j2 Rotary Process

Before any model is built there is a definition of requirements and data gathering. The requirements define the overall master objectives as to what the simulator is intended to be used for and what it is to demonstrate. These can be in the form of a series of use cases or definition of objectives and what in turn is expected of j2 with respect to delivery. The requirements can eventually get written into part of the acceptance tests.

As much information as possible is gathered about the helicopter. These include geometry and mass/mass distribution if possible and details regarding the rotors, type, airfoil section, speed, stiffness, geometry etc. Whilst not all information may be known the more data that is available the better the rotor model that can be created. Any flight manuals, maintenance or documentation describing the system are gathered along with documents detailing the engine performance.

An initial Alpha model will focus on a base weight, cg and inertia and will be used to primarily develop the rotors along with the core airframe aerodynamics. The rotor and gearbox characteristics, found through the data gathering activity, are added into HeliSIM through the HeliSIM Engineering environment to create a Blade Element Rotor Model (BERM). The BERM models the complex airflow around the rotating airfoils and accurately simulates the blade hinge and hub articulation, as well as all of the power-drive linkages. The simulation of blade malfunctions is an integral part of the BERM. Outputs from the BERM include the forces and moments of the rotors and gearbox at their relevant locations, the downwash characteristics, rotors speeds and torque requirements. At the same time, the gearbox will establish which, if any, engine is driving the rotors. This can account for engine split and rotor separation. For the Alpha model the rotors can use an “ideal” engine that means the rotors are held at a constant speed, the torque required by the rotors always being matched by the engines. This prevents situations, where the engines are not operating correctly causing variations in rotor speed from interfering with the flight characteristics.

The airframe model is built in the j2 Universal Tool-Kit using a combination of data tables entered into j2 Builder containing mass and inertia information along with global aerodynamic corrections and the integrated Aerodynamic Strip Theory (AST) built into j2 Elements. With the j2 Elements and AST it is possible to enter the local aerodynamic characteristics of the fuselage or empennage and the software automatically calculates the local velocities, including rotor downwash, and as such the local aerodynamic forces and moments on each item. These are then summated to a total at the reference location. The rotors and gearbox are integrated through the j2 Rotors interface that communicates seamlessly between the j2 modelling environment and the HeliSIM BERM.

Based upon the aircraft states, atmosphere and inputs into the helicopter, the forces and moments from the airframe are combined with those from the rotors and gearbox and totalled at the calculated CG. These along with the airframe mass and inertias and kinematics are put into the equations of motion to calculate the movement of the complete helicopter.

A series of Steady flight and gentle manoeuvring flight are used to cross check the basic control settings and general performance. These are analysed offline using j2 Freedom. Initially a series of steady state cases are evaluated to ensure that the controls (cyclic, pedals, and collective) are all following the appropriate trends and their magnitude is comparable to that provided as part of the acceptance tests.

A reversionary or backup SAS system is developed to manage the basic behaviour along with any mechanical linkages/coupling of the rotors to enable stable flight to be achieved. The SAS gains can be developed using simple response analyses looking at gentle manoeuvring flight to ensure stable flight can be achieved and the appropriate response to pilot inputs is obtained.

The SAS is developed as an external component integrated as a j2 Developer item. The SAS can be maintained within the j2 modelling environment or integrated into the overall controller to enable a smoother transition to higher order functions.

An interface is developed to integrate the FDEM with the Client Host/Manager. Once the Alpha model and interface have been built they are tested. Any variations and adjustments necessary to achieve the acceptance tests will be completed prior to acceptance.

With the steady state and gentle manoeuvre dynamics satisfied along with the basic SAS tuned in the Alpha model, the functionality is extended into the Beta model. The engine is based on Design and off Design characteristics along with dynamic responses to model the responsiveness and Power/Torque of the engine. This can then be integrated with the airframe and a Governor developed for basic control. The engine and Governor will be integrated into the model through j2 Developer. Hard points will be added to represent the skids. The skids will take individual inputs for the height of the terrain at the location of the skid such that the helicopter can be landed on a mountainside, or perched half on a building.

Additional flight test information can be used to enable the model to be developed to an acceptable level. However, this is not intended to be the final version but a “good” model that can be further tuned and refined to meet the full simulator requirements.

The Final interface is developed in parallel with the Beta model to enable the complete system to be tested. The complete system is tested against the Beta Model and Final Interface acceptance criteria and any adjustments necessary made prior to final acceptance.

Once the Beta has been completed then any final tuning activity can take place. This is performed within the j2 Universal Tool-Kit. The initial tuning on the Beta model can be to any standards. This requires compliance in several areas including:

  • Performance
    For these tests, the focus is on the engine torque and speed, helicopter attitude and Pilot Control positions. Tolerances are the same across all standards. These tests include:

    • Engine Start
    • Steady State Engine Speeds
    • Turbine Speeds
    • Engine Governor (Torque and Speed control)
    • Take-Off (Ground)
    • Take-Off Climb
    • Hover
    • Vertical Climb
    • Level Flight
    • Climb
    • Descent
    • Autorotation Entry
    • Landing
  • Handling Qualities
    The handling qualities tests include Stability Augmentation on and off.

    • Longitudinal
      • Long Term (Phugoid) Response
      • Short Term (Short Period) Response
    • Lateral
      • Roll Control response
      • Directional Control Response
      • Directional Static Stability
      • Lateral Directional Oscillation

This will require flight testing of the manoeuvres over the required points in the flight envelope, and Mass and CG range. J2 Aircraft Dynamics can support the flight testing in terms of planning and data collection to ensure the appropriate information is recorded to support the tuning. Additional flight tests may be recommended to further support the model development.

J2 can provide all training and knowledge transfer activities along with additional software licenses should for clients to develop their own in house model build and test capability.