Vibration Checks for Safe Helos

By the early 1970s, several vibration analysis and basic monitoring systems were available, but all had shortcomings. In particular, they were difficult, if not impossible, to use on a helicopter in flight, while their analogue design meant that analysis of problems was lengthy, limited, often subjective and provided no trend data for future consideration.

Today, the causes of helicopter vibration are much better understood and more easily corrected as a result of new methods of recording and analysing the performance of the dynamic components.

Several companies have played a major role in bringing about these changes. One of the leading firms, Helitune of Fleet in southern England, was started in 1978 by Noel Trigg who ended a long career in the RAF as Chief Helicopter Engineer.

Trigg’s experience with helicopter vibration led him to develop a comprehensive yet fully portable vibration analysis system now marketed as the Helitune Rotortuner. It is based on a portable data processor unit, linked to an infra-red camera, that monitors blade track and a number of accelerometers mounted inside the cabin to measure vertical and lateral balance and in other areas as necessary.

Magnetic pick-ups on the main rotor shaft provide the system’s heartbeat, a one-per-revolution pulse detecting any rotor mass imbalance and giving a vibration signature, as well as driving the camera. The latter, patented by Helitune, can measure the track of an individual rotor blade to within 1. 6mm, so long as it is pointed out by the operator to within 1o degrees of the tip. The infrared line-scan uses the centre of gravity of the passing blade as the datum, comparing any up or down variations against the mean path.

By programming the Rotortuner with information such as the number of rotor blades, gear ratios, number of teeth on each gear and so on, the system is able to pinpoint vibration sources and analyse engine and gearbox performance. Known characteristics of the dynamics system of a particular helicopter type can be built into the analysis, enabling the Rotortuner to identify acceptable limits and inform the operator accordingly.

All data, whether collected in ground running. or in flight, is received by the compact computer, analysed and displayed on a knee pad console. After each test the computer generates hard copies with corrective instructions for the ground engineers. In addition, the data is stored on an individual cassette for that particular aircraft, enabling an historical file to be built up for trend reference.

To extract maximum benefit from such trending data, Helitune also invites each Rotortuner operator to submit the tapes for central analysis, the results then being shared among all contributing operators.

The benefits of the Rotortuner are perhaps obvious. The rapid identification of vibration sources and the ease with which the operation can be carried out by relatively inexperienced operators – in the field if necessary- leads to less maintenance, less premature component removals and longer lives for the various components.

As a result the safety factor is markedly increased, while direct operating costs are reduced. All these benefits are already being demonstrated by current Rotortuner operators, including the Royal Air Force, British International Helicopters, the Italian Army, the Canadian Armed Forces and the United States Coast Guard.

Several of the major helicopter manufacturers also now use and recommend the system. In France, Helitune has signed agreements with SEMIA to market the Rotortuner locally and also to Aerospatiale customers worldwide. Ansofone Elettronica in Italy is to produce up to 200 of the latest Dash 5 systems under licence.

To date more than 40 helicopter types have been configured to the Rotortuner. Data tapes readily available for use range from such giants as the Boeing Chinook and Sikorsky CH-53E to the smaller Aerospatiale Alouette and the Bell AH-1 Cobra. Meanwhile, Helitune is developing a permanent on-board version of the system for application to new generation helicopters.

Meanwhile, the United Kingdom’s Civil Aviation Authority has been watching the development of Rotortuner with more than average interest, seeing it as one answer to current helicopter safety and pilot fatigue problems. New UK airworthiness requirements making such monitoring mandatory may well bejust round the corner.