Wind-tunnel data have been obtained from a tapered tanker wing and receiver aircraft model at varying vertical separation. The data are presented in derivative form and compared with theory using a flat vortex sheet model of the tanker wing wake to determine the induced angle-of-attack variation on the receiver wing, fin, and tailplane due to the tanker wing. Aerodynamic loads on the receiver are obtained by the vortex lattice method, with an allowance made for the vertical displacement of the tanker wake in the estimation of the fin side force. In the experiments, the lateral aerodynamic interference between tanker and receiver was determined by banking the tanker wing and displacing it sideways, and by yawing the receiver aircraft model. Data were obtained from open and closed test sections in order to assess the significant boundary interference effect and corrections estimated from the image vortex system of the tanker and receiver wings in the test section. In general, the theory compares favorably with the experimental data. The most significant terms are the rolling moments due to sideways and bank displacements. Significant side forces are produced due to sidewash on the fin from the tanker and receiver wings and, when displaced in yaw, the receiver experienced a loss in directional stability.

M. G. West

Papers

Lateral Aerodynamic Interference Between Tanker and Receiver in Air-to-Air Refueling

Interference between tanker wing wake with roll-up and receiver aircraft