ENGINE AND ROTOR GOVERNING
The engine and rotor governing system of a helicopter is designed to regulate the flow of fuel to the engine(s) to provide the pilot with the correct amount of power at a suitable rotor speed for the demands being made with the flight controls. In addition the system is required to provide control over the starting and stability of the combustion process. The aim of the test programme therefore, is to assess the operation of the governing system over as wide a range as possible of operationally relevant conditions. Since the main focus of this book is flight testing, the qualification testing of gas turbine powerplants [7.10] will not be covered in any detail. The results from flight tests are used to make recommendations for improvements to the system as well as defining system limitations. These limitations include restrictions on the rate of conducting power changes, residual engine temperature limits for starting, and altitude and airspeed limits for engine re-lights whilst airborne.
7.4.1 Trials planning
To conduct a full engine and governing system assessment a comprehensive instrumentation package is fitted to the aircraft. The parameters recorded typically include:
• the state of the aircraft (airspeed, altitude, OAT, RoC/RoD, sideslip);
• the position of relevant aircraft controls (collective lever, SSL or ‘beep’ control, throttle);
• the state of the engine(s) (compressor speed, power turbine speed, engine temperatures, engine pressures, rotor speed, positions of any anti-surge devices).
For trials involving digital engine control systems the inputs to the digital engine control unit (DECU) and the outputs from it are recorded. For more limited trials where it is not practical to fit a full instrumentation suite, it is possible to obtain useful data by using a video camera trained on the instrument panel. As part of the trials planning process the environmental conditions required and how they will be achieved need to be decided upon. For example, it is necessary to conduct engine starting tests with the aircraft both hot – and cold-soaked. This can either be achieved by placing the aircraft in an environmental chamber or by conducting the tests during cold weather and/or hot and high trials in suitable locations.
Before the trial begins the aircraft is checked to make sure that the engine(s) and the rotor are set-up in accordance with the manufacturer’s instructions. This may include such tests as determining the engine power output and FIG or autorotative descents to establish that the rotor has been correctly rigged. As the trial progresses these settings are checked periodically to make sure that engine and rotor have not shifted excessively from these baseline settings. Usually the engine(s) are bench tested and calibrated both prior to the trial and post-trial.
7.4.2 Cockpit assessment
The controls and indicators associated with the engine(s) and rotor governing system are assessed in the same way as any cockpit assessment. Particular emphasis is placed on the sensible grouping of items so that tasks such as engine and rotor starting or stopping are easy to accomplish. For example, it is common to find that the start switch is remote from the igniter indication which is also remote from the engine temperature gauge; this can make it difficult for the pilot to monitor the start adequately and may lead to late intervention during a hot start. Emergencies that require the use of the engine(s) and rotor controls are also studied to determine if all the controls can be reached and operated easily. For these evaluations the normal and emergency drills contained in the aircraft checklists are commonly used.