Mohammad M. Lone

Review of pilot modelling techniques

As aircraft increase in size, unsteady aerodynamics and aeroelasticity pose new handling qualities challenges. These require a greater understanding of the pilot-vehicle system as a whole. The level of interaction between the flying control system and the pilot increases as structural mode frequencies enter into the manual control frequency range. Therefore, accurate physiological and control-theoretic models of the pilot are crucial. Interactions range from conscious changes in pilot gain and equalisation to unconscious biodynamic feedthrough of structural modes. So far, use of theoretical pilot models has been an effective way of gaining handling qualities insight. Pilot modelling can be split into three areas: human sensory modelling, biomechanical modelling and control-theoretic modelling. To date, a review that brings together these aspects and provides a holistic view of pilot modelling has not been presented. This paper aims to address this literature gap and presents a review of the state-of-the-art. he design of civil aircraft raises interesting issues involving weight and efficiency tradeoffs. The more passengers a civil aircraft carries the more efficient it becomes, but at the same time its size and weight increase. These have to be compensated for by the design of a lightweight airframe. The end product is a large lightweight airframe that is characterised by reduced rigidity and consequently increased levels of aeroelasticity. Aeroelasticity can affect aircraft stability and control in ways which are often not fully appreciated or understood. As structural modes enter into the rigid-body dynamic frequency range, the flight control systems (FCS) relying on feedback from the various sensors around the airframe are also affected. These sensors can no longer distinguish between aeroelastic and rigid body dynamics and this can lead to unexpected FCS behaviour. This inability to distinguish between rigid body dynamics and aeroelastic effects is also inherent to the human sensory dynamics. Now as airframe structural modes frequencies encroach into the frequency ranges of human senses, biodynamics and control, traditional assumptions in handling qualities analysis no longer remain valid. Pilot’s perception of aircraft states is corrupted just like the onboard FCS sensors. Therefore, pilot introduced gain and equalisation in the pilot-vehicle system may no longer be appropriate. During routine flying, pilots tend to command the aircraft with airframe structural limits in mind. However, now there are possibilities of certain scenarios where the pilot and FCS combination may load the aircraft beyond its limits. Such a scenario may be caused by various triggers and the pilot may either be aware or unaware of the scenario. Triggering events can range from changes in pilot’s strategy to extreme atmospheric conditions. Investigation of such scenarios require first an understanding of aircraft manual control. Current civil aircraft effectively have three modes of operation. Aircraft control can be achieved through complete manual control with objectives from the pilot’s mind or manual control with objectives from a flight director. The aircraft can also be controlled via the mode control panel which commands the various autopilot modes; the pilot plays a more supervisory role here. Figure 1 presents the key components involved in the manual control mode. The system is driven by an objective (derived consciously) that is subconsciously processed by higher brain functions to derive a control action; a function of pilot experience and skill. This control action is applied through the neuromuscular system that is in turn affected by the human body’s response to the environment. Then it goes through the flying control system and the aircraft responds accordingly.

Pilot-model-in-the-loop simulation environment to study large aircraft dynamics

Effects of aeroservoelasticity on the manual control of large aircraft are investigated through a pilot modelling approach based on the modified optimal control model. A synopsis of modelling techniques is presented followed by the description of the adopted method. A simulation environment suitable for investigating pilot–vehicle dynamics in the longitudinal axis has been developed which couples an aeroelastic model of a large transport aircraft with the modified optimal control pilot model. The pilot model parameter selection was based on limiting the bandwidth. A comparison between a conventionally designed height hold autopilot and the pilot model control in continuous turbulence is made to demonstrate this simulation capability.

Modelling framework for flight dynamics of flexible aircraft

The flight dynamics and handling qualities of any flexible aircraft can be analysed within the Cranfield Aircraft Accelerated Loads Model (CA2LM) framework. The modelling techniques and methods used to develop the framework are presented. The aerodynamic surfaces were modelled using the Modified Strip Theory (MST) and a state-space representation to model unsteady aerodynamics. With a modal approach, the structural flexibility and each mode’s influence on the structure deflections are analysed. To supplement the general overview of the framework equations of motion, models of atmosphere, gravity, fuselage and engines are introduced. The AX-1 general transport aircraft model is analysed as an example of the CA2LM framework capabilities. The results showed that, according to the Gibson Dropback criterion, the aircraft with no control system lacks the stability and its longitudinal handling qualities are unsatisfactory. Finally, the steps for future developments of the CA2LM framework are listed within conclusions.

Entropy Generation Minimisation and Exergy analysis approaches for aerospace applications - A review

Based on the first and second law of thermodynamics, exergy analysis provides a method to optimise future aircraft by mapping the energy use through the entire system. By highlighting areas where useful energy is destroyed through entropy generation, exergy analysis can focus a designer on the design aspects that require improvement. This also provides a common energy-based metric allowing comparison of dissimilar technologies and cost analysis throughout a system. The paper will focus on the origins and make up of this analysis technique, and how this method may have the potential to develop new and innovative concepts that do not just marginally improve performance but may enable the realization of entirely new regimes of performance and operability. It will also discuss why second law derived methods are not being widely adopted outside the classic thermodynamic systems such as engines, and will look at areas for future development in the aerospace domain are also discussed.

Neural network-based dynamic model and gust identification system for the jetstream G-NFLA

Artificial neural networks are an established technique for constructing non-linear models of multi-input-multi-output systems based on sets of observations. In terms of aerospace vehicle modelling, however, these are currently restricted to either unmanned applications or simulations, despite the fact that large amounts of flight data are typically recorded and kept for reasons of safety and maintenance. In this paper, a methodology for constructing practical models of aerospace vehicles based on available flight data recordings from the vehicles’ operational use is proposed and applied on the Jetstream G-NFLA aircraft. This includes a data analysis procedure to assess the suitability of the available flight databases and a neural network based approach for modelling. In this context, a database of recorded landings of the Jetstream G-NFLA, normally kept as part of a routine maintenance procedure, is used to form training datasets for two separate applications. A neural network based longitudinal dynamic model and gust identification system are constructed and tested against real flight data. Results indicate that in both cases, the resulting models’ predictions achieve a level of accuracy that allows them to be used as a basis for practical real-world applications.

Evaluating the Rationale for Folding Wing Tips Comparing the Exergy and Breguet Approaches

The design and development processes for future aircraft aims to address the environmental and efficiency challenges needed to facilitate the engineering of concepts that are far more integrated and require a multidisciplinary approach. This study investigates the benefit of incorporating span extension wing tips onto future aircraft configurations as a method of providing improved aerodynamic efficiency, whilst allowing the extension to fold on the ground to meet airport gate size constraints. Although the actuated wing tips are not studied in detail, the focus of this study is to compare two different methods of analysis that can be used to identify the benefit and limitations of adding such devices. The two methods considered are a quasi-steady implicit energy analysis based on the Breguet Range Equation and an explicit energy analysis based on the first and second laws of thermodynamics known as Exergy Analysis. It has been found that both methods provide agreeable results and have individual merits. The Breguet Range Equation can provide quick results in early design, whilst the Exergy Analysis has been found to be far more extensive and allows the complete dynamic behaviour of the aircraft to be assessed through a single metric. Hence, allowing comparison of losses from multiple subsystems.

Towards the design of a pilot-induced-oscillation detection and mitigation scheme

Preliminary development of a real time pilot-induced oscillations detection and mitigation system that consists of a detector, based on Short Time Fourier Transform and autoregressive model, and an adaptive mitigation system is presented. The system is not only capable of detecting Type I and II pilot-induced oscillation characteristics but also focuses on the pilot’s behavioural trend by calculating the rate of change of the open-loop pilot-vehicle system crossover frequency. The detection system employs a sliding Short Time Fourier Transform method to identify frequency and phase characteristics by monitoring pilot input and aircraft states. An autoregressive model with exogenous inputs was also applied to identify pilot characteristics and estimate the rate of change of the crossover frequency. After detection, a pilot-induced oscillation cue is shown on the primary flight display and the adaptive controller is used to provide mitigation. The system was assessed through a series of desktop based piloted simulations where subjects conducted a number of tracking tasks. Results have shown that presenting the pilot with the state of the mitigation system via the visual PIO cue led to the opinion of reduced PIO tendencies. However, the lack of this information led to cases of over-control and increased workload, which in turn resulted in higher PIO ratings. The main factor affecting the results was the inability to induce the levels of urgency in pilot control behaviour for the assessing the impact of including the rate of change of open-loop PVS crossover frequency as a PIO feature. As a result only some of the test data showed its effectiveness. The real time calculation speed was constrained by the computer and the calculation algorithm.

Method to assess lateral handling qualities of aircraft with wingtip morphing

The potential of dihedral morphing wingtips for gust loads alleviation of high aspect ratio wings is currently under investigation. The impact of such devices on the handling qualities of an aircraft is investigated in this article. A specific morphing mechanism is introduced and implemented on a conventional long range flexible aircraft. Specific lateral-directional aerodynamic derivatives of the morphed vehicle were identified using the outputs of a 6 degrees of freedom flexible aircraft simulation. The modifications to aerodynamic derivatives with morphing were then implemented within the aircraft model of a real-time engineering flight simulator to evaluate the handling qualities and flight dynamics of the vehicle. Modifications made to the simulator and the development of the test campaign are given and discussed in this paper. The feasibility and relevance of each flight task was verified using the modified flight simulation framework. Modifications to CLp with wingtip morphing were verified to be tolerable using pilot in the loop simulations. Certified test pilots have been invited to participate in this study. A handling quality assessment will be made based on pilot qualitative and data post processing feedbacks. The morphing mechanism and flight control schemes will then be modified using these results to optimise the design.

Modelling Framework for Handling Qualities Analysis of Flexible Aircraft

Accurate prediction of interactions between aeroelasticty effects, flight control and flight dynamics are critical for the evaluation of handling qualities for future aircraft concepts. A simulation framework for such analyses, known as CA2LM, is presented in this paper together with the modelling techniques and methods used to develop the framework. Aerodynamic surfaces were modelled using the Modified Strip Theory (MST) approach and a state-space representation of the Leishmann-Beddoes unsteady aerodynamics model. A modal approach allows the analysis of structural flexibility and each mode’s influence on structural deflections. The derivation of equations of motion that provide a new approach to modelling flexible aircraft is presented. This takes into account the variations in mass and inertia properties due to structural deformations. The AX-1 generic large transport aircraft model is analysed as an example of the CA2LM framework capabilities. Results show that according to the Gibson Dropback Criterion longitudinal handling qualities of the AX-1 with no flight control system are unsatisfactory. The analysis of the structural stiffness influence shows that increasing flexibility of an aircraft structure deteriorates the longitudinal handling qualities. The short-period damping ratio is within Level 2 acceptability requirements. In contrast, the short-period frequency and acceleration sensitivity graph shows that the AX-1 model handling qualities are within Level 1 acceptability requirements. Finally, steps towards the future development of the CA2LM framework are listed.

Effect of wingtip morphing on the roll mode of a flexible aircraft

It is well known that increasing wing span leads to improved aerodynamic performances. To comply with airport infrastructure limits, ground folding wingtips are implemented as a solution for wing span extension. To further justify the mechanism’s weight penalty the concept of in-flight folding is investigated here. A time domain aeroservoelastic simulation framework is used to asses its impact on lateral flight dynamics. An established system identification method, was used to derive key lateral aerodynamic derivatives and investigate the aircraft’s roll handling qualities. A range of wingtip deflections and various flight conditions were used to generate a sufficiently large database of coefficients to assess the effect of wingtip morphing as a function of airframe flexibility and flight conditions. Results show that overall, small changes in lateral aerodynamic derivatives are introduced with wingtip morphing. Different trends in aerodynamic derivatives were identified as a function of flight condition and wingtip deflection, leading to the derivation of prediction models to replace the aerodynamic derivatives database.