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Helicopter tail boom vibration analysis and suppression

Funnell, M. E. (Marc Edwin) (2002-04)

Thesis (MScEng)--University of Stellenbosch, 2002.


ENGLISH ABSTRACT: An experimental investigation was conducted usmg a cantilever beam to illustrate an understanding of vibration techniques and phenomena. The [Tee and forced vibration of the cantilever beam was investigated using various modelling strategies. Theoretical models such as the Lumped parameter, Myklestad, Distributed Parameter and Finite Element Methods were analysed and compared with experimental measurements. Excellent agreement of the natural frequencies, mode shapes and the harmonic transfer functions were found. To investigate tail boom vibration, a Finite Element Model of a simplified helicopter tail boom design was tested against experimentally measured data. The results correlated accurately and the model was used to evaluate the effectiveness of a vibration suppression system designed for the tail boom. The vibration suppression system was designed using two, parallel dynamic absorbers to reduce tail boom vibrations at its two dominant frequencies. For the purposes of this thesis, the dynamic absorbers were tuned to reduce the resonant peaks at the dominant excitation frequencies of the excitation force in-line with the thrust of the tail rotor only. This excitation force was estimated using a simplified tail rotor dynamic model, which when compared with actual vibration data was assumed to predict the basic frequency trends of the force, reliably. Incorporating the designed components into an accurate Lumped Parameter model of a cantilever beam structure and simulating the response of the structure at the dominant excitation frequencies of the calculated excitation force, tested the vibration suppression system. The results of this investigation proved the effectiveness of the design procedure and optimisation process of the design parameters. A similarly designed suppression system was incorporated into the Finite Element tail boom model and from the analytical results obtained it was shown to cause a fair reduction in the vibration response at the two dominant frequencies of the excitation force. Finally, two possible applications were suggested from the research presented in this thesis. The first application was the simplified configuration of a vibration monitoring system for the tail boom. The second application was the possible development of a system using the response of vibration suppression absorbers to predict the magnitudes of the excitation forces on the tail boom.

AFRIKAANSE OPSOMMING: 'n Experimentele ondersoek is ondemeem deur gebruik te maak van 'n ingeklemde balk om vibrasie tegnieke te illustreer. Die vrye en geforseerde vibrasie van die ingeklemde balk is ondersoek deur verskillende model tegnieke te gebruik. Teoretiese modelle soos die Lumped parameter, Myklestad, Distributed Parameter en Eindige Element Metodes was geanaliseer en vergelyk met die eksperimentele waardes. Uitstekende ooreenstemmings was gevind tussen die natuurlike frekwensies, die eievektore en die harmoniese oordragsfunk:sie. Om stert vibrasie te ondersoek, was'n eindige element model van 'n helikopter stert getoets teenoor die eksperimentele data. Die resultate het akuraat gekorreleer en dus was die model gebruik om die effektiwiteit van 'n vibrasie supressie sisteem te evalueer. Die vibrasie supressie sisteem was ontwerp deur gebruik te maak van twee parallele dinamiese absorbeerders om die helikopter stert se eerste twee dominante vibrasie frekwensies te minimeer. Vir die doel van hierdie tesis was die dinamiese absorbeerders ingestel om net die resonante pieke van die opgewende krag wat in lyn is met die drykrag van die stert rotor te minimeer Hierdie opgewende krag was, deur gebruik te maak van 'n basiese stert rotor dinamiese model, aanvaar om die basiese frekwensie tendense van die krag te voorspel. Die supressie sisteem wat ontwerp was, was getoets op die lumped parameter balk en het veroorsaak dat die voorspelde opgewende krag by al twee van sy dominante . frekwensies baie geminirneer was. Die effektiwiteit van die sisteem was dus bewys en die sisteem was gebruik om die respons van die eindige element model van die stert te verbeter. Die resultate het 'n aanvaarbare reduksie in die respons by die twee dominante frekwensies van die opgewende krag, gewys. Laastens, deur gebruik te maak van die resultate in hierdie tesis. was twee moontlikhede voorgestel. Die eerste een was die konfigurasie van 'n vibrasie moniterings sisteem vir die helikopter stert. Die tweede mmontlikheid was die moontlike ontwikkeling van 'n sisteem wat die grootte van die opgewende krag by die stert rotor kon voorspel, deur gebruik te maak van die vibrasie suppressie absorbeerders.

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