ESTRUCTURA CURRICULAR DEL CICLO SUPERIOR EN EDUCACIÓN FÍSICA

For the purposes of the pilot model development exercise described in Chapter 7, a simulation model representing a light aircraft was required. The aircraft library of the HELIFLIGHT simulator included a model of the Grob Tutor; a single-engine, two-seat light training aircraft [69].

Figure 47. Grob 115 Tutor. Image from Ref. 69.

A prerequisite for the implementation of one of the pilot models described in Chapter 7 was the presence of a stability control augmentation system (SCAS) on this aircraft model to give the appropriate response type. Specifically, it was necessary for the aircraft to feature an Attitude Command Attitude Hold (ACAH) response type in the roll axis. As such a system was not present on the standard aircraft model, it was deemed necessary to implement a simple SCAS. In order to allow rapid prototyping of such a system, a linear approximation of the Grob Tutor FLIGHTLAB model was developed in Simulink (Figure 48).

Figure 48. Simulink implementation of linear Grob Tutor aircraft model.

Figure 48 shows that inputs from any of the four controls (lateral and longitudinal stick, throttle and rudder pedals) were passed to a state-space model of the aircraft dynamics. This block contained matrices describing the linear dynamics of the aircraft model for a particular flight condition, relating each of the inputs to a number of model outputs. These matrices were obtained from the full, non-linear FLIGHTLAB model of the Grob Tutor through the linearization process defined in Ref. 62. Note that in this instance the control inputs fed to the aircraft dynamics block were purely a function of their respective stick inputs, with no feedback from the aircraft response. For this reason, it is referred to as an open-loop system. The model outputs shown in Figure 48 include aircraft body rates ( , , ), angles ( , , ) and velocities ( , , ). From these, it was possible to

obtain the response of the unagumented baseline aircraft model to a step input in any of the four controls. An example is shown in Figure 49 for a unit amplitude step in lateral stick from straight and level flight at 90kts.

Figure 49. Response of linear and non-linear aircraft models to a unit step in lateral stick.

Figure 49 shows the response of both the nonlinear FLIGHTLAB model and the linear Simulink model to a unit step in lateral stick. The results demonstrate that this lateral stick input caused the aircraft to roll at an approximately constant rate of 0.4deg/sec following a short transient period. This shows that the unagumented aircraft featured a rate response type in the roll axis, as a constant stick displacement resulted in a constant roll angle. The close concurrence of the linear and nonlinear results demonstrates that the linear model shown in Figure 48 was an appropriate basis for further controller development.

It should be noted that it is beyond the scope of this study to undertake a full controller design exercise, with its associated measures of controllability, observability and robustness. Instead, the intention was to implement a simple system which would sufficiently augment the flight dynamics of the aircraft model for the purposes of the pilot modelling

exercise described in Chapter 7. As such, the SCAS described in this Section is not proposed as an alternative to more sophisticated existing solutions, but rather as a simplified version. The fundamental design principle for this simplified roll axis SCAS can be summarised as follows:

1. Measure current roll angle

2. Compare current roll angle with commanded roll angle

3. Generate a command signal as a function of the difference between the current roll angle and the commanded value

4. Feed control signal to the lateral control channel

5. Return to step 1

Fundamentally, therefore, if the difference between the lateral stick command and the roll angle is minimised by the SCAS, a constant amplitude lateral stick input would result in a constant roll angle being held. This is referred to as Attitude Command Attitude Hold (ACAH). Figure 50 shows the Simulink implementation of the basic system architecture (defined by Ref. 15) required to achieve this form of aircraft response.

Figure 50. Simulink implementation of ACAH SCAS to linear Grob Tutor aircraft

model.

Figure 50 shows that the control inputs to the aircraft dynamics block were augmented with signals fed back from the outputs. For example, the roll axis input signal contains feedback from both the roll rate and roll angle in addition to the lateral stick signal. Typically both of these loops are used to give a stable ACAH type response [15]. The behaviour of this closed-loop system was determined by the values of the controller gains in the rate ( ) and attitude ( ) channels. These determine the relative

amplitude of the three contributions to the control signal; small controller gains typically give a system which is responsive to pilot control input but lightly damped. Conversely, high controller gains increase the relative amplitude of the stabilising command, thus reducing the responsiveness to

pilot control input but increasing stability. On this basis, it was possible to tune the system to give an appropriate response for the pilot model development exercise described in Chapter 7. Note that for reasons of simplicity, the controller gains used for this exercise were defined as proportional gains, whereas a more advanced system would typically feature more sophisticated controller types such as proportional-integral (PI) [15]. However, it was found that a satisfactory aircraft response could be achieved without the need for this added complexity (Figure 51).

Figure 51. Response of baseline and ACAH- enabled aircraft models to a unit step in

lateral stick.

Figure 51 shows a comparison between the response of the baseline and ACAH-enabled aircraft models to a unit step in lateral stick. The results demonstrate that the ACAH SCAS suppressed the roll rate response of the aircraft after an initial increase. Consequently, the roll attitude achieved and sustained a constant value of approximately 1°, essentially emulating the shape of the control input. Thus, it was demonstrated that the dynamics

of the aircraft model had been augmented to give an attitude response type with a 1:1 relationship between lateral stick and roll angle (i.e. 1% stick resulted in 1° of roll angle). This SCAS was subsequently implemented in CSGE to enable its use with the nonlinear FLIGHTLAB model for the pilot modelling exercise described in Chapter 7.

3.3.2. Visual Model

In order to enable real-time simulation of the pilot model concepts described in Chapter 7, the Grob Tutor FLIGHTLAB model was used to drive a visual representation (Figure 52).

Figure 52. Visual representation of the Grob Tutor test aircraft [70].

The purpose of the visual representation shown in Figure 52 was to provide a visualisation of the route flown by the aircraft during the circuit of the airfield (a requirement defined in Section 7.1.2). This was a useful method for subjectively verifying that the aircraft was behaving correctly during the manoeuvres controlled by the pilot models, as undesirable behaviour was not always easily identifiable during the (offline) development phase. The scenery database was provided through use of the Visual Flight Rules (VFR) photographic database of the UK [70]. This provided the aircraft

with a suitable test environment consisting of scenery that was both textured and contained representative geographical features such as hills, coastline and sea. Elevation data from the visual database was fed back into the FLIGHTLAB model in order to allow ground interaction. This allowed the aircraft to take off and land on any areas of interest within the scenery database; a requirement for the circuit task defined in Section 7.1.2.

C h a p t e r 4

FLARE INITIATION

One of the key conclusions of the Technical Review (Chapter 2) was that the flare manoeuvre is difficult to execute, and that the consequences of incorrect execution can be very serious; for example hard landings or runway overshoots. Training literature suggests that there are two aspects of the flare that must be performed correctly to ensure a safe touchdown [5]. Firstly, the flare must be initiated at the appropriate moment; and secondly the correct amount of aft longitudinal stick must be applied. This Chapter is concerned with the former of these two aspects, and outlines an experiment which was undertaken to investigate how pilots determine the appropriate point for flare initiation.