IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Use of Lighter-Than-Air (LTAs) vehicles for the telecommunication and surveillance purposes has attained a great deal of interest in recent years. This platform is very attractive because of its long endurance and cost effectiveness. The shape of airship envelope has huge impact on its performance. Literature suggests that shape of the airship should be such that it should have significant maneuvering capabilities and stability in the stratosphere. This paper presents open loop and closed loop stability analysis of Gertler shape of stratospheric airship which has minimum circumferential stresses and minimum drag characteristics. The complete 6-DOF nonlinear mathematical model of Gertler shape has been developed for the analysis of stability. As airship is a buoyant vehicle, added mass effect is taken into account while deriving the equations of motion. Stability analysis is carried out using the linearized model at a desired operating condition using both time domain and frequency domain techniques. Different motions of the airship with its control characteristics is explained at the end of paper. This analysis is to be used to develop Guidance, Navigation and Control (GNC) strategy for the stratospheric airship.

GENERAL

Department of Aerospace Engineering

30th Congress of the International Council of the Aeronautical Sciences, ICAS 2016

Dynamic modeling and stability analysis of a high altitude airship

In this brief, a new guidance law for the defense missile of nonmaneuverable aircraft is formulated based on dynamic game considerations. First, a simple differential game of protecting a static target in 2-D, involving simple motions for the attacker and defender, is introduced. The analysis is then extended to a moving noncooperative target in 2-D, in view of the fact that a nonmaneuverable aircraft would not be able to cooperate with the defender. A heuristic solution for the game is proposed and tested, and the results of the 2-D analysis are then extended to 3-D to formulate a new guidance law for the defense missile called the command to optimal interception point (COIP) guidance law. The validity of the new guidance law is checked using trajectory and envelope simulations, built with high-fidelity 6-DOF models using the computer-aided design of aerospace concepts in C++ framework. Performance comparisons are shown between the COIP guidance law and the recently proposed airborne command to line-of-sight (A-CLOS) guidance law. The results show that the performances of COIP and A-CLOS guidance laws are almost identical in a coplanar engagement scenario, but the COIP law has the additional advantage of working with only position information, without the knowledge of motion of the players. In addition, in a noncoplanar engagement case studied, the defense missile is shown to achieve intercept using the COIP guidance law, but fails when using the A-CLOS guidance law. © 2015 IEEE.

IEEE Transactions on Control Systems Technology

A New Guidance Law for the Defense Missile of Nonmaneuverable Aircraft

Deploying flare decoys against heat-seeking threats involves various parameters such as flare timing, flare ejection velocity, direction of ejection and the number of flares used. In this study, an attempt has been made to identify the key parameters among these that impact the performance of flares the most. For this, engagement studies involving sixdegrees- of-freedom models for an air-to-air heat-seeking missile and a fighter aircraft were carried out using the CADAC++ simulation environment. The effect of flare parameters was studied using their impact on missile envelopes. Studies show that the effectiveness of the flare decoys is a strong function of the flare timing and the number of flares used. © 2013 The Society for Modeling.

Journal of Defense Modeling and Simulation

Key factors that affect the performance of flares against a heat-seeking air-to-air missile

Thrust vector nozzles are finding place on modern fighter airplanes because of the benefits they provide and also due to diminishing weight penalty of such nozzles. They offer additional benefits in the case of a twin-engine airplane. Different vectoring configurations such as multi-axis vectoring, single-axis pitch vectoring and single-axis vectoring with canted nozzles have been studied with respect to twin-engine airplane configuration. Modeling and integration of thrust vector nozzles with rigid airplane six-degrees-of-freedom equations of motion have been carried out in this article. Using the integrated model, a comparative study is carried out to summarize the capabilities and limitations of various nozzle configurations with respect to performance of an airplane in velocity vector roll and in Herbst maneuvers. The airplane model used in this work is the F-18/HARV and all simulation results have been produced using a nonlinear dynamic inversion controller developed in Matlab/Simulink environment. Results show that a multi-axis thrust vectoring provides additional benefits as compared to single-axis vectoring with canted nozzles in high angle of attack velocity vector roll and in Herbst maneuvers. The single-axis pitch only vectoring has roll control power and lacks in yaw control power, to execute the velocity vector roll maneuver. © IMechE 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.

Fulltext

Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering

An assessment of thrust vector concepts for twin-engine airplane

In this paper, the target guarding problem, posed by Rufus Isaacs in his seminal textbook is revisited. Unlike the simplified version of the problem solved by Isaacs, optimal strategies are discussed for the pursuer and the evader having simple motions with different speeds. Analysis is also done for the case when the intercept occurs if the pursuer reaches within a certain distance of the evader, instead of a strict coincidence of their positions. The importance of the study is that, it is fundamental to building autonomous systems that would be used for protecting a target, for example, unmanned vehicles used in anti-poaching operations.

World University root node

IFAC Proceedings Volumes

The Target Guarding Problem Revisited: Some Interesting Revelations

A systematic methodology for conceptual aircraft design using constrained bifurcation and continuation technique is studied. The methodology uses a full six-degrees-of-freedom nonlinear mathematical model for flight dynamic stability analysis; stability and control derivatives required for analysis are estimated through standard empirical relations available in design books. To illustrate the design approach using the constrained bifurcation and continuation method, a vertical tail sizing problem of a generic six-seater airplane is investigated. Stability analysis of the aircraft lateral-directional modes with variation in vertical tail size is carried out, and a comparison of computed stability parameters is made with flying quality specifications to optimally size the vertical tail.

Journal of Aircraft

Aircraft design using constrained bifurcation and continuation method

The short take-off capability is of paramount importance for a fighter airplane to enable its operation from short and damaged runways. This paper analyses the airplane take-off process from the viewpoint of reducing the ground roll/take-off distance with the use of thrust vectoring. The airplane take-off is modelled incorporating the ground reactions on the landing gear and the thrust vector forces and moments. The take-off problem is formulated as an optimal control problem with appropriate constraints. Though many researchers have applied optimal control techniques for designing airplane manoeuvres, its application to the airplane take-off problem is rarely available in the open literature. It is expedient to use such methodology to understand the use of thrust vectoring features of an aircraft to maximise the benefits in shortening the ground roll/take-off distance. An optimal control methodology has been applied in this paper with the objectives stated above to a twin-engine fighter nonlinear aircraft model popularly known as F-18/HARV. Computation of flight path and control schedules using optimal control has been carried out with and without the use of vector nozzles. A reduction of about 6% in take-off distance and about 29% in ground roll distance is obtained with the use of thrust vector for the configuration studied.

Aeronautical Journal

Optimal aircraft take-off with thrust vectoring

Design of maneuvers for carefree access of an aircraft to its complete flight envelope including poststall regimes is useful not only from a combat strategy point of view, but also for devising recovery strategies from an accident scenario. Maneuvers for an aircraft can be efficiently designed if a priori knowledge of its maneuverability characteristics is available to the control designers. Different types of agility metrics that characterize aircraft maneuvering capabilities have been proposed in literature based on different criteria. The concept is based on the assumption that the boundary of the AER is defined by saturation of one or more control surfaces. Therefore, a point lying on the boundary of AER is initially located using a continuation method. Using the trim point on the boundary as a starting condition, a separate continuation procedure is carried out, with the saturated control fixed at its limit value to obtain the envelope containing attainable equilibrium points.

Journal of Guidance, Control, and Dynamics

Accessible regions for controlled aircraft maneuvering

Various shortcomings and inaccuracies in the existing approach and presentation of airplane flight dynamics in pedagogy have been identified. An improved pedagogical presentation is offered which is clean - uses the two timescales in longitudinal dynamics to derive short period and phugoid mode parameters, simple - starts with the longitudinal dynamics equations rather than the complete six degree of freedom formulation, and correct - uses a physically correct model of the dynamic (rate) derivatives and employs the concept of a static residual in deriving the slower (phugoid) mode approximations. The redundant and confusing concepts of dimensional derivatives and static stability are junked. A companion paper presents a similar approach to the lateral-directional dynamics.

AIAA Atmospheric Flight Mechanics (AFM) Conference

A Simple, correct pedagogical presentation of airplane longitudinal dynamics

In this paper, bifurcation analysis-based methodology is used in conjunction with a sliding-mode-based control algorithm to construct and simulate maneuvers for a nonlinear, 6 degree-of-freedom, F-18 high-alpha research vehicle aircraft model. Three different types of maneuvers, namely, a minimum-radius level turn, velocity vector roll, and spin recovery to a level flight condition, are attempted to demonstrate the usefulness of the proposed approach. The procedure involves constructing the desired maneuvers using constrained bifurcation analysis-based methodology. The results obtained from bifurcation analysis provide the reference inputs for the sliding mode controller to switch the aircraft between desired flight conditions. Robustness of the sliding mode controller is also examined by introducing uncertainties in aerodynamic parameters. Closed-loop simulation results are later presented to show the effectiveness of the proposed technique. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc.

Aircraft maneuver design using bifurcation analysis and sliding mode control techniques

Spin dynamics of F/A-18HARV at a high angle of attack is investigated from the bifurcation analysis results and a brief description of modeling of wind into aircraft equations is presented along with the procedures used to carry out analysis of numerical simulation results for different wind speeds. When the wind speed is increased to a value of 35 m/s, numerical simulation results appear to show aperiodic and uncorrelated motions. The phase portrait resembles a thread bundle, which shows that the trajectories are scattered around and rarely visit any particular region on the phase portrait. The positive slope of the straight line on the curve represents positive Lyapunov exponent, which implies that the motion is chaotic. Evolution of autorotational spin motion of a nonlinear model of F/A-18 HARV in changing mean wind condition is reported.

Engineering notes: Autorotational spin evolving toward chaos

In this paper, we present a methodology using bifurcation analysis and nonlinear control techniques to design aircraft maneuvers. As an illustrative example, maneuvering of an aircraft from a steady level flight to level turn flights is attempted. A constrained bifurcation analysis based procedure is used to determine the level flight and level turn steady states for an F-18/HARV model. Using the available states from bifurcation analysis results as setpoints, a Nonlinear Dynamic Inversion (NDI) technique based controller is designed to perform the closed loop simulation of a constant speed, level, coordinated turn maneuver from a straight and level flight to the maximum sustained turn rate flight conditions. Successful implementation of the technique while satisfying the prescribed constraints shows usefulness of the approach.Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition

Aircraft Maneuver Design using Bifurcation Analysis and Nonlinear Control Techniques

A sliding-mode (SM) controller designed for recovery of an aircraft from flat oscillatory spin is discussed. Aircraft controls aileron δa, rudder δr, and elevator δe were used as active controls within the limits. A sliding surface was designed depending upon available control authority and acceptable error dynamics. SM controller based on variable-structure control technique for nonlinear systems is suitable for all nonlinear controllers. Limits on the control surfaces, both in position and in rate, are maintained during the computation of control commands. The low-angle-of-attack segment is the stable level-trim state. The SM controller recovers the aircraft to the level flight trim state within the limits specified on the rudder saturation rates, which could be achieved using the nonlinear dynamic inversion (NDI) controller.

Aircraft spin recovery using a sliding-mode controller

In this paper, we present a variable structure based sliding mode controller for recovery of an aircraft from spin. The spin recovery problem is formulated as a two point boundary value problem where the two points, different steady states of the aircraft, are obtained from a bifurcation analysis of the aircraft model. Using the bifurcation analysis results of the aircraft, the spin states of aircraft are identified. Once the aircraft enters into spin, the controller is activated to bring it back to a desired state which is a level flight trim solution also found from a bifurcation map of the aircraft model.

Proceedings of the 11th WSEAS International Conference on Mathematical Methods, Computational Techniques and Intelligent Systems, MAMECTIS '09, Proc. 8th WSEAS NOLASC '09, Proc. 5th WSEAS CONTROL '09

Controller design using bifurcation map for aircraft spin recovery

Computational bifurcation analysis of multiparameter dynamical systems has been reported. Bifurcation methods have been widely used, to study nonlinear phenomena in aircraft flight dynamics. A bifurcation analysis must begin by computing all equilibrium and periodic solutions of that system along with information about the stability of these solutions. The bifurcation analysis of a multiparameter dynamical system involves solving a series of one-parameter problems of various forms. It is possible to capture multiple bifurcation points with a single computation, and this is illustrated with an example of a five-state, three-parameter dynamical system from aircraft flight dynamics. The key to our approach is to recognize that multiple bifurcations in the state-parameter space may be captured by a single computation solving. There is scope for devising better and more rigorous algorithms to arrive at the parameter constraint function, which forms the underpinning of the approach.

Computational bifurcation analysis of multiparameter dynamical systems

In this work, we present a speaker-normalization method based on the idea that the speaker-dependent scale-factor can be separated out as a fixed translation factor in an alternate domain. We also introduce a non-linear frequency-scaling model motivated by the analysis of speech data. The proposed shift-based normalization approach is implemented using a maximum-likelihood (ML) search for the translation factor in the alternate domain. The advantage of our approach is that we are able to show the relationship between conventional frequency-warping based vocal-tract length normalization (VTLN) methods and the methods based on shifts in psycho-acoustic scale thus providing a unifying frame-work for speaker-normalization. Additionally, in our approach it is simple to show that the shifting required for normalization can be expressed as a linear transformation in the cepstral domain. This is important for computational efficiency since we do not have to recompute the features by re-doing the signal processing for each scale/translation factor as is usually done in conventional normalization. We present recognition results using our proposed approach on a digit recognition task and show that the non-linear scaling model provides relative improvement of 4% for adults and 7.5% for children when compared to the linear-scaling model. © 2007 Elsevier B.V. All rights reserved.

Department of Electrical Engineering

Speech Communication

A shift-based approach to speaker normalization using non-linear frequency-scaling model

In this paper, we study the effect of filter bank smoothing on the recognition performance of children's speech. Filter bank smoothing of spectra is done during the computation of the Mel filter bank cepstral coefficients (MFCCs). We study the effect of smoothing both for the case when there is vocal-tract length normalization (VTLN) as well as for the case when there is no VTLN. The results from our experiments indicate that unlike conventional VTLN implementation, it is better not to scale the bandwidths of the filters during VTLN - only the filter center frequencies need be scaled. Our interpretation of the above result is that while the formant center frequencies may approximately scale between speakers, the formant bandwidths do not change significantly. Therefore, the scaling of filter bandwidths by a warp-factor during conventional VTLN results in differences in spectral smoothing leading to degradation in recognition performance. Similarly, results from our experiments indicate that for telephone-based speech when there is no normalization it is better to use uniform-bandwidth filters instead of the constant- like filters that are used in the computation of conventional MFCC. Our interpretation is that with constant- filters there is excessive spectral smoothing at higher frequencies which leads to degradation in performance for children's speech. However, the use of constant- filters during VTLN does not create any additional performance degradation. As we will show, during VTLN it is only important that the filter bandwidths are not scaled irrespective of whether we use constant- or uniform-bandwidth filters. With our proposed changes in the filter bank implementation we get comparable performance for adults and about 6% improvement for children both for the case of using VTLN as well as the for the case of not using VTLN on a telephone-based digit recognition task. © 2006 IEEE.

IEEE Transactions on Audio, Speech and Language Processing

A study of filter bank smoothing in MFCC features for recognition of children's speech

Aeolian vibrations in the frequency range of 10-50 Hz are ubiquitous in the conductors of transmission lines. They are usually not perceptible to the casual observers due to their small amplitudes, nevertheless, they are important as they may lead to failure of the lines through material fatigue. These problems become more severe for long-span lines. Since the conductors are fabricated in finite lengths, every once in a while two conductors have to be connected in the field, using appropriate fittings. A particular case of fitting, considered in this paper, is the compression splice bushing (CSB), which is sometimes used in long spans to join the ends of two conductors. It is desired to estimate the values of the maximum bending stress developed in the conductors and in related fittings due to aeolian vibrations. An obvious approach would be to solve the boundary value problem for the conductors with the CSB; here a model is presented that avoids the solutions of the complete boundary value problem and results in solving a set of linear algebraic equations. © 2006 Elsevier Ltd. All rights reserved.

Journal of Sound and Vibration

Wind-excited overhead transmission lines: Estimation of connection stresses at junctions

Illustrative examples are presented in this paper to show the applications of the continuation-algorithm-based, recently developed extended bifurcation analysis (EBA) methodology to compute optimum flight parameters of an F/A-18 high-angle-of-attack research vehicle aerodynamic model. A complete model describing the six-degree-of-freedom motion of aircraft is used in this analysis to compute trims for the aircraft in constrained flights, for example, straight and level and steady climb flights. Using the results of such computations, one can plot the performance curves for aircraft which are in contrast to the performance curves usually available in the aircraft design text books. Special features of the performance curves computed here are high-lighted and the reasons for the contrasting features between the performance curves obtained here and the corresponding ones presented in the aircraft design text books are discussed. Primary objective of this work is to show the efficiency of the EBA methodology in evaluating the optimum values of multiple objective functions which usually requires the use of the optimization theory-based numerical techniques and can be generally computationally exhaustive in terms of time and efforts. © IMechE 2007.

Optimum flight parameters and stability computations using continuation technique

In this paper, we study non-linear frequency-warping functions that are commonly used in speaker normalization. This study is motivated by our recently proposed affine transformation model for speaker normalization [1] which has provided improved recognition performance when compared to uniform scaling model [1, 2]. In this work, using formant data from Peterson & Barney and Hillenbrand vowel databases, we analyze the behavior of scale factor as a function of frequency. The empirical observation [3, 4] shows that while uniform scaling assumption may be valid at higher frequencies, there are significant deviations at low frequencies. We show that while our recently proposed model has behavior similar to the empirical result, the behavior of many of the commonly used non-linear models (including that of Eide-Gish, power law and bilinear transformation) differ significantly from the empirical result. This difference in behavior from the empirical observation may explain the limited improvement in recognition performance provided by these non-linear models when compared to conventional uniform-scaling model. We also show that our proposed model does better fitting to the formant data than these non-linear models. We, therefore, conclude that the affine-transformation model may be a more appropriate non-linear model for speaker normalization. © 2006 IEEE.