The full non-linear model of the BO105 was created using FLIGHTLAB. The reference data used to develop the model are shown in Appendix A. FLIGHTLAB is a commer- cial modelling software that can be used for the construction and application of vehicle simulation models [106]. Developed by Advanced Rotorcraft Technology (ART), the modelling software has been tailored towards the helicopter market. Models are devel- oped using a library of physically based modelling components. Each component is an independent dynamic element such as a spring, a damper, etc. [106]. The general vehi-
cle model is constructed by interconnecting these components to create one structure. Each component can be tailored to include as much (or as little) ‘real world’ effects as is desired for the simulation model.
Components are included within a library and can be selected and combined for different types of aircraft model. These include aerodynamics, control, structure and propulsion [106]. Within the components, aircraft specific data can be added in order to create accurate simulation models of ‘real’ rotorcraft. An advantage to the component method during model development is the fact that the fidelity of each component can be modified depending on the available data. In this way, models can be developed and ‘fine-tuned’ without the need to ‘rebuild’ simulation models.
When building a simulation model, FLIGHTLAB Model Editor (FLME) can be used in order to tailor the individual components. FLME is structured as a tree, where the user can select which components they wish to include in their simulation model. FLME is also used to position components and to define the control systems.
The BO105 model generated consists of 44 states; 18 translational and rotational body states, four propulsion states and 22 rotor states, incorporating flap and lead-lag rotation for each individual rotor blade. The model uses a Peters-He Six State Inflow model. Real-time computation of rotor forces and moments is achieved through the use of dynamic look-up tables. These tables incorporate additional elements of realism, such as rotor stall. No rotor interference is included in the model in its current form. The tail rotor was modelled as a Bailey type rotor. Aerodynamic surfaces include non-linear effects, and stall.
Main Rotor and Tail Rotor
The BO105 features a hingeless rotor. In order to model the response of a hingless rotor to pilot control, one must build a full elastic model of the vehicle’s blades. This presents two problems: the real-time simulation of such a blade is computationally expensive, which would lead to a low frame rate; and unrealistic simulated experience and there is a requirement for detailed data of the blade properties. For these reasons, for use in real-time simulation, the rotor was modelled using the equivalent hinge offset approach, which is described in detail in Ref. [111]. The hingeless rotor allows pilot control of the vehicle through flexing of rotor blades, attached ‘rigidly’ to the rotorcraft hub. Low mechanical complexity leads to fast rotor response, which leads to a more agile and manoeuvrable helicopter. As a result, the rotor-type features generally improved HQs [112].
For low frequency investigations, such as ones conducted in this study, the hingeless rotor can be modelled as an equivalent articulated rotor system, using the centre- spring equivalent rotor approach [111]. An equivalent hub offset is introduced, to replicate the forces at the hub of a hingeless type. This assumes that the blade has rigid segments beyond the equivalent offset. The implementation of an elastic rotor blade was not necessary for two reasons. The first was that there was no requirement in this investigation to observe the high-frequency (> 1.5 Hz) response of the aircraft. The second was that there was a requirement only for an aircraft akin to a BO105. Therefore, if the characteristics were shown to be similar to the actual aircraft, the model could be considered representative of a small, agile helicopter.
Control System
The control system was modelled using FLIGHTLAB Control System Graphical Ed- itor (CSGE). The system uses a block diagram structure to enable elements of the vehicle control systems to be modelled. As the study was based around the response of a simple, un-augmented helicopter model, the control system employed was simple, featuring a direct link between pilot input and vehicle swashplate pitch. As mentioned above, the BO105 aircraft features a hingeless rotor system. This rotor type make the aircraft very responsive to control inputs, allowing for high agility during flight. As swashplate responds to pilot demands faster than a conventional articulated rotor, the phase distortion between pilot input and vehicle output is lower. This results in lower RPC susceptibility than for an equivalent articulated rotor.
For this reason, in order to engineer RPC susceptibility, Rate Limiting Elements (RLE) and transport delays were added within the forward control system. The RLE is positioned after any control system gain and any saturation elements. These represent triggers that could occur within the system actuator, prior to the vehicle swashplate. Figure 3.2 displays the elements of the control system between the pilot input and vehicle swashplate.