On-ground and in-flight vibratory tests of an AS350 helicopter rear-view mirror

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Vicoter (www.vicoter.it) is an Italian company that for more than ten years has been operating in various sectors (aerospace, automotive, energy) for both static and dynamic measurement of mechanical structures. Company’s main expertise concerns the execution of modal analyses and the detection of operational vibrations, which are carried out with state-of-the-art methodologies and tools.
To get the flight certification of new equipment to be installed on a helicopter, the aeronautical standards require to experimentally verify that no natural frequency falls within a band of “± 5% of 1 / rev” and “number blades / rev”, considering both the main rotor and the tail rotor. In addition to this, it is also necessary to provide a measurement of the acceleration levels on the equipment in various flight conditions.
As part of a multi-year cooperation with Boggi Aeronautics (http://www.boggiaeronautics.eu), Vicoter carried out all the necessary measurements on an adjustable rear-view mirror to be mounted on an AS350 helicopter. The aircraft was supplied and set up by Elifriulia (www.elifriulia.it), while the mirror was tested at the Ronchi dei Legionari airport (GO), Italy.

Helicopter prepared for the flight test.

With a well-established methodology, Vicoter first carried out the modal analysis of the mirror on the ground, and then moved on to flight tests. Ten triaxial accelerometers were placed on the structure so that various modal shapes could be reconstructed. Particular attention was paid to the survey of the participation of the three attachment points to the helicopter for the future correlation of the finite element model. The transfer functions (FRF) were acquired using an instrumented hammer for excitation and impacting in various points and directions. The experimental modes were then identified by means of a MIMO (Multi Input Multi Output) technique to obtain the best reliability of the obtained values.

Modal analysis instrumentation setup (left) and identified up-down mode (right).

The second phase of the certification procedure concerned the measurement of accelerations in different flight phases: take-off, level flight at various speeds, turn, dive. The number of sensors has been reduced to 5, to allow the use of on-board instrumentation. An additional accelerometer was placed in the cabin on the pilot’s seat, to verify the absence of abnormal vibrations introduced by the equipment under test. The frequency colormaps of the various accelerometers were created to study the vibrational behaviour of the rear-view mirror and identify the importance of the various phenomena.
Both tests were performed successfully.

Example of colormap of a sensor installed on the rearview mirror.