Gareth D. Padfield

Integrating CFD and piloted simulation to quantify ship-helicopter operating limits

This paper describes a study which has been concerned with numerical predictions of the airwakes resulting from two simplified ship geometries: the internationally agreed Simple Frigate Shape, SFS1, and its successor, SFS2. Extensive steady-state simulations have been carried out for a wide range of wind conditions using Fluent, a commercially available Computational Fluid Dynamics (CFD) code. The CFD predictions have been partially validated against wind tunnel data produced by the National Research Council of Canada (NRC) and have shown good agreement. The resulting airwake velocity components have been exported from Fluent, interpolated onto suitable grids and attached to the FLIGHTLAB flight-simulation environment as look-up tables; piloted flight trials were then carried out using the Liverpool full-motion simulator. The pilot workload and helicopter control margins resulting from a range of wind-over-deck conditions have been used to develop the Ship-Helicopter Operating Limits (SHOL) for a Lynx-like helicopter and the SFS2. The workload was compared to the pilot’s experiences on a similar aircraft and a Type 23 Frigate and the simulated SHOL compared with SHOLs derived from sea trials. The results are very encouraging and open up further the long awaited prospect of such simulations being used in the future to reduce at-sea trials, and to provide a safe environment for pilot training.

Flight simulation in academia HELIFLIGHT in its first year of operation at the University of Liverpool

The challenges of helicopter simulation are being tackled across a broad front as technology is developed to meet the needs of Industry. Traditionally the most powerful driver has been the training community and this is likely to continue for some time as simulation technology advances at increasing pace, raising fidelity standards. The development of PC-based simulation technologies is providing a significant spur in this development and lowering the cost, making complete simulation systems of reasonably high fidelity available to smaller organisations. This paper describes the first year of operation with such a system at the University of Liverpool – HELIFLIGHT - developed by the Motionbase plc/ART Inc. partnership. With its full motion, wide field-of-view visuals, programmable force feel system and the comprehensive FLIGHTLAB modelling environment, we describe the system as high fidelity and the first year of utilisation has shown that such a facility is fit for extensive use in a variety of handling qualities and pilot-vehicle technology research and teaching. From a year of many highlights, the EU-funded programme to develop handling qualities for a civil tilt rotor aircraft is selected to demonstrate the capability of the HELIFLIGHT system.

First Steps in the Development of Handling Qualities Criteria for a Civil Tilt Rotor

This paper describes an approach to the development of Handling Qualities criteria for a civil tiltrotor. The work presented is the outcome of a combined effort of European research organizations and helicopter industries, working together in an European Commission sponsored critical technology program – RHILP (Rotorcraft Handling, Interactions and Loads Prediction). The RHILP approach to Handling Qualities has developed from the systems methodology in ADS-33. An important exercise has involved assembling a consistent set of Handling Qualities criteria appropriate to flight in rotorcraft, conversion and fixed-wing aircraft modes. Within this process a compatibility analysis has revealed inconsistencies and, more critically, gaps in the available data, particularly relating to flight in conversion mode. The paper outlines the approach taken and presents results from a series of piloted simulation trials carried out to provide supporting data which can be used to fill the gaps. The results presented are considered to be unique since there are no open publications available to date on tiltrotor aircraft Handling Qualities criteria.

Investigation of the Flare Maneuver Using Optical Tau

Airline transport operations are carried out in a wide range of visual and instrument meteorological conditions. However, the pilot can choose to land the aircraft manually using the visual cues available for all but the most limiting of degraded visibilities. How this is achieved may seem rather obvious, but has challenged researchers for some time. Optical flow theory offers a solution: pilots detect motion from the surfaces over which they move. In a relatively recent incarnation, flow theory transforms motion into the temporal, time-to-contact parameter, tau. Research conducted at Liverpool has applied this theory to low-level helicopter flight. The present paper extends the application to the fixed-wing flare. Flight simulation results show that tau-guidance strategies exist for this maneuver. It is shown that, as expected from tau theory, the pilot-selected values of the rate of change of tau with time, tau-dot, and the tau-guide coupling constant, directly influence the acceptability of the touchdown rate achieved. Degraded visual environments are shown, under certain circumstances, to cause a breakdown in the tau relationships observed. Potential uses of these results are presented in terms of application to future pilot vision aids, which is the planned next stage of this work.

How Do Helicopter Pilots Know When to Stop, Turn or Pull Up? (Developing Guidelines for Vision Aids)

The title of this paper, posed as a question, reflects the current interest in gaining an improved understanding of visual perception in flight control to inform the development of design guidelines for future pilot vision aids. The paper develops the optical flow theory of visual perception into its most recent incarnation, tau-coupling, where tau is the time to closure to surfaces at current velocity. General tau-theory posits that the closure of any type of gap, using any form of sensory input, is guided by sensing and constantly adjusting the tau of the gap. According to the theory, and contrary to what might be expected, information about the distance to obstacles or the landing surface, for example, and about the speed and deceleration of approach, are not necessary for precise control of landing or stopping. Analysis is presented that supports the importance of tau-coupling in flight control. Results from simulation trials conducted at DERA and at The University of Liverpool demonstrate the considerable power of what we describe as tau-guides, that lead the pilot to adopt a prospective flight control strategy.

Rotorcraft simulation modelling and validation for control law design

This paper describes the development and validation of a high fidelity simulation model of the Bell 412 helicopter for handling qualities and flight control investigations. The base-line model features a rigid, articulated blade-element formulation of the main rotor, with flap and lag degrees of freedom. The Bell 412 HP engine/governor dynamics are represented by a second-order system. Other key features of the base-line model include a finite-state dynamic inflow model and lag damper dynamics. The base-line model gives excellent agreement with flight-test data over the speed range 15-120kt for on-axis responses. Prediction of off-axis responses is less accurate. Several model enhancement options were introduced to obtain an improved off-axis response. It is shown that the pitch/roll off-axis responses in transient manoeuvres can be improved significantly by including wake geometry distortion effects in the Peters-He finite-state dynamic inflow model.

Acceptance testing and commissioning of a flight simulator for rotorcraft simulation fidelity research

The rotorcraft industry faces a number of challenges today regarding the replacement of ageing airframes, an expansion in the operational roles of helicopters and a requirement to improve safety whilst reducing the environmental impact of rotorcraft operations. The quantification of simulation fidelity underpins the confidence required for the expanding use of modelling and simulation to develop solutions to these challenges in a timely and cost-efficient manner. Current simulator certification standards do not provide a fully quantitative method for assessing simulation fidelity, especially in a research environment. This article details the commissioning and acceptance process of the new research flight simulation facility at the University of Liverpool, HELIFLIGHT-R, and its subsequent use in a research project ‘Lifting Standards: A Novel Approach to the Development of Fidelity Criteria for Rotorcraft Flight Simulators’ aimed at developing new predicted and perceptual measures of simulator fidelity. Some initial results from both piloted simulation and flight tests using the Bell 412 Advanced Systems Research Aircraft are reported within the context of the rotorcraft simulation fidelity project.

Ship-Helicopter Operating Limits Prediction Using Piloted Flight Simulation and Time-Accurate Airwakes

This paper gives an overview of the ship―helicopter dynamic interface simulation facility at the University of Liverpool, with an emphasis on recent improvements made through the inclusion of unsteady computational fluid dynamics (CFD) ship airwake data. A FLIGHTLAB model of an SH-60B Seahawk helicopter has been flown in a full motion base simulator to the deck of a Type 23 frigate and a Wave class auxiliary oiler, under the influence of unsteady airwakes derived from CFD. Pilot workload ratings have been obtained for the deck landing task, using both the Bedford workload rating scale and the deck interface pilot effort scale, from which fully simulated ship―helicopter operating limits have been derived. Analysis of pilot ratings, comments, and control inputs has also enabled both subjective and objective assessments of workloads at various wind-over-deck conditions, highlighting the dominant aerodynamic airwake features which contribute to the difficulty of the landing task. Having access to the underlying CFD data allows the aircraft handling qualities and pilot workload to be correlated with the aerodynamic characteristics of the airwake and identification of the geometric features of the ship that cause them.

Load alleviation in tilt rotor aircraft through active control; modelling and control concepts

This paper presents the first results from research into active control of structural load alleviation (SLA) for Tiltrotor aircraft carried out in the European ‘critical technology’ RHILP project. The importance of and the need for SLA in Tiltrotors is discussed, drawing on US experience reported in the open literature. The paper addresses the modelling aspects in some detail; hence forming the foundation for both the FLIGHTLAB simulated XV-15 and EUROTILT configurations. The primary focus of attention is the suppression of in-plane rotor yoke loads for pitch manoeuvres in airplane mode; without suppression these loads would result in a very high level of fatigue damage. Multi-variable control law design methods are used to develop controller schemes and load suppression of 80-90% is demonstrated using rotor cyclic control, albeit at a 20-30% performance penalty. However, rotor flapping transients tend to increase by the action of the SLA system. A dual-objective control design approach demonstrates the effectiveness of suppressing both loads and flapping simultaneously. Symbols a1, a2 Left and right rotor gimbal longitudinal tilt 2 1 a , a & & Left and right rotor gimbal longitudinal tilt rates an Normal acceleration Iβ Flap moment of inertia of a rotor blade Mz In-plane moment Mzb,1,2,3 In-plane moment at the blade root

Tau Guidance in Boundary-Avoidance Tracking: New Perspectives on Pilot-Induced Oscillations

DOI: 10.2514/1.54065 Tau theory, introduced to the flight control discipline as a model for natural guidance, is shown to provide an approach to predicting a class of adverse aircraft-pilot couplings described as boundary-avoidance tracking events and pilot-induced oscillations. These have previously been modeled a posterior as discrete events using timedependent feedback gains. Drawing on the prospective nature of the time-to-contact variable optical tau � , a new method is proposed for modeling such phenomenon and also for determining the critical incipience for this class of aircraft-pilotcoupling.Inthepresentstudy,theapproachhasbeenappliedtotauguidanceinarotorcrafttrajectory tracking maneuver, to predict the conditions under which aircraft-pilot couplings may occur. In addition, a strong correlation between motion and control activity and the derivatives of tau adds substance to the hypothesis that the pilot’sperceptualsystemworksdirectlywithinvariantsintheoptical flowduringvisualguidance.Resultsfrom flight simulation tests conducted at the University of Liverpool and complementary flight tests carried out with the National Research Council (Canada) advanced systems research aircraft in-flight simulator support the tau control hypothesis. The theory suggests ways that pilots could be alerted to the impending threat of such adverse aircraftpilot couplings.