Compliance with the paragraph ‘CS 27/29.251 – Vibrations requirements’ has the objective of preventing the risk of excessive vibration and to mitigate the risk of damaging primary structures and systems. Conformity must be addressed at Rotorcraft level and at Assembly/Part level (regardless its criticality) and it can be demonstrated by:
- Vibration identification by Resonance Assessment Profile (RAP) testing.
- Flight tests as needed.
Vicoter (www.vicoter.it) is an Italian company that has been operating for 15 years in the sector of experimental measurements of vibrations on helicopters, performing on-ground Impact (Hammer) tests and in-flight acceleration acquisitions both. Working under the administrative supervision of a DOA, Vicoter carries out the technical activity in compliance with the higher standards using certified and cutting-edge instrumentation and methodologies.
Due to their skills, Vicoter’s engineers were called by TPS Group (www.tps-group.it), a DOA specialized in creating and producing modifications for small and large airplane and small and large helicopters, to measure the acceleration transmissibility of two STCs on an AS332-Super Puma and on a MD902-Explorer. Modification consists for both cases in the substitution of the original Ni-Cd main battery with a new Li-ion one. EASA requires in such circumstance the verification of the absence of modes of the really installed changed part concurrent with the frequencies excited by the main and tail rotors. Apart from this, a quantitative evaluation of the possible impact on the whole dynamic behavior of the aircraft is mandatory.
To satisfy the given requirements, Vicoter installed some sensors on the main battery, in its vane and on the structure around it, performing the prescribed RAP tests. They were realized in two configurations, with the original battery and with the new one, to verify by FRFs (Frequency Response Functions) comparison the entity of the dynamic change introduced by the battery substitution. Agreement between the two data sets at low frequency guarantees that the vibratory behaviour of the helicopter is not influenced by this modification. 9 triaxial accelerometers were installed on AS332 and 5 on MD902. All the channels were acquired simultaneously together with the instrumented hammer excitation, given in various locations and directions to realize a MIMO (Multi Input Multi Output) data set without biases due to possible nodes of the modes. Battery resonance frequencies were identified by the state-of-art Polymax™ algorithm.
Vicoter warmly thanks all the staff of TPS for the highly qualified suggestions given during the test and all the staff of Heli Austria (www.heli-austria.at) for the technical support.
