Close formation flight control

: Automatic formation flight involves controlling multiple wing aircraft equipped with standard Mach-hold, altitude-hold, and heading-hold autopilots in order to maintain a desired position relative to a lead aircraft throughout formation maneuvers. Changes in the lead aircraft's states, including formation heading, velocity, altitude, and geometry changes, are treated as disturbance and are rejected by the formation flight control system. The work in this thesis is a continuation of five previous theses, dealing with the design of formation flight control systems. The goal of the optimal formation flight control design is to achieve robust formation maintenance in the face of formation maneuvers and the presence of full system nonlinearities. Second-order aircraft/autopilot models are included in the design and a new control law is employed. A constrained optimization for determining the optimal controller gains of fixed structure controllers is employed. The two controllers considered are a Proportional plus FeedForward (PFF) controller, previously developed at AFIT, and a new Proportional plus Integral and FeedForward (PIFF) controller, which uses less feedforward information. Finally, the constrained optimization is applied to a wide variety of formation maneuvers and geometry changes initiated from both the tight, closely spaced, and loose, more widely dispersed, types of formations.

Flight control system design for two F/A- 18 aircraft flying in closed-coupled formation is the subject of this paper. Developed by Boeing - Phantom Works as an integral part of NASA revolutionary concept program called Autonomous Formation Flight (AFF), the Formation Flight Control System (FFCS) provides autonomous aircraft operation and flight testing capability. Using two specially instrumented Boeing-built F/A-18 Hornets research aircraft, AFF Phase I flight demonstrations are scheduled for the fall of 2002 at NASA's Dryden Flight Research Center, Edwards Air Force Base, in California. It is expected that the trailing aircraft equipped with FFCS would sustain 15-20% drag reduction if it is flown in the wake vortex created by the lead aircraft wingtips. This paper presents the baseline FFCS design, analysis, and evaluation using high fidelity 6-DoF nonlinear aircraft simulation environment. Aircraft vortex modeling is discussed. An alternative adaptive formation flight control system design is proposed. The adaptive system is shown to improve the formation tracking performance.

Birds fly in formation to benefit from the upwash wake vortices created by their peer leader. Similar strategies can be employed on aircraft for fuel saving or for range extension. However, further understanding of wake vortices in the near field and mid field is needed for close formation flight (< 10 wingspans). This paper introduces a new method for wake vortex detection using small UAV formation. A NonLinear Dynamic Inversion (NLDI) based formation control law is adapted to the WVU Phastball UAV platform. The designed controller showed good tracking performance during close formation flight (5-10 wingspans) with typical errors of less than 3 meters during straight legs. Then, multiple sensor measurements from leader-follower UAV formation flights were used for the wake vortex detection, including angle of attack, angle of sideslip, accelerometers, and gyros. The collected flight data showed the feasibility of using UAV formation for the wake encounter detection.

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 crosstrack 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 leads 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.

This paper reports on in-flight tests conducted to evaluate a method aimed at controlling the formation of autonomous unmanned aerial vehicles (UAVs) in a leader-follower configuration. The study introduces a two-stage formation flight controller designed to address the challenges encountered in controlling the position and velocity errors inherent in the leader-follower formation flight. In particular, difficulties arise when activating the formation flight mode with considerable distances between the leader and the follower, as well as substantial disparities in their heading angles. The proposed formation flight control scheme comprises two stages: coarse guidance on the leader and precise leader following. The efficacy of this control system was assessed by in-flight testing, in which flight parameters of both the leader and the follower were recorded and scrutinized. The analysis demonstrates that the proposed control algorithm significantly enhances the organization of formation flight. The results obtained from the tests validate the effectiveness of this method, showcasing improvements in formation flight organization and ensuring collision-free conditions. The described algorithm presents a promising approach toward enhancing formation flight control for autonomous UAVs.

This paper investigates the position-tracking and attitude-tracking control problem of close formation flight with vortex effects under simultaneous actuator and sensor faults. On the basis of the estimated state and fault information from unknown input observers and relative output information from neighbors, an integration of decentralized fault estimation and distributed fault-tolerant control is developed to deal with bidirectional interactions and to guarantee the asymptotic stability and H_\\infty performance of close formations.

Formation flying techniques, demonstrated by migratory birds, have the potential to significantly save energy when two or more transport aircraft fly the same route at the same time. In fuel-saving formations, aircraft are flying straight trajectories for most of the time. For proof of concept, however, how vertical and lateral maneuvers can be flown has to be investigated. Those investigations require a flight simulator that is equipped with adequate flight mechanical models. Not much research exists on maneuvers of formation. The paper addresses the components that are needed to perform this research with real-time flight simulations. They are, first, a method to model wake vortices on curved trajectories; second, a formation flight control system, which comprises a method for the relative position determination, the design of control laws, and suitable cockpit displays; and third, techniques that allow vertical and lateral flight maneuvers. Two different strategies to position the following aircraft relative to the wake vortex during turns are described, and simulation results show their differences in terms of fuel savings and passenger comfort. Piloted flight simulator tests confirmed the usability of the proposed flight techniques for formation maneuvers, and the participating airline pilots indicated general consent to the concept.

Experimental study of the torsional aeroelastic instability of single-axis solar trackers under different turbulence intensities

Current trends in turbomachinery design significantly reduce the mass ratio of structure to air, making them prone to flutter by aerodynamic coupling between mode shapes, also called coupled-mode flutter. The p–k method, which solves an aeroelastic eigenvalue problem for frequency and damping, respectively, excitation of the aerodynamically coupled system, was adapted for turbomachinery application using aerodynamic responses computed in the frequency domain (FD). A two-dimensional (2D) test case is validated against time-marching fluid–structure coupled simulations for subsonic and transonic conditions. A span of mass ratios is investigated showing that the adapted p–k method is able to predict the transition between aeroelastically stable and unstable cascades depending on the mass ratio. Finally, the p–k method is applied to a low mass ratio fan showing that the flutter-free operating range is significantly reduced when aerodynamic coupling effects are taken into account.

A chained method to compute the single mode vibration of mistuned bladed disk assemblies is proposed. Aerodynamic coupling effects are first evaluated by computing unsteady aerodynamic coefficients for a quasi-3D geometry. The code is parallelized with a decomposition domain method and validated with the 4th Aeroelastic Standard Configuration. Aerodynamic coefficients are then included in a mass-spring model of the bladed disk assembly. The chained method is applied to the last row blades of a low pressure steam turbine in which localized high levels of vibrations have been observed. The phenomenon of vibration localization found by experiments is predicted with a reasonably good accuracy. Numerical results indicate that aerodynamic coupling is predominant and highly influences mistuning sensitivity. It is also shown that the mistuned row is very sensitive to structural damping. Intentional mistuning is used as a mean to reduce the amplitude magnification on the mistuned assembly. Modifying the frequency of every two blades of the assembly has proved to efficiently reduce localization. A very similar result is obtained by locally modifying the characteristics of a few blades located in high magnification zones.

This paper presents identification, control synthesis and simulation results for an YF-22 aircraft model designed, built, and instrumented at West Virginia University. The ultimate goal of the project is the experimental demonstration of formation flight for a set of 3 of the above models. In the planned flight configuration, a pilot on the ground maintains controls of the leader aircraft while a wingman aircraft is required to maintain a pre-defined position and orientation with respect to the leader. The identification of both a linear model and a nonlinear model of the aircraft from flight data is discussed first. Then, the design of the control scheme is presented and discussed with an emphasis on the amount of information, relative to the leader aircraft, needed by the wingman to maintain formation. Using the developed nonlinear model, the control laws for a maneuvered flight of the formation are then simulated with Simulink® and displayed with the Virtual Reality Toolbox®. Simulation studies have been performed to evaluate the effects of specific parameters and the system robustness to atmospheric turbulence. The conclusions from this analysis have allowed the formulation of specific guidelines for the design of the electronic payload for formation flight.

The nonlinear dynamic inversion (DI) control techniques are used in leader-follower unmanned aerial vehicle (UAV) dynamics modelling considering new input variables in the present paper. Three separate DI controllers are developed for holding the velocity, heading and flight path angles of both UAVs in a synchronous manner. A numerical simulation is performed at the end for validating this formation control mission of multiple UAVs. The quantitative estimation of reduction in follower drag while in close formation flight is also calculated along with the controller design.

This paper presents a trajectory optimization study that has been conducted using a recently developed tool for the synthesis and analysis of extended flight formations of long-haul commercial aircraft, with the aim to minimize overall fuel consumption. In extended flight formations, trailing aircraft can attain an appreciable reduction in induced drag and associated reduction in fuel burn by flying in the upwash of the lead aircraft’s wake. In the present study, a previously developed multi-phase optimal control (MOC) framework for the synthesis of two-ship flight formations has been extended to include the assembly of three-ship flight formations. Using the extended tool, various numerical experiments have been conducted in relation to the assembly of two-ship and three-ship flight formations in long-haul operations across the North-Atlantic Ocean. Additionally, numerical experiments have been carried out to examine the impact of wind fields on the synthesis and performance of flight formations. Additionally, a parametric investigation has been conducted to assess the sensitivity of the solutions with respect to the degree of the induced drag reduction that might be attained by the trailing aircraft in a formation. The results of the various numerical experiments reveal that formation flight can result in appreciable reductions in fuel burn in comparison to flying solo—particularly when larger formation strings are permitted.

This paper deals with a behavioural decentralized approach for UAV formation flight to carry out a waypoint-passing mission. The objective of the proposed approach is to make each UAV in the formation fly through predefined waypoints while maintaining the distance from other UAVs. To apply the behavioural decentralized controller based on the coupled dynamics, a feedback linearization technique with a diffeomorphic transfer map is derived for a three-dimensional UAV kinematics model. The behavioural approach in which the control input is decided by the relative weight of each UAV's desired behaviour is considered so that each UAV can promptly react on various situations. Optimization techniques of the gain matrices are also performed to improve the performance of the formation flight using eigen-structure analysis and Cholesky decomposition. To verify the performance of the proposed controller, numerical simulation is performed for a waypoint-passing mission of multiple UAVs.

A new set of analytical range equations (a modification of the traditional Bréguet range equation) suitable for formation flight at transonic flight speeds under realistic operating conditions (constant Mach number and altitude) is derived. Formations of two aircraft of the same type are analyzed to determine the effects of 1) weight differences between the aircraft, 2) altitude, and 3) the formation flight range on the potential fuel benefits and the associated optimum Mach number. In the case of a weight difference, the lightest aircraft should lead the formation to realize the largest fuel benefits. Overall, fuel savings of 6 to 12% for the total formation can be realized at the expense of a reduction in cruise Mach number from 0.85 to 0.80. The fuel benefit is much less (2 to 8%) when the formation is flown at the original design cruise Mach number. In terms of fuel benefits and Mach number, it is beneficial to fly in formation at higher altitudes. Formation flight step climb procedures are possible, but the additional fuel savings are minimal compared to a constant-altitude formation flight.