Daniel J. Walker

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.

Helicopter Flight Control Law Design Using H ∞ Techniques

This paper describes the development of H∞controllers for two helicpter models. The study was first carried out using rigid body state feedback only and was then extended to include the rotor flap and lag states. It was observed that the H∞controllers gave better robustness and performance than the baseline controllers. Moreover, using rotor state feedback, it was posible to design high bandwidth controllers.

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

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

Load alleviation for a tilt-rotor aircraft in airplane mode

Results from research into active structural load alleviation are presented for a tilt-rotor aircraft. The work formed part of the European Commission’s Fifth Framework “critical technology” Rotorcraft Handling, Interactions, and Loads Prediction project. Results are presented from an analysis of the structural loads of a tilt-rotor aircraft maneuvering in airplane mode. The potential for suppression of structural loads through active control is assessed. The study has addressed modeling aspects, particularly the nature of the buildup of dynamic loads during maneuvers. The Eurocopter EUROTILT configuration has been used as the test aircraft and a model is developed within the FLIGHTLAB simulation environment. Mathematical analysis and simulation of the rotor in-plane moments and gimbal flapping during longitudinal and lateral maneuvers are presented. The problem also addressed the torque split on the interconnect drive shaft in airplane mode for maneuvers involving large roll and yaw rates. Attempts were made to design an “ideal” controller that minimizes the gimbal flap excursions and the loads using a linear quadratic Gaussian formulation. In this preliminary conceptual study, robustness, actuator requirements, or the availability of measurements were not considered. The main objective of the controller design study was to investigate the effectiveness of cyclic controls to achieve simultaneous suppression of the loads and flapping. Evaluations performed using the controller show that use of cyclic controls is very effective in suppressing the buildup of in-plane loads, gimbal flapping, and interconnect drive shaft torque split.

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.

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.

An LPV control law design and evaluation for the ADMIRE model.

This chapter presents the design and evaluation of an LPV control law for the ADMIRE model over a specified wide flight envelope, including subsonic, transonic and supersonic regions. The design of the LPV control law is based on the parameter-dependent Lyapunov function approach with gridding of the parameter space. It is demonstrated that by using a linear piece-wise interpolation of the aircraft model the LPV approach allows the design of a controller for the whole flight envelope (including the transonic region) with satisfactory performance and robustness characteristics. The longitudinal LPV controller provides an automatic transition from the α-demand system at Mach numbers M < 0.58 to the nz-demand system at M < 0.62; in the intermediate region a mixed control principle is implemented. A thorough evaluation of the designed LPV controllers is performed using a number of methods, including time and frequency domain criteria, linear and nonlinear simulation tests, and also piloted simulation in real time on the HELIFLIGHT simulator at the University of Liverpool. The performed evaluation clearly demonstrates that the designed LPV control laws satisfy most of the design requirements. Ways of further improving the performance of the LPV controller are discussed at the end of the chapter.

Helicopter load alleviation using active control

This paper discusses how structural load objectives can be included in a rotorcraft flight control system design in an efficient and straightforward way using multivariable control techniques. Several research studies have indicated that pitch link loads for various rotorcraft types can reach high or even unacceptable values, both in steady state and maneuvering flight. This is especially the case for high-speed aggressive maneouvers. Pitch link loads at high-speed flight are therefore taken as a case study. A novel longitudinal control system is presented, designed to reduce helicopter pitch-link loads during high-speed longitudinal manoeuvres whilst providing a pitch attitude command attitude hold response type. The design is based on a high-order model of a helicopter representative of the UH-60 Black Hawk. New metrics are presented for the analysis of structural loads that can be used in combination with ADS-33 handling qualities requirements.