Rotary Wing Design
Hover and Autorotation From the Very Beginning
Helicopters have additional complications over fixed wing aircraft due to the additional degrees of freedom presented by the rotors. In order to accurately model the rotors, a Blade Element Rotor Model (BERM) is required. This is already built into the j2 Universal Tool-Kit through the j2 Rotary plug-in. As soon as the rotor characteristics have been entered it's possible to start analysing the Helicopter. This removes the need to start coding up rotor dynamics or to work with momentum theory and provides the ability to hover and autorotate straight out of the box. Using the j2 Universal Tool-Kit no coding is necessary, flight dynamics and simulation can start from the very beginning of a project providing the ability to investigate stability and control matters as well as pilot in the loop simulations instantly. As the aircraft design evolves so the data is updated and a new set of results generated. Everything tracked and traced with integrated version control.
The hierarchical structure for developing the models that is present in j2 Builder enables the aircraft and proposed systems to be laid out in the model as in the real aircraft. As the aircraft evolves so whole sub-assemblies can be relocated quickly and easily and the software will take care of all changes in locations and moment arms to recalculate contributions. The Main Rotor, Tail Rotor, and Gearbox are added as a rotor assembly using j2 Rotary. Engine characteristics are added through Drive Shafts and can start with ideal (torque matching) engines and developed later to cover higher fidelity engine performance characteristics.
The airframe can be kept separately from the rotors and it too can be built in the hierarchical structure. With limited information relating to fuselage, horizontal, and vertical tail geometry and location, the complete aircraft model can be developed in j2 Elements. This provides detailed dynamic characteristics based upon the local flow at any point on the wing. These items can include the downwash and sidewash characteristics from the rotors. Legacy tools and existing data frameworks can easily be added into the j2 model framework using j2 Developer.
Preliminary Handling through to Certification
From initial investigations into the stability of the helicopter across the complete flight envelope, identifying the basic pilot control settings through trim analysis in j2 Freedom, dynamic manoeuvres can be analysed using response analysis. These can start with initial disturbances from the trimmed condition to establish basic stability through to more test based manoeuvres and controls inputs such as 3211’s and doublets. The primary Stability Augmentation System (SAS) can be developed and tested with the Washout Filters, PID Controllers and Rate Limiters built into j2 Builder. More complex control laws and logic can be developed through the j2 Matlab Toolbox. This enables the complete helicopter model to be dropped into a Simulink model as a completely self contained block. The AFCS is then wrapped around the Airframe and the Simulink model file is integrated back into the j2 Universal Tool-Kit as a Matlab Aircraft Model. This now provides a consistent environment to develop and test the rotors, the airframe, and the AFCS simultaneously in a version controlled and configuration managed system.
From the very beginning of the project, as soon as an aircraft model has been developed and saved it can be flown real-time with a pilot in the loop using the j2 Pilot plug-in. This enables the test pilot to provide early opinions of the design and handling qualities, leading to an improved design. Pilots can test and evaluate failure modes and envelope excursions without risk and can feedback information to the design team.
As the aircraft develops, so the aircraft certification manoeuvres can be tested and evaluated across the complete flight regime. These can be tested ahead of flight test to establish a safe test envelope. Results can be brought back into the j2 Universal Tool-Kit with j2 Flight and the model can be automatically tuned. The separation of the rotors from the aerodynamic characteristics means that corrections are not being made in the wrong area and leads to improved tuning.
From the initial tests, the model can be reviewed and updated. This provides improved confidence in the model and enables the next set of manoeuvres to be tested. As the flight test program expands so the model becomes more accurate. This means that the model can be used to demonstrate stability and control for certification in support of validation and verification flights.