Authors: Eng. Potito Cordisco, Senior Project Manager, Vicoter
Eng. Mauro Terraneo, Chief Technical Officer, Vicoter
Nowadays, one of the main limitations to the intensive use of helicopters over inhabited centres is the large amount of noise they emit during operations. Therefore, helicopter noise reduction is a topic of research into designing helicopters which can be operated more quietly, reducing the public-relations problems with night-flying or near a heliport.
An ad hoc retrofit, named “Silent Eco Wing” (http://www.silentecowing.com/), has been designed for such purpose. It allows, after the installation at the tip blade, to optimize the behaviour of boundary vortexes, reducing the emitted noise and improving the fuel consumption. To experimentally evaluate the benefits of this device, Heli Austria (https://www.heli-austria.at/en/index.html) has started a flight test campaign installing this retrofit on one of the AS350-Ecureuil of its fleet.
To be compliant with the paragraph ‘CS 27/29.251 – Vibrations requirements’, requiring the evaluation and possible mitigation of the risk of damaging primary structures and systems by vibration, TPS Group (www.tps-group.it), a DOA specialized in creating and producing modifications for small and large airplane and helicopters, entrusted Vicoter to address vibration conformity by the execution of Resonance Assessment Profile (RAP) tests.
Vicoter (www.vicoter.it) is an Italian company that has been operating for 15 years in the sector of experimental measurements of vibrations on aircrafts, performing on-ground bonk (hammer) tests and/or in-flight acceleration acquisitions. 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.
To verify that the impact on the dynamic behaviour of the blade due to the addition of the “Silent Eco Wing” was negligible, experimental modal analyses were carried out on ground. Two configurations were analysed. The first analyses was performed on the classical blade, while the second one on blade with the “Silent Eco Wing” equipped at the tip (see Figure 2).The goal of the tests was to measure the modes, with and without the new device, and to compare the FRFs (Frequency Response Functions) and the modes of the blade, in terms of frequencies, modal shapes and generalized mass.
A single blade was tested mounted on the main rotor of the helicopter, as shown in Figure 3, while the helicopter was grounded on its landing gear.
To obtain the modes of the structure, a MIMO test in impact roving modality was carried out. The roving technique is based on the reciprocity Maxwell’s theorem stating that for a LTI (Linear and Time Invariant) system a FRF does not change forcing in a point ‘i’ and reading in a point ‘j’ or forcing in ‘j’ and reading in ‘i’. Therefore, it is possible to acquire the FRFs needed for the modal identification installing only a limited number of accelerometers and impacting, from time to time, in all the selected wireframe points. In this method, the accelerometric signals becomes the input and the load cell ones, the output. Two accelerometers were used as reference to reduce possible biases related to the installation of the sensors in nodal points of the modes. Wireframe was selected to obtain information regarding the first out-of-plane bending and torsional modes. Eleven locations have been chosen.
Modal identification was performed by the state-of-art Polymax™ algorithm.
Dynamic analyses of the blade recovered twenty modes and up to 150 Hz, showing that no significant variations of the modal frequencies and shapes of the blade was introduced by the new device, as shown in Figure 4. In such way, safe installation of the “Silent Eco Wing” was demonstrated.
Vicoter wishes this innovation a great success and seizes the opportunity to warmly thank all the staff of TPS and Heli Austria for their support during the execution of the experimental activity.
