Ranjith Mohan

Ranjith Mohan
Indian Institute of Technology Madras | IIT Madras · Department of Aerospace Engineering

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40
Publications
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151
Citations
Introduction
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Publications

Publications (40)
Article
The paper investigates the unpowered descent of a rotor system through the upper atmosphere. Axial and helical trajectories are investigated in the context of fixed points as well as an optimal control problem for maximizing flight time. The mathematical model considered in the paper incorporates the fuselage degrees of freedom, dynamic inflow mode...
Article
Dynamic soaring is a technique for extending flight times by utilising wind gradients. It was originally observed amongst birds like albatrosses and has huge potential for high-endurance UAV flights. In this paper, we investigate the stability of dynamic soaring orbits from a linear periodic system perspective and explore the feasibility of using a...
Article
The paper demonstrates applications of pseudospectral-method-based framework to rotorcraft dynamics. The framework is developed to incorporate blade-to-blade symmetry (fast-Floquet) to reduce computational time. The methodology is applied to compute Floquet exponents of an isolated rotor in forward flight using the periodic eigenvector equations, a...
Article
The advent of efficient numerical algorithms and powerful computing resources have made real-time optimization a reality. However, for systems like 6DoF aircraft, the problem remains challenging due to the complexity and fast dynamics of the system. Smaller optimization problems with fewer constraints can be obtained from a differential flatness-ba...
Article
Numerical solutions of flatness-based reformulation of optimal control problems lead to an optimization problem with fewer variables and constraints. However, the expressions for the states and controls in terms of the flat output variables can be highly nonlinear and complicated. Hence, this may lead to expensive function evaluations within the op...
Preprint
Full-text available
In this paper, a nonlinear model predictive controller for path following of a surface vessel in the presence of regular waves is studied. A model predictive controller is developed for a 3DoF unified manoeuvring model of the KVLCC2 crude oil tanker. The surge, sway and yaw motions of the ship are influenced by the second order wave drift forces an...
Article
Full-text available
We present compact closed-form expressions of the lift and moment coefficients predicted by thin-airfoil theory for camber line geometries of the NACA 4 and 5-digit airfoils and a generalized polynomial thin-airfoil. For the generalized camber curve, the solution is achieved by means of the integral form of the beta function. These general solution...
Article
This paper investigates three-dimensional aerodynamic effects due to radial flow on lead–lag damping of a rotor in forward flight conditions. Three-dimensional effects in this study are restricted to yawed flow aerodynamics and radial flow coupling between blade segments. These effects are included in the ONERA dynamic stall model, and lead–lag dam...
Article
Periodic trajectories that satisfy stability-based requirements are useful in many engineering applications. In this work, a time-domain approach using Fourier collocation for solving the Floquet eigenvector boundary value problem, and consequently estimating Floquet exponents and eigenvectors, is introduced. This method is incorporated into a Four...
Preprint
Full-text available
The development, validation, and applications of an object-oriented free-wake solver for multi-rotor and fixed-wing systems are outlined here. Advantages of utilizing the object-oriented philosophy for modeling the multi-rotor/wing free-wake problem are described. To explore the feasibility of utilizing conventional desktop workstations, the vortex...
Conference Paper
In this paper, we use a flatness-based approach for computing constrained trajectories while optimizing an objective function for a six degree of freedom aircraft. The resulting optimal control problem in flat output space is numerically solved using Legendre-Gauss-Lobatto pseudospectral method. A real-time linear optimal controller based on flatne...
Article
Aeromechanical instabilities arising due to frequency coalescence of different modes of a helicopter rotor and fuselage system is investigated. The focus is on removing the small angle approximation typically used for ground resonance analysis and perform simulation of the resulting nonlinear model. The results reveal significantly different charac...
Conference Paper
This paper studies aerodynamic effects of ground on regressive lag mode damping during ground resonance. The experimental investigation is performed on a scaled helicopter model built to simulate the ground resonance scenario. The study involves both stationary as well as dynamic (oscillating) ground conditions. Experiments are conducted by measuri...
Article
The flatness of a six-degree-of-freedom (6DoF) aircraft model with conventional control surfaces – aileron, flap, rudder and elevator, along with thrust vectoring ability is established in this work. Trajectory optimisation of an aircraft can be cast as an inverse problem where the solution for control inputs that yield desired trajectories for cer...
Conference Paper
Full-text available
Cheeseman's factor for ground effect-based on image sources-has widely been used to account for the increase in thrust experienced by a lifting rotor in ground effect. However, for elevations lesser than half the rotor radius, the factor is known to drastically over predict this thrust augmentation. Even though these elevations may not be viable to...
Conference Paper
Full-text available
A well-known phenomenon in rotorcraft, the ground effect, occurs when a rotor is at close proximity to the ground creating an obstruction to the trailing wake from the rotor. This results in an increase in thrust and a decreased inflow through the rotor disk, in turn affecting handling qualities. Until recently, analytical modelling of ground effec...
Article
Full-text available
This paper addresses how dynamic inflow, while improving the correlation with ground-resonance measurements, could make the mode identification of the rotor-body-inflow system demanding. Specifically, the paper demonstrates dynamic inflow effects on the modes of two earlier tested configurations: configuration 1 with a nonmatched stiffness rotor (n...
Conference Paper
Full-text available
Rotorcraft flying in close proximity to ground are known to generate a larger thrust due to the shed wake being obstructed by the ground. The effect has been well researched and empirical corrections are available in literature. However, studies on the influence of side walls on ground effect is still lacking. Such studies may be applied to full sc...
Article
Full-text available
Dynamic soaring is the rationale behind the prolonged flights of a seabird Albatross. It involves utilization of energy from the wind shear present near the earth surface. Small unmanned aerial vehicles (UAVs) can be kept loitering without any external power input by dynamic soaring. In this work, dynamic soaring is used to power UAVs for the surve...
Conference Paper
Ground resonance is a type of aeromechanical instability that occurs when the helicopter is in contact with the ground. It may occur due to coalescence between frequencies of two modes of the system if damping is insufficient. In this paper, we analyze the phase relations between the fuselage states corresponding to the least damped mode (regressiv...
Conference Paper
Dynamic inflow effects on stability and modes of a coupled rotor-fuselage system are investigated. Dynamic inflow effects are incorporated using Peters-He model. Characteristics of different modes associated with a coupled rotor-fuselage system are investigated. In addition to the magnitude of the states in the eigenvector (periodic eigenvector in...
Conference Paper
Full-text available
The influence of rotor wake and ground effect on aeromechanical instabilities, specifically ground resonance is investigated. The analytical model considered has fuselage pitch, roll and blade flap, lag degrees of freedom. To analyze the system stability at different rotor rotational speeds, modal damping and frequency is computed using Floquet met...
Conference Paper
Full-text available
The paper focuses on analysis of aeromechanical instabilities, specifically ground resonance in helicopters and active control methods for improving the existing stability margins. First, a simplified model of coupled rotor-fuselage system with translational fuselage degrees of freedom and blade lead-lag degree of freedom is considered. Anisotropy...
Article
We describe the use of a spectral collocation method to compute the characteristics of incompressible, viscous flow in a lid driven right triangular cavity with wall motion away from the right angle. First, the 2-D Gauss–Lobatto grid on the square [-1,1]×[-1,1][-1,1]×[-1,1] is mapped to a triangular domain by means of a singular mapping. A singular...
Article
The current predictive capability for modal frequencies, particularly in the presence of frequency coalescence and branching, merits significant improvement. No different is the current appreciation of the role and effectiveness of fast- Floquet, generalized-Floquet and generalized-fast-Floquet methods as feasible alternatives to regular Floquet me...
Article
A framework is presented for extracting interpretive models of airwake autocorrelation and autospectrum as well as crosscorrelation and cross-spectrum froma database. These models have a simple analytical structure that aids routine simulation and application as a predictive tool. Airwake refers to turbulence shed from the ship superstructure, and...
Article
The dissertation investigates helicopter trim and stability during level bank-angle and diving bank-angle turns. The level turn is moderate in that sufficient power is available to maintain level maneuver, and the diving turn is severe where the power deficit is overcome by the kinetic energy of descent. The investigation basically represents desig...
Article
The measurements of regressive lag mode (RLM) damping are correlated with theories that include dynamic inflow, quasi-steady stall drag, and three dynamic stall lift models: Leishman-Beddoes, ONERA-EDLIN (Équation Differentielle Linéaire), ONERA-BH (Bifurcation de Hopf). The database refers to an isolated hingeless rotor with three torsionally stif...
Article
In stability analysis from Floquet theory, the current predictive capability for mode identification merits significant improvement, particularly in rotor-fuselage or aeromechanical stability of hingeless and bearingless helicopters. Accordingly, this mode identification is addressed under ground contact, hovering and forward flight conditions from...
Article
A framework based on perturbation theory is presented for extracting crosscorrelation and cross-spectrum models from a database from experimental and CFD-based investigations. This framework covers all three velocity components at two arbitrarily chosen points on a horizontal plane and for arbitrary orientation of relative wind. These extracted mod...
Article
This paper presents the strengths and weaknesses of an earlier-proposed framework based on perturbation theory for extracting approximate analytical models of airwake from a database. Accordingly, the framework is tested against a comprehensive database of autospectra from both experimental and CFD-based investigations. The database covers a wide r...

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