During the flight operations with an helicopter is necessary to have a certain degree of confort inside the cockpit; this is not possible if the vibrations generated from the rotor are too wide. The rotor, indeed, generates vibrations that are transmitted to the fuselage. The rotor works as a filter and transmit only harmonics at some frequencies multiple of the frequency at which the rotor rotates. These vibrations could be damped in three different ways: directly on the fuselage, insulating the fuselage from the rotor or directly on the rotor. This last chance is the one that will be enterprised in this thesis. Previously some passive vibration absorbers have been developed, but this kind of devices have the great problem that they usually damp harmonics only in a certain range of frequencies. This fact has been accepted for a long time because the angular velocity, from which the transmitted vibrations depend, was fixed, but now the situation is changing and some helicopters with varying rotor velocity have been developed. So presently it is required to create an active vibration absorber that can follow the variations of speed of the rotor and damp harmonics at different frequencies. In this thesis this matter has been faced: in the first part some solutions, the most different from previous solutions as possible, are exposed and in the second part a device, that seems to have better developments and that can solve definitively the problem, has been designed. This device generates forces able to damp the vibrations making use of the rotation of masses and the centrifugal acceleration generated by them. The power required to reach this target is taken directly from the main rotor using a transmission; some motors are necessary to regulate the position of the masses and the amplitude of the force generated.
Durante le operazioni di volo su un elicottero ́e necessario un certo grado di comfort e poter comunicare all’interno dell’abitacolo; questo non è possibile qualora le vibrazioni generate dal rotore siano eccessive. Il rotore, infatti, crea delle vibrazioni, che vengono poi trasmesse alla fusoliera. Il rotore agisce, però, da filtro e trasmette solo armoniche a determinate frequenze multiple della frequenza a cui esso ruota. Tali vibrazioni possono essere smorzate in tre modi differenti: direttamente sulla fusoliera, isolando la fusoliera dal rotore o direttamente sul rotore. Quest’ultima strada è quella che viene seguita in questa tesi. In precedenza sono stati sviluppati svariati smorzatori passivi, ma questi presentano il grosso limite che generalmente riescono a smorzare armoniche solo in un determinato range di frequenze. Questo fatto è stato tollerabile fino al giorno d’oggi essendo la velocità di rotazione del rotore, dalla quale dipendono le vibrazioni trasmesse, generalmente fissa, ma attualmente si tende ad avere rotori con velocità di rotazione variabile. Da ciò nasce l’esigenza di avere un dispositivo attivo in grado di seguire le variazioni di tale velocità e smorzare armoniche in un range di frequenze più ampio. In questa tesi viene affrontato questo problema: nella prima parte vengono proposte soluzioni che si distacchino dallo stato dell’arte attuale e nella seconda parte si procede alla progettazione di un dispositivo che più degli altri è sembrato poter avere sviluppi e risolvere definitivamente il problema. Tale dispositivo genera forze in grado di smorzare le vibrazioni mettendo in rotazione delle masse e sfruttando la forza centrifuga da esse generata. La potenza necessaria per raggiungere questo scopo viene ottenuta direttamente dal rotore attraverso una trasmissione; sono comunque necessari alcuni motori per regolare la posizione delle masse e il modulo della forza da esse generata.
Sviluppo di un sistema attivo per la riduzione delle vibrazioni associate al rotore principale di un elicottero
LAFFRANCHI, ALESSANDRO;MISSAGLIA, ANDREA
2015/2016
Abstract
During the flight operations with an helicopter is necessary to have a certain degree of confort inside the cockpit; this is not possible if the vibrations generated from the rotor are too wide. The rotor, indeed, generates vibrations that are transmitted to the fuselage. The rotor works as a filter and transmit only harmonics at some frequencies multiple of the frequency at which the rotor rotates. These vibrations could be damped in three different ways: directly on the fuselage, insulating the fuselage from the rotor or directly on the rotor. This last chance is the one that will be enterprised in this thesis. Previously some passive vibration absorbers have been developed, but this kind of devices have the great problem that they usually damp harmonics only in a certain range of frequencies. This fact has been accepted for a long time because the angular velocity, from which the transmitted vibrations depend, was fixed, but now the situation is changing and some helicopters with varying rotor velocity have been developed. So presently it is required to create an active vibration absorber that can follow the variations of speed of the rotor and damp harmonics at different frequencies. In this thesis this matter has been faced: in the first part some solutions, the most different from previous solutions as possible, are exposed and in the second part a device, that seems to have better developments and that can solve definitively the problem, has been designed. This device generates forces able to damp the vibrations making use of the rotation of masses and the centrifugal acceleration generated by them. The power required to reach this target is taken directly from the main rotor using a transmission; some motors are necessary to regulate the position of the masses and the amplitude of the force generated.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/131307