Bifurcation analysis has previously been applied to the NASA Generic Transport Model (GTM) to provide insight into open-loop upset dynamics and also the impact on and sensitivity of such behaviour to closing the loop with a flight controller. However, these studies have not considered time delay in the system: this arises in all feedback controllers and has specific relevance when remotely piloting a vehicle such as the NASA AirSTAR GTM with ground-based controllers. Developments in the AirSTAR programme, in which a sub-scale generic airliner model will be tested for loss-of-control conditions over long ranges, raise the prospect of increased adverse effects of time delay relative to previous testing. This paper utilises bifurcation analysis, supplemented with time histories, on the GTM numerical model with a LQR-PI controller to evaluate the sensitivity of the closed-loop system stability to time delay. In this paper, the impact of time delays in both a fixed-gain and a gain-scheduled version of the controller is presented in terms of stability of nominal and off-nominal solutions.

Sensitivity of the Generic Transport Model upset dynamics to time delay

Loss of control is a major factor in fatal aircraft accidents. Although definitions of loss of control remain vague in analytical terms, it is generally associated with flight outside of the normal flight envelope, with nonlinear influences, and with a significantly diminished capability of the pilot to control the aircraft. Primary sources of nonlinearity are the intrinsic nonlinear dynamics of the aircraft and the state and control constraints within which the aircraft must operate. This paper examines how these nonlinearities affect the ability to control the aircraft and how they may contribute to loss of control. Specifically, the ability to regulate an aircraft around stall points is considered, as is the question of how damage to control effectors impacts the capability to remain within an acceptable envelope and to maneuver within it. It is shown that, even when a sufficient set of steady motions exist, the ability to regulate around them or transition between them can be difficult and nonintuitiv...

Nonlinear Analysis of Aircraft Loss of Control

Neural network modeling of unsteady aerodynamic characteristics at high angles of attack

This paper reports the latest development of several techniques for adaptive flight envelope estimation and protection system for aircraft under damage upset conditions. Through the integration of advanced fault detection algorithms, real-time system identification of the damage/faulted aircraft and flight envelop estimation, real-time decision support can be executed autonomously for improving damage tolerance and flight recoverability. Particularly, a bank of adaptive nonlinear fault detection and isolation estimators were developed for flight control actuator faults; a real-time system identification method was developed for assessing the dynamics and performance limitation of impaired aircraft; online learning neural networks were used to approximate selected aircraft dynamics which were then inverted to estimate command margins. As off-line training of network weights is not required, the method has the advantage of adapting to varying flight conditions and different vehicle configurations. The key benefit of the envelope estimation and protection system is that it allows the aircraft to fly close to its limit boundary by constantly updating the controller command limits during flight. The developed techniques were demonstrated on NASA s Generic Transport Model (GTM) simulation environments with simulated actuator faults. Simulation results and remarks on future work are presented.

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Methodologies for Adaptive Flight Envelope Estimation and Protection

Aircraft Loss of Control Problem Analysis and Research Toward a Holistic Solution

An aircraft's performance and maneuvering capabilities in steady flight conditions are usually analyzed considering the steady states of the rigid-body equations of motion. A systematic way of computation of the set of all attainable steady states for a general class of helical trajectories is presented. The proposed reconstruction of attainable equilibrium states and their local stability maps provides a comprehensive and consistent representation of the aircraft flight and maneuvering envelopes. The numerical procedure is outlined and computational examples of attainable equilibrium sets in the form of two-dimensional cross sections of steady-state maneuver parameters are presented for three different aircraft models.

Evaluation of Aircraft Performance and Maneuverability by Computation of Attainable Equilibrium Sets

Pushing Ahead - SUPRA Airplane Model for Upset Recovery

Multiple equilibrium solutions of aircraft motion equation are investigated using nongradient based numerical methods and computation of attainable equilibrium sets. The proposed approach may be applied to realistic industrial scale aircraft aerodynamic models based on look-up data tables. Advantages of this method and its joint use with predictorcorrector techniques for continuation and bifurcation analysis of aircraft nonlinear dynamics are discussed. A number of computational examples for aircraft dynamics investigation are presented for a generic airliner aerodynamic model developed within the SUPRA research project – “Simulation of Upset Recovery in Aviation” – funded by the European Union 7 th Framework Program.

Analysis of Aircraft Nonlinear Dynamics Using Non-Gradient Based Numerical Methods and Attainable Equilibrium Sets

A control law design approach that robustifies the nonlinear dynamic inversion method for an aircraft performing nonlinear maneuvers is proposed. The robustness condition is based upon Lyapunov stability theory and multiple time scale dynamic inversion. The algorithm is analytically developed to be robust to a specified level of parameter uncertainty. A smoothed sliding mode control condition is introduced to provide linear stability of a closedloop system and maintain a specified level of command trajectory tracking accuracy. The method is demonstrated via an application to a twin-engined high performance aircraft performing the Cobra maneuver and a 360 degrees stability axis roll at high angle of attack while asymmetrically dropping external stores or experiencing thrust loss of one engine. Simulation results are presented to show the closedloop system dynamics using both the proposed algorithm and the conventional nonlinear dynamic inversion method.

Robust nonlinear dynamic inversion method for an aircraft motion control

In this chapter, the manoeuvring capabilities of the ADMIRE model are analyzed using qualitative methods via computation of attainable equilibrium sets, local stability maps and two-dimensional cross-sections of stability regions for stable equilibria considering the velocity-vector roll manoeuvre. A nonlinear dynamic inversion control law is implemented as a prototype of a control and stability augmentation system. The analysis of the closed-loop dynamics allows one to assign the airplane manoeuvre limitations via specification of flight envelope critical boundaries. A functional interconnect between control inputs helps to avoid control surfaces saturation and to significantly enhance the ADMIRE manoeuvrability at high angles of attack. A number of computational examples for attainable equilibrium sets and regions of attraction illustrate the applied investigation methodology.

E. N. Kolesnikov

Papers

Evaluation of Aircraft Performance and Maneuverability by Computation of Attainable Equilibrium Sets

Pushing Ahead - SUPRA Airplane Model for Upset Recovery

Analysis of Aircraft Nonlinear Dynamics Using Non-Gradient Based Numerical Methods and Attainable Equilibrium Sets

Robust nonlinear dynamic inversion method for an aircraft motion control

Investigation of the ADMIRE Manoeuvring Capabilities Using Qualitative Methods