Arthur W. Gubbels

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.

Rotorcraft simulation fidelity: new methods for quantification and assessment

Flight simulators are integral to the design/development, testing/qualification, training and research communities and their utilisation is ever expanding. The use of flight simulation to provide a safe environment for pilot training, and in research and development, must be underpinned by quantification of simulator fidelity. While regulatory simulator standards exist for flight training simulators and new standards are in development, previous research has shown that current standards do not provide a fully quantitative approach for assessing simulation fidelity, especially in a research environment. This paper reports on progress made in a research project at the University of Liverpool (Lifting Standards), in which new predicted and perceptual measures of simulator fidelity have been developed. The new metrics have been derived from handling qualities engineering practice. Results from flight tests on the National Research Council (Canada) Bell 412 ASRA research aircraft and piloted simulation trials using the HELIFLIGHT-R simulator at Liverpool are presented to show the efficacy of adopting a handling qualities approach for fidelity assessment. Analysis of the new metrics has shown an appropriate degree of sensitivity to differences between flight and simulation.

Multivariable Control of the Bell 412 Helicopter

In December 2005 the first flight-test took place of an H-infinity controller on the National Research Council of Canadas Bell 412 Advanced Systems Research Aircraft (ASRA). The ASRA is a helicopter which has been modified for research use; it is equipped with a programmable digital full-authority fly-by-wire control system. This paper presents results from the test, and discusses the collaborative project within which it took place. The aim of the project is to develop control laws meeting handling qualities criteria and providing structural load alleviation and flight envelope protection. Piloted simulation using Liverpools advanced flight simulator is a feature of the research. One of the first goals of the project was to produce simulation models of the test aircraft. The paper discusses how the new models were used in a control law design. Results from flight test of the control law are presented. Using linear analysis, closed-loop bandwidths of 4.2 rad/s in roll and 3.0 rad/s in pitch were accurately predicted

Helmet-mounted display research activity on the NRC Bell 205 airborne simulator

The National Research Council (NRC) of Canada, in conjunction with the Canadian Department of National Defence, is investigating the use of helmet-mounted displays as an aid in improving pilot situational awareness in all- weather search and rescue helicopter operations. For over 30 years, the NRC Bell 205 Airborne Simulator has been an integral part of valuable research programs. Equipped with a full authority fly-by-wire control systems, the Bell 205 has variable stability characteristics, which makes the airborne simulator the ideal platform for the integrated flight testing of helmet-mounted displays in a simulated operational environment. This paper will describe the test facility in detail, including a description of the airborne simulator, the fiber-optic helmet-mounted display hardware, the camera system, the head tracking system, and the sensor platform. In addition, the paper will provide a complete description of the symbology overlaid on the camera image that was developed for use with the helmet mounted display. The paper will conclude with the description of a planned preliminary handling-qualities investigation into the effects of using helmet-mounted displays on the pilots workload and performance while performing standard maneuvers.

Simulation of automatic helicopter deck landings using nature inspired flight control

The landing of a helicopter on a ship is one of the most dangerous of all helicopter flight operations. The Bell 412 advanced systems research aircraft is subject to a torque oscillation issue which increases pilot workload significantly when operating with low power margins and/or whilst performing tasks that require accurate torque control. This makes the deck landing task with this helicopter even more difficult. An automatic deck landing system was therefore developed. This system makes use of a novel control strategy for vertical control based on optical flow theory. Furthermore, it incorporates a torque envelope protection system. A successful automatic landing was performed in the flight simulator at the University of Liverpool. The novel control strategy created a very natural motion of the helicopter, similar to how a real pilot would fly it. The same control technique was subsequently applied to the simulation of an automatic lateral repositioning of a UH60 like helicopter in order to prove the generality of the technique. This manoeuvre was simulated successfully within level 1 handling qualities boundaries.

A rating scale for the subjective assessment of simulation fidelity

A new rating scale for capturing pilot subjective assessment of simulation fidelity is described in this paper. The scale has been developed through a series of flight and simulation trials using six test pilots from a variety of backgrounds, and is based on the methodology utilised with the Cooper-Harper Handling Qualities Rating scale and the concepts of transfer of training, comparative task performance and task strategy adaptation. The development of the new rating scale has been undertaken using simulations of rotary-wing aircraft on the University of Liverpool’s HELIFLIGHT-R research simulator, in conjunction with the Canadian Flight Research Laboratory’s Bell 412 ASRA in-flight simulator. The utility of the scale applied to locating fidelity boundaries for quantitative metrics is illustrated for an inter-axis coupling criterion. The work described in this paper is preliminary in nature, and research activities are on-going to continue the validation of the fidelity rating scale.

Nonlinear Attitude Control Laws for the Bell 412 Helicopter

Helicopters generally exhibit a ratelike response type in pitch and roll axes, and when feedback control is used to increase the level of augmentation to provide attitude command and attitude hold, there is generally a reduction in performance. Use of nonlinear elements in the control system can lead to recovery of some of this performance. The paper investigates such use of nonlinearities in the pitch control loop of a helicopter with a full-authority digital fly-by-wire control system. The nonlinear elements are used to specify the rate of response and thus the attitude quickness. Describing function analysis was used to test compliance with the relative stability requirements of MIL-F-9490D. The control laws were successfully flight-tested on a Bell 412 modified for fly-by-wire research and results from those tests are presented. Control laws of the type presented here can potentially be optimized to maximize agility within the available actuator limits. The first control law presented was intended to test the concept; modest pitch-axis performance was therefore specified. The second control law was designed to provide ADS-33E-PRF level-1 handling qualities for noncombat-mission task elements. Both controllers gave a stable closed loop and provided the required response type. Closed-loop bandwidth predictions based on analysis of linear models were close to the bandwidths achieved in flight. Likewise, the attitude quickness achieved in flight was very close to that specified via the nonlinear element.

In-flight evaluation of a fiber optic helmet-mounted display

The National Research Council of Canada (NRC), in conjunction with the Canadian Department of National Defence (DND), is investigating the use of helmet-mounted displays (HMD) to improve pilot situational awareness in all-weather search and rescue helicopter operations. The National Research Council has installed a visually coupled HMD system in the NRC Bell 205 Airborne Simulator. Equipped with a full authority fly-by-wire control system, the Bell 205 has variable stability characteristics, which makes the airborne simulator the ideal platform for the integrated flight testing of HMDs in a simulated operational environment. This paper presents preliminary results from flight test of the NRC HMD. These results are in the form of numerical head tracker data, and subjective handling qualities ratings. Flight test results showed that the HMD degraded handling qualities due to reduced acuity, limited field-of-view, time delays in the sensor platform, and fatigue caused by excessive helmet inertia. Some evidence was found to support the hypothesis of an opto-kinetic cervical reflex whereby a pilot pitches and rolls his head in response to aircraft movements to maintain a level horizon in their field-of- view.

The NRC Bell 412 ASRA safety system: a human factors perspective on lessons learned from an airborne incident

Abstract The National Research Council (NRC) Bell 205 Airborne Simulator is a full authority fly-by-wire (FBW) research helicopter. On 24 May, 1996 this aircraft underwent a failure which drove all four flight control actuators to full extension shortly after engagement of the FBW system, with nearly catastrophic results. The sound design inherent in the original Bell 205 safety system allowed the safety pilot to override the FBW system and prevented the loss of aircraft and crew. This incident, however, led to the realization that the existing safety system configuration in the Bell 205 was only marginally acceptable, and that this same system would be inadequate for the next generation FBW aircraft, the NRC Bell 412 Advanced Systems Research Aircraft (ASRA). Experience gained from the Bell 205 incident, and historical experience, has driven the design process of the safety systems for ASRA, with a particular view toward the capabilities and limitations of the operators.