Douglas Thomson

The principles and practical application of helicopter inverse simulation

Inverse simulation is a technique whereby the control actions required for a modelled vehicle to fly a specified manoeuvre can be established. In this paper the general concepts of inverse simulation are introduced, and an algorithm designed specifically to achieve inverse simulation of a single main and tail rotor helicopter is presented. An important element of an inverse simulation is the design of the input functions i.e. manoeuvre definitions, and the methods used are also detailed. A helicopter mathematical model is also discussed along with the validation and verification of the inverse simulation. Finally, the applicability of the method is demonstrated by illustration of its use in two flight dynamics studies.

Helicopter Inverse Simulation Incorporating an Individual Blade Rotor Model

Inverse simulation is used to calculate the control displacements required for a modeled vehicle to perform a particular maneuver. As with all simulations, the usefulness of the technique depends on the validity of the mathematical model used. To incorporate the latest forms of helicopter model (individual blade models) in an inverse framework, it has been necessary to modify existing inverse techniques. Such a model is described in this paper along with an inverse algorithm capable of accommodating it, The resulting simulation is validated against flight data and comparisons are made with a more basic model. It is shown that the basic disk model compares well with the more comprehensive individual blade model until the severity of the maneuver is increased to encompass flight states where nonlinear aerodynamics effects are prevalent.

Mathematical Definition of Helicopter Maneuvers

Helicopter performance and handling qualities are now routinely assessed in relation to specific maneuvers. The use of inverse simulation is then attractive as it allows a helicopter mathematical model to be driven by a pre‐defined maneuver, and the control histories required to achieve this trajectory to be found. The manner in which maneuver is defined is then of great importance, and this paper aims to outline some appropriate techniques for modelling helicopter maneuvers. The basic principles and requirements for inverse simulation are introduced, and several methods of defining trajectories are described and examples given. Finally, for validation purposes, models of the Rapid Sidestep and Transient Turn Mission Task Elements (MTEs) are compared with flight data.

Model Predictive Control Architecture for Rotorcraft Inverse Simulation

A novel inverse simulation scheme is proposed for applications to rotorcraft dynamic models. The algorithm adopts an architecture that closely resembles that of a model predictive control scheme, where the controlled plant is represented by a high-order helicopter model. A fast solution of the inverse simulation step is obtained on the basis of a lower-order, simplified model. The resulting control action is then propagated forward in time using the more complex one. The algorithm compensates for discrepancies between the models by updating initial conditions for the inverse simulation step and introducing a simple guidance scheme in the definition of the tracked output variables. The proposed approach allows for the assessment of handling quality potential on the basis of the most sophisticated model, while keeping model complexity to a minimum for the computationally more demanding inverse simulation algorithm. The reported results, for an articulated blade, single main rotor helicopter model, demonstra...

Issues of numerical accuracy and stability in inverse simulation

Abstract Inverse simulation algorithms based on integration have been widely applied to predict the control input time histories required for aircraft to follow ideally defined manoeuvres. Several different inverse simulation algorithms are available but these different methods are all subject to a number of numerical and stability problems, such as high frequency oscillation effects and also lower frequency oscillatory phenomena termed “constraint oscillations”. Difficulties can also arise in applications involving discontinuous manoeuvres, discontinuities within the model or input constraints involving actuator saturation. This paper has shown that the dynamic response properties of the internal system are the cause of the so-called “constraint oscillation” phenomenon. In addition, a new inverse simulation approach based on the constrained derivative-free Nelder–Mead search-based optimisation method has been developed to eliminate problems of discontinuities and saturation. Simulation studies involving nonlinear ship models suggest that this new approach leads to improved properties in terms of convergence and numerical stability.

Sensitivity-analysis method for inverse simulation application

An important criticism of traditional methods of inverse simulation that are based on the Newton–Raphson algorithm is that they suffer from numerical problems. In this paper these problems are discussed and a new method based on sensitivity-analysis theory is developed and evaluated. The Jacobian matrix may be calculated by solving a sensitivity equation and this has advantages over the approximation methods that are usually applied when the derivatives of output variables with respect to inputs cannot be found analytically. The methodology also overcomes problems of input-output redundancy that arise in the traditional approaches to inverse simulation. The sensitivityanalysis approach makes full use of information within the time interval over which key quantities are compared, such as the difference between calculated values and the given ideal maneuver after each integration step. Applications to nonlinear HS125 aircraft and Lynx helicopter models show that, for this sensitivity-analysis method, more stable and accurate results are obtained than from use of the traditional Newton–Raphson approach.

Hybrid inverse methods for helicopters in aggressive manoeuvring flight

Abstract The need to implement a process of constraints handling into conventional inverse simulation represents an important problem. To achieve this, a modification of the conventional inverse simulation technique which includes predictive capability is proposed in this paper. After giving details of the development of the predictive inverse simulation algorithm, the paper demonstrates the applicability of the developed methodology on two aggressive helicopter manoeuvres: pop-up and lateral realignment.

Flight Dynamics Investigation of Compound Helicopter Configurations

Compounding has often been proposed as a method to increase the maximum speed of the helicopter. There are two common types of compounding known as wing and thrust compounding. Wing compounding offloads the rotor at high speeds, delaying the onset of retreating blade stall, and hence increasing the maximum achievable speed, whereas with thrust compounding, axial thrust provides additional propulsive force. There has been a resurgence of interest in the configuration due to the emergence of new requirements for speeds greater than those of conventional helicopters. The aim of this paper is to investigate the dynamic stability characteristics of compound helicopters and compare the results with a conventional helicopter. The paper discusses the modeling of two compound helicopters, which are named the coaxial compound and hybrid compound helicopters. Their respective trim results are contrasted with a conventional helicopter model. Furthermore, using a numerical differentiation technique, the dynamic stabil...

A Simulation Study of Helicopter Ship Landing Procedures Incorporating Measured Flow-Field Data:

Abstract The aim of this article is to investigate the use of inverse simulation to help identify those regions of a ships flight deck which provide the safest locations for landing a rotorcraft in various atmospheric conditions. This requires appropriate information on the wind loading conditions around a ship deck and superstructure, and for the current work, these data were obtained from wind tunnel tests of a ship model representative of a typical helicopter carrier/assault ship. A series of wind tunnel tests were carried out on the model in the University of Glasgows 2.65 × 2.04 m wind tunnel and three-axis measurements of wind speed were made at various locations on the ship deck. Measurements were made at four locations on the flight deck at three different heights. The choice of these locations was made on the basis of preliminary flow visualization tests which highlighted the areas where the most severe wind effects were most likely to occur. In addition, for the case where the wind was from 30° to starboard, measurements were made at three further locations to assess the extent of the wake of the superstructure. The generated wind profiles can then be imposed on the inverse simulation, allowing study of the vehicle and pilot response during a typical landing manoeuvre in these conditions. The power of the inverse simulation for this application is demonstrated by a series of simulations performed using configurational data representing two aircraft types, a Westland Lynx and a transport helicopter flying an approach and landing manoeuvre with the worst atmospheric conditions applied. It is shown from the results that attempting to land in the area aft of the superstructure in a 30° crosswind might lead to problems for the transport configuration due to upgusts in this area. Attempting to perform the landing manoeuvre in an aggressive manner is also shown to lead to diminished control margin in higher winds.

Application of Inverse Simulation to a wheeled mobile robot

This paper presents the application of Inverse Simulation to the control of a mobile robot. The implementation of this technique for motion control has been found to provide highly accurate trajectory tracking. Since the input to the Inverse Simulation is a time history of the desired response, then greater control over the position and orientation of the mobile robot can be achieved. There are many situations where the desired path of a mobile robot is known e.g. planetary rover navigation, factory or warehouse floor, bomb disposal. Typically the robot is either controlled remotely or runs an online controller to navigate the desired path. For a given path, a navigation system generates the desired drive parameters (i.e. heading and velocity) and the associated controllers drive the corresponding actuators. Traditionally the controllers are required to be tuned using knowledge of the limitations of the mobile robot. Inverse Simulation provides a means of generating the required control signals with no need for controller tuning. The use of Inverse Simulation is suitable in cases where the cost of the mobile robot or actuators is high, desired drive requirements need to be met or for situations where tight tolerances on the trajectory are to be achieved. In this paper the benefits of applying Inverse Simulation to the control of a mobile robot are discussed and appropriate results presented.